Category: Embryology

Week 7 of development

◦CONNECTION BETWEEN THE GUT AND YOLK SAC consists of only the small yolk stalk

• Umbilical herniation occurs; intestines during rotation of the gut enter the extraembryonic coelom in the proximal portion of the umbilical cord

◦THE LIMBS change markedly during this week

• The forelimbs project over the heart
• Notches are seen between the rays in the hand plates indicating future fingers

Week 8 of development

This is the final week of the embryonic period

◦THE FINGERS are noticeably webbed and short

◦NOTCHES now are seen between the toe rays, and the tail bud is still visible

◦THE LIMB REGIONS are clear, fingers lengthen, toes are distinct, and the tail bud disappears by the end of this week

◦THE EMBRYO has human characteristics, but the head is distinctly large (about one-half of embryo)

• The neck region is established, and the eyelids are obvious
• The abdomen is less bulging, and the umbilical cord is reduced in size
• The intestine is still within the proximal portion of the umbilical cord

◦THE EYES usually open, but near the end of week 8 the eyelids begin to meet and fuse

◦THE EXTERNAL EARS (AURICLES) assume their final shape but are still low set

◦THE EXTERNAL GENITALIA are not distinct enough for accurate sexual identification

External Embryo Appearance

◦BY THE END OF WEEK 4 the embryo has about 28 somites, the ventral body wall has closed, and the major external features are somites and pharyngeal arches. The age of the embryo now is often expressed in somites or as the crown-rump (CR) length (sitting height) in millimetres. The standing height or crown-heel (CH) length is sometimes used for 8-week and older specimens, but these are hard to make accurately

• The CR length is measured from the skull vertex to the midpoint between the apices of the buttocks. Variations in flexion result only in approximate real age measurements
  • 5 weeks: 5-8 mm; 6 weeks: l0-14 mm; 7 weeks: l7-22 mm; 8 weeks: 28-30 mm

◦THE EXTERNAL APPEARANCE during month 2 changes due to the great size of the head and formation of the limbs, ears, nose, and eyes. By the beginning of week 5, the fore- and hind-limbs appear as paddle-shaped buds

◦AGE ESTIMATION usually relies on 2 commonly used references, namely, the onset of the last menstrual period (LMP) and the time of fertilization

• Since the zygote does not form until the second week after the onset of the last normal menstrual period, 14 ? 2 days are deducted from the so-called menstrual age to get the actual or fertilization age of the embryo
• Foot-length correlates with CR length and is used in aging an incomplete or macerated foetus
• Foetal weight is not too accurate for use in age determination in light of any maternal metabolic disturbance

During this relatively short embryonic period (weeks 4 to 8), one sees the beginnings of all major internal and external structural (organ and organ systems) develop during which time the 3 germ layers give rise to specific tissues and organs – the period of organogenesis. The shape of the embryo changes, and major features of the external body form (morphogenesis) become recognizable by the end of month 2. In addition, major congenital malformations can occur due to exposure of the embryo to teratogens during this developmental period.

Week 4 of development

◦ABOUT DAYS 22 TO 23: embryo is almost straight or slightly curved, and somites create conspicuous surface elevations. The neural tube is closed opposite the somites but is open at its caudal and rostral neuropores

◦ABOUT DAY 24: the first (mandibular) and second (hyoid) branchial arches become distinct

• Most of the mandibular process of the first arch gives rise to the lower jaw and a rostral extension of it; the maxillary process helps form the upper jaw
• Head- and tailfolds cause a slight curvature of the embryo
• The heart produces a large ventral prominence

◦ABOUT DAY 26: 3 pairs of branchial arches are seen and the rostral neuropore closes

• The forebrain creates a distinct elevation on the head and, with longitudinal folding, the embryo now has a distinct C-shaped curvature to it
• Transverse folding causes a narrowing of the connection between yolk sac and embryo
• The arm buds are now recognizable as small swellings on the body wall’s ventral surface
• The otic pits, the primordia for the inner ears, are clearly seen

◦ABOUT DAY 28 (END OF WEEK 4): the fourth pair of branchial arches and the leg buds are seen

• The lens placodes (ectodermal thickenings) represent the future lenses on the side of the head

Week 5 of development

There are fewer body form changes this week

◦HEAD GROWTH is accelerated as a result of rapid brain development

• The face contacts the heart prominence
• The second (hyoid) branchial arch overgrows arches 3 and 4 to form an ectodermal depression, the cervical sinus
• The forelimbs show some regional differentiation as the hand plates develop

Week 6 of development

1. LIMB BUDS, especially the forelimbs, show regional differentiation. Hind limbs develop later

• Elbow and wrist areas are identifiable
• Paddle-shaped hand plates develop digital ridges, the finger rays, for future fingers

2. SOME SMALL SWELLINGS appear around the groove between the first branchial arches. The groove becomes the external auditory meatus, the swellings the ear auricle

3. THE EYE becomes obvious due to appearance of retinal pigment

4. THE HEAD appears larger relative to the trunk and bends even farther over the heart prominence

• Bending is due to the cervical flexure as a result of bending of the brain in the cervical region
• The trunk and neck begin to straighten out

5. SOMITES are visible in the lumbosacral region by the middle of this week

Trilaminar Germ Disk Embryo Formation and Gastrulation

week 3 is a period of rapid development of the conceptus coinciding with the first missed menstrual period.

By days 15-16, the embryo is 1.5 mm long, and one clearly sees the primitive streak, Hensen’s node, and the notochordal process- All morphologic indications characteristic of gastrulation. The latter is the formation of the third embryonic layer, the mesoderm

The term gastrulation means the formation of gut (Greek, gastrula = belly), but has now a more broad sense to to describe the formation of the trilaminar embryo. The epiblast layer, consisting of totipotential cells, derives all 3 embryo layers: ectoderm, mesoderm and endoderm. The primitive streak is the visible feature which represents the site of cell migration to form the additional layers.

primitive node – region in the middle of the early embryonic disc epiblast from which the primitive streak extends caudally (tail)

◦nodal cilia establish the embryo left/right axis

◦axial process extends from the nodal epiblast

—primitive streak – region of cell migration from the epiblast layer forming sequentially the two germ cell layers (endoderm and mesoderm)

Epithelial to Mesenchymal Transition

Epithelial cells (organised cellular layer) which loose their organisation and migrate/proliferate as a mesenchymal cells (disorganised cellular layers) are said to have undergone an Epithelial Mesenchymal Transition (EMT).

Mesenchymal cells have an embryonic connective tissue-like cellular arrangement, that have undergone this process may at a later time and under specific signalling conditions undergo the opposite process, mesenchyme to epithelia. In development, this process can be repeated several times during tissue differentiation.

This process occurs at the primitive streak where epiblast cells undergo an epithelial to mesenchymal transition in order to delaminate and migrate.

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– Period: The third week (week 3) following fertilization or clinical gestational age GA week 5, based on last menstrual period.

Note that during this time the conceptus cells not contributing to the embryo are contributing to placental membranes and the early placenta.

During the third week of development conceptus implantion in the uterus wall is complete and trophoblast cells continue to invade uterine wall in the process of early placentation (villi formation). Within the conceptus, gastrulation converts the bilaminar embryo into the trilaminar embryo (ectoderm, mesoderm, endoderm). Morphological changes include an epithelial to mesenchymal cell transition and folding of the embryonic disc.

Outside of the embryo, the extraembryonic spaces (chorionic, amniotic, yolk) and intraembryonic spaces (coeloms) continue to develop. Trophoblast cells continue to invade uterine wall and the emdometrium is being converted into the decidua, the process of villi formation and early placentation has begun.

Week 3 of Development: The Notochord, Neural Tube, and Allantois

Formation of the 3 germ layers occurred in a cephalocaudal direction, this means that the 3 germ layer are establish first in the head region then the tail region. Also tissues and organs develop in a cephalocaudal direction.

Notochord development:  in the human, the notochord is a cellular rod that develops from the prochordal process and forms the first longitudinal midline axis around which the vertebral bodies are organized and is the basis for the axial skeleton. It will later regress.

By day 12 or 13, the notochord is visible throughout the length of the embryo and around it are layered concentrations of cells, representing the primordia of the future vertebral bodies.

Notochord Development

1. STAGE OF THE NOTOCHORDAL PROCESS (entire area cephalic to the primitive streak):  about day 17

• The floor of the notochordal process fuses with the underlying endoderm as it undergoes preferential growth. Hensen’s node seems to recede toward the caudal end.

2. PROCHORDAL STAGE: about day 19

• Degeneration of the fused region takes place, and openings appear in the floor of the notochordal process (resorption of the floor), opening a communication between the yolk sac and the notochordal canal, a lumen which is formed as the primitive pit extends into the notochordal process during its development

The openings become confluent, and the floor of the notochordal canal disappears. A small passage, the neurenteric canal, temporarily connects the yolk sac and the amniotic cavity

3. NOTOCHORD STAGE: about day 20

• The notochord process remains and forms a grooved, flattened plate, the notochordal plate, which, beginning at its cranial end, infolds to form the notochord. The embryonic endoderm again forms a continuous layer below the notochord The latter is thus the primary skeleton of the 3-layer embryo.

Neural tube development (neurulation)

1. THE EMBRYONIC ECTODERM over the developing notochord thickens to form a neural plate (about day 18) which apparently is enduced by the developing notochord and paraxial mesoderm on either side

• The plate first appears cranial to the primitive knot and dorsal to the notochordal process with mesoderm adjacent to it
• With the elongation of the notochordal process, the neural plate broadens and extends cranially to the oropharyngeal membrane
• The ectoderm of the plate is called neuroectoderm and eventually gives rise to the central nervous system (brain and spinal cord)

2. THE NEURAL PLATE, on about day 20, invaginates along its central axis to form the neural groove with neural folds created on each side of the groove

• By the end of week 3, the neural folds move together, fuse, and convert the neural plate into the neural tube. Closure begins in the middle of the embryo and progresses toward both cephalic and caudal ends. It begins on day 21
  • Closure of the neural groove is more rapid toward the cephalic (anterior) end than toward the caudal (posterior) end
    • The anterior or cranial neuropore closes in week 4 (day 26), whereas the posterior or caudal neuropore closes near day 28
  • Neuroectodermal cells at the lateral edge of the neural plate -do not become part of the tube but form a neural crest over the neural tube and give rise to the neural crest cells

The allantois

appears on day 16 as a small, finger like outpouching or diverticulum from the caudal wall of the yolk sac. It remains small in the human embryo, is involved with early blood formation, and is related to the development of the urinary bladder.

Functions of The Notochord

1.It forms the basis of the axial skeleton (bones of the head and vertebral column).

2. It induces the overlying ectoderm to thicken and form the neural plate; the primordium  of the central nervous system (Notochord is the organizer for nervous system formation) .

3. The notochord degenerates and disappears as the bodies of the vertebrae form. Its remnant is the nucleus pulposus of the intervertebral discs.

4. It functions as the primary inducer in the early embryo i.e. it is a prime mover in a series of signal-calling episodes that ultimately transform unspecialized embryonic cells into definitive adult tissues and organs

Clinical Application

Third week of development is a very sensitive period in foetal development. Many factors such as drugs, alcohol or irradiation to the mother may cause congenital anomalies to her embryo.

Week 3 of Development: Intraembryonic Mesoderm, Somite Development, and The Intraembryonic Coelom

Intraembryonic Mesoderm

The cells of the primitive streak& notochordal process proliferate—->2ry mesoderm —->migrate laterally & cranially between ectoderm& endoderm except oral membrane ( cranially) & cloacal membrane  (caudally).

The embryo now is called gastrula.

AS THE NOTOCHORD AND NEURAL TUBE FORM, the intraembryonic mesoderm on each side forms longitudinal columns, the paraxial mesoderm, each in turn being continuous laterally with the intermediate mesoderm, and the latter gradually thinning out further laterally into the lateral mesoderm.

Paraxial Mesoderm and Somite Formation

somite development begins about day 20 and is the result of segmentation of the paraxial mesoderm

A. The paraxial mesoderm thickens and fragments metamerically, dividing into paired cuboid bodies called somites which give rise to most of the axial skeleton and associated musculature as well as much of the dermis of the skin

B. The first pair of somites develops just caudal to the cranial end of the notochord (future occipital area), and subsequent pairs form in a craniocaudal sequence after the appearance of the first somites

C. About 38 somite pairs form during days 20-30, the so-called somite period. Eventually about 42-44 somite pairs develop by the end of week 5

1. The somites form distinct surface elevations and are triangular in shape when seen in a transverse section
2. Each somite develops a slitlike cavity, the myocele, which eventually is occluded
3. Somites give origin to the sclerotome, whose cells condense around the notochord and give rise to the vertebral primordia and the myotome, which gives rise to the vertebral muscles
   • The myotome with the somatopleure gives origin to the muscles of the limbs and the anterior lateral body walls

The intraembryonic coelom

The intraembryonic coelom first appears as many small isolated coelomic spaces in the lateral mesoderm and cardiogenic mesoderm (between the 2 layers of the lateral plate mesoderm) which coalesce to form a horseshoe-shaped cavity, the intraembryonic coelom, which is lined by flattened epithelial (mesothelial) cells. It will become the pleuropericardial-peritoneal cavity

◦THE COELOM DIVIDES the lateral mesoderm into 2 layers: a somatic (parietal) layer continuous with the extraembryonic mesoderm over the amnion and a splanchnic (visceral) layer, which is continuous with the extraembryonic mesoderm over the yolk sac

• Somatic mesoderm plus overlying embryonic ectoderm forms the body wall or somatopleure
• Splanchnic mesoderm plus embryonic entoderm forms the wall of the primitive gut and is called the splanchnopleure

◦DURING THE SECOND MONTH, the intraembryonic coelom is divided into the body cavities, namely, the pericardial cavity, the pleural cavities, and the peritoneal cavity

Week 3 of Development: Cardiovascular System Development

A.  Angiogenesis or blood formation begins in the extraembryonic mesoderm of the yolk sac, connecting stalk, and chorion during days 13-15. The embryonic vessels begin to develop approximately 2 days later

◦MESENCHYMAL CELLS or angioblasts aggregate to form isolated masses and cords called blood islands

1. Spaces accumulate in the islands, angiob1asts arrange themselves around the cavities to form the primitive endothelium, then isolated vessels fuse to form networks of endothelial channels

2. Vessels continue to extend into adjacent areas by endothelial budding and fusion with other vessels being formed independently

B. Blood cells and primitive plasma develop from the endothelial cells as the vessels develop on the allantois and yolk sac

1. BLOOD FORMATION begins in week 5, occurring in various portions of the embryonic mesenchyme, particularly in the liver, then later in the spleen, bone marrow, and lymph nodes

2. MESENCHYMAL CELLS around the primitive endothelial vessels differentiate into the muscles and connective tissue of the blood vessels

3. THE PRIMITIVE ENDOTHELIAL CARDIAC TUBES form from mesenchymal cells in the cardiogenic area

• Longitudinally paired endothelial channels, the heart tubes, develop before the end of week 3 and begin to fuse into the primitive heart tube
• By day 21, the paired tubes link up with blood vessels in the embryo, connecting stalk, chorion, and yolk sac to form a primitive cardiovascular system. The cardiovascular system is the first organ system to reach a functional state
• Blood circulation usually is started by the end of week 3

Week 3 of Development: Trophoblast and Villus Development

1. THE TROPHOBLAST is characterized by many primary stem villi, consisting of a cytotrophoblast core covered by a syncytial layer, at the beginning of week 3

• With development, mesodermal cells from the extraembryonic somatopleuric mesoderm or cytotrophoblast penetrate the core of the primary villi and grow in the direction of the decidua to form the secondary stem villi which consist of a loose connective tissue core covered by a cytotrophoblastic layer which, in turn, is covered by a thin syncytial layer

2. BY THE END OF WEEK 3, mesodermal cells in the villus core differentiate into blood cells and small blood vessels, forming the villous capillary system, and thus create the tertiary villi

• By week 4, the tertiary villi are seen over the entire surface of the chorion
• The capillaries in the tertiary villi contact capillaries developing in the mesoderm of the chorionic plate and in the connecting stalk, eventually contact the intraembryonic circulatory system, and connect the placenta and the embryo. Thus, in week 4, when the heart begins to beat, the villous system is able to supply the embryo with oxygen and nutrients, whereas prior to that time it was all done by diffusion

3.  CYTOTROPHOBLAST CELLS in the villi penetrate the overlying syncytium to reach the maternal endometrium

• They establish contact with similar extensions of neighboring villous stems to form a thin outer cytotrophoblast shell
  • The cytotrophoblast shell is seen on the embryonic pole initially and then expands toward the abembryonic pole until it covers the entire trophoblast, thus attaching the chorionic sac firmly to the maternal endometrial tissue
• Villi attached to the maternal tissues via the trophoblastic shell are called stem or anchoring villi
• Villi that grow from the sides of the stem villi are called branch villi, and it is through these that the major exchange of materials between the mother and the embryo takes place

4. BY DAYS 19 AND 20, the extraembryonic coelom or chorionic cavity enlarges, and the embryo is attached to its trophoblast shell only by a narrow connecting stalk

• The stalk is composed of extraembryonic mesoderm which is continuous with the chorionic plate and is attached to the embryo at its caudal end

The connecting stalk or body stalk later develops into the umbilical cord to connect the placenta and the embryo

Functions of the villi

1.Nutrition of the embryo (free villi).

2.Fixation of the embryo (anchoring villi).

3.Respiration of the embryo.

4.Excretion of the embryo.

– With development of the embryo, the chorionic villi toward the decidua basalis grow and become well developed. So the chorion there is called chorion frondosum.

– While the villi toward the decidua  capsularis become poorly developed, the chorion there is called chorion leava.

Week 2 is about the implantation process and blastocyst differentiation. Note that all cells produced from the initial fertilization event are defined as the “conceptus” and will include cells with both embryonic and extraembryonic features.

In the conceptus, this is a period of blastocyst “hatching” rapid blastocyst differentiation into extraembryonic and embryonic tissues and proliferation. In placental animals, this is the first physical interaction between the conceptus and the maternal uterine wall with adplantation and the commencement of implantation.

The implanting conceptus releases a hormone (human Chorionic Gonadotropin or hCG) that initiates maternal hormonal changes, stopping the menstrual cycle. Detection of hCG in maternal urine or blood is also the basis of many modern pregnancy tests.

The second week of human development is concerned with the process of implantation and the differentiation of the blastocyst into early embryonic and placental forming structures.

Normal Stages of week 2 development

Day 08

1. Cells of the inner cell mass (embrioblast) differentiate into 2 distinct layers

• Endodermal (entodermal) germ layer: layer of small, cuboidal cells
• Ectodermal germ layer: layer of high columnar cells

2. The cells of each germ layer form a flat disk and together are known as the bilaminar germ disk

3. Cells of the ectodermal layers, initially firmly attached to the cytotrophoblast, develop small clefts between their layers as development proceeds

• The clefts coalesce and form a cavity, the amniotic cavity
• Amnioblasts, large, flattened cells, are seen along the trophoblastic border of the newly formed amniotic cavity (probably derived from trophoblast)
  •   The cells are continuous with the ectoderm and together line the amniotic cavity
• Endometrial stroma adjacent to the implantation site is edematous, highly vascular, with large tortuous glands that secrete glycogen and mucus

Day 09

◦blastocyst embeds deeper into endometrium, and a fibrin coagulum “plug” (blood clot and cellular debris) closes the penetration defect in uterine epithelial surface – interstitial implantation

1.Trophoblast progresses in development, especially at the embryonic pole, and vacuoles appear in the syncytium. The vacuoles fuse to form large lacunae (lakes), and we have the lacunar stage of trophoblast development

2.Endometrial stroma around the trophoblast has vascular congestion, and the cells are rich in glycogen

3.Flattened cells delaminate from the inner surface of the cytotrophoblast, at the embryonic pole, and form a thin membrane called Heuser’s exocoelomic membrane which is continuous with the edges of the entodermal layer. Together, they form the lining of the exocoelomic cavity or the primitive yolk sac.

Day 10, 11 and 12

1.The blastocyst becomes completely embedded in the endometrial stroma, and the uterine surface epithelium almost entirely covers the original epithelial lining defect of the mucosa Only a slight protrusion is seen in the uterine lumen

2.The trophoblast is characterized by lacunar spaces in the syncytium, and they form an interconnecting network, particularly at the embryonic pole

3.At the embryonic pole, the trophoblast consists of cytotrophoblastic cells and only a few lacunar spaces

4. The syncytial cells penetrate deep into the stroma and erode the endothelial lining of maternal congested and dilated capillaries called sinusoids

• The syncytium becomes continuous with the endothelial cells of the vessels, and maternal blood enters the lacunar system
• With more and more sinusoid invasion by the trophoblast, the lacunae eventually become continuous with the arterial and venous systems. Pressure differences between arterial and venous capillaries result in maternal blood flowing through the trophoblastic lacunar system to form the uteroplacental circulation

5. Cytotrophoblast also differentiates. On its inner surface, cells delaminate to form a fine, loose tissue, the extraembryonic mesoderm, which fills the space between external trophoblast and amnion and internal yolk sac

• Large cavities develop in this extraembryonic mesoderm, become confluent, and form the extraembryonic coelom, which surrounds the primitive yolk sac and amniotic cavity, except where the extraembryonic mesoderm forms the future connection between the germ disk and the trophoblast
  • The extraembryonic mesoderm lining the cytotrophoblast   and   amnion is called the extraembryonic somatopleuric mesoderm; that   covering the yolk sac is called   the extraembryonic splanchnopleuric   mesoderm

6. The bilaminar germ disk grows slowly compared to the trophoblast, but by the end of day 12, entodermal cells begin to spread over the inside of Heuser’s membrane

• Endometrial cells become polyhedral, are loaded with glycogen and lipids, and the intercellular spaces fill with extravasate; the tissue is edematous – all a process of the decidual reaction, initially at implantation site but then throughout endometrium

Day 13 and 14

the endometrial surface defect is usually healed, but there may be occasional bleeding at the site of implantation due to the increased blood flow in the lacunar spaces at the abembryonic pole. Can be confused with menstrual bleeding

1.The trophoblast shows more organization at the embryonic pole

2.The cytotrophoblast cells proliferate, penetrate the syncytium, and form cellular columns surrounded by syncytium-together forming the primary stem villi

3.The entodermal germ layer proliferates and newly formed cells gradually line a new cavity known as the secondary or definitive yolk sac (smaller than original)

4.The extraembryonic coelom expands to form the chorionic cavity

5.The extraembryonic mesoderm then lines the cytotrophoblast and is called the chorionic plate. It also forms a covering layer for the secondary yolk sac and amnion

  – The extraembryonic mesoderm only traverses the chorionic cavity in the connecting stalk (connecting the embryo with the trophoblast)

  – With development of blood vessels, the stalk becomes the umbilical cord

6.By the end of week 2, the germ disk consists of two apposed cell disks: the ectodermal germ layer, forming the floor of the expanding amniotic cavity, and the entodermal germ layer, forming the roof of the secondary yolk sac

  – In its cephalic region, the entodermal disk is thickened to form the prochordal plate, an area of columnar cells attached to the overlying ectodermal disk

7.The primitive streak appears and indicates the onset of gastrulation

The first week of human development begins with fertilization of the egg by sperm forming the first cell, the zygote. Cell division leads to a ball of cells, the morula. Further cell division and the formation of a cavity in the ball of cells forms the blastocyst. 

Initially, there is a halving of chromosomal content in the gametes (spermatozoa and oocyte) by the process called gametogenesis. Chromosomal content is then restored by fertilization, allowing genetic recombination to occur.

This is then followed by a series of cell divisions without cytoplasmic growth. During this first week the egg, then zygote, morula then the blastula is moving along the uterine horn into the uterus for implantation in the uterine wall.

Summary

• The conceptus proceeds through embryonic cell cycle rounds of mitosis still enclosed within the zona pellucida. Progressing from a zygote, to blastomeres, then to a morula and finally to a blastocyst.
• The uterine tube epithelium consists of ciliated cells that are moving the secreted fluid towards the uterine body. The conceptus is floating “boat-like” within this fluid and moved in the same direction.
• Towards the end of the first week the blastocyst has reached the uterine body and from about day 5 onwards, the blastocyst “hatches” from the surrounding zona pellucida.
• The conceptus can now receive nutrition directly and can commence the process of implantation in week 2.

Molecular Changes

There are several important changes that occur in this new diploid cell beginning the very first mitotic cell divisions and expressing a new genome.

The oocyte arrested in meiosis is initially quiescent in terms of gene expression, and many other animal models of development have shown maternal RNAs and proteins to be important for early functions.

The new zygote gene expression is about cycles of mitosis and maintaining the totipotency of the stem cell offspring cells.

The morula gene expression supports the formation of two populations of cells the trophoblast (trophectoderm) and embryoblast (inner cell mass), each having different roles in development, while maintaining the totipotency of these populations.

Current research is now also pointing to non-genetic mechanisms or epigenetics as an additional mechanism in play in these processes.

Cleavage

It is the repeated mitotic divisions of the zygote, resulting into a rapid increase in the number of cells that are called blastomeres.

Site: The uterine tube medial to the ampula.

DIVISION OF THE ZYGOTE into 2 daughter cells, the blastomeres, takes place by 30 hours. Further divisions follow rapidly upon one another, forming progressively smaller and smaller blastomeres: 4 are seen in 40-50 hours, 8 by 60 hours, and 12-16 by day 3 or 4

The 12-16 blastomere stage, arrived at by cleavage of the fertilized ovum, is a solid ball resembling a mulberry and is called a morula (morula stage). As it forms, the morula enters the uterine cavity from the tube.

Stages of cleavage

a- Morula: It has 16 cells(blastomeres). It has mulberry appearance. It is surrounded by Zona pellucida. It enters the uterus nearly 3 days after fertilization.

b- ABOUT DAY 4, fluid enters the morula from the uterine cavity and occupies the intercellular spaces. The fluid-filled spaces fuse to form a single, large cavity, the blastocele, and the morula is now called a blastocyst (blastocyst stage)

• As fluid increases, the cells separate into 2 major areas
  • An outer cell layer, the trophoblast, which gives rise to the placenta
  • A group of centrally located cells, the inner cell mass or embryoblast, which gives rise to the embryo proper
• The free blastocyst is seen in the uterine cavity on day 4 or 5, from this time the zona pellucida disappears rapidly

Week 1 abnormalities

Dizygotic Twinning

• Dizygotic twins (fraternal, non-identical) arise from separate fertilization events involving two separate oocyte (egg, ova) and spermatozoa (sperm).

Monoygotic Twinning

• Monozygotic twins (identical) produced from a single fertilization event (one fertilised egg and a single spermatazoa, form a single zygote), these twins therefore share the same genetic makeup. Occurs in approximately 3-5 per 1000 pregnancies, more commonly with aged mothers. The later the twinning event, the less common are initially separate placental membranes and finally resulting in conjoined twins.

Implantation is the stage in embryonic development in which the blastocyst hatches as the embryo, and adheres to the wall of the uterus. Once this adhesion is successful, the female is considered to be pregnant and the embryo will receive oxygen and nutrients from the mother in order to grow.

We can learn this lesson under the following topics.

1. Preimplantation
2. Implantation
3. Hormonal Aspect of Implantation
4. Hypophyseal – ovarian relationship
5. Abnormal implantation sites

Preimplantation

After a successful fertilization, which takes place in the ampullary part of the fallopian tube, the embryo migrates through the tube into the uterine cavity. This migration takes six days. Along the way, the zygote divides several times, initially without increasing its volume because it is still enveloped by the pellucid zone. Daughter cells are engendered and one speaks now of the blastomere stage. After around 16 cells (morula) the compaction occurs in which the outer cells, the trophoblasts, form a compact epithelial structure. 

Implantation

Implantation occurs at the 6th day after fertilization and is completed about the 11th day.

The normal site is the endometrium of the posterior wall of the fundus of the uterus in or near the middle line. The endometrium after implantation is called decidua.

In the human, the trophoblast cells over the embryonic pole of the blastocyst penetrate the epithelial cells of the uterine mucosa at about day 6 or 7 or about 20 days after the beginning of the last menstruation

• Penetration and erosion of the epithelial cells of the mucosa result from proteolytic enzymes produced by the trophoblast  itself
• The uterine mucosa also promotes proteolytic action of the blastocyst so implantation is a mutual action of the endometrium and trophoblast

1. As invasion of uterus proceeds, the trophoblast differentiates into 2 layers

• An inner cytotrophoblast or cellular trophoblast
• An outer syncytiotrophoblast or syncytial trophoblast consisting of a multinucleated protoplasmic mass in which intercellular boundaries are absent

2. The finger like processes of the syncytial trophoblast grow into the endometrial epithelium and invade the endometrial stroma

3. By the end of week 1, the blastocyst is superficially implanted in the compact layer and the conceptus derives its initial nourishment from endometrial tissues. Later it receives nutrients directly from maternal blood.

4. At the time of implantation, the uterine mucosa is at day 21 of the menstrual cycle and is richly vascularized, oedematous, and secreting mucus and glycogen – all favouring implantation of the blastocyst

6. As the blastocyst is implanting, early differentiation of the inner cells mass occurs, forming bilaminar Embryo (Human development about day 8 to 9 )

• The embryoblast (inner cell mass) forms the epiblast and hypoblast layers. This early stage of embryo development is referred to as the bilaminar embryo.
• The inner cell mass forms an inner layer of larger cells is also called the “embryoblast” is a cluster of cells located and attached on one wall of the outer trophoblast layer. In week 2 this mass will differentiate into two distinct layers the epiblast and hypoblast, also called the bilaminar embryo.
• Epiblast
• The epiblast layer will form the entire embryo and undergoes gastrulation in week three to form the 3 germ layers. It also forms an epithelial layer lining the amniotic cavity.
• Hypoblast
• The hypoblast (or primitive endoderm) is a transient epithelial layer facing towards the blastoceol, it is replaced in week three by the gastrulation migrating endoderm cells.
• Concomitantly to the morphological blastocyst changes, the amniotic cavity, yolk sac, connecting stalk, and chorion develop.

Hormonal Aspect of Implantation

◦ACTION OF OVARIAN HORMONES ON THE ENDOMETRIUM

• During each menstrual cycle, the uterine mucosa undergoes preparation for implantation which is directly conditioned by the ovarian hormones oestrogen and progesterone
• The theca interna of the Graafian follicle is the major source of oestrogen, and the corpus luteum of pregnancy is the principal source of progesterone
• Morphologic changes in the uterine mucosa during the menstrual cycle result in proliferation of the endometrium involving not only the epithelium, the glands, and the stroma, but also, in a very essential way, the blood vessels

Hypophyseal-ovarian relationship

1. HYPOPHYSIS: endocrine activity of the ovary is under the control of the anterior lobe of the pituitary gland. The latter, in the human, secretes 2 gonad-stimulating hormones called gonadotropins or gonadotropes

• Follicle-stimulating hormone (FSH) is secreted from the very beginning of the menstrual cycle and determines the growth of the ovarian follicle
• Luteinizing hormone (LH) is secreted in the middle of the cycle and acts synergistically with FSH to provoke ovulation. LH stimulates the development of the corpus luteum

2. THE OVARY: the endocrine activity of the ovary, under the influence of the hypophyseal gonadotropins, is diphasic

• During phase 1, it secretes oestrogen
• During phase 2, both oestrogen and progesterone are secreted
  • The secretion of progesterone is detectable even before the formation of the
  • corpus luteum

Abnormal Implantation Sites

1.THE HUMAN BLASTOCYST normally implants in the endometrium along the posterior wall of the body of the uterus, where it becomes attached between the openings of the endometrial glands or occasionally in the mouth of a glandular duct

2.NOT INFREQUENTLY, THE BLASTOCYST IMPLANTS IN ABNORMAL LOCATIONS outside the uterine body. This usually leads to the death of the embryo and severe hemorrhage of the mother during the second month of pregnancy. Such an implantation is called an extrauterine or ectopic pregnancy and may occur in the abdominal cavity, the ovary, the uterine tube or pelvis. Rarely does an extrauterine embryo come to full term

1. Tubal pregnancy is the most frequent ectopic sit The tube usually ruptures during the second month of pregnancy, resulting in severe internal hemorrhaging
2. Abdominal pregnancy: the peritoneal lining of the rectouterine cavity is the most frequent implantation sit Also on peritoneum of the intestinal tract or omentum

3.OCCASIONALLY, IMPLANTATION IN THE UTERUS ITSELF may lead to serious complications, particularly if implantation occurs near the internal os (low uterus). The placenta then bridges the os and we have what is called placenta previa which results in severe bleeding in the latter or second part of pregnancy and during delivery

4.FERTILIZED OVUM MAY ABNORMALLY MOVE to contralateral tube

Fertilization is the union of male(sperm) and female(oocyte) gametes to form a zygote and marks the beginning of the pregnancy. Embryonic life begins with fertilization. Fertilization process require 24 hours.

During this lesson we need to discuss about

1. Ovulation
2. Getting the spermatozoa ready
3. Key steps of fertilization
4. The formation of the zygote
5. Abnormal Fertilization

Ovulation

The fertilization takes place in the female genital track.

Roughly a week before the midpoint of the menstrual cycle the dominant follicle develops in one of the two ovaries. This grows faster than the other tertiary follicles and prepares itself for ovulation. It reaches a diameter of up to 25 mm and is also known then as the graafian follicle.

Since at this moment the cumulus oophorus has detached itself from the granulosa, the oocyte “swims” surrounded by its cumulus cells (so-called corona radiata) in the follicle fluid.
The graafian follicle bulges out the ovary’s surface. It is ready to tear open, i.e., its wall and the peritoneal covering will soon rupture and its watery contents will pour into the infundibulum of the tuba, together with the oocyte.

A hormonal regulation is responsible for follicle maturation. Approximately one-and-a-half days before the midpoint of the cycle, the concentration of the luteinizing hormone (LH) rises steeply.

In the first week of the cycle the maturation of the oocyte in its associated follicle depends on the progress of the maturation of the surrounding follicle cells.

The fittest follicle with its oocyte becomes the dominant follicle in the second cycle week and, later, a graafian follicle.

Up to just under two days before ovulation, the maturation of the oocyte consists in its ingestion of substances (growth of the yolk) that are supplied by the surrounding granulosa cells that are anchored through the pellucid zone at the oocyte surface.

The oocyte nucleus [2n, 4C] is also matured in the last days before the LH peak.

Up to that point it was arrested in the extremely elongated prophase (= dictyotene) of the first meiosis.

Through the «maturation» the nucleus changes in the diakinesis (of the prophase) and prepares itself for the completion of the first meiosis, which is triggered by the LH peak.

With the end of the first meiosis the name of the oocyte changes from primary oocyte to secondary oocyte.

Through the effects of LH on the granulosa cells, these have begun to loosen their cellular bonds and to multiply. They now also produce progesterone that is released into the follicle fluid.

Through the separation of the homologous chromosomes in the first meiosis a haploid (reduplicated) set of chromosomes (1n, 2C) is now to be found in the secondary oocyte. The first polar body also contains 1n, 2C.

Via a fine cytoplasmic connection the polar body and oocyte remain bound together following the meiotic division, similar to what takes place when male gametes are formed.

Besides the hormones, the granulosa cells also secrete an extra-cellular matrix, mainly hyaluronic acid, into the follicle fluid. Before ovulation the follicle fluid volume increases markedly. The cumulus cell bonds loosen further. In this way, together with the enclosed oocyte, they free themselves from where they were attached to the follicular wall and now swim in the follicle fluid. The wreath of granulosa cells that enclose the oocyte is called the corona radiata.

The follicle and the oocyte are now ready for ovulation that takes place roughly 38 hours after the LH peak.

Ovulation, i.e., the emergence of the secondary oocyte from the follicle, depends on the disintegration of the follicle wall and the rupture of the ovarian surface.

The hyaluronic acid has the property that it binds water molecules. So, tension to the follicle wall increases.

On the ovarian surface above the follicle that is about to burst, a white point forms shortly before the rupture (due to compression of the blood vessels), the so-called stigma.

When the surface ruptures the mass of cumulus cells, which are saturated with hyaluronic acid and which shelter the oocyte, reach the fallopian tube together with serous yellow follicle contents.

In the fallopian tube, the secondary oocyte is surrounded by the corona radiata and scattered parts of cumulus cells (so-called cumulus cell cloud). The fluid that lies in between is sticky and stringy (effect of the hyaluronic acid) with a high concentration of progesterone (to attract the spermatozoa).

Getting the Spermatozoa Ready

There are parallels between getting the spermatozoa ready and the maturation of an oocyte but there are also clear differences.

The spermatozoa have to go through several temporal maturation steps in a series of different locations in order to be capable of penetrating into the oocyte. While the oocyte’s maturation steps involve the storing of yolk and the process of meiosis, functional maturation steps are required with the spermatozoa, which mainly involve their motile abilities along with their ability to penetrate through the egg covering.

he spermatozoa experience an initial maturation step during the time they are “stored” in the epididymis. When the ejaculation occurs, a second step follows that leads to a sudden activation of their motility.

The third step takes place during their stay in the female genital tract, especially during the ascension towards the ovary through the uterus and fallopian tube. The spermatozoa experience thereby the so-called capacitation. Finally, the last activation step follows: the acrosome reaction in the immediate vicinity of the oocyte.

The maturation and activation of the spermatozoa occur in the following four steps:

• Storage in the epididymis ==> Maturation
• Ejaculation ==> Activation
• Ascension to the ovary ==> Capacitation
• Near the oocyte ==> Acrosome reaction

Key Steps of Fertilization

The process of fertilization in humans involves a number of key processes

1. Capacitation
2. Acrosome Reaction
3. Cortical Reaction

Capacitation

Capacitation occurs after ejaculation, when chemicals released by the uterus dissolve the sperm’s cholesterol coat

• This improves sperm motility (hyperactivity), meaning sperm is more likely to reach the egg (in the oviduct)
• It also destabilises the acrosome cap, which is necessary for the acrosome reaction to occur upon egg and sperm contact

Acrosome Reaction

When the sperm reaches an egg, the acrosome reaction allows the sperm to break through the surrounding jelly coat

• The sperm pushes through the follicular cells of the corona radiata and binds to the zona pellucida (jelly coat)
• The acrosome vesicle fuses with the jelly coat and releases digestive enzymes which soften the glycoprotein matrix
• The sperm then pushes its way through the softened jelly coat and binds to exposed docking proteins on the egg membrane
• The membrane of the egg and sperm then fuse and the sperm nucleus (and centriole) enters the egg

Cortical Reaction

The cortical reaction occurs once a sperm has successfully penetrated an egg in order to prevent polyspermy

• Cortical granules within the egg’s cytoplasm release enzymes (via exocytosis) into the zona pellucida (jelly coat)
• These enzymes destroy sperm binding sites and also thicken and harden the glycoprotein matrix of the jelly coat
• This prevents other sperm from being able to penetrate the egg (polyspermy), ensuring the zygote formed is diploid 

The Formation of The Zygote

After the spermatozoon has impregnated the oocyte, i.e., has delivered the paternal portion of the genetic material, things are now set into motion within the oocyte so that the paternal as well as the maternal genetic information are put into a form that allows both to be brought together in a proper way. The unpacked DNA is enclosed in the slowly forming paternal and maternal pronuclei. 

Along side several morphological changes can be seen.

1. Sperm passes through the corona radiata

• this is the result of enzymatic action of tubal mucosa and semen. Sperm tail movements also help penetration of corona and zona pellucida

2. Sperm penetrates the zona pellucida: digests a path by action of enzymes released from its acrosome

• Only 1 sperm enters the oocyte and fertilizes it, even though several may penetrate the zona pellucida
• Two sperm may take part in fertilization during an abnormal process called dispermy resulting in a triploid embryo (69 chromosomes), but it nearly always aborts or dies shortly after birth
• If 2 female pronuclei take part in fertilization, it is called polygyny

3. Sperm head attaches to surface of the oocyte, plasma membranes of oocyte and sperm fuse, and then break at contact point

• Head and tail of sperm enter oocyte cytoplasm with sperm’s plasma membrane being attached to oocyte’s plasma membrane Once inside the cytoplasm of the oocyte, the sperm tail degenerates

4. Oocyte responds by

• Zonal reaction: change in zona pellucida inhibits entry of more sperm, due to substance of oocyte cytoplasm

• Secondary oocyte completes second meiotic division and its chromosomes (22 plus X) arrange themselves in a vesicular nucleus called the female pronucleus. The second polar body is extruded

5. Sperm head enlarges and forms the male pronucleus

6. The male and female pronuclei approach each other in the oocyte center, meet, and lose their nuclear membranes. They resolve their chromatin into a complete single haploid set of chromosomes which become organized on a spindle

7. After the maternal and paternal chromosomes intermingle, metaphase of the first cleavage mitosis takes place, and the normal chromosome number is reconstituted

8. Anaphase of the first cleavage mitosis then occurs

9. The first 2 blastomeres are next seen, following cell division, and they are surrounded by the zona pellucida

After the two pronuclei have come as close together as they can, no merging of them takes place, i.e., a fitting together of the chromosomes of the two pronuclei within a single nucleic membrane does not happen. It is much more accurate to say that the nucleic membranes of both pronuclei dissolve and the chromosomes of both align themselves on the spindle apparatus at the equator.

The zygote, the first cell of a new organism with an individual genome (2n4C) is created by the alignment of the maternal chromosomes together with the paternal ones on a common spindle apparatus.

Abnormal Fertilization

◦PARTHENOGENESIS: oocyte is activated without sperm penetration and development may begin. No record of viable birth via this method

• Cleaving oocytes in ovary may develop into an ovarian teratoma

◦SUPERFECUNDATION may follow polyovulation. An oocyte is fertilized by spermatozoa from one male and another oocyte is fertilized by a second male. Seen in various mammals, not usual in man.

◦SUPERFETATION: ovulation and fertilization occur during an established pregnancy

Gametogenesis is the production of gametes from haploid precursor cells. In animals and higher plants, two morphologically distinct types of gametes are produced (male and female) via distinct differentiation programs. Animals produce a tissue that is dedicated to forming gametes, called the germ line.

Now we are going to learn about the,

1. The Germ Line
2. Sexual Differentiation
3. Spermatogenesis
4. Oogenesis

The Germ Line

The primordial germ cells(PGCs) are the ancestors of the germ line. They are diploid.

In the second week of the human embryo these can be already found in the primary ectoderm (epiblast).

In the third week, the PGCs wander from the primary ectoderm to yolk sac wall. And they collect near the allantois. Now they are extraembryonal.

Between the fourth and the sixth week PGCs wander back into the embryo. They move along the yolk sac wall to the vitelline and into the wall of the rectum. After crossing the dorsal mesentery they colonize the gonadal ridge.

For both sexes the gonads arise in the gonadal ridges. They are generated in the 5th week. At this point, the gonadal ridge represents the primitive gonadal primordium.

Sexual Differentiation

The gender of an embryo is determined at the moment of fertilization and depends on whether the spermatozoon carries an X or a Y chromosome.

In XX embryos the germinal cords do not grow as far as the medulla and the cortical cords envelop the oogonia.

In XY embryos, on the other hand, the medullary cords become the testicular cords that also grow into the depths and establish contact with the mesonephros.

Spermatogenesis

Spermatogenesis is initiated in the male testis with the beginning of puberty.

This comprises the entire development of the spermatogonia (former primordial germ cells) up to sperm cells.

The gonadal cords develop a lumen. They then transform themselves into spermatic canals.

They are termed convoluted seminiferous tubules (Tubuli seminiferi contorti).

They are coated by a germinal epithelium that exhibits two differing cell populations: some are sustentacular cells (= Sertoli’s cells) and the great majority are the germ cells in various stages of division and differentiation.

The maturation of the germ cells begins with the spermatogonia at the periphery of the seminal canal and advances towards the lumen over spermatocytes I (primary spermatocytes), spermatocytes II (secondary spermatocytes), spermatids and finally to mature sperm cells.

The epithelium consists of Sertoli’s sustentacular cells and the spermatogenic cells.

Spermatogenesis is thus accomplished in close contact with the Sertoli’s cells, which not only have supportive and nourishing functions, but also secrete hormones and phagocytize cell fragments.

Along the course of spermatogenesis the germ cells move towards the lumen as they mature. The following developmental stages are thereby passed through:

1. A-spermatogonium
2. B-spermatogonium
3. Primary spermatocyte (= spermatocyte order I)
4. Secondary spermatocyte (= spermatocyte order II)
5. Spermatid
6. Sperm cell (= spermatozoon)

The spermatogenesis can be subdivided into two successive sections:

The first comprises the cells from the spermatogonium up to and including the secondary spermatocyte and is termed spermatocytogenesis.

The second one comprises the differentiation/maturation of the sperm cell, starting with the spermatid phase and is termed spermiogenesis (or spermiohistogenesis).

The approximate 64 day cycle of the spermatogenesis can be subdivided into four phases that last differing lengths of time:

Mitosis of the spermatogonia	16 Days	Up to the primary spermatocytes
Meiosis I	24 Days	For the division of the primary spermatocytes to form secondary spermatocytes
Meiosis II	Few Hours	For engendering the spermatids
Spermiogenesis	24 Days	Up to the completed sperm cells
Total	64 Days

Among the spermatogonia that form the basal layer of the germinal epithelium, several types can be distinguished: 1. Type A cells that undergo homonymous division. 2. Type A cells undergo heteronymous division.

After a further mitotic division type B spermatogonia divide into primary spermatocytes (I) which enter first meiosis.

They then go immediately into the S phase, double their internal DNA.

Following the S phase, these cells attain the complex stage of the prophase of the meiosis and become thereby noticeably visible with a light microscope.

This prophase, which lasts 24 days, can be divided into five sections:

Leptotene
Zygotene
Pachytene
Diplotene
Diakinesis

After the long prophase follow the metaphase, anaphase and telophase that take much less time. One primary spermatocyte yields two secondary spermatocytes.

The secondary spermatocytes go directly into the second meiosis, out of which the spermatids emerge. In the secondary spermatocytes neither DNA reduplication nor a recombination of the genetic material occurs.

Through the division of the chromatids of a secondary spermatocyte, two haploid spermatids arise that contain only half the original DNA content. In a process called spermiogenesis they are transformed into sperm cells with the active assistance of the Sertoli’s cells.

In examining a cross-section of a convoluted seminiferous tubule sometimes it is noticeable that cells appear in groups having the same maturation stages. Because, daughter cells generated by each meiotic step remain bound together by thin cytoplasmic bridges.

Thus with each meiotic step the following generation is twice as large, until the cells have formed a relatively complex network.

The result is that cells of the same development stages are seen there in groups.

Thus, it is highly improbable that all of the development stages will be seen in a single section at the same time. However, not all the spermatogenesis stages are found in a cross-section.

The differentiation of the spermatids into sperm cells is called spermiogenesis. It corresponds to the final part of spermatogenesis and comprises the following individual processes that partially proceed at the same time:

• Nuclear condensation: thickening and reduction of the nuclear size, condensation of the nuclear contents into the smallest space.
• Acrosome formation: Forming a cap (acrosome) containing enzymes that play an important role in the penetration through the pellucid zone of the oocyte.
• Flagellum formation: generation of the sperm cell tail. Four parts of the finished flagellum can be distinguished: The neck, the mid piece, the principle piece, and the tail.
• Cytoplasm reduction: elimination of all unnecessary cytoplasm

Leydig’s interstitial cells are endocrine cells that mainly produce testosterone. An initial active stage of these cells occurs during the embryonic development of the testis.

Oogenesis

Following the immigration of the primordial germ cells into the gonadal ridge, they proliferate. They form germinal cords. Now a cortical zone and a medulla can be distinguished. Because, in females the germinal cords never penetrate into the medullary zone.

In the genital primordium the following processes then take place:

1. A wave of proliferation begins that lasts from the 15th week to the 7th month: primary germ cells arise in the cortical zone.
2. With the onset of the meiosis (earliest onset in the prophase in the 12th week) the designation of the germ cells changes. They are now called primary oocytes. The primary oocytes become arrested in the diplotene stage of prophase I. Shortly before birth, all the foetal oocytes in the female ovary have attained this stage. The meiotic resting phase that then begins is called the dictyotene and it lasts till puberty. Only a few oocytes (secondary oocytes plus one polar body), though, reach the second meiosis and the subsequent ovulation. The remaining oocytes that mature each month become atretic.
3. While the oogonia transform into primary oocytes, they become restructured so that at the end of prophase I (the time of the dictyotene) each one gets enveloped by a single layer of flat, follicular epithelial cells. (oocyte + follicular epithelium = primordial follicle).

The developmental sequence of the female germ cells is as follows:
Primordial germ cell – oogonium – primary oocyte – primary oocyte in the dictyotene
Birth
The continuation of the development / maturation of the oocyte begins again only a few days before ovulation (see fertilization module).

The developmental sequence of a follicle goes through various follicle stages:
Primordial follicle – primary follicle – secondary follicle – tertiary follicle (graafian follicle)
Since a follicle can die at any moment in its development (= atresia), not all reach the tertiary follicle stage.

An ovary is subdivided into cortical (ovarian cortex) and medullary compartments (ovarian medulla).
Both blood and lymph vessels are found in the loose connective tissue of the ovarian medulla.
In the cortical compartment the oocytes are present within the various follicle stages.

The sex hormones influence the primordial follicles to grow and a restructuring to take place. From the primordial follicles the primary follicles, secondary follicles, and tertiary follicles develop in turn. Only a small percentage of the primordial follicles reach the tertiary follicle stage – the great majority meet their end beforehand in the various maturation stages. Large follicles leave scars behind in the cortical compartment and the small ones disappear without a trace.
The tertiary follicles get to be the largest and, shortly before ovulation, can attain a diameter up to 2.5 mm through a special spurt of growth. They are then termed graafian follicles.

During the foetal period, the count of germ cells in the female organism is subject to large variations. These arise due to the fact that the phases of proliferation and decomposition of oocytes take place partially stepwise and partially in parallel.

The normal, common fate of a follicle or female germ cell is known as atresia – ovulation represents an exceptional destiny.

The number of germ cells decreases from the 20th week in order that they are all gone by about 50 years of age. Even though the decrease actually proceeds continuously, three moments in the life of a woman are apparent in which this takes place more rapidly. The largest decrease occurs in the 20th week after the maximum number of 7 million germ cells (per ovary) is reached, thus still in the foetal period. Immediately following birth a further, short period of accelerated decline happens. The third, temporally longest period, of increased decline takes place during puberty.

One terms the decline or the regression of follicles of each stage at every time in the life of a woman follicular atresia. These follicles do not ovulate and the name is derived from that fact. Follicle atresia occurs more intensely, though, at certain moments (foetal period, early postnatal, begin of the menarche).

Of the roughly 500’000 follicles that are present in the two ovaries at the beginning of sexual maturity, only around 480 reach the graafian follicle stage and are thus able to release oocytes (ovulation). Ovulation represents an exceptional fate of a follicle.

Cyclic changes in the hormonal cycle are responsible for the periodicity of the ovulation. In a woman, the rhythmic hormonal influence leads to the following cyclic events:

1. the ovarian cycle (follicle maturation) that peaks in the ovulation and the subsequent luteinization of the granulose cells
2. cyclic alterations of the endometrium that prepare the uterine mucosa so fertilized oocytes can “nest” there. In the absence of implantation, the mucosa will be eliminated (menstrual bleeding).

In the centre of this hormonal control is the hypothalamamics-hypophysial (pituitary gland) system with the two hypophysial gonadotropins FSH and LH. The pulsating liberation of GnRH by the hypothalamus is the fundamental precondition for a normal control of the cyclic ovarian function.

This cyclic activity releases FSH and LH, both of which stimulate the maturation of the follicles in the ovary and trigger ovulation. During the ovarian cycle, estrogen is produced by the theca interna and follicular cells (in the so-called follicle phase) and progesterone by the corpus luteum (so-called luteal phase).

As a rule, the ovarian cycle lasts 28 days. It is subdivided into two phases:

• Follicular phase: recruitment of a so-called follicle cohort and, within this, the selection of the mature follicle. This phase ends with ovulation. Estradiol is the steering hormone. Normally, it lasts 14 days, but this can vary considerably!
• Luteal phase: progesteron production by the “yellow body” (= corpus luteum) and lasts 14 days (relatively constant)

During the follicular phase, a cohort of follicles is recruited from which a dominant follicle is selected that will develop into a De Graaf follicle. The cohort of tertiary follicles that will deliver the de Graaf follicle that will ovulate on day 14 of a cycle has started its maturation about 6 months earlier

• The development of the primary follicle into a secondary follicle takes more than 120 days.
• The development of the secondary follicle into a tertiary (or cavitary or antral) follicle takes about 71 days.
• The terminal development of the tertiary follicle into a Graaf’s follicle lasts 2 weeks and starts at D0 of the menstrual cycle and ends at D14 with ovulation. This last phase is dependent on gonadotropins (LH and FSH) and starts as soon as the corpus luteum of the preceding cycle has regressed.

Embryology is the branch of biology that studies the prenatal development of gametes, fertilization, and development of embryos and foetuses. Additionally, embryology encompasses the study of congenital disorders that occur before birth, known as teratology.

Embryology can be mainly studied under two main divisions. They are EMBRYOGENESIS and ORGANOGENESIS.

Embryogenesis is the process of initiation and development of an embryo from a zygote (zygotic embryogenesis) or a somatic cell (somatic embryogenesis). Embryo development occurs through an exceptionally organized sequence of cell division, enlargement and differentiation.

Studying embryogenesis can be easier if we go through the topics below.

01 GAMETOGENESIS

• Origin and migration of the germ cells
• Male gonadal primordium
• Female gonadal primordium
• Spermatogenesis
• Oogenesis

02 FERTILIZATION

• Explaining the ovulation process
• Knowing the necessary steps which lead to spermatozoa being ready
• Describing how the enabling of the spermatozoa takes place
• Describing how the spermatozoon penetrates into the oocyte
• Knowing the process whereby a zygote is formed

03 IMPLANTATION

• Describe the histological structures of the endometrium
• Explain the phases of endometrial changes during the menstruation cycle
• Know the effects of the hypophysial hormones in the regulation of the menstruation cycle
• Explain the various stages of implantation
• Know the fundamental mechanisms of the implantation at the molecular level
• List the normal types of implantation and the anomalies of the extra-uterine pregnancies
• List the various possibilities for hindering an implantation and thus a pregnancy

04 EMBRYONIC DISC

• The differentiation of the embryonic germ layers, emanating from the trilaminar embryo
• The mechanism of gastrulation and especially the morphogenetic role of the primitive streak
• The arrangement of the intraembryonic mesoblast, its segmentation and the formation of the intraembryonic coelomic cavity
• The formation of the notochord and its role in the differentiations of nerve tissue
• The stages of neurulation and the first steps in the genesis of the central and peripheral nervous system

05 EMBRYONIC PHASE

• The differentiations of the germinal layers during the fourth week of development that lead to an individualization of the embryo.
• The key concepts of the embryonic period that describe the first stages of organogenesis.
• The various types of congenital abnormalities and be able to cite a few characteristic examples.

06 FOETAL PHASE

• The duration of the pregnancy and its various developmental stages.
• Various techniques of prenatal diagnostics.
• The differences among premature, full-term and post-term births.
• The intrauterine development of the child.
• Positions of the child during birth.
• Swiss legal aspects of pregnancy termination (abortion).
• Various causes of embryo-/ fetopathies and possibilities for therapy.
• Sensitivity of the embryo or foetus to teratogenic substances.

07 FETAL MEMBRANES AND PLACENTA

• name the foetal membranes and cavities together with their components and functions
• distinguish between the maternal and foetal parts of the placenta
• describe the macroscopic morphology of the placenta
• explain the development of the placental structures during pregnancy and their influence on the physiologic functions of the placenta
• name the structural and functional characteristics of the foetal blood circulation and the properties of the hemato-placental barrier
• list the endocrine functions of the placenta
• describe the peculiarities of twin pregnancies
• name the pathologies of embryonic development (ectopic pregnancy, hydatid mole, foetal erythroblastosis) in connection with the foetal membranes

08 CHROMOSOMAL AND GENE ABERRATION

• The difference between various kinds of chromosomal aberrations and gene mutations
• Possible causes of such disorders
• Interactions between genotype and the environment
• Polygeny and abnormalities
• General clinical symptoms of chromosomal aberrations

ORGANOGENESIS, in embryology, the series of organized integrated processes that transforms an amorphous mass of cells into a complete organ in the developing embryo. The cells of an organ-forming region undergo differential development and movement to form an organ primordium, or anlage. Organogenesis continues until the definitive characteristics of the organ are achieved.

01 MUSCULAR SYSTEM

• The origin of the three muscle types
• The development of the hypaxial and epaxial parts of the muscles based on the development of the somites and their differing innervation
• The histological development of muscle fiber to maturity
• The approximate segment level of the innervation of large muscle groups as well as the partial displacement
• Congenital muscle ailments and their causes which can be understood by knowing muscle development

02 CARDIOVASCULAR SYSTEM

• The first signs of heart development as well as the location of the cardiogenic tissues
• How the serial blood circulation system is converted to a parallel one during the course of embryonic development and which factors promote this development
• The processes that occur in the partitioning of the atria and ventricles.
• An enumeration of the arterial and venous systems with their various components that are near the heart
• The relationships of the pericardial cavity in adults, taking into account pericardial development
• The various nerves that are responsible for cardiac innervation

03 BLOOD AND LYMPHATIC TISSUES

• know the development from stem cells to differentiated blood cells
• know the location where erythropoiesis occurs
• have a concept of the functions of the various blood cells both before and after birth.
• know the organs of the lymphatic system
• know how they arise
• know the difference between cell-derived and humoral immunity
• have a concept of how immunological competence arises

04 RESPIRATION TRACT

• know the various prenatal stages of lung development.
• be able to list and localize the various cells that are typical for lung tissue.
• know the components of the blood-air barrier.
• be able to describe the development of the various somatic cavities.
• know where the pericardio-peritoneal duct lies.
• know the difference between the vasa publica and privata in the lungs.
• be able to explain the occurrence of fistulas between the esophagus and trachea based on your knowledge of the development of the two structures.
• know the various mechanisms in charge of the switch of the circulation systems at birth.

05 DIGESTION TRACT

• describe the various parts that are involved in forming the face.
• trace the development of the teeth.
• explain the innervation of the tongue from an embryologic point of view.
• list the derivatives of the individual pharyngeal arches.
• construct the relationship between the aortic and pharyngeal arches.
• describe the individual portions of the intestine and know their definitive location in the abdomen.
• describe the mesenteric relationships with the associated intestinal sections and blood vessels.
• determine which blood vessel is responsible for which intestinal portion.
• map out the course
• of the portal vein and explain it from an embryologic point of view.
• know the individual parts of the pancreas and explain their derivation.
• draw the relationships of the duodenal loops in a fetus.
• discuss the development of the urogenital sinus with respect to the formation of the hind gut and anus.

06 URINARY SYSTEM

• Describe the sequence of transitory and definitive anlagen of the upper urinary tract as well as their functions over the course of their development.
• Describe how the lower urinary tract forms from the cloaca.
• Explain some of the basic mechanisms that can lead to pathological development of the urinary system.

07 GENITAL SYSTEM

• list the genetic and hormonal factors that lead to sexual differentiation
• describe the steps that occur in the differentiation of the testicles and ovaries
• explain the formation of the internal and external genitals of both sexes
• name the abnormalities that indicated disorders in the most important mechanisms of genital development

08 NERVOUS SYSTEM

• describe typical features of the central and peripheral nervous systems
• distinguish between primary and secondary neurulation
• summarize the molecular mechanisms that underlie the development of the nervous system
• correlate the formation of the brain vesicle with the structures of the completely developed brain
• name the main functional divisions of the brain and the peripheral nervous system
• explain the histological and functional differentiation of nerve tissue cells (neurons and glial cells)
• describe and interpret the importance of the basic phenomena that occur during brain development (apoptosis, cell migration, splicing)
• explain the structural equivalents between embryonic development of the spinal cord and supraspinal centers
• sketch out blood circulation in the brain