Week 2 of Embryonic Development

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