Implantation

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