Week Two of Development

The process of implantation occurs by the end of the first week of development. However, for implantation to take place successfully, the uterus needs to be ready to receive the developing embryo. This preparation takes places through the following stages:

  1. Proliferative Stage

    • This occurs prior to the process of ovulation, in the second week of a menstrual cycle. Here, the endometrial lining of the uterus begins to thicken and increase in vascularization.

    • Follicle stimulating hormone (FSH) initiates the maturation of some follicles within the ovaries.

    • Luteinising hormone (LH) is released prior to ovulation to ensure the ovary releases an ovum.

    • The empty follicle then matures and produces estrogen which further prompts the production of FSH and LH which in turn prompt the production of estrogen. This loop allows for the eventual thickening of the endometrial lining of the uetrus.

  2. Secretory Stage

    • This occurs when secretions increase to maintain the thickness of the endometrial lining of the uterus.

    • The follicle matures into a structure known as corpus luteum which produces estrogen and progesterone hormones. Progesterone ensures the maintenance of the endometrial lining and increases secretions of the uterine glands. Progesterone and estrogen eventually inhibit the continued production of FSH and LH.

    • Following implantation, trophoblast cells produce human chorionic gonadotrophin (hCG) which is a hormone that ensures the corpus luteum continues produces progesterone.

Mechanisms of Implantation

The mechanisms of implantation include:

  1. Hatching

    • This occurs when the developing blastocyst is able to hatch from its surrounding zona pellucida.

  2. Apposition

    • This occurs when the trophoblast layer of cells communicate with the endometrial lining of the uterus.

  3. Adhesion

    • This occurs when there is communication between the blastocyst and the endometrial lining of the uterus on a molecular level.

  4. Invasion

    • This occurs when the trophoblast layer of cells begins to invade the endometrial lining of the uterus.

Formation of the Bilaminar Disc

By the second week of development, the cells within the developing blastocyst begin to reposition themselves.

The previously formed inner cell mass differentiates into an epiblast and hypoblast layer forming the bilaminar disc together.

The epiblast layer of cells will form the floor of the amniotic cavity, and the hypoblast layer of cells will surround the primitive yolk sac.

Meanwhile, the previously formed outer cell mass differentiates into an inner cytotrophoblast layer and an outer syncytiotrophoblast layer.

Formation of the Placenta

By the ninth day of development, the cytotrophoblast and syncytiotrophoblast layers are growing quicker than the bilaminar disc layer. Eventually, as the syncytiotrophoblast layer continues to further invade the endometrial lining of the uterus, it will encounter blood vessels and begin to form pools of maternal blood.

Simultaneously, chorionic villi will begin to grow to line the surface of the chorion to allow for the passage of blood between the capillaries and the villi. Eventually, once the placenta is formed, the embryo will have acquired a primitive circulatory system.

The placenta functions in the following processes:

  1. Exchange of gases between the developing fetus and its mother

  2. Waste removal

  3. Receiving nutrients

  4. Receiving antibodies and hormones

Clinical Notes:

  1. Placenta previa - this occurs when implantation occurs near the internal os of the cervix (low implantation).

  2. Pre-eclampsia - this occurs due to a placenta that is too low and may lead to hypoxia and the initiation of an immune response from the mother.