Embryonic Development: Week 0 to Week 8

Embryonic Development (Weeks 0-8)

Early Mitotic Division

  • Within approximately 36 hours after fertilization, the zygote undergoes its first mitotic division, resulting in two cells.

72 Hours (3 Days)

  • Cell division continues rapidly.
  • At around 72 hours, the developing structure consists of 16 or more cells.
  • This stage is referred to as the Moorula.

Days 4-5: Blastocyst Formation

  • Around days 4 to 5, the developing structure contains approximately 100 cells.
  • It is now called a Blastocyst.
  • The blastocyst at this stage is considered an early blastocyst.

Day 7: Blastocyst Arrival in Uterus

  • By day 7, the fully developed blastocyst arrives in the uterus, after traveling through the fimbria, infundibulum, and uterine tube.
  • The blastocyst is equipped with adhesion molecules (proteins) on its surface.

Adhesion Molecules and Implantation

  • Adhesion molecules help the blastocyst find an appropriate implantation site in the uterus.
  • The uterine lining (endometrium) is in the secretory phase, prepared for implantation.
  • The endometrium is rich in glycogen, nutrients, and blood vessels, providing a nourishing environment for the blastocyst.

Embryonic Disc (Embryoblast)

  • Adhesion molecules are specifically located above the embryonic disc or embryoblast. The embryoblast contains only 20-30 cells at this stage, which will eventually develop into the entire embryo.

Trophoblast

  • Surrounding the embryoblast is the trophoblast.
  • The trophoblast has several key functions, including:
    • Differentiating into various layers.
    • Facilitating the implantation/burrowing process into the endometrium.

Blastocyst Cavity

  • The blastocyst cavity is a fluid-filled space within the blastocyst.
  • It imbibes fluid, providing nutritional support to the developing embryonic tissue even before the placenta is fully formed.
  • Nutrients, hormones, and other essential substances are transferred to the embryo, while waste products are removed.

Inner Cell Mass

  • The embryoblast is also referred to as the inner cell mass or embryonic plate.

Trophoblast Function

  • The trophoblast, located on the outer layer of the blastocyst, contains adhesion molecules that mediate attachment to the uterine wall.
  • The trophoblast facilitates the burrowing process, embedding the blastocyst within the endometrium.