Blastulation

Timeline of Early Embryonic Development

Cleavage Divisions

  • Occurs right after fertilization (approximately within four days).

  • The fertilized zygote travels down the uterine cavity.

  • Undergoes cleavage divisions, where cell numbers double with each division.

    • Each division occurs approximately every twelve hours.

  • At the stage of morula, approximately 16 to 32 cells are present.

  • The zona pellucida weakens, leading to the hatching of the blastocyst from it, leaving behind debris.

Formation of the Blastocyst

  • Blastulation: The process where the morula develops into a blastocyst after cleavage divisions.

  • The number of cells in the blastula ranges from about 64 to 100 cells, which may still be considered a morula if contained within the zona pellucida.

  • The blastula is formed from many blastomeres and breaks free from the zona pellucida.

  • Subsequently, the cells continue to divide:

    • From 200 cells → 400 cells → 800 cells → 1,600 cells.

  • Results in a large cell mass with round, cuboidal cells.

    • These cuboidal cells are efficient at secreting substances that push other cells aside.

  • Fluid-filled cavity develops leading to the formation of:

    • Inner Cell Mass: Group of cells that eventually forms the embryo.

    • Trophoblast Cells: Outer layer of cells ("tropho" means nutrient).

Components of the Blastocyst

  • Blastocoel: The fluid-filled cavity within the blastocyst.

  • The blastocyst continues to move down the uterine wall.

Hormonal Changes During Uterine Cycle

  • At ovulation:

    • Estrogen levels:

    • Low initially, then increase around day fourteen (ovulation).

    • Estrogen dips due to damage to granulosa cells after peak.

    • Progesterone levels:

    • Increase about 1-5 days post ovulation, correlating with the development of the uterine lining.

  • The window for fertilization is within 24 hours post-ovulation peak determined by luteinizing hormone (LH).

  • The blastocyst typically enters the uterine cavity between day four and five post-fertilization.

Uterine Lining Development

  • The uterine lining undergoes distinct phases:

    • Menstrual Phase: Declining lining.

    • Proliferative Phase: Thickening of the lining.

    • Secretory Phase: Lining becomes rich and supportive for potential implantation.

    • The trophoblast cells have integrin and L-selectin molecules which aid in adhesion to the uterine lining's glycoproteins.

The Implantation Process

  • Implantation: Begins when the blastocyst's inner cell mass approaches the uterine wall.

    • The uterine lining's functional layer includes:

    • Spiral arteries and glycoproteins that support implantation.

  • If the uterine environment is not yet ready, the blastocyst may continue traveling downward.

  • Generally, for implantation to occur, it must be prepared before the average cycle day 28 to avoid menstruation.

  • The blastocyst usually implants high within the uterus as the inner mass tumbles due to its l-selectins and integrins.

  • Implantation process duration: Approximately one week.

Changes in Trophoblast Cells

  • Upon contact with the uterine wall, trophoblast cells undergo differentiation into:

    • Outer Layer: Syncytiotrophoblasts (fusion of trophoblasts).

    • Inner Layer: Cytotrophoblasts.

  • Trophoblasts rapidly divide, increasing cell type numbers during implantation.

  • Blastulation occurs from day six to day seven leading up to a rapid increase in cells for implantation, spanning from day seven to day fourteen.