Early Embryonic Development and Implantation

Mural Trophectoderm and Implantation

• Mural trophectoderm cells replicate their DNA without cell division, leading to the formation of trophoblast giant cells that invade the uterus during implantation.
• The remaining mural trophectoderm grows to form the ectoplacental cone and extra-embryonic ectoderm, which contribute to the formation of the placenta.

Primitive Endoderm and Parietal Endoderm

• Cells from the primitive endoderm migrate to cover the inner surface of the mural trophectoderm, becoming the parietal endoderm.
• The remaining primitive endoderm cells form the visceral endoderm, which covers the elongating egg cylinder containing the epiblast.

Embryo Development

• By 6 days after fertilization, an internal cavity has formed inside the epiblast, which becomes cup-shaped or U-shaped in cross-section.
• The embryo proper develops from this curved layer of epithelium, containing around 1000 cells.

Implantation and Uterine Changes

• The first visual sign of implantation is an increase in endometrial vascular permeability at the site of blastocyst apposition.
• The attachment reaction occurs at midnight (22:00-23:00 h) on the fourth day of gestation, leading to stromal edema and a marked increase in uterine weight.
• The receptive state is defined as the "window" of limited time when the uterine environment is conducive to blastocyst acceptance.

Blastocyst Development

• Early cleavages are slow, with the first occurring around 24 hours after fertilization and subsequent cleavages at 12-hour intervals.
• The morula contains around 10 internal cells and over 20 outer cells by the 32-cell stage.
• The internal cells of the morula give rise to the inner cell mass, while the outer cells form the trophectoderm.

Uterine Decidualization

• After the attachment of the blastocyst to the luminal epithelium, endometrial stromal cells (fibroblasts) proliferate and differentiate into decidual cells.
• The decidual cells produce hormones, growth factors, and matrix components to support implanting and post-implantation embryos.
• The primary decidual zone (PDZ) forms at 6 days of gestation, followed by the secondary decidual zone (SDZ).

Learn about the early stages of embryonic development, including the formation of trophoblast giant cells and the role of the trophectoderm in implantation. Understand how the primitive endoderm contributes to the development of the placenta.