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).