Embryonic Development Day 8

Blastocyst Development

• By the 8th day, the blastocyst is partially embedded in the endometrial stroma.
• The trophoblast differentiates into two layers: the cytotrophoblast (inner layer of mononucleated cells) and the syncytiotrophoblast (outer multinucleated zone).
• Mitotic figures are found in the cytotrophoblast, but not in the syncytiotrophoblast.
• Cells in the cytotrophoblast divide and migrate into the syncytiotrophoblast, where they fuse and lose their individual cell membranes.

Inner Cell Mass Differentiation

• The inner cell mass or embryoblast differentiates into two layers: the hypoblast layer (small cuboidal cells) and the epiblast layer (high columnar cells).
• Together, these layers form a flat disc.
• A small cavity appears within the epiblast, which enlarges to become the amniotic cavity.

Amniotic Cavity and Epiblast

• Epiblast cells adjacent to the cytotrophoblast are called amnioblasts; they line the amniotic cavity.
• The endometrial stroma adjacent to the implantation site is edematous and highly vascular.

Development by Day 9

• The blastocyst is more deeply embedded in the endometrium, and the penetration defect in the surface epithelium is closed by a fibrin coagulum.
• The trophoblast shows considerable progress in development, particularly at the embryonic pole, where vacuoles appear in the syncytium.
• These vacuoles fuse to form large lacunae, marking the lacunar stage of trophoblast development.

Exocoelomic Membrane

• At the abembryonic pole, flattened cells from the hypoblast form a thin membrane, the exocoelomic (Heuser's) membrane, which lines the inner surface of the cytotrophoblast.
• This membrane, together with the hypoblast, forms the lining of the exocoelomic cavity or primitive yolk sac.

Development by Day 11-12

• The blastocyst is completely embedded in the endometrial stroma, and the surface epithelium almost entirely covers the original defect in the uterine wall.
• The trophoblast is characterized by lacunar spaces in the syncytium that form an intercommunicating network.
• The syncytial lacunae become continuous with the sinusoids, and maternal blood enters the lacunar system, establishing the uteroplacental circulation.

Extraembryonic Mesoderm

• A new population of cells appears between the inner surface of the cytotrophoblast and the outer surface of the exocoelomic cavity.
• These cells, derived from yolk sac cells, form a fine, loose connective tissue, the extraembryonic mesoderm.
• The extraembryonic mesoderm eventually fills the space between the trophoblast and the amnion and exocoelomic membrane.
• Large cavities develop in the extraembryonic mesoderm, forming the extraembryonic cavity or chorionic cavity.