Chapter 15. Week 2 of Development: Bilaminar Germ Disk Embryo* - Review of Medical Embryology Book

Introduction: on day 7, endometrial invasion has begun, and the trophoblast differentiates into 2 layers: a cytotrophoblast and a syncytiotrophoblast or syncytium. The active erosive trophoblast invades the endometrial stroma containing capillaries and glands, and the blastocyst slowly sinks into the endometrium. The syncytium at the embryonic pole, near the developing embryo, rapidly becomes a thick, multinucleated protoplasmic mass with no cell boundaries. It is invasive, ingestive, and digestive, and the conceptus derives its initial nourishment from endometrial tissues. Later it receives nutrients directly from maternal blood. The cytotrophoblast is mitotically active and forms new cells that migrate into the increasing mass of the syncytiotrophoblast.
In week 2 of implantation, the trophoblast penetrates deeper into the endometrium, and the blastocyst changes morphologically. The inner cell mass produces a bilaminar embryonic disk composed of epiblast (future embryonic ectoderm and mesoderm) and embryonic endoderm. Concomitantly, the amniotic cavity, yolk sac, connecting stalk, and chorion develop.
Normal stages of week 2 development

The cells of each germ layer form a flat disk and together are known as the bilaminar germ disk
Cells of the ectodermal layers, initially firmly attached to the cytotrophoblast, develop small clefts between their layers as development proceeds

The clefts coalesce and form a cavity, the amniotic cavity
Amnioblasts, large, flattened cells, are seen along the trophoblastic border of the newly formed amniotic cavity (probably derived from trophoblast)

The cells are continuous with the ectoderm and together line the amniotic cavity

Endometrial stroma adjacent to the implantation site is edematous, highly vascular, with large tortuous glands that secrete glycogen and mucus

DAY 9: blastocyst embeds deeper into endometrium, and a fibrin coagulum "plug" (blood clot and cellular debris) closes the penetration defect in uterine epithelial surface - interstitial implantation

Trophoblast progresses in development, especially at the embryonic pole, and vacuoles appear in the syncytium. The vacuoles fuse to form large lacunae (lakes), and we have the lacunar stage of trophoblast development
Endometrial stroma around the trophoblast has vascular congestion, and the cells are rich in glycogen
Flattened cells delaminate from the inner surface of the cytotrophoblast, at the abembryonic pole, and form a thin membrane called Heuser's exocoelomic membrane which is continuous with the edges of the entodermal layer. Together, they form the lining of the exocoelomic cavity or the primitive yolk sac

*Embryos of the same fertilization size do not necessarily develop at the same rate. Considerable differences are seen in early developmental stages.