Chapter 16. Week 2 of Development: Days 10 to 14 - Review of Medical Embryology Book

The blastocyst becomes completely embedded in the endometrial stroma, and the uterine surface epithelium almost entirely covers the original epithelial lining defect of the mucos Only a slight protrusion is seen in the uterine lumen
The trophoblast is characterized by lacunar spaces in the syncytium, and they form an interconnecting network, particularly at the embryonic pole
At the abembryonic pole, the trophoblast consists of cytotrophoblastic cells and only a few lacunar spaces
The syncytial cells penetrate deep into the stroma and erode the endothelial lining of maternal congested and dilated capillaries called sinusoids

The syncytium becomes continuous with the endothelial cells of the vessels, and maternal blood enters the lacunar system
With more and more sinusoid invasion by the trophoblast, the lacunae eventually become continuous with the arterial and venous systems. Pressure differences between arterial and venous capillaries result in maternal blood flowing through the trophoblastic lacunar system to form the uteroplacental circulation

Cytotrophoblast also differentiates. On its inner surface, cells delaminate to form a fine, loose tissue, the extraembryonic mesoderm, which fills the space between external trophoblast and amnion and internal yolk sac

Large cavities develop in this extraembryonic mesoderm, become confluent, and form the extraembryonic coelom, which surrounds the primitive yolk sac and amniotic cavity, except where the extraembryonic mesoderm forms the future connection between the germ disk and the trophoblast

The extraembryonic mesoderm lining the cytotrophoblast and amnion is called the extraembryonic somatopleuric mesoderm; that covering the yolk sac is called the extraembryonic splanchnopleuric mesoderm

The bilaminar germ disk grows slowly compared to the trophoblast, but by the end of day 12, entodermal cells begin to spread over the inside of Heuser's membrane
Endometrial cells become polyhedral, are loaded with glycogen and lipids, and the intercellular spaces fill with extravasate; the tissue is edematous - all a process of the decidual reaction, initially at implantation site but then throughout endometrium

DAYS 13 AND 14: the endometrial surface defect is usually healed, but there may be occasional bleeding at the site of implantation due to the increased blood flow in the lacunar spaces at the abembryonic pole. Can be confused with menstrual bleeding

The trophoblast shows more organization at the embryonic pole
The cytotrophoblast cells proliferate, penetrate the syncytium, and form cellular columns surrounded by syncytium-together forming the primary stem villi
The entodermal germ layer proliferates and newly formed cells gradually line a new cavity known as the secondary or definitive yolk sac (smaller than original)
The extraembryonic coelom expands to form the chorionic cavity
The extraembryonic mesoderm then lines the cytotrophoblast and is called the chorionic plate. It also forms a covering layer for the secondary yolk sac and amnion

The extraembryonic mesoderm only traverses the chorionic cavity in the connecting stalk (connecting the embryo with the trophoblast)

By the end of week 2, the germ disk consists of two apposed cell disks: the ectodermal germ layer, forming the floor of the expanding amniotic cavity, and the entodermal germ layer, forming the roof of the secondary yolk sac

In its cephalic region, the entodermal disk is thickened to form the prochordal plate, an area of columnar cells attached to the overlying ectodermal disk