As the trophoblast invades deeply into the endometrium, the maternal capillaries in the vicinity become congested with blood cells and dilate to form sinusoids. The syncytial trophoblast erodes the wall of the sinusoids and becomes continuous with their endothelial lining.
Spaces called lacunae develop in the syncytial trophoblast by fusion of intracytoplasmic vacuoles. The lacunae join together to form an intercommunicating network that becomes continuous with the maternal blood vessels. Maternal blood begins to flow through the lacunar network, establishing the early uteroplacental circulation.
The cellular trophoblast is located on the inner or embryo side of the syncytial trophoblast. Cells called extraembryonic mesoblasts form along the inner surface of the cellular trophoblast. Their origin is controversial.
CHORION
By the end of the second week the extraembryonic mesoblasts fuse with the overlying trophoblast to form a membrane called the chorion. The chorion encloses the embryo proper and all of the other extraembryonic membranes.
Only a portion of the chorion together with the adjacent endometrium form the placenta in subsequent weeks.
Fingerlike projections called villi appear in the chorion. They develop in the following manner. Small areas of the single-layered cellular trophoblast begin to proliferate into cell masses that extend into strands of the syncytial trophoblast. Such a cellular mass is called a primary villus. Toward the end of the second week some of the cell masses change into columns of cells that have a core of extraembryonic mesoblasts. For a short period the core is avascular and is called a secondary villus. During the third week blood vessels differentiate in the extraembryonic mesoblast and connect the villus with the vascular system of the embryo proper. A villus with blood vessels in its core is called a tertiary villus.
YOLK SAC
Extraembryonic mesoblasts line the entire blastocoele converting it into the primary yolk sac cavity. These cells form a thin membrane on their inner aspect called the exocoelomic or Heuser’s membrane that is continuous with the endoderm of the embryonic disc. The primary yolk sac cavity is enclosed for a short period by the exocoelomic membrane and embryonic endoderm.
In the second half of the second week the endoderm at the periphery of the embryonic disc gives rise to cells that migrate from the disc to completely line the inner aspect of the exocoelomic membrane. This lining comprises the extraembryonic endoderm.
The extraembryonic mesoblasts produce a reticulum between the outer aspect of the exocoelomic membrane and the cellular trophoblasts. Large spaces develop within the reticulum and coalesce to form the extraembryonic coelom or chorionic cavity. Extraembryonic mesoblasts comprise the outer covering of the primary yolk sac.
By the end of the second week the lower part of the primary yolk sac pinches off, resulting in a smaller sac next to the embryonic disc called the secondary (definitive) yolk sac. The detached remnant in the extraembryonic coelom is called the exocoelomic cyst (Fig. 2–6, Sect. 1).
AMNION
Early in the second week another membrane known as the amnion begins to form by delamination from the cellular trophoblasts adjacent to the amniotic cleft.
By the end of the week the amnion is composed of an inner thin layer of squamous cells called extraembryonic ectoblasts and an outer covering of extraembryonic mesoblasts. The inner layer is continuous with the epiblast of the embryonic disc and encloses a space above the disc called the amniotic cavity. A small diverticulum called the amniotic duct is a transient structure in the dome of the amnion and has no known function (Figs. 2–5, 2–6, Sect 3).
YOLK SAC
Extraembryonic mesoblasts accumulate around the caudal end of the embryonic disc and join it and the adjacent amnion to the cellular trophoblast. The junction area is called the connecting (body) stalk, which is the forerunner of the umbilical cord. It is surrounded by the extraembryonic coelom.
ALLANTOIS
A tiny diverticulum called the allantois appears in the caudal wall of the secondary yolk sac. It extends into the connecting stalk and, for the most part, will remain a rudimentary structure.
Source: Atlas of Human Embryos.