The Second Week of Embryo Development

Day 8[edit | edit source]

The blastocyst is partially incorporated into the endometrium of the uterus (into which it enters due to the proteolytic enzymes of the syncytiotrophoblast cells), and in order to fit in, the blastocyst must partially collapse and change its shape.

The trophoblast differentiates into two layers (in the area around the inner cell mass) - an inner layer = cytotrophoblast and an outer multinucleated layer with no visible boundaries between the cells = syncytiotrophoblast. The cells of the cytotrophoblast divide; mitotic figures are found here. The cells of the cytotrophoblast travel to the syncytiotrophoblast where they fuse and lose their individual plasma membrane. We do not find mitotic figures in syncytiotrophoblast cells.
The cells of the embryoblast (inner cell mass) also differentiate into two layers - a layer of cuboidal cells adjacent to the blastocyte cavity = hypoblast and a layer of tall cylindrical cells = epiblast.

Together, the hypoblast and epiblast form a flat structure. Inside the epiblast, forms a cavity, which enlarges and becomes the amniotic cavity. The cells of the epiblast adjacent to the cytotrophoblast are called the amnioblast. The amniotic cavity is bounded by the amnioblast together with the epiblast. Stromal cells in the endometrium fill with glycogen and lipids, take on a polyhedral shape, and thus transform the endometrium into the decidua (themselves into decidual cells). The decidual cells near the syncytiotrophoblast degenerate and are then engulfed by the embryo (histiotrophy).

Day 9[edit | edit source]

The blastocyst is deeper in the endometrium. The place where the blastocyst entered the endometrium is closed by a fibrin coagulum = operculum deciduae. The trophoblast continues to develop mainly at the embryonic pole, where cavities form in the syncytium. Eventually these cavities fuse to form larger lacunae. At the same time, at the abembryonic pole, the flat cells form a thin exocoelom = Heuser's membrane, which lines the inner surface of the cytotrophoblast. The Heuser membrane together with the hypoblast delimits the exocoelomic cavity = primitive yolk sac.

11th and 12th day[edit | edit source]

From day 11th or 12th, the blastocyst is completely embedded in the endometrium and the endometrial surface epithelium is completely re-epithelialized at the site of blastocyst entry. The lacunae in the syncytium form an intercommunicating network, mainly at the embryonic pole, while at the abembryonic (opposite) pole the trophoblast is mainly composed of cytotrophoblast cells. Syncytiotrophoblast cells penetrate deeper into the endometrium where they disrupt the endothelial layer of blood capillaries (congested and dilated sinusoids). Eventually, the sinusoids fuse with the lacunae and maternal blood enters the lacunar system. Maternal blood begins to flow through the trophoblastic system of the lacunes, and uteroplacental circulation begins. Meanwhile, a new population of cells derived from the yolk sac cells appears between the inner surface of the cytotrophoblast and the outer surface of the exocoelomic cavity. These cells form a fine connective tissue = extraembryonic mesoderm. The extraembryonic mesoderm forms large cavities, when these cavities fuse, a cavity is formed = extraembryonic coelom = chorionic cavity. The chorionic cavity surrounds the primitive yolk sac and the amniotic cavity, except the place where the germinal target is connected to the trophoblast by a germinal stalk (connecting stalk).

The extraembryonic mesoderm covering the yolk sac is referred to as extraembryonic splanchnopleuric mesoderm.
The extraembryonic mesoderm below the cytotrophoblast and around the amniotic sac is referred to as the extraembryonic somatopleural mesoderm.

The tertium remains relatively small.

Day 13[edit | edit source]

By day 13, the superficial defect in the endometrium is usually healed. Sometimes bleeding occurs at the implantation site (increased blood flow in the lacunar spaces). This bleeding can occur around day 28 of the menstrual cycle and can therefore be mistaken for normal menstrual bleeding. The trophoblast has a characteristic wedge-shaped structure. The cells of the cytotrophoblast proliferate locally and penetrate the syncytiotrophoblast to form cell columns surrounded by syncytia = primary villi. The hypoblast produces additional cells, which migrate along the inner wall of the Heuser's membrane. These cells proliferate and eventually form a cavity within the exocoelomic cavity of the secondary yolk sac (definitive yolk sac). This is much smaller than the exocoelomic cavity (primitive yolk sac). During the formation of the definitive yolk sac, much of the exocoelomic cavity is separated. The separated portion represents the exocoelomic cyst, which is often found in the extraembryonic coelom (chorionic cavity). While the extraembryonic coelom expands to form a large cavity (chorionic cavity), the extraembryonic mesoderm lies on the inner side of the cytotrophoblast as a chorionic plate. The only place where the extraembryonic mesoderm crosses the chorionic cavity is the germinal stem. With the development of blood vessels, the germinal cord becomes the umbilical cord^[1].