The formation of the bilaminar germ disc and the development of adnexal structures

During the second week of development, the embryo goes through major events, although it is important to note that not all embryos develop at the same rate even during this early time.

At the end of the first week, the blastocyst attaches to the uterine wall (nidation), leading to the proliferation of the trophoblast at the embryonic pole. The trophoblast differentiates into two layers: an outer layer called syncytiocytotrophoblast, which is a syncytium without distinct cell boundaries, and an inner layer called cytotrophoblast.

Between days 6-9, the syncytiocytotrophoblast becomes invasive and secretes hydrolytic enzymes that allow it to penetrate the endometrium. As implantation progresses, the syncytiocytotrophoblast surrounds the blastocyst, except for a small portion at the non-embryonic pole, which is covered by a coagulation plug.

From day 8, the embryoblast cells differentiate into two layers: the outer layer called the epiblast or primary ectoderm, and the inner layer called the hypoblast or primary endoderm. This bilaminar embryoblast is known as the bilaminar germ disc.

During this week, the amniotic cavity forms. On day 8, fluid collects between the epiblastic cells, pushing them towards the embryonic pole. These cells will differentiate into a thin membrane called the amniotic membrane, which separates the new cavity from the cytotrophoblast. The amniotic cavity initially smaller than the blastocyst cavity, gradually expands and eventually surrounds the entire embryo, particularly in the eighth week of development.

The formation of the primary yolk sac and chorionic cavity involves the migration of two groups of cells from the hypoblast towards the periphery. These cells will eventually form two membranes that border the blastocyst cavity. The first membrane transforms the blastocyst cavity into the primary yolk sac, while the second membrane forms the definitive yolk sac. The chorionic cavity is formed between the embryo, the amniotic cavity, and the yolk sac on one side, and the cytotrophoblast on the other side. The mechanism of formation for the chorionic cavity and definitive yolk sac is not fully understood, but the following stages of development are known:

• Formation of the primary yolk sac: On the 8th day, cells from the hypoblast migrate to the inner surface of the cytotrophoblast, forming a thin membrane called the exocoelomic membrane or Heuser's membrane. This membrane consists of flattened cells that delimit the forming primary yolk cavity until the 12th day of development. The blastocyst cavity is now referred to as the primary yolk sac or exocoelomic cavity. During the formation of the primary yolk sac, an acellular reticular material called extraembryonic reticulum is secreted between Heuser's membrane and the cytotrophoblast.
• Formation of the chorionic cavity: This occurs simultaneously with the appearance and development of the extraembryonic mesoderm. Between days 12-13, a population of distinct extraembryonic mesodermal cells appears within the mass of extraembryonic reticulum containing some hypoblastic cells. These mesodermal cells are believed to originate from the epiblast and the caudal portion of the bilaminar germ disc. They form two layers: one lining the outer surface of Heuser's membrane and another lining the inner surface of the cytotrophoblast.
  
  The extraembryonic reticulum between these two layers is replaced with fluid, resulting in the formation of the chorionic cavity. The chorionic cavity expands during the second week, gradually separating the amnion from the cytotrophoblast. By the 13th day, the embryonic disc, with the amnion dorsally and the yolk sac ventrally, is suspended in the chorionic cavity by a mesodermal stalk known as the connecting stalk.

Some authors propose that the extraembryonic mesoderm is not derived from the embryonic germ disc but from the delamination of the cytotrophoblast by Heuser's membrane. According to this perspective, the chorionic cavity appears through a process of vacuolization of the extraembryonic mesoderm, dividing it into an inner and outer layer.

The formation of the definitive yolk sac begins on the 12th day when the hypoblast cells start to proliferate and migrate. This causes a detachment of cuboidal hypoblastic cells from the inner surface of the extraembryonic mesoderm. Consequently, the old primary yolk sac is pushed towards the embryonic pole, although the exact mechanism of this replacement remains unclear.

As a result, the primary yolk sac narrows and disintegrates into a collection of extracoelomic vesicles that are no longer connected to the embryo. These extracoelomic vesicles become visible at the embryonic pole from the 13th day but eventually degenerate. The blastocyst cavity, which initially transformed into the primary yolk sac, now becomes the definitive yolk sac. This structure remains important for the embryo until the fourth week and serves several crucial primary functions.

The outer layer of the wall of the definitive yolk sac, known as the extraembryonic splanchnopleura, is formed by the extraembryonic mesoderm. It acts as a major site of hematopoiesis. On the other hand, the inner endodermal layer of the yolk sac secretes serum proteins and gives rise to germ cells that will populate the developing gonads. After the fourth week, the yolk sac is invaded by the developing embryonic disc. Normally, the yolk sac disappears after birth, but in some cases, it persists as an abnormal digestive tract known as Meckel's diverticulum.

During the first week of development, the embryo relies on simple diffusion for nourishment and waste elimination. However, as the embryo grows, more efficient exchange mechanisms are required. This is achieved through utero-placental circulation, a process in which maternal and fetal blood exchange through the placenta to facilitate gas and metabolic exchanges. On the 9th day, vacuoles or lacunae appear in the syncytiotrophoblast, forming the exchange system. Maternal capillaries near the syncytiotrophoblast extend and create maternal sinuses that penetrate the syncytiotrophoblast lacunae.

Between days 11-13, the cytotrophoblast proliferates and forms buds of chorionic villi that grow into the surrounding syncytiotrophoblast. The presence of extraembryonic mesoderm (extraembryonic somatopleura) bordering the chorionic cavity (extraembryonic celom) seems to reduce the appearance of these primary chorionic villi buds. From the 16th day, the extraembryonic mesoderm and cytotrophoblast penetrate the core of the primary chorionic villi buds, transforming them into secondary chorionic villi buds.

By the end of the third week, blood vessels develop in the mesoderm (somatopleura of the chorionic cavity) that penetrates the core of the villous buds, connecting with the embryo's own vessels. This establishes utero-placental circulation. These villi, which contain blood vessels, are referred to as tertiary villi.

At the end of the second week, the germ disc consists of two overlapping cellular discs: the epiblast and the hypoblast. The epiblast forms the floor of the expanding amniotic cavity, while the hypoblast forms the roof of the definitive yolk sac cavity. In the cephalic region of the hypoblastic disc, there is a small thickening called the precordial plate, which is a tightly attached cell group above the epiblastic disc.

References

1. Anatomia Omului, Volumul I: Embriologie; Semestrul I; Prof. univ. dr. med. Andrei Motoc; Editura „Victor Babeș”, Timișoara, 2018

2. Embriologie - Sinteze didactice; Romeo Chiriac, Anca Indrei ;Editura Fundatiei Chemarea, Iasi, 1997

3. Embriologie Umană; Lidia Chircor, Loredana Surdu;Editura Ex Ponto, Constanța, 2014

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