Human Development Week 2 - Formation of the Embryonic Disc
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Introduction
Implantation is completed in the beginning of week 2. The invading syncytiotrophoblast encounters maternal blood vessels, forming lacunae that will nourish the embryo.
By day 10 the embryo is completely embedded. The syncytiotrophoblast, is solid cell mass but begins encountering the many glands and vessels of the endometrium in secretory phase. This creates lacunae that fill with maternal blood and surround the embryo.
The endometrium behind the site of implantation forms a surrounding layer of decidual cells (the decidua basalis). This forms an immune-priviledged site for the embryo.
The Embryo
Morphological changes occur in the embryoblast to form the embryonic disc, a bilaminar plate of cells consisting of:
- epiblast: pluripotent columnar cells which give rise to the embryo proper. The epiblast forms the floor of the amnionic cavity and is continuous with the amnion
- hypoblast: forms from the inner cell mass and forms the extra-embryonic endoderm surrounding the blastocyst cavity. The primary yolk sac forms from the hypoblast.
The Yolk Sac
The yolk sac, which is responsible for nutrient and gas transfer, develops from the hypoblast. It becomes surrounded by the exocoelomic membrane.
Development of the Chorionic Sac
The chorionic sac, which is the precursor to the placenta, develops from the trophoblastic syncytiotrophoblasts and cytotrophoblasts, and as implantation is complete envelops the embryo. By the end of the second week, primary chorionic villi form from cytotrophoblasts. This growth is thought to be induced by underlying extraembryonic somatic mesoderm.
With further development, mesenchymal cells invade the primary chorionic villi, resulting in secondary choronic villi. These cells go on to form fetal blood vessels, at which point tertiary chorionic villi are present.
The chorionic cavity, also known as the extraembryonic coelom, contain the embryo and yolk sac attached to the rest of the chorion by the connecting stalk.
During week three, the embryo begins rapidly developing, with primitive organ systems emerging. The chorionic sac continues to develop secondary and then tertiary villi.
Hyperplasia of trophoblasts within the uterine, together with loss of the embryo, leads to hyadiform mole formation.
Choriocarcinomas are cancers arising from trophoblastic cells that may occur following pregnancy, abortion, or hyadiform mole.
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