Introduction to the developmental processes during the third week (stages 6 to 7; ca. 17-19 days)
This chapter is devoted to the most important events during the embryonic period. All of the details of organogenesis were thoroughly treated in the corresponding chapters.
The third week of embryonic development was discussed in the trilaminar embryonic disk module. We come back to it here briefly in order to speak about angiogenesis and the formation of the embryonic blood circulation system
The angioblasts already appear early in the mesoblast (stage 5). The first blood vessels (angiogenesis) become visible in the third week in the splanchnopleura that adjoins the umbilical vesicle and the allantois (stage 7). They are also found in the extraembryonic part of the somatopleura at the level of the body stalk (stage 6) and again in the vicinity of the tertiary villi of the placenta (stage 6b).
The endocardiac tubes form in the middle of the third week in the cardiogenic zone above the prechordal plate.
- The extraembryonic mesoblast differentiates into the angioblast in that it forms angiogenetic islands (aggregates of angioblasts). Within these blood islands lacunae form.
The cells in the periphery become flattened and differentiate into endothelial cells.
The blood islands merge, gradually forming tubes that develop into vessels. The embryonic vessels possess great plasticity. The blood vessels inside the embryo also form in the way just described, starting in the intraembryonic mesoblast. As soon as the two circulation systems meet with one another, they join together to form the feto-maternal circulation system - The hematopoiesis begins in the third week in the extraembryonic section of the splanchnopleura (stage 7). It proceeds from the endothelial cells (hematoblasts). After the fourth week the liver takes over the task of generating the blood cells (stage 12).
The mesoblast cells that surround the blood vessels differentiate and form smooth muscle- and connective tissue cells. The latter participate in the formation of the adventitia (outer layer of the blood vessel).