7.5 Brief summary

This module describes the stages of the differentiation of the embryonic disk from the 2nd to the 4th week of its development.

In the course of the 2nd week (7.1) the embryoblast differentiates itself into two germinal layers: the epiblast, out of which the tissue of the embryo as well as the amniotic epithelium will arise, and the hypoblast, which forms the umbilical vesicle.

During the 3rd week of its development, the epiblast experiences a number of complex changes that lead to the differentiation of the three embryonic germinal layers. It all begins with the appearance of the primitive streak (7.2), which is an accumulation of cells along the midline. This streak is the location where laterally immigrating cells sink down to form the deep layers of the mesoblast and endoblast.
So it comes to the formation of the trilaminar germ disk.
The mesoblast is divided into three parts: paraxial, intermediate and lateral plate mesoderm (7.2).
The paraxial part surrounds the neural tube and later forms the somites, in that it becomes segmented. The intermediate mesoderm gives rise to the urogenital system. The lateral plate mesoderm becomes divided into the somatopleural and splanchnopleural mesoderm. Together, they enclose the intraembryonic coelom (7.2).
During this time a cylinder-shaped , medially-located structure, the notochord, induces the differentiation of the neuroblast. This process is called neurulation (7.2). The median part of the epiblast thickens and forms a groove and afterwards a tube (neural tube), out of which the central nervous system will arise (7.2).
From the edges of the neural groove the neural crest cells are released, out of which the largest part of the peripheral nervous system is generated.

Two transitional structures, the notochord and the primitive streak, can lead to developmental anomalies, when they are not completely resorbed (7.3).
The sacro-coccygeal teratoma forms itself from remainders of the primitive streak, the chordoma from that of the notochord. Caudal dysplasia comprises a group of syndromes that affect the lower extremities and the intestines. An incomplete closure of the cranial folds of the neural tube leads to an anencephalia. When the same phenomenon happens in the caudal part of the neural tube, various forms of spina bifida result.

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