Icon module 14

Genesis of the middle germinal layer (mesoderm)

With only a few exceptions the musculature stems from mesoblast cells that – for their part – come originally from a population of epiblast cells (ectoderm cells), which immigrate via the primitive streak (stages 6-11) between the hypoblast and the remaining epiblast cells. They thus form a middle germinal layer, the mesoderm layer.

Fig. 1 - The streaming in of epiblast cells via the primitive streak
Stage 8, ca. 23 days

  1. Prechordal plate
  2. Primitive nodes
  3. Primitive groove with entrance into the axial canal (canalis neuro-entericus)
  4. Primitive streak

Fig. 1

The left image shows an embryo dorsally. The epiblast cells stream in via the primitive streak and nodes and form the mesoderm layer. In the right image the embryo is shown in a median longitudinal section. The region of the prechordal plate is colored in red. The area where the cardiogenic plate arises is indicated by a red arrow.

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Detailed information about the structures in this diagram.

Differentiation of the mesoderm layer into muscle tissue

Except at two places, the cloacal and oropharyngeal membranes, the mesoderm layer separates the hypoblast completely from the ectoderm. The embryonic disk is now trilaminar (stage 8). Consequently, the cells of this mesoderm layer are called mesoblast cells until their specific mesenchymal or epithelial allocations are clear. Over the course of their further development, the mesoblast cells become clearer. A portion of these cells on both sides of the neural tube differentiate into somites and lateral plate (stage 9). The intermediary mesoderm (stage 10) becomes visible somewhat later.

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One supposes that somite formation is multi-factorial and is regulated by various genes. According to mutation analysis in animal experiments, the following genes are known: Fgfr1, Tbx6 and Wnt3a. It is assumed that such genes are activated mainly during the inflowing of cells via the primitive streak and nodes. The gene expression on presegmental mesoderm tissue is also studied. In none of these investigations, though, could a clear connection be found between the gene expression and the somite pattern in the same embryo. Thus it can only be surmised that additional, regional gene activities are responsible for forming the somites. The hox, pax and bHLH gene families are possibly involved.

The main things to be aware of are:

  • Mediolateral differentiation of the mesoderm in that the cells which are found closest to axial structures are subject to segmentation, whereas the more lateral mesoderm does not exhibit this pattern. (interactive diagram)
  • Dorso-ventral differentiation in mesoderm. It could be shown that a regional pattern exists already in epithelial somites before the cells differentiate. Depending on which region is involved, various inducing proteins work together on this dorso-ventral differentiation. (interactive diagram)
Fig. 2 - Section through an embryo in stage 10, ca. 28 days

Paraxial mesoderm
Intermediary mesoderm
Lateral plate mesoderm
Neural tube

Fig. 2

Dorsal view of the trilaminar embryo in stage 10. On the right side the ectoderm has been omitted, so the segmentation of the medially located somites is visible.

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Detailed information about the structures in this diagram.

Among other things, the skeletal musculature arises from the paraxial mesoderm. The intermediary mesoderm is involved in the formation of the urinary system. From the unsegmented lateral plate mesoderm, the somato- and splanchnopleura form with the coelom that lies in between them. Other mesoblast cells settle together to form the cardiogenic and the myocardial plates. This is a connected group of cuboid cells in the ventral mesoderm in front of the prechordal plate (see Fig. 1). From this ensemble of cells arises the cardiac musculature.