The intraembryonic mesoderm can be broken into paraxial, intermediate and lateral mesoderm relative to its midline position. During the 3rd week the paraxial mesoderm forms into "balls" of mesoderm paired either side of the neural groove, called somites. Somites appear bilaterally as pairs at the same time and form earliest at the cranial (rostral,brain) end of the neural groove and add sequentially at the caudal end. This addition occurs so regularly that embryos are staged according to the number of somites that are present. Different regions of the somite differentiate into dermomyotome (dermal and muscle component) and sclerotome (forms vertebral column).
see also Larsen Movie- Fate of the Somite (315Kb)
from Kyoto Collection
A brief Figure overview is shown below. More detailed overviews are shown on other notes pages (Mesoderm and Somite, Vertebral Column, Limb) in combination with serial sections and Carnegie images.
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Cells migrate through the primitive streak to form mesodermal layer. Extraembryonic mesoderm lies adjacent to the trilaminar embryo totally enclosing the amnion, yolk sac and forming the connecting stalk. |
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Paraxial mesoderm accumulates under the neural plate with thinner mesoderm laterally. This forms 2 thickened streaks running the length of the embryonic disc along the rostrocaudal axis. In humans, during the 3rd week, this mesoderm begins to segment. The neural plate folds to form a neural groove and folds. |
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Segmentation of the paraxial mesoderm into somites continues caudally at 1 somite/90minutes and a cavity (intraembryonic coelom) forms in the lateral plate mesoderm separating somatic and splanchnic mesoderm.
Note intraembryonic coelomic cavity communicates with extraembryonic coelom through portals (holes) initially on lateral margin of embryonic disc. |
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Somites continue to form. The neural groove fuses dorsally to form a tube at the level of the 4th somite and "zips up cranially and caudally and the neural crest migrates into the mesoderm.
Next Somite Development
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Mesoderm beside the notochord (axial mesoderm) thickens, forming the paraxial mesoderm as a pair of strips along the rostro-caudal axis. Paraxial mesoderm towards the rostral end, begins to segment forming the first somite. Somites are then sequentially added caudally. The somitocoel, is a cavity forming in early somites, which is lost as the somite matures. |
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Cells in the somite differentiate
medially to form the sclerotome (forms
vertebral column) and laterally to form
the dermomyotome. The dermomyotome then
forms the dermotome (forms dermis) and
myotome (forms muscle). Neural crest cells migrate beside and through somite. |
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The myotome differentiates to form 2 components dorsally the epimere and ventrally the hypomere, which in turn form epaxial and hypaxial muscles respectively. The bulk of the trunk and limb muscle coming from the Hypaxial mesoderm. Different structures will be contributed depending upon the somite level.
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Note these are very simplified figures. |
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Week 3 is about trilaminar embryo development and the beginning of each layer differentiation. Outside of the embryonic disc, early placentation is occurring.
At the trilaminar embryonic disc stage, the middle mesoderm layer will form the connective tissues oof the body.
A key early step is the formation of the notochord within the mesoderm, which does not contribute to the nervous system, but is a "patterning" embryonic structure that regulates the development of surrounding structures (such as the neural plate and somites).
Please email Dr Mark Hill if you wish to make a comment about this current project.
