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| *Somites are simple at this stage, with epithelial cells surrounding an inner cavity that is filled with mesenchymal pre - somitic mesoderm cells. These cells are called '''somitocoel cells'''. | | *Somites are simple at this stage, with epithelial cells surrounding an inner cavity that is filled with mesenchymal pre - somitic mesoderm cells. These cells are called '''somitocoel cells'''. |
| *All cells are undergoing mitosis. | | *All cells are undergoing mitosis. |
| + | |
| + | ===Somites IV - X=== |
| + | *Epithelial cells on the ''dorsal'' aspect of the somite develop into the dermomyotome. |
| + | *Epithelial cells of the ''ventral - medial'' portion of the somites develop into the sclerotome. |
| + | **Cells undergo an epithelial to mesenchymal transition. |
| + | **Somitocoel cells join these new mesenchymal cells to also become part of the sclerotome. |
| + | *The prospective sclerotome migrates to surround the notochord and neural tube. |
| + | *''The scelerotome needs to be at the midline to form the vertebrae.'' |
| + | |
| + | ===Somites VI Onward=== |
| + | *At the dorsal aspect of the somite, some cells of the dermomyotome are ingressing to form a new layer between the dermomytome and sclerotome; the '''myotome'''. |
| + | *Ingression occurs at two points; ventral - laterally and dorsal - medially. |
| + | :'''Ventral - lateral ingression develops into ''hypaxial muscle''. This forms muscle of the body and limb.''' |
| + | :'''Dorsal - medial ingression develops intp the ''epaxial muscle''. This forms the muscles of the dorsum.''' |
| + | *Myotomal cells do not have the ability to undergo mitosis. To increase cell numbers, cells are recruited from the dermomyotome. |
| + | *After all the myotomal cells have ingressed what remains of the dermomyotome becomes the dermotome. |
| + | *''The dermotome, which lies dorsally in the somite, forms the dermis.'' |
| + | |
| + | |
| + | ===Somites XI Onward=== |
| + | *The sclerotome surrounds the notochord and breaks it down. |
| + | *''Thus far, somite development has been segmented. Cells need to be adjacent to each other to function appropriately, so the sclerotome, in the ventral part of the somite, resegments.'' |
| + | #A fissure develops in the middle of the sclerotome, dividing each somite into an anterior and posterior portion. |
| + | #The anterior portion of one somite unites with the posterior portion of the adjacent somite - '''resegmentation'''. |
| + | #*But; because only the sclerotome resegments, the muscle derived from the myotome remains to unite the original anterior and posterior pairs together. |
| + | #The sclerotome surrounds the neural tube and differentiates firstly to cartilage and secondly bone. It forms the vertebrae. |
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| ==Species Differences== | | ==Species Differences== |