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| ::'''Dermotome''' - develops into the dermis. | | ::'''Dermotome''' - develops into the dermis. |
| *Somites develop in an identical manner, although those at the posterior develop after those at the anterior. | | *Somites develop in an identical manner, although those at the posterior develop after those at the anterior. |
| + | *Somitic derivatives form according to the somite's position along the anterior - posterior axis. |
| + | **Formation of sclerotome, myotome and dermotome is not intrinsic, but governed by '''environmental signals'''. |
| + | **However, positional information is '''predetermined and intrinsic''' to the somite. Positional information is important to form the appropriate type of vertebrae, muscle and skin along the anterior - posterior axis. |
| + | **Positional information is governed by HOX gene expression. Each somite expresses a unique number of HOX genes. |
| *The chick has 12 somites. | | *The chick has 12 somites. |
| | | |
− | ''Stages of somite maturation as observed along the anterior - posterior axis:'' | + | ''Stages of somite maturation as observed along the anterior - posterior axis of the chick embryo:'' |
| | | |
| ===Somites I - III=== | | ===Somites I - III=== |
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| ===Somites IV - X=== | | ===Somites IV - X=== |
− | *Epithelial cells on the ''dorsal'' aspect of the somite develop into the dermomyotome. | + | *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. | + | *Epithelial cells of the ''ventral - medial'' portion of the somites develop into the '''sclerotome'''. |
| **Cells undergo an epithelial to mesenchymal transition. | | **Cells undergo an epithelial to mesenchymal transition. |
| **Somitocoel cells join these new mesenchymal cells to also become part of the sclerotome. | | **Somitocoel cells join these new mesenchymal cells to also become part of the sclerotome. |