Axial Skeleton: Ossification

Though the standard histology text carries an explanation of endochondral ossification, the long bone is used as a model. This process as it applies to long bones is explained here. The irregular bones of the axial skeleton have a very different structure than the long bones, with only a thin layer of compact bone as compared to the strong, dense bone found in the shaft of long bone, implying a different ossification mechanism. While both would require an initial surfacing of perichondral bone to cut off the exchange of nutrients and gasses between cartilage and blood vessels, I could find no explanation of how the perichondral bone forms in the axial skeleton, as the bones do not have long shafts around which it can form. As perichondral ossification precedes endochondral ossification in evolution (showing up in some of the earliest jawless vertebrates) and has not been lost in the formation of the appendicular skeleton, it should theoretically play some role in the ossification of the axial skeleton as well. [D]

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The axial skeleton becomes ossified through chondral ossification. Primordial cartilaginous elements are laid down first, beginning in two centers on either side of the notochord. These grow up and around the notochord. Second and third cartilaginous centers arise dorsally and laterally to form the vertebral arches and the costal process.

Ossification in the vertebrae begins in these three different centers in each vertebrae: one for each vertebral arch and one for the body. These centers grow towards each other and at birth still have not fused, and complete fusion does not occur until the end of the first year of life. [B].

From an evolutionary standpoint, the axial and appendicular skeletons exist for very different purposes and were not simultaneous in their derivation. They are of different embryonic origins and are induced in very different ways, in addition to the disparity between the two stucturally.

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