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Developmental stability in the mouse vertebral column.

The effects of six mutant genes (Sd, T, vt, Sp, un, Ph) on the axial skeleton of the mouse were studied both alone and in combination with 24 hours starvation on Day 8 of gestation. Normal controls showed a craniocaudal gradient of size variation and relatively low variability towards the boundaries of the cervical, thoracic and lumbar vertebral classes. Gene-induced craniocaudal gradients of abnormality were observed in response to Sd and T, and to Ph, which have direct and indirect effects respectively on the primitive streak and/or notochord. By contrast, vt, which influences the paraxial mesoderm, and Sp and un, which act later, during somite formation and sclerotome differentiation respectively, did not produce craniocaudal gradients of abnormality. For all genes there were indications of relative resistance to their effects towards the vertebral class boundaries. Starvation did not materially influence these patterns of abnormality. The division of the axial skeleton of the mouse into its morphologically distinct classes of vertebrae thus appears to originate at the earliest stage of axial development, even before somite formation, with the positions of the future vertebral class boundaries established both axially and paraxially as regions of relative stability that persist into later stages of development. The primary craniocaudal gradient associated with the advancing primitive streak, from which the vertebral class boundary positions may be derived, appears to be restricted to the axial structures themselves and to be lost after the somites appear.