Notochord action on spinal development. A histologic and morphometric investigation

Forty-seven human embryos and fetuses of a gestational age from 5 to 13 weeks were studied histologically and histomorphometrically in an attempt to elucidate the possible action of the notochord on spinal development, in particular as it relates to segmentation. Segmentation occurred first in the tissue surrounding the notochord and not in the notochord itself; notochordal development at its most cranial part lagged behind that of its surrounding tissue. We also found that in later stages the decrease of notochordal cells at the level of the body was due to degeneration and not to migration of cells toward the intervertebral space. This observation puts into doubt the theory of squeezing or migration of notochordal cells from their vertebral position into the intervertebral disc space.