Kinetics and differentiation of somite cells forming the vertebral column: studies on human and chick embryos

We have studied the kinetics of somite cells with an antibody against proliferating cell nuclear antigen (PCNA/cyclin) in human and chick embryos, and with the BrdU anti-BrdU method in chick embryos, to investigate whether the metameric pattern of the developing vertebral column can be explained by different proliferation rates. Furthermore we applied antibodies against differentiation markers of chondrogenic and myogenic cells of the somites in order to study the correlation between proliferation and differentiation. There are no principal differences in the proliferation pattern of the vertebral column between human and chick embryos. In all stages examined, the cell density is higher in the caudal sclerotome halves than in the cranial halves. Laterally, the caudal sclerotome halves, which give rise to the neural arches, are characterized by a higher proliferative activity than the cranial halves. Although there is a high variability, the labelling indices show significant differences between the two halves with both proliferation markers. With the onset of chondrogenic differentiation, only the perichondrial cells retain a high proliferation rate. During fetal development, the neural arches and their processes grow appositionally. Even at the earliest stages, there is practically no immunostaining for PCNA or BrdU in the desmin-positive myotome cells of human and chick embryos. Axially, a higher proliferation rate is found in the condensed mesenchyme of the anlagen of the intervertebral discs than in the anlagen of the vertebral bodies. During fetal development, cells at the borders between vertebral bodies and intervertebral discs proliferate, indicating appositional growth. Our results show that local differences in the proliferation rates of the paraxial mesoderm exist, and may be an important mechanism for the establishment of the metameric pattern of the vertebral column in human and chick embryos.