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 em bryos, and with the BrdU anti-BrdU method in chick embryos, to investi gate whether the metameric pattern of the developing vertebral column can be explained by different proliferation rates. Furthermore we appl ied antibodies against differentiation markers of chondrogenic and myo genic cells of the somites in order to study the correlation between p roliferation and differentiation. There are no principal differences i n the proliferation pattern of the vertebral column between human and chick embryos. In all stages examined, the cell density is higher in t he caudal sclerotome halves than in the cranial halves. Laterally, the caudal sclerotome halves, which give rise to the neural arches, are c haracterized by a higher proliferative activity than the cranial halve s. Although there is a high variability, the labelling indices show si gnificant differences between the two halves with both proliferation m arkers. With the onset of chondrogenic differentiation, only the peric hondrial cells retain a high proliferation rate. During fetal developm ent,; the neural arches and their processes grow appositionally. Even at the earliest stages, there is practically no immunostaining for PCN A or BrdU in the desmin-positive myotome cells of human and chick embr yos. Axially, a higher proliferation rate is found in the condensed me senchyme of the anlagen of the intervertebral discs than in the anlage n of the vertebral bodies. During fetal development, cells at the bord ers between vertebral bodies and intervertebral discs proliferate, ind icating appositional growth. Our results show that local differences i n the proliferation rates of the paraxial mesoderm exist, and may be a n important mechanism for the establishment of the metameric pattern o f the vertebral column in human and chick embryos.