ABSTRACT
Chick embryos ranging in age from 24 to 72 h were treated with 3H-thymidine to examine proliferation, formation and differentiation of the various somite components. The embryos were fixed 1 – 10 h after treatment, sectioned at 4 μ and processed for radioautography. The following observations were made:
Before the formation of the paraxial mesoderm, the DNA-synthesizing mesoderm cells are randomly distributed throughout the tissue. When the paraxial mesoderm cells become arranged in epithelial fashion, DNA synthesis occurs predominantly in the nuclei at the periphery of the epithelium, while the unlabeled nuclei and the mitotic figures are adjacent to the central space. Since 3 h after labeling the mitotic figures are labeled and 5 h later the labeled nuclei are again at the periphery, it is concluded that the nuclei in the walls of the somite undergo interkinetic migration.
The cells formed by the ventral and major portion of the medial somite walls are deposited in the central cavity of the somite, where they lose their epithelial shape and become mesenchyme-like. They rarely synthesize DNA. With further increase in the number of the cells in the center of the somite, the ventral and medial walls lose their epithelial continuity and blend with the centrally located cells. Together they form a loosely woven tissue, referred to as the sclerotome.
After formation of the sclerotome, the dorsal somite wall, now referred to as the dermatome, is the main source of a layer of cells with pale nuclei and darkly stained nucleoli. This layer, which is directly in contact with the inner surface of the dermatome, is referred to as the myotome. DNA synthesis is never observed in the cells of this layer.
After the myotome has extended in a ventro-lateral direction to the coelomic cavity, the dermatome, with the exception of its most dorsal portion, loses its epithelial structure and the cells begin to form the dermis.