title: Thymidine-3H electron microscope radioautography of osteogenic cells in the fetal rat.
abstract: The proximal tibial epiphyses of 18-21-day-old fetal rats have been studied by thymidine-(3)H radioautography. The results reveal that the label is incorporated into two types of osteogenic cells: (a) a spindle cell type (A cells) with characteristics generally associated with matrix production, including an extensive development of the endoplasmic reticulum and the presence of large intracellular accumulations of a dense, finely granular material, morphologically identifiable as glycogen; and (b) a rounded cell type (B cells) with morphological features similar in degree and kind to those of the developing neutrophilic leukocyte, including an abundance of free ribosomes and mitochondria and a complex Golgi apparatus associated with dense specific granules, morphologically identifiable with primary lysosomes. These results, along with the occurrence of recognizable, labeled, immature, perivascular forms of both of these A and B type cells, lead to the conclusion that the specialization of osteogenic cells into osteoclasts and osteocytes may involve separate pathways of cytodifferentiation.
mesh_terms: D000818:Animals; D001345:Autoradiography; D002454:Cell Differentiation; D003239:Connective Tissue Cells; D004721:Endoplasmic Reticulum; D004838:Epiphyses / Q000378:metabolism; D005333:Fetus / Q000378:metabolism*; D006003:Glycogen / Q000378:metabolism; D008854:Microscopy, Electron; D010010:Osteoclasts; D010011:Osteocytes; D010012:Osteogenesis / Q000502:physiology*; D051381:Rats; D013936:Thymidine / Q000378:metabolism; D014316:Tritium
chemical_list: D014316:Tritium; D006003:Glycogen; D013936:Thymidine