Endothelial cell division in metaphyseal capillaries during endochondral bone formation in rats.

Endothelial cell division in the metaphyseal capillaries of growing rats was studied by serial sectioning and electron microscopic examination. The endothelium of these capillary sprouts forms a continuous attenuated squamous lining. During endochondral bone formation these growing vessels possess a region of endothelial cell division which is located behind the sprout tip in an area where the microvascular wall consists of an endothelium and a discontinuous layer of perivascular cells. Examination of this region has shown the presence of junctional attachments between daughter cells even before cell separation is complete. Thus, the integrity of the vascular wall is not compromised during cell division. Junctional complexes with adjacent endothelial cells are also formed along the cleavage plane prior to the completion of cytokinesis. Numerous microvilli and from both the daughter cells and adjacent endothelial cells often make contact and form junctions with the plasma membrane of the dividing cells. A model for endothelial junction formation between daughter cells during cytokinesis and the role that microvilli play in the process is proposed.