THE OSTEOCLAST

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Abstract
Summary: Osteoclasts, Kölliker's universal agent of bone destruction, must still be regarded as enigmatical structures. It is likely that their life span is limited to a few days. Regarded by some as resulting from fusion of relatively immobile individuals of connective tissue type, and by others of mobile cells such as macrophages, it seems a possibility that they may form from any cell, indifferently, which has assumed the ‘histiocytic state’. Osteoclasts resemble histiocytes in their reaction to supravital neutral red, in their brisk motility and in the differentiation of a superficial ecto‐plasmic layer into an ‘undulating membrane’ in tissue culture. There is no direct evidence that osteoclasts erode bone, but their constant occurrence in zones where absorption is taking place, together with the associated histological picture and especially their striated border, suggest that their presence is more than incidental. A linear orientation of cytoplasmic ultramicrons may be reflected in the striated border, which may thus constitute a cytoplasmic zone adapted for the transfer of material into or out from the cell. There is practically no evidence upon which to base an explanation of how osteoclasts could erode bone. The cells form in response to such a wide variety of normal and experimental stimuli that at least in part a local action on the bone seems likely. Local humoral factors, emanating from the bone, may be responsible for the determination of osteoclasts, and the formation of giant cells under the influence of injected phthioic acid seems to provide a valid analogy. It may be that the factors reside in the ground substance. The process of osteoclastic absorption does not appear to be regulated by the nervous system. Further experiments should be undertaken to test the ability of osteoclasts to erode bone and to ascertain whether bone about to be absorbed differs constitutionally from normal bone.
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