The biology of macrophages in osteopetrosis. Structure and function.

Osteoclasts from ia rats are present in excessive numbers, contain an excessive amount of the lysosomal enzyme acid phosphatase, and show reduced-bone-resorptive function due to lack of ruffled borders. Given the current theory of a mononuclear phagocytic cell origin for osteoclasts, it was anticipated that the presence or absence of similarities between macrophages of both phenotypes might address the relatedness of these two cells. Thus, peritoneal macrophages from littermate normal and incisors-absent (ia) osteopetrotic rats were examined for differences in structure, acid phosphatase content, and phagocytic activity. The results of this investigation did not reveal any significant differences in these parameters in mutants compared with normal littermates. From these data it is suggested that macrophages are distinct, differentiated cells with functions separate from those of osteoclasts but that they may be related through a common stem cell early in development. Use of more immature cells or monocyte subpopulations may represent better choices for future studies of defects within the mononuclear phagocyte system in the ia mutation.