Bone resorption in calcium-deficient rats

Abstract

The cellular basis of bone resorption is a topic of continued interest and debate. Many studies, since the early work of Kolliker, have emphasized the important role of osteoclasts. More recently, following the work of Belanger (1969), the concept of osteocytic osteolysis has been developed (Baud, 1968; Baylink et al., 1973; Bernard and Meunier, 1975; Baud and Boivin, 1978) although some authorities are unconvinced of the importance, or even the existence, of this process (Parfitt, 1977; Boyde, 1980). It is well known that rats fed a calcium-deficient diet develop osteoporosis, presumably the result of exaggerated bone resorption produced by parathyroid overactivity resulting from nutritional hypocalcemia (Harrison and Frasel, 1960; Gershon-Cohen et al, 1962; Salomon. 1972; de Winter and Steendrjk, 1975; Rasmussen, 1977) and this provides a convenient experimental system for studying the cellular processes concerned in the alteration of bone structure. Osteoclastic bone resorption has been observed in the bones of calcium-deficient rats by several workers (de Winter and Steendijk, 1975; Rasmussen, 1977), but others have invoked osteocytic osteolysis as a factor producrng the bony rarefaction (Salomon, 1972; Salomon and Volpin, 1970; Lok and Jaworski, 1976). Their conclusion is based on the observation of large osteocyte lacunae in the bones of calcrumdeficient animals when compared with those of normally fed controls. This evidence, however, IS insufficient to establish the role of osteocytic osteolysis. In order to do this it is necessary, in addition to finding abnormally large osteocyte lacunae, to demonstrate that these are the result of enlargement of normally sized lacunae and not lacunae of newly formed bone tissue that may have been large from their inception. This has not been shown In calcium-deficient osteoporosis or in other circumstances where osteocytic osteolysis has been postulated (Parfitt, 1977; Marotti et al., 1977; Boyde, 1980). In addition, when the dimensions of osteocyte lacunae are considered, attention must be paid to the type of bone tissue concerned and to the orientation of the lacunae with respect to the adjacent bone lamellae if misleadrng results are to be avoided (Marotti, 1979).