Osteopetrosis in the toothless (tl) rat: Presence of osteoclasts but failure to respond to parathyroid extract or to be cured by infusion of spleen or bone marrow cells from normal littermates

Osteoclasts have been observed for the first time in toothless (tl ) rats, a mutation which inherits osteopetrosis as an autosomal recessive. The ability of tl rats to raise the serum calcium concentration after injection of parathyroid extract was severely limited when compared with normal littermates. In addition, osteopetrosis in tl rats is not cured by radiation and infusion of normal spleen or bone marrow cells from normal littermates, a method known to cure osteopetrosis in mutants of this and other species. This indirect evidence for a reduction in bone resorption as a cause of osteopetrosis in this mutation and the failure of transplanted cells to cure the disease are discussed in relation to the development and function of osteoclasts.