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Stock of the lethal osteopetrotic mutation in the rabbit has been outbred to robust New Zealand White stock. Features of this new stock include skeletal sclerosis and failure of tooth eruption characteristic of osteopetrosis, together with hypocalcemia, hypophosphatemia, and osteoclasts of reduced numbers and abnormal cytology. No abnormalities of carbonic anhydrase II were detectable by electrophoresis. The commercial availability of mutants of this new stock makes them a potentially important source of information about both osteopetrosis and osteoclast biology.

Congenital mammalian osteopetrosis is characterized by a generalized skeletal sclerosis due to reduced bone resorption by osteoclasts. This condition can be cured in several mutant strains of mice and rats by transplantation of spleen or bone marrow cells from normal littermates. The ability of this regimen to cure osteopetrosis in oplop mice was examined in 23 mice treated with spleen or bone marrow cells from normal littermates and followed for up to 80 days. In no instance was there radiographic or histologic evidence of removal of the excessive skeletal mass characteristic of the disease. These data show that spleen or bone marrow cells do not cure osteopetrosis in this mutation. Recent demonstrations that not all children with congenital osteopetrosis are cured by bone marrow transplants from HLA-matched donors suggest that the oplop mouse mutation may be a useful model system in which to develop alternate treatments.

Thioglycollate was introduced into the peritoneal cavity of 32-, 50- and 80-day-old normal (ia/+) and incisor absent (ia/ia) osteopetrotic rats, and 3 days later macrophages were harvested. The number of macrophages recovered from 32- and 50-day-old ia rats was significantly less than that elicited from corresponding normal littermates. The macrophages from mutants also showed a significantly impaired ability to migrate in vitro. At 80 days, the number of macrophages obtained from ia rats was not significantly different from those elicited from control rats, and their migratory function was also close to those obtained from controls. The increase in macrophage number and migrating capacity noted in 80-day-old ia rats correlates with increased marrow development at this age as described by Marks (Pathogenesis of osteopetrosis in the ia rat. Reduced bone resorption due to reduced osteoclast function, Am. J Anat 1973;138:165-190.

The osteosclerotic (oc) mouse is an osteopetrotic mutation that has recently been identified as having rickets associated with its osteopetrosis. The presence of this rachitic lesion, unexplainable from a nutritional standpoint, prompted an investigation into the vitamin D endocrine system in these animals. The developmental appearance of vitamin D-dependent calcium-binding protein (calbindin-D9k) and alkaline phosphatase was studied in oc mutant and normal mice from birth to weaning, as were serum concentrations of 25-hydroxyvitamin D3 (25OHD3), 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], calcium, and phosphorus. Intestinal and renal calbindin-D9k levels were markedly and precociously elevated (4- to 9-fold) in young suckling, but not newborn, mutant mice compared to values in normal controls. Serum 25OHD3 levels were very low to undetectable in 2-week-old mutant mice compared to normal values, while 1,25-(OH)2D3 levels were 6 times higher in mutants. The exact cause of this premature induction in mutants is unknown, but may be due to elevated circulating levels of 1,25-(OH)2D. Alkaline phosphatase activity was similar between phenotypes at all ages. These studies indicate that the rachitic lesion present in oc mutants may be the result of some inherited disorder in vitamin D metabolism in these animals. Alternatively, these data are also consistent with a normal appropriate response to hypocalcemia and hypophosphatemia resulting from decreased osteoclastic bone resorption.

A new mouse osteopetrotic mutation, osteosclerosis, has been examined with respect to rickets. These osteopetrotic mice were hypocalcemic and hypophosphatemic, and had greatly thickened epiphyseal plates with abnormalities in matrix vesicles when compared with normal littermates. Such biochemic and morphologic manifestations of rickets in this osteopetrotic mutation may explain the failure of osteosclerotic mice to be cured by transplantation of bone marrow from normal littermates and indicate that vitamin D metabolism and matrix vesicle biochemistry warrant further study.

Osteosclerotic (oc/oc) and osteopetrotic (op/op) mice are not cured by bone marrow transplantation from normal littermates. The possibility that this is due to production of poorly resorbable bone was examined by comparing the fate of mutant and normal bone particles implanted subcutaneously in normal hosts. Bone, removed aseptically from calvarial and tibial sites of normal littermates and mutants, was cleaned of adherent soft tissue, ground and sieved to a particle size of 70-300 micron. Aliquots (17-20 mg) of bone from each phenotype of each stock were pelleted and implanted beneath the anterior thoracic skin of normal littermates for two weeks. Particle density in tissue sections was determined as percent of field by a point-counting method. Giant cell response was recorded as number per high-power field. Percent bone present initially was determined in pellets implanted for less than 24 hr. Bone particles were reduced in each pellet with time, about 25% of the original volume being removed in two weeks. No statistically significant differences were noted in the rates of disappearance of mutant and normal bone or in the percentage or number of giant cells in implants of mutant and normal bone in either stock. Furthermore, these values were not different from identical studies in microphthalmic mice, an osteopetrotic stock cured by bone marrow transplantation. These data suggest that the failure of osteopetrotic and osteosclerotic mice to be cured by bone marrow transplants from normal littermates is not due to the presence of unresorbable bone.

The osteosclerotic mouse is a new, lethal recessive skeletal mutation which inherits osteopetrosis as an autosomal recessive. Affected mice are hypocalcemic and also have rickets. Osteoclasts are small and numerous with little evidence of function. This report examines the effects of bone marrow and spleen cell transplantation from normal littermates on survival and the radiographic appearance of the skeleton in 29 mutants. This procedure did not significantly prolong life or effect radiographic changes in the skeleton of most (27) recipients. However, two treated mutants lived for almost a year with radiographically normal skeletons. These variable responses to stem cell transplantation in an osteopetrotic mutant with rickets deserve further study.