Morphological and Histochemical Observations on the Lack of Osteoclasis in the “tl” Strain of Rat 1 2

Abstract

A spontaneous mutation has been recently described in a colony of Osborne-Mendel rats which may provide a unique model for the study of calcified tissue (1). The animals (designated TLOM/Ndri) are characterized by retarded growth, normal longevity, osteopetrosis, un-erupted dentition, extramedullary hemopoiesis and periorbital encrustations. Preliminary evidence indicates that “tl” bone is characterized by a lack of physiological resorption.

The “tl” rat is similar to the “ia” rat (1), except that in the latter there is a transitory lack of bone resorption after which there is a partial eruption of teeth (2). The osteopetrotic bone of the “tl” and “ia” rats appears to be histologi-cally similar, except that the “tl” bone appears devoid of osteoclasts (1), while the “ia” bone has been found to display osteoclasts in normal or increased numbers (3-8). The present study was undertaken in an attempt to identify osteoclasts that may be present within “tl” bone, by the application of histochemical methods.

Alveolar and long bones were obtained from a 275-day-old “tl” rat and from two normal Osborne-Mendel animals of 256 and 286 days of age. The animals were killed by an anesthetic overdose and hard tissue specimens were removed and fixed immediately. All tissues were fixed for 3 hr in cold 4% glyoxal in 0.2 M cacodylate buffer and rinsed in chilled Holt's sucrose solution. Specimens were decalcified in buffered EDTA after Fullmer and Link (9). Frozen sections were cut in a cryostat, picked up on clean dry slides and either incubated directly or placed in slide boxes, wrapped in plastic and stored at 0-4°. Some sections were stained with H&E. Acid phosphatase activity was demonstrated by Barka's (10) azo-dye method using either naphthol AS-BI or AS-TR phosphate as substrate and hexazonium pararosaniline as coupler.