Ontogenesis of chondro/osteoclasts and their precursors in the mandibular condyle of the mouse

Tartrate-resistant acid phosphatase (TRAP) histochemistry has been used to follow chondro/osteoclasts and their precursors during ontogenesis of the mandibular condyle of the mouse (from day 16 of gestation until day 15 of neonatal life). TRAP+ mono-, bi-, and multinuclear cells were counted separately in the perichondrium, along the resorption front and in the subchondral spongiosa. Index of cellular density was calculated by dividing the absolute numbers of cells by the length of resorption front or the area of spongiosa, respectively. The study revealed that TRAP+ cells are present in the perichondrium of the mandibular condylar cartilage from the first day of its existence as an organ, namely day 17 of gestation. These cells are more numerous in the posterior part of the condyle, and reach their maximal number on the third day of neonatal life. Along the resorption front, mono-, bi-, and multinucleated TRAP+ cells were counted. Their total number and their respective indexes of cellular density changed variably during the observation period: (a) the number of mononuclear cells increased gradually and, by the end of the observation period, was 13 times greater than it was at the beginning; (b) the number of binuclear cells increased threefold; and (c) the number of multinuclear cells stayed constant, except for a small peak around the time of birth. The changes in the spongiosa showed an opposite trend. In the spongiosa, the index of cellular density of the mononuclear TRAP+ cells increased slightly during ontogenesis. The number of binuclear cells increased twofold, while the number of multinuclear cells increased 17-fold. The results of the study suggest that both mono- and multinuclear TRAP+ cells are involved in the resorption process, whereby the cartilage is remodeled into bone by the mechanism of endochondral ossification. Moreover, TRAP+ cells make their appearance right from the onset of the organization of the mandibular condyle as an organ.