In order to elucidate the cytological features of preosteoclasts and the process of their differentiation into osteoclasts, fetal rat parietal bone was examined using light microscopy, organ culture, electron microscopy and histochemical methods.
1. Parietal bones of rat fetuses from 15 to 21 days of gestational age were examined light microscopically. A solid bone plate was found in 19 day old fetuses, but no multinucleated giant cells were observed in either the ecto- or endocranial periosteal surfaces. They were first observed at the endocranial periosteal surface in 20 day old fetuses, and increased in number in 21 day old fetuses.
2. Parietal bones of fetuses from 15 to 19 days of age were cultured and the possible occurrence of preosteoclasts prior to the appearance of osteoclasts was examined. During organ culture, eosinophilic multinucleated cells appeared in the parietal bones from 17, 18 and 19 day old fetuses, and increased in those from 19 day old fetuses.
3. Electron microscope observation of the parietal bones in 19 day old fetuses revealed moderate numbers of mononuclear cells identified as preosteoclasts (SCOTT, 1967) principally among the osteoblasts and preosteoblasts at the endocranial periosteal surface. Preosteoclasts with ill-developed cell organelles tended to be located between blood vessels and active osteoblasts, and sometimes located close to the bone surface with only the thin cytoplasmic processes of adjacent osteoblasts intervening. On the other hand, well-developed preosteoclasts tended to be located close to flattened osteoblasts and came into direct contact with the exposed mineralized bone between them. Preosteoclasts were not clustered together but were usually found in contact with adjacent osteoblasts and/or preosteoblasts. Membrane fusion between a preosteoclast and a flattened osteoblastic cell was observed. Multinucleated cells were principally preosteoclastic in appearance but some were both osteoclastic and osteoblastic. The multinucleated cells with ruffled borders identified as active osteoclasts increased in number over a particular time span.
4. The cytochemical localizations of ALPase, ALPase and peroxidase activities in the preosteoclasts resembled those in the osteoclasts but differed from the osteoblasts and preosteoblasts with respect to the ALPase activity. An intense peroxidase activity was detected only in monocytes and neither in preosteoclasts nor in osteoclasts.
These results suggest that the cytodifferentiation of preosteoclasts into osteoclasts may be induced by their direct contact to the mineralized bone surface exposed by detachment of osteoblasts, and that the detached osteoblasts may also serve as either an inducer or a constituent of the forming osteoclasts.