Cytochemical studies of the cellular events sequence in bone remodeling: Cytological evidence for a coupling mechanism

Abstract

In order to elucidate the sequence of the cellular events in the bone remodeling process which may contribute to the coupling mechanism between bone resorption and formation, this study was attempted to observe the cytochemical localization of acid phosphatase (ACPase) and alkaline phosphatase (ALPase) activities in the remodeling bone cells. The ACPase activity in osteoclasts could be demonstrated in the vacuoles, extracellular channels of the ruffled border and spaces between cells and bone. In addition, the reaction product was observed at the resorbed bone surface, which was continued to the cement line resulting in new bone formation. Attached to the resorbed bone surface and newly formed cement line, ALPase-positive osteoblastic cells appeared and formed new bone matrix. These suggest that osteoclasts may move as resorbing bone and form a new cement line, which may be responsible for coupling of bone resorption and formation. Moreover, the ACPase-positive mononuclear cells could be observed on the resorbed bone surface adjacent to the resorbing osteoclast. They were characterized by numerous cytoplasmic extensions, sparse r-ER and mitochondria, and many lysosomes. However, the osteocytes being excavated by osteoclasts also showed the similar appearance. In the cytochemical study, these osteocytes showed ACPase activity in lysosomes in the same way as the mononuclear cells on the resorbed bone surface. Therefore, it is suggested that the ACPase-positive mononuclear cells principally originate from the osteocytes which are excavated by osteoclasts from bone matrix, and these cells are identified with the mononuclear phagocytes in reversal phase which have been postulated.