Individual osteoblasts in the developing calvaria express different gene repertoires

Several studies in vitro and a few in vivo have suggested that mature osteo blasts heterogeneously express osteoblast markers. In one recent study of t he osteoblasts associated with bone nodules formed in vitro in rat calvaria cell populations, extensive diversity was documented in the overall gene r epertoires expressed. To address whether comparable heterogeneity is eviden t in vivo, we investigated the expression of nine osteoblast lineage marker s by both in situ hybridization and immunohistochemistry. At 21 days of fet al rat development, the calvaria is a rapidly growing bone with distinct ma turational zones that are readily observed in coronal sections; that is, an osteogenic front emerging at sagittal and coronal sutures is adjacent to a reas of growing trabeculae of bone, followed by more mature areas of remode ling bone. Based on expression patterns, markers can be divided into two ca tegories. One category comprises markers that are globally expressed by all osteoblasts irrespective of their position in the calvaria, Of those teste d, only two, alkaline phosphatase and the pth/pthrp receptor, fit into this category. Ail other markers analyzed, including transcription factors (c-f os and msx-2), matrix molecules (bone sialoprotein, osteopontin, and osteoc alcin), and a hormone (pthrp), were differentially expressed only in subpop ulations of osteoblasts, based on cell maturational status, environment (ec tocranial vs. endocranial surfaces), and microenvironment (adjacent osteobl asts), Preosteoblasts and osteocytes in different regions of the calvaria a lso expressed different subsets of the lineage markers, Mechanisms responsi ble for generating differential gene expression profiles appear to be both transcriptional and posttranscriptional, These results indicate that postpr oliferative, morphologically indistinguishable osteoblasts are not a homoge neous class of cells, but instead are molecularly diverse. The present resu lts also raise the possibility that lineage progression and/or maintenance of the differentiated state may be adaptable in the calvaria. (C) 2001 by E lsevier Science Inc, All rights reserved.