Recognition of discrete commitment and differentiation stages requires characterization of changes in proliferative capacity together with the temporal acquisition or loss of expression of molecular and morphological traits. Both cell lines and primary cultures have been useful for analysis of transitional steps in the chondroblast (CB) and osteoblast (OB) lineages. One striking feature is that OBs and CBs share expression of some molecules, including newer markers such as ϵBP (galectin-3), while also having unique markers. The fact that hypertrophic chondrocytes appear able to downregulate cartilage markers and upregulate OB markers also points to an interesting lineage relationship that needs to be explored further. Recently, we have focused on the osteoprogenitors that divide and differentiate into mature OBs forming bone nodules in fetal rat calvaria cell cultures. We use cellular, immunocytochemical, and molecular approaches, including PCR on small numbers of cells, to discriminate stages. Nodule formation is characterized by loss of proliferative capacity and sequential increased marker expression, that is, alkaline phosphatase (AP), followed by bone sialoprotein (BSP), and osteocalcin. Upegulation of collagen type I and biphasic expression of osteopontin, with two peaks corresponding to proliferation and differentiation stages, also occurs. A variety of other molecules are also upregulated in the mature OB, including ϵBP and CD44s. By replica plating and PCR, we have begun to study the expression of the messenger RNAs (mRNAs) for potential regulatory molecules (e.g., PTHrP) and their receptors (e.g., PTHR, FGFR-1, and PDGFRα) and have found all to be modulated during the progression from committed osteoprogenitor to mature OB. Strikingly, FGFR-1 mRNA is expressed earlier than that for PDGFRα, and before detectable AP. PTHrP mRNA appears prior to its receptor, PTHR. Our data suggest that expression of regulatory molecules such as hormones, growth factors, and their receptors will serve as useful markers for osteoprogenitor cell less mature than those already expressing AP and other markers of the OB phenotype.