Expression of the basement membrane heparan sulfate proteoglycan (HSPG), perlecan (Pln), mRNA, and protein has been examined during murine development. Both Pln mRNA and protein are highly expressed in cartilaginous regions of developing mouse embryos, but not in areas of membranous bone formation. Initially detected at low levels in precartilaginous areas of d 12.5 embryos, Pln protein accumulates in these regions through d 15.5 at which time high levels are detected in the cartilage primordia. Laminin and collagen type IV, other basal lamina proteins commonly found colocalized with Pln, are absent from the cartilage primordia. Accumulation of Pln mRNA, detected by in situ hybridization, was increased in d 14.5 embryos. Cartilage primordia expression decreased to levels similar to that of the surrounding tissue at d 15.5. Pln accumulation in developing cartilage is preceded by that of collagen type II. To gain insight into Pln function in chondrogenesis, an assay was developed to assess the potential inductive activity of Pln using multipotential 10T1/2 murine embryonic fibroblast cells. Culture on Pln, but not on a variety of other matrices, stimulated extensive formation of dense nodules reminiscent of embryonic cartilaginous condensations. These nodules stained intensely with Alcian blue and collagen type II antibodies. mRNA encoding chondrocyte markers including collagen type II, aggrecan, and Pln was elevated in 10T1/2 cells cultured on Pln. Human chondrocytes that otherwise rapidly dedifferentiate during in vitro culture also formed nodules and expressed high levels of chondrocytic marker proteins when cultured on Pln. Collectively, these studies demonstrate that Pln is not only a marker of chondrogenesis, but also strongly potentiates chondrogenic differentiation in vitro.

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Osteoblast Ca2؉ channels play a fundamental role in controlling intracellular and systemic Ca 2؉ homeostasis. A reverse transcription-polymerase chain reaction strategy was used to determine the molecular identity of voltage-sensitive calcium channels present in ROS 17/ 2.8 osteosarcoma cells. The amino acid sequences encoded by the two resultant PCR products matched the The balance between osteoclastic bone resorption and osteoblastic bone formation determines skeletal mass and composition. 1,25-Dihydroxyvitamin D 3 (1,25(OH) 2 D 3 ) 1 has long been appreciated as a hormonal modulator of osteoblast function and bone remodeling. 1,25(OH) 2 D 3 , classically considered a resorptive hormone, has the paracrine effects on osteoclasts that are believed to be mediated through separate membrane and nuclear osteoblastic 1,25(OH) 2 D 3 receptor systems (1). Paradoxically, 1,25(OH) 2 D 3 has anabolic effects on osteoblasts, stimulating production of the noncollagenous matrix proteins, osteopontin and osteocalcin (2-4). The exact mechanism by which the bone-forming and -resorbing aspects of the remodeling process are regulated and coupled remains unclear.