SPARC (Secreted Protein that is Acidic and Rich in Cysteine), a Ca++-binding glycoprotein also known as osteonectin, is produced in significant amounts by injured or proliferating cells in vitro. To elucidate the possible function of SPARC in growth and remodeling, we examined its distribution in embryonic and adult murine tissues. Immunohistochemistry on adult mouse tissues revealed a preferential association of SPARC protein with epithelia exhibiting high rates of turnover (gut, skin, and glandular tissue). Fetal tissues containing high levels of SPARC included heart, thymus, lung, and gut. In the 14-18-day developing fetus, SPARC expression was particularly enhanced in areas undergoing chondrogenesis, osteogenesis, and somitogenesis, whereas 10-day embryos exhibited selective staining for this protein in Reichert's membrane, maternal sinuses, and trophoblastic giant cells. SPARC displayed a Ca++-dependent affinity for hydrophobic surfaces and was not incorporated into the extracellular matrix produced by cells in vitro. We propose that in some tissues SPARC associates with cell surfaces to facilitate proliferation during embryonic morphogenesis and normal cell turnover in the adult.

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Morphogenesis and maturation of the sagittal suture in newborn C57B1/6J strain mice were studied using light and electron microscopy. Morphodifferentiation of the murine parietal bones progresses radially with the interposed sagittal suture, assuming a greater level of maturity at birth at a midpoint along its length. The presumptive suture develops in a sulcus, deeper posteriorly, more shallow anteriorly. Cells at the osteogenic front (OF) are distinguished from the surrounding fibrocytic cells by a number of distinctive characteristics: 1. increased cytoplasmic density; 2. extensive endoplasmic reticulum; 3. dispersed nuclear chromatin aggregates; 4. extensive surface projections; 5. close approximation. Mineralization of the developing parietal bone occurs extracellularly with the initial deposits of apatite crystals exhibiting no oriented relationship to either cellular or extracellular fibrillar elements. The majority of collagen fibers lie superior and inferior to the presumptive suture, oriented anteroposteriorly with their long axes parallel to the ectocranial surface. Other fiber bundles more intimately associated with the developing suture display a more random orientation.