De novo cellular synthesis of sulfated proteoglycans of the developing renal glomerulus in vivo

The site of cellular synthesis of glomerular proteoglycans was investigated in developing glomeruli of 4- to 5-day-old rats. [³⁵S]Sulfate was administered intravenously and animals were sacrificed 15 min to 12 hr later. The outermost layers of the kidney cortices were utilized for characterization of proteoglycans and electron microscopic autoradiography. Sepharose CL-6B chromatography and cellulose acetate electrophoresis revealed that most (≃96%) of the radioactivity was associated with heparan sulfate-proteoglycan synthesized during maturation of glomerular capillaries. Tissue autoradiography revealed the following: (i) during the S-shaped body stage, there is rapid incorporation of [³⁵S]sulfate by mesenchymal cells into the cleft region (site for development of future glomerular extracellular matrices); (ii) during the precapillary stage, mesenchyme-derived cells showed higher incorporation of radioisotope than did epithelial cells; and (iii) during the mature capillary stage, all glomerular cell types (mesangial, endothelial, and epithelial) incorporated [³⁵S]sulfate, incorporation by mesangial cells being the greatest. Radiolabeling was also higher in the mesangial matrix than in the glomerular basement membrane of peripheral capillary loops. Synthesis of a single major species of sulfated glycosaminoglycan by cells of different embryologic origin may be unique to glomerular capillaries.