MORPHOGENESIS OF THE GLOMERULAR FILTER - THE SYNCHRONOUS ASSEMBLY ANDMATURATION OF 2 DISTINCT EXTRACELLULAR MATRICES

The morphogenesis of the glomerular filtration apparatus during pre-an d postnatal development in the rodent involves the coordinated assembl y of two closely apposed but morphologically different extracellular m atrices, the glomerular capillary basement membrane and the mesangial matrix. The cellular origin of these matrices is known to be distinct and complex; however, the mechanisms by which these matrices are assem bled during morphogenesis are not entirely understood. It has been sho wn that in the earliest stages of glomerular morphogenesis the nascent glomerular basement membrane exists as a four-layered structure, the product of both the visceral epithelium and capillary endothelium. Dur ing the latter stages of glomerular development, the quadrilaminar str ucture becomes a trilaminar basement membrane, the event thought to oc cur by fusion of closely apposed basement membrane layers. In subseque nt stages of maturation and throughout the life of the animal, the vis ceral epithelial cells, which line the periphery of the glomerular cap illary, are the primary source of newly synthesized basement membrane material. The mesangial matrix, which lacks the specific organization of a basement membrane, first occurs in the developing glomerulus as a diffuse matrix central to the developing glomerular capillaries. Duri ng glomerular maturation the mesangial matrix undergoes a compaction/a rborization coincident with the ramification of the vascular histoarch itecture of the glomerular tuft. Recent advances in the cell biology o f basement membrane now demonstrate that there is a divergence in isof orms of the molecules that comprise the glomerular capillary basement membrane and mesangial matrices during development, possibly coinciden tal with functional specialization during the process of glomerular ma turation. (C) 1997 Wiley-Liss, Inc.