Studies of the human fetal kidney: I. Development of the glomerulus

Abstract

Summary Specimens of kidney from 20 humanfetuses, ages 1 1/2 to 5 months' gestation, have been studied by light and electron microscopy. Glomeruli of all stages of maturity from the most primitive glomerular anlage to the mature, adult type of glomerulus were found in the renal cortex of fetuses of all ages. Glomerular maturation was noted to proceed progressively from about 1 1/2 month (origin of metanephros) to about 35 weeks of gestation. Of the total glomerular population, about 30 per cent were mature at 5 months of age. It was shown that the glomerular capillaries form in situ in the endothelial cell mass, rather than from ingrowth of capillaries of the primitive afferent arteriole. Branches of the afferent arteriole connect with the previously formed capillaries of the glomerulus, and continuity of circulation is thus established. No evidence was obtained of in situ formation of blood cells within the glomerulus. The primitive capillaries are surrounded by a very thin basement membrane which consists only of the opposed plasma membranes of the epithelial and endothelial cells, separated by an essentially empty space of 500 Angstrom units. As the development of the glomerulus progresses, the width of the basement membrane increases to about 1,100 A and a dense fibrillar layer (the lamina densa, about 700 A) appears in the middle of the space separating the limiting membranes of the epithelial and endothelial cells. Evidence is presented that the source of the lamina densa is primarily the epithelial cells. Foot processes form by infoldings of the plasma membranes of the epithelial cell, and a normal complement of the adult-type foot processes are present in 30 per cent of the glomeruli in fetuses of 5 months of age. Thinning of the luminal endothelium results in the formation of endothelial fenestra, which reesmble the adult structure, but are fewer in number. An additional endothelial cell type, the intercapillary cell, is readily identified in fetal glomeruli. The structure of this cell and its relationship to other cells in the glomerulus are described. Thus, many glomeruli from fetuses of 5 months' gestation, near the period of viability of premature infants, appear structurally fully capable of function. The implications of the findings to thestructure and formation of glomeruli in the infant and children in health and disease, are discussed.