Development and ultrastructure of glomerular capillaries in human foetus

Abstract

Glomerulus is an important filtrating apparatus in the body. Three types of cells - endothelial, mesangial and visceral epithelial cells can be identified in the capillary tuft. Glomeruli develop during nephrogenesis which starts in the 8th week and ends between the 32nd and 36th week of gestation. The nephron develops through stages described as the vesicle, the comma-shaped, S-shaped with the developing glomerulus and the mature glomerulus. Glomerular differentiation involves the expansion of the original capillary component into the plexus that consists of 6-8 loops and the migration of podocytes that are arranged around these glomerular capillaries. Glomerular capillary differentiation represents a set of developmental changes of the glomerular endothelial and epithelial cells. The active differentiation of glomerular capillaries starts in the hemisphere of an inferior arm of S-shaped bodies. Endothelial precursors unite into precapillaries devoid of lumen. Further differentiation includes the flattening of endothelial cells on the basement membrane, the loss of superfluous cells, the development of lumen and the formation of fenestrae. The glomerular basement membrane is differentiated by the fusion of epithelial and endothelial basement membrane. The differentiation of visceral epithelial cells includes the development of cytoplasmic processes and the flattening of cell bodies. Primary cytoplasmic processes are formed from the podocyte bodies and develop secondary and tertiary processes or foot processes. Foot processes from one podocyte interdigitate with foot processes from other podocytes. In the developing glomeruli, there is a difference in the level of differentiation of visceral epithelial cells. Cells with differentiated foot processes and cells with no cytoplasmic processes are observed within the same glomerulus.