A COMPARED TEM SEM INVESTIGATION ON THE PERICYTIC INVESTMENT IN DEVELOPING MICROVASCULATURE OF THE CHICK OPTIC TECTUM/

The pericytes have been ultrastructurally and morphometrically analyze d in the neural vessels of the chick embryo optic tectum, under the tr ansmission and scanning electron microscopes. The observations demonst rated that shape and surface features of pericytes, as well as their s patial relation with the endothelium, remarkably change during develop ment, whereas their ultrastructure does not substantially modify from the early to the late embryonic stages. The pericytes have an ovoid bo dy, broad processes, smooth surfaces, and are closely applied to the e ndothelial tube on days 5-7; they show convolute shape, highly irregul ar surfaces, and are complicately interdigitated with the endothelial cells, when a vivacious vessel growth takes place, on days 12-14; fina lly, they are flattened, smooth, highly branched, and completely enclo sed in the basement lamina on days 20-21, when a definitive vascular p attern is established. The contribution of pericytes to the formation of the basement lamina has been confirmed by the detection of exocytot ic vesicles discharging their content toward the subendothelial cleft. The morphometric evaluations revealed that pericytes provide the grow ing neural microvessels with an almost continuous coverage which, howe ver, undergoes a significant reduction within hatching time. On the wh ole, the results suggest that the pericytes are as active as the endot helial cells during the vessel growth acid play an inhibitory role on the endothelial proliferation only later on, when they are closely adh erent to the endothelium and are encompassed by the basement lamina.