Common morphogenetic aspects of various organotypic microvascular patterns

Abstract

The development of the terminal vascular bed in various organs of rats has been reinvestigated mainly using ink injection of the vascular system, emphasizing those stages during which organotypic patterns develop. Results from bradytrophic tissues and organs with epi- or endoparenchymal capillaries confirm that during all developmental stages elongation, dilation, and sprouting of terminal vessels are produced by endothelial mitosis and flattening. Microvessels growing in close relation to the parenchyma develop their final shape and arrangement due to the spatial reorganization of the surrounding tissue, thus changing the spatial relationship of arterioles, venules, and capillaries. The present observations suggest that the varying complexity of the terminal vascular patterns in different organs depends on variations in three consecutive developmental processes: (1) the formation of an epiparenchymal vascular plexus 2+ and its transformation into a postcapillary or terminal venular plexus; (2) the formation of a-v capillaries 2++ that are arranged in series between terminal arterioles and venules and/or the epiparenchymal plexus; (3) the development of netcapillaries 2+++, mostly arising from a-v capillaries, but sometimes also from arterioles (sphincter capillaries). These capillaries then fuse with venules and/or the epiparenchymal plexus. Therefore most of the net capillaries are arranged parallel to a-v capillaries. Especially in highly branched networks this arrangement may temporarily and locally represent an equivocal position within the a-v pressure gradient. On the other hand, the final development of net capillaries seems to be suppressed or not induced in those organs that are supplied mainly by capillary loops, such as bradytrophic tissues and many organs in poikilotherms.