[Vascular endothelium (review). I. General morphology. 2A: histogenesis of the vascular endothelium]

Determination, differentiation, and specialization of vascular endothelium are the main steps of its histogenesis. The source of the first determined endothelioblasts are condensed mesenchymal cells (blood islands) at the yolk sac (extraembryonal) and also in the intraembryonal mesenchyme. The term of determination is not yet known. The marginal cells of the blood islands differentiate into primordial endothelial cells forming primitive vessels by migration, proliferation, fusion, and selection. This "vasculogenesis" is induced by specific matrix components produced by the endothelioblasts themselves and other not known factors. Formation of secondary capillary plexuses is related to organogenesis and takes place by sprouting from preexisting endothelium ("angiogenesis"). Factors which induce and promote angiogenesis were isolated from different embryonic organs. Migration, proliferation, and tube formation are regulated by extracellular matrix components (fibronectin, laminin). Main features of primordial endothelium of protocapillaries are: irregular profile, abundance of synthetic organelles, lack of plasmalemmal vesicles and basement membrane, production of specific matrix components. Specialized endothelium (continuous, fenestrated, discontinuous a.s.o.) develops from secondary plexuses influenced by factors of the specific organ tissue. The probably mechanism by which the endotheliocytes reach their final shape and behavior is discussed; some morphological and functional properties during maturation are documented. The maturation of endothelium is related to establishing of the specific blood-tissue barrier.