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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.

The vascular endothelium as a monolayer interposed between blood/lymph and interstitial fluid realizes different functions as continuous circulation of blood/lymph, processes of clotting, fibrinolysis and antithrombotic surface properties, some aspects of defense, inflammation, different synthetic activities, and establishing of exchange pathways and barriers for several substances. This survey will be presented as a sequence of 6 single articles. The 1st one deals with the general morphology of vascular endothelium. Heteromorphism of endothelium means variability of shape and orientation as a result of different functional conditions, mediated by the cytoskeleton. "Contactons" are units of interconnected cells; each cell exhibits 4 zones of different structural and functional specialization: nuclear-, organelle-, peripheric-, and contact zone. Membrane associated structures of the surface are the glycocalix and the subplasmalemmal subcortical layer. Composition and function of these including the plasmalemma itself are explained. Structures formed by the endothelial plasmalemma are vesicles, fenestrations, pores, gaps, and microvilli. Arrangement, function, dynamics, and their relationships to the cytoskeleton are referred including TEM, SEM, and Freeze Etching techniques. Concerning interendothelial contacts, different types of junctions and 4 types of junctional fibrils are described. A short structural description of the basement membrane and of the organelles of endothelium is given. Some new informations of the endothelial cytoskeleton, concerning composition, structure, arrangement, properties, and relationships to other subcellular constituents are presented, completed by impressive SEM-photographs.