The production of a basal lamina by microvascular endothelial cells (MEC) cultured on various substrata was examined . MEC were isolated from human dermis and plated on plastic dishes coated with fibronectin, or cell-free extracellular matrices elaborated by fibroblasts, smooth muscle cells, corneal endothelial cells, or PF HR9 endodermal cells. Examination of cultures by electron microscopy at selected intervals after plating revealed that on most substrates the MEC produced an extracellular matrix at the basal surface that was discontinuous, multilayered, and polymorphous . Immunocytochemical studies demonstrated that the MEC synthesize and deposit both type IV collagen and laminin into the subendothelial matrix . When cultured on matrices produced by the PF HR9 endodermal cells MEC deposit a subendothelial matrix that was present as a uniform sheet which usually exhibited lamina raraand lamina densa-like regions . The results indicate that under the appropriate conditions, human MEC elaborate a basal lamina-like matrix that is ultrastructurally similar to basal lamina formed in vivo, which suggests that this experimental system may be a useful model for studies of basal lamina formation and metabolism.Capillary endothelium lie on a basal lamina that has a number of important functions. Not only does it provide the substratum for endothelial cell attachment and tissue organization (52), but it also has a role in blood vessel permeability (15), in the initiation of blood clotting (2), and provides a barrier to cellular migration, the violation of which has importance for angiogenesis (17), leukocyte emigration (55), and tumor metastasis (35,38). Alterations in vascular basal lamina have been implicated in a number of pathological states, including diabetes mellitus, thrombogenesis, atherosclerosis, and renal failure (3,15,28,53).The vascular basal lamina is a complex structure . Because of difficulties in isolating sufficient quantities of purified material, biochemical studies of its molecular composition and structure have been limited . It is known, however, that vascular basal lamina is composed of a number of macromolecules including type IV collagen, proteoglycans, and glycoproteins (reviewed in 32, 37, 50). Studies on the synthesis of basal lamina macromolecules and their assembly into mature basal lamina by endothelia has been even more difficult. Recent advances in the isolation and long term culture of microvascular endothelial cells (3,10,17,40) provides the opportunity for the study of these events.
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