Immunocytochemical Studies of Endothelial Cells In Vivo. I. The Presence of Desmin Only, or of Desmin Plus Vimentin, or Vimentin Only, in the Endothelial Cells of Different Capillaries of the Adult Chicken

It is currently believed that the intermediate filaments of endothelial cells contain vimentin subunits exclusively. This inference, however, is derived from studies of only a few types of endothelial cells. By double indirect immunofluorescence and immunoelectron microscopy, we have now examined the endothelial cells of the microand macrovasculature of a variety of tissues and organs of adult chicken in vivo for their content of desmin and vimentin. Endothelial cells of the peritubular capillary in the renal cortex, the hepatic sinusoid, and the splenic sinusoid were found to contain only desmin; those of the exocrine pancreas capillary contained both desmin and vimentin; and the endothelial cells of the macrovasculatures and of all the other microvasculatures examined, including the vasa recta of the renal medulla, contained only vimentin. Such heterogeneity suggests that different types of adult chicken endothelial cells may have different embryological origins. To the extent that desmin and vimentin intermediate filaments may be functionally distinct, these results also suggest that different capillary endothelial cells may have different functional properties. T hE endothelial cells that form the innermost layer of blood vessels and lymphatics are of great physiological importance. They regulate the traffic of molecules and cells between the tissues and the circulation, control vascular tone by synthesizing vasoactive agents, and maintain a nonthrombogenic surface throughout the vasculature (27). Despite this importance, endothelial cells have until recently been among the least well investigated cell types. Although readily accessible endothelial cells, such as those of the large blood vessels like the aorta, have been studied in vivo and have been placed in culture in vitro, less accessible endothelial cells, such as those of the smaller vessels and capillaries, have not as yet had their molecular ultrastructure extensively investigated. This is because capillary endothelial cells are not only difficult to isolate and culture (38), but even in vivo they are often so thin that they are difficult to study by conventional immunofluorescence microscopy. However, there is evidence that endothelial cells are not all alike, and are differentiated to meet specific demands from their tissue environment (20, 30, 34); therefore, results obtained with aorta endothelium in vivo or in culture do not necessarily apply to all (e.g., capillary) endothelial cells. Under these circumstances, a general and powerful approach to the study of the molecular ultrastructure of endothelial cells is immunoelectron microscopy, using techniques of frozen sectioning and immunolabeling developed in this laboratory (31, 32). In this paper, we begin a systematic immunocytochemical study of the structural proteins of endothelial cells of different tissues and organs, with an investigation of the intermediate filament proteins desmin and vimentin. From studies of endothelial cells of large vessels in vivo (4) or in culture (4, 12, 17, 25) that were found to contain only vimentin and no desmin, it has been assumed that vimentin is generally the only intermediate filament protein in endothelial cells. We show, however, that depending on the chicken organ, endothelial cells of different capillaries contain either desmin only, vimentin only, or desmin and vimentin. These results indicate that endothelial cells of different vasculatures are differentiated in their cytoskeletons. This suggests that they may have different embryological origins, as well as different functional characteristics. Materials and Methods lmmunochemical Reagents Guinea pig antibodies against chicken erythrocyte vimentin and rabbit antibodies against chicken gizzard desmin were afffinity-purified and crossabsorbed with the heterologous antigen as described (33). It is important to emphasize that cross-reactivity between these resultant antibodies is absent (33). For the secondary antibodies used in immunofluorescence, affinity-purified and cross-absorbed goat antibodies to guinea pig IgG and to rabbit IgG were conjugated to rhodamine and fluorescein (5). For double © The Rockefeller University Press, 0021-9525/86/12/2775/12 $1.00 The Journal of Cell Biology, Volume 103 (No. 6, Pt. 2), Dec. 1986 2775-2786 2775 on O cber 8, 2017 jcb.rress.org D ow nladed fom