DEVELOPMENT OF THE AORTIC VESSEL WALL AS DEFINED BY VASCULAR SMOOTH-MUSCLE AND EXTRACELLULAR-MATRIX MARKERS

The building of the vessel wall from its cellular and extracellular ma trix (ECM) components is a critical event in the development and matur ation of the cardiovascular system. However, little is known about the events that occur after the initial vascular network, a nascent endot helium, is established. The proper recruitment of vascular smooth musc le cells (VSMCs) to the endothelium is one such critical event. Althou gh the majority of VSMCs are of mesodermal origin, it is not understoo d which populations of embryonic cells are capable of following the VS MC differentiation pathway. Previous studies, which have focused on th e VSMC component of vessel wall development, have been limited by the use of markers that are not smooth muscle specific, or have focused on events that occur after a multilayered wail has been established. The refore, the initial goal of this study was to define when overtly iden tifiable VSMCs were first associated with the vascular endothelium. Mo noclonal antibodies (MAbs) were generated from embryonic vessel wall a ntigens in order to circumvent problems of cell specificity associated with the use of previously available markers to VSMCs. Critical to th is study is our MAb, 1E12, which unlike other antibody markers, is smo oth muscle specific. Using 1E12, we defined a pattern for recruitment and differentiation of the VSMC component of the descending aorta in s tage 12 to stage 20 (Hamburger and Hamilton, 1951) quail embryos. Immu nofluorescent labeling of quail embryos with 1E12 and a MAb to smooth muscle alpha-actin (SM alpha A) shows that the first mesodermally deri ved cells to associate with the aortic endothelium do so at the ventra l surface. Recruitment of these cells, which we believe to be primordi al VSMCs, proceeds in a ventral to dorsal direction along the aorta an d in a radial direction, emanating from the endothelium. Additionally, we have determined the distribution of several ECM proteins, during t he initial events of vessel wall development. Our studies show that fi bulin-1 is expressed surrounding the primordial VSMCs of the vessel wa ll before elastin precursors are present and suggest that differential expression of the JB3 antigen (Wunsch et al., 1994) may be indicative of early diversity among embryonic VSMCs. (C) Academic Press, Inc.