Ultrastructure of zebrafish dorsal aortic cells

Expression of vascular smooth muscle cell (VSMC) markers such as serum response factor (SRF) is complicated in zebrafish because of the ill-defined histology of the dorsal aorta and the presence of perivascular pigment. We report the ultrastructure of aortic cells in 7-day, 1-month, and 3-month-old zebrafish and provide clear evidence for the presence of perivascular melanocytes harboring an abundance of melanin. In 7-day-old larvae, endothelial cells (EC) and synthetic mural cells that display little evidence of VSMC differentiation comprise the dorsal aorta. The latter mural cells appear to fully differentiate into VSMC by 1 month of age. In 3-month-old adult zebrafish, EC exhibit greater differentiation as evidenced by the accumulation of electron-dense bodies having a diameter of approximately 200 nm. Adult zebrafish aortae also exhibit at least one clear layer of VSMC with the characteristic array of membrane-associated dense plaques, myofilament bundles, and a basement membrane. Subjacent to VSMC are collagen-producing adventitial fibroblasts and melanocytes. These studies indicate that fully differentiated VSMC occur only after day 7 in zebrafish and that such cells are arranged in at least one lamellar unit circumscribing the endothelium. These findings provide new data about the timing and accumulation of VSMC around the zebrafish aorta, which will be useful in phenotyping mutant zebrafish that exhibit defects in blood circulation.