MORPHOLOGY AND TYROSINE-HYDROXYLASE IMMUNOHISTOCHEMISTRY OF THE SYSTEMIC SECONDARY VESSEL SYSTEM OF THE BLUE CATFISH, ARIUS-GRAEFFEI

Fish have a secondary vessel system which emerges from the primary vas culature via large numbers of coiled origins. The precise role of this vessel system is unknown. Vascular casting techniques and scanning el ectron microscopy reveal that the secondary vessels of the blue catfis h, Arius graeffei, originate from dorsal, lateral, and ventral segment al primary arteries and from the caudal dorsal aorta. These vessels an astomose with each other to form larger secondary arteries which paral lel the primary vessels for their entire length. Secondary vessels do not appear to form a capillary bed in the skin in A. graeffei as they do in some fish species. Coiled secondary vessel origins are abundant within the tunica media and adventitia of the primary vessels from whi ch they emerge. The origins of the secondary vessels are surrounded by the extensive cytoplasmic processes of specialized endothelial cells. These processes extend for up to 6 mu m into the lumen of the primary vessel. Ultrastructurally the coiled secondary capillaries consist of an endothelial cell tube which is surrounded by a single layer of per icytes. These endothelial cells extend large numbers of microvilli int o the lumen of the coiled secondary capillary. Nerve terminals are com monly associated with the coiled secondary capillaries. Immunohistoche mistry has revealed the presence of tyrosine-hydroxylase, an enzyme in volved in catecholamine synthesis in nerve varicosities close to secon dary vessels in A. graeffei. This vessel system could therefore be reg ulated by adrenergic nerves. (C) 1996 Wiley-Liss, Inc.