The earliest stages in the development of the circulatory system of the Rainbow Trout Oncorhynchus mykiss.

The older literature suggests that the development of the blood vascular system in teleosts differs from that of other vertebrates. The evidence, however, came mostly from studies of salmonid embryos beyond the stages when blood cells had begun to circulate, which overlooked earlier developmental stages. The development of the blood vessels of the rainbow trout are illustrated from the time of the first heartbeat to the stage of eye pigment formation. Unless they can be injected with a dye solution, the earliest vessels to develop remain invisible until blood flow makes them visible. By the time of the first heartbeat stage, the embryo has a dorsal aorta, caudal artery and vein, a few transverse vessels, and even the beginning of a vitelline network. One feature peculiar to teleosts is the development of the intermediate cell mass, from which the erythrocytes and a temporary capillary network are formed rather than from the yolk sac. Development of the early posterior cardinal and the subintestinal vein occurs much as in other vertebrates. Previous investigators missed these earliest phases of development because of the difficulty of making them visible. Early formation and transformation of the vascular system of the rainbow trout generally conforms to that seen in vertebrates, except as modified by the temporary presence of the intermediate cell mass and the specialized teleostean yolk mass. With the reduction of the intermediate cell mass, the primary circulatory system for yolk utilization is transformed into a secondary one for respiratory and metabolic functions, as happens usually among vertebrates. J. Morphol. 233:215-236, 1997. © 1997 Wiley-Liss, Inc.