Patterns of angiogenic and hematopoietic gene expression during brown trout embryogenesis

In this article, whole mount in situ hybridization is used to examine early blood vessel and blood cell development in the embryos of the brown trout Salmo trutta lacustris. cDNAs encoding for the angiogenic markers fli1 and flk1, and for the hematopoietic markers gata1 and gata2, were identified from an expressed sequence tag library of rainbow trout. Results show that fli1, flk1 and gata2 are activated in bilateral bands of the lateral trunk mesoderm before the onset of somitogenesis, shortly followed by gata1. These bands then converge toward the ventral midline to form the intermediate cell mass (ICM) (anterior ICM). Subsequent axial vasculogenesis and initial blood cell formation involve a clear spatial separation of fli1 and gata gene expression. Fli1 staining is most intense within the axial vessel (dorsal aorta, posterior cardinal vein) forming and lateral ICM cells, whereas binding of gata1 and gata2 probes becomes confined to the central portion of ICM cells beneath the dorsal aorta. This is followed by a first wave of angiogenesis, indicated by expression of fli1 and flk1. This gives rise to the intersegmental, dorsal longitudinal anastomotic and intestinal vessels. Further angiogenesis and hematopoiesis are activated in the "posterior ICM" of the tail. Here, the absence of gata1 indicates that hematopoiesis in this tissue generates myeloid rather than erythroid cells. The results supplement and validate previous, now historical morphological work in salmonids, thus aiding the elucidation of a comprehensive general scheme of angiogenic and hematopoietic development in the teleost embryo.