Zebrafish alk5a and alk5b are not expressed in the endothelium and activities are not necessary for vessel development.alk5a is expressed in the eye, brain, pharyngeal arches, and endoderm at 24 and 48 hpf (A-D). alk5b is also expressed in these tissues, as well as in the spinal cord and ventral somites (E-H). Neither seems to be expressed in blood vessels (compare A-H with I-L, vecad expression). Exposure of phenotypically wild-type zebrafish embryos to 100 μM SB-431542 beginning at the 8- to 10-somite stage had no effect on trunk (M,N) or cranial (O,P) vascular anatomy at 24 or 48 hpf, respectively. This same exposure regimen did not exacerbate the cranial vascular phenotype in alk1^−/− embryos (Q,R). Comparing panels Q and R with O and P, note enlargement of basal communicating artery (asterisk), posterior connecting segments (arrows), and primordial hindbrain channel (arrowhead). (A-L) In situ hybridization, lateral views, anterior to the left. First and third rows, head; second and fourth rows, trunk and tail. M-R, 2-dimensional reconstructions of laser scanning confocal Z-series of TG(flk1:GFP)^la116 embryos. (M,N) Lateral views of the trunk, anterior to the left. (O-R) Dorsal views of the head, anterior to the left.