TAL1 expression is down-regulated as part of vascular morphogenesis/maturation.

Panels A through D depict a segment of a 9.0-dpc aorta triple immunolabeled with TAL1, Flk1, and PECAM antibodies. In panel A, PECAM immunofluorescence is associated with endothelial cells of the dorsal aortae (da), the intersomitic vessels (isv), and the intervertebral arteries (iva). In panel B, TAL1 immunofluorescence is associated with uniformly round cells within the vessel lumen (blood cells). In panel C, a superimposition of the PECAM and TAL1 immunostaining patterns shows that endothelial cells lack detectable TAL1 expression; this is most apparent in the vessel segment indicated by the arrow. In panel D, Flk1 immunofluorescence (arrowheads) is associated with endothelial cells including those in the vessel segment indicated by the arrow. Magnification bar = 25 μm.