For all panels, the allantoic rod and PECAM1 midline vessel are encased by dashed lines, the allantois is outlined by dotted lines, and the proximal ACD is indicated with a white asterisk. (A-T) Frontal optical sections through the fetal-placental interface of a wildtype T+/T+ allantois (A-E) and a TC/TC mutant allantois (K-O) aided by higher magnification views of the white-boxed allantoic region in all of these panels (F-J, P-T), showing AX contribution to the allantoic rod (A-J) and failure of the TC/TC mutants to form a rod (K-T). White arrowheads (P-T), disorganized clusters of PECAM1- and COLIV-positive cells. Scale bar (T): 10 μm (F-J, P-T); 35 μm (A-E, K-O). al, allantois; da, dorsal aortae; ys, yolk sac.