2.5-3 Days Post-Fertilization
By 2.5 dpf, the trunk and tail Se are usually all patent [2.5 dpf overview]. The arterial-venous identity of these vessels does not follow a fixed pattern from embryo to embryo, except for the first 4 sets of Se, which do apparently have an invariant pattern (unpublished results- see also discussion). Other SeV and SeA do not alternate between arteries and veins from one segment to the next, and the A-V identity of a given Se in a given segment is different from embryo to the next. The DLAVs are reticular throughout the tail and caudal half of trunk (but not in the cranial trunk). They continue to "zipper" together in a caudal to rostral progression. As noted above, by this stage of development the DLAVs are also connected to the PHBC and BA in the head (see [head-trunk connections] for an explanation of changes in head-trunk connections). Beginning at 2.5-3 dpf, a small vessel begins to extend caudally from the caudalmost end of the CA into the base of the caudal fin [2.5 dpf overview]. Initially this vessel extends as a single tube but at later stages it splits into a pair of vessels, forming a caudal circulatory loop ([4.5 dpf overview], for example). Increased D-V separation begins to appear between the axial vessels (DA and PCV), a process that will continue at least through 4.5 dpf. The CV plexus continues to slowly condense down into a single more ventral channel.

The large CCV or duct of Cuvier is narrows and moves to a more cranial position as it fans out across the yolk ball. The right PCV continues to become more dominant, now carrying most of the venous return from the trunk, whereas the left PCV drains primarily SeV from only the first 1-4 segments [later 2.5 dpf head lateral]. At approximately 2.5-3 dpf a new sprout also takes off caudally from the AMA just after it exits the DA. This vessel is the arterial branch to the swim bladder (SBA). The AMA continues on to become the SIA (see [AMA & derivatives]). Several (2-4) posterior mesenteric arteries (PMA) branch ventrally from the DA and anastomose longitudinally along the dorsal wall of the hindgut, forming the caudal part of the SIA (these vessels, and the SIA, are not well visualized in most of our angiograms). The cranial portion of the SIA extends caudally from the anterior mesenteric artery (AMA) and joins to the caudal SIA of hindgut, completing the full extent of the SIA (reasonably well visualized in [4 dpf overview] and diagrammed in [intestinal vasculature]. The AMA and its branches and connections are detailed further in the next section and in [AMA & derivatives]. The vascular plexus of the mid- and hind-gut walls also appears by 3 dpf. The right and left SIV, which drain this plexus, are by 3 dpf continuous across most of the dorsal-lateral aspect of the yolk ball [intestinal vasculature], although they are not always continuous over the yolk extension at 2.5-3 dpf (compare [2.5 dpf overview] and [3.5 dpf overview]). The most rostral end of the left SIV begins to divide into separate smaller channels. These numerous elaborating branches represent the forming reticular hepatic sinusoids (see [3.5 dpf hepatic vessels] for a diagram of liver vascularization at 3.5 dpf). Just caudal to this, the most rostral unbranched portion of the left SIV will become the hepatic portal vein. Multiple venous connections (hepatic veins) appear linking the forming reticular hepatic sinusoids to the left PCV and CCV (mostly to the left CCV), draining blood from the developing liver. At the caudalmost end of both SIVs some minor drainage routes also form from these vessels to the PCV. The pectoral (subclavian) artery (PA) and pectoral vein (PV) come on line at late 2.5 to 3 dpf. The PA takes off directly from the lateral wall of the DA caudal to the pronephric glomus, at the same level as the root of the second SeA (see Fig. 6F). The PV drains into the ACV just cranial to its junction with the CCV (see 4 dpf section and {4.5 dpf pectoral fin vessels] for diagram of pectoral vessels).

Despite massive elaboration of smaller caliber vessels in the head through 7 dpf (the latest stage examined in this study), the overall "wiring pattern" of major head vessels is largely unaltered after 2-2.5 dpf. There are a few significant changes at this stage, however. With the development of the fifth and the sixth aortic arches, the complete set of aortic arches is generally present by 2.5 dpf (see [early 2.5 dpf head arches], [later 2.5 dpf head arches] for intermediate and "final" stages of arch formation). The connections between the aortic arches and the cranial circulatory system are detailed more fully in the next section (see [3.5 dpf arch-head]). The caudal two aortic arches (AA5 and AA6) drain by a common route into a separate, more superficial branch of the LDA [2.5 dpf head dorsal multilayer composite] (lowest layer). This separate branch flows to the radix of the dorsal aorta in the cranial trunk, and AA5 and AA6 eventually are the major (if not sole) aortic arches supplying the trunk and tail circulation [3.5 dpf arch-head], [3.5 dpf head dorsal multilayer composite] (lowest layer). AA3 and AA4 flow into the more medial original LDA. Blood from the medial LDA flows into both the PICA and radix of the aorta at this stage, but at later stages flows almost entirely into the PICA [4.5 dpf head dorsal multilayer composite]. Thus, AA3 and AA4 become the major aortic arches supplying the cranial circulation. The ventral aorta is now prominent [2 dpf head, ventral-anterior-lateral], but has not yet shifted anteriorly and lengthened (see [3 dpf head, ventral-lateral]). Before 2.5 dpf the PHS is complete through to the large PMBC-PHBC junction point. The completed PHS carries most of the venous return from the head, subsuming in large part the early role of the PHBC. The branches of the PICA are basically the same as those at 2 dpf, although the MsA is now complete to the MsV, rising up through most of the D-V depth of the head continuously to the DLV [2.5 dpf overview],[early 2.5 dpf head lateral] and [2.5 dpf head dorsal multilayer composite]. The PCeV generally become complete at or shortly after this stage, running bilaterally from the DMJ/DLV caudally and ventrally to a junction with the PHBC, and from there through a short segment to the ACV [early 2.5 dpf head lateral], [later 2.5 dpf head lateral] and [2.5 dpf head dorsal multilayer composite]