1-1.2 Days Post-Fertilization
Circulation begins in the zebrafish embryo at approximately 24-26 hours post-fertilization (hpf). Initially blood flows through a simple single circulatory loop [24 hpf overview]. Blood exits the heart through the bulbus arteriosus and the ventral aorta then branches to the right and left into the mandibular (first) aortic arches. The arches empty directly into the right and left lateral dorsal aortae (LDA), which run caudally, approaching one another and then merging in the rostral trunk to form a single medial dorsal aorta (DA). The single dorsal aorta continues into the tail (the portion caudal to the anal pore is designated the caudal artery (CA)) and then turns 180 degrees at its caudalmost end to empty into the caudal vein (CV). At or after this stage, the CV becomes a sinus or a braided plexus of vessels rather than a single distinct channel like the dorsal aorta or caudal artery. The CV remains in this form for many days hence. Circulation from this caudal venous plexus continues into the trunk, becoming a single distinct posterior cardinal vein (PCV) crania; to the anal pore. The cardinal vein splits into a pair of vessels in the rostral trunk, just caudal to the radix of the aorta. The posterior cardinals each empty into a large sinus, the Duct of Cuvier (DC) or future common cardinal vein (CCV), which fans out across the yolk cell on either side, coming together rostr-ventrally at the sinus venosus of the heart. There are no vitelline arteriolae (transverse vessels) per se in the zebrafish; the DC presumably fulfills this function. It should be noted that, unlike many other vertebrates, in zebrafish embryos the DA and PCV are single medial unpaired tubes at the time circulation begins in the trunk, rather than a pair of more lateral vessels. Amphibians and reptiles also have a medial DA, although like mammals and avians their PCV is present as paired lateral vessels throughout the trunk.

Shortly after the commencement of this initial circulatory loop a second, rostral loop comes on line [26 Hour Overview]. Blood exiting the first aortic arches now flowsbidirectionally, caudally into the lateral dorsal aortae (LDA) and rostrally into the primitive internal carotid arteries (PICA) [28 hpf head, dorsal-lateral]. Each PICA initially divides into two branches. The caudal division of the internal carotid artery (CaDI) dives deep inside the embryo, looping dorsally and caudally to join the equivalent branch from the other side via the basal communicating artery (BCA) at the brain midline [26 Hour Overview],[1.5 dpf head / trunk],[1-2 dpf head vessel changes]. The cranial division of internal carotid artery (CrDI) goes anterior to the optic capsule, then curves caudally, emptying into the primordial midbrain channel (PMBC) [28 hpf head, dorsal-lateral], [28 hpf head, dorsal]. The PMBC continues caudally to the primordial hindbrain channel (PHBC). The anterior cerebral vein (ACeV) also branches from it at 1.3 dpf.

The mid-cerebral veins (MCeV) take off from the PHBC-PMBC junction and extend bilaterally up along the midbrain-hindbrain boundary toward the dorsal midline of the head [28 hpf head, dorsal-lateral], [28 hpf head, dorsal]. At this stage these vessels do not yet meet at the midline, and have no arterial feed. The PHBC are located medial to the cranial nerves and the otic capsule, running caudally from the base of the MCeV to the base of the future posterior cerebral veins (PCeV). At the base of the future PCeVs blood then flows ventro-laterally from the PHBC, via short segments near the caudal end of the otic capsule, down to the anterior cardinal veins (ACV). The ACV joins with the PCV at approximately the same antero-posterior level as the radix of the aorta to form the CCV/DC. The more rostral portion of the ACV (primary head sinus, PHS) located ventro-laterally to the otic capsule, has not yet formed.