| Summary: | Vertebrates differ in how the internal carotid arteries (ICAs) connect to the aortic arches and how the basilar artery and its penetrating branches arise. To test phylogenetic patterns in brain vasculature, the carotid/basilar arterial system in embryonic and adult Squalus acanthias were examined using serial sections, vascular fills and microdissection. Dissections of other cartilaginous fish such as Hydrolagus , Raja , Mustelus and Scyliorhinus allowed identification of general patterns within Chondrichthyes for comparison to other vertebrates. At their earliest appearance, both the primitive ICA and a pial hindbrain angiogenic network contributed to the carotid‐basilar arterial tree. The paired ICAs are extensions of a midline sinus cephalicus which transforms into an unpaired encephalic trunk that enters the hypophyseal fossa and branches into paired cerebral carotids that are joined by the efferent pseudobranchial arteries. Within the fossa, the carotids divide into anterior and posterior divisions. The posterior run caudally along the lateral walls of the brainstem, giving off diencephalic, mesencephalic and cerebellar branches before fusing in the midline to form an unpaired basilar artery. The basilar gives off a quasi‐segmental series of transverse rhombencephalic branches. Fusion of the initially paired basilars is often incomplete beneath the medulla, resulting in a fenestrated adult basilar. Despite the significant diversity in adult brain vasculature, a highly conserved framework of brainstem vessels and morphogenetic processes is evident when comparing development of distantly related vertebrates such as dogfish sharks, zebrafish and humans.
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