Gradual establishment of dominant longitudinal vessels leads to regression of most transverse segmental arteries, except at some levels where such vessels persist in supplying the longitudinal artery. This pattern is seen throughout the body, but is somewhat easier to recognize in the vertebrospinal arterial system, where it gives rise to adult anterior spinal artery and numerous extradural longitudinal segmental connections which will be discussed below. When its limits are reached (200 micrometers perhaps), a primitive vascular system consisting of paired dorsal and ventral aortae (longitudinal vessels) and transversely oriented segmental arteries come into play to vascularize the developing tissue of the embryo.Īs the tissue of spinal cord continues to enlarge, new longitudinal connections form between the transverse segmental arteries, most likely to facilitate distribution of blood within the vascular system. In the early human embryo, the neural tube is first supplied by simple diffusion. It is perhaps easiest to appreciate this concept at the thoracic level, where each rib, vertebral body, and other elements constitute the prototyical segment. Each vertebral body, its ribs, muscle, nerves, and dermatome, correspond to one level or segment. Just like the fly and the worm, the human body consists of metameric segments, with ecto-, meso-, and endodermal elements. Millions of years of speciation have taken place upon a basic building block of the organism - the metameric segment. This simple bit of knowledge goes a long way in understanding spinal anatomy. The basic arrangement of the spinal system consists of a metameric grid of trasversely oriented segmental vessels, connected by various longitudinal channels. A more complete discussion is found in the dedicated section of n eurovascular embryology. Development - this abbreviated, important section precedes discussion of adult anatomy.
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