In the fifth week, cells originating in the neural crest of the thoracic region migrate on each side of the spinal cord toward the region immediately behind the dorsal aorta (Fig. 19.42). Here they form a bilateral chain of segmentally arranged sympathetic ganglia interconnected by longitudinal nerve fibers. Together they form the sympathetic chains on each side of the vertebral column. From their position in the thorax, neuroblasts migrate toward the cervical and lumbosacral regions, extending the sympathetic chains to their full length. Although initially the ganglia are arranged segmentally, this arrangement is later obscured, particularly in the cervical region, by fusion of the ganglia.
Some sympathetic neuroblasts migrate in front of the aorta to form preaortic ganglia, such as the celiac and mesenteric ganglia. Other sympathetic cells migrate to the heart, lungs, and gastrointestinal tract, where they give rise to sympathetic organ plexuses (Fig. 19.42).
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