Development of the spinal nerves of the larval lamprey: IV. Spinal nerve roots of 21‐mm larval and adult lampreys, with special reference to the relation of meninges with the root sheath and the perineurium

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Abstract
Spinal nerve roots of 21‐mm larval and adult lampreys were electron microscopically studied. In 21‐mm larval lampreys, each ventral and dorsal rootlet contains axons of various diameters enclosed together as groups in individual troughs of a Schwann cell cytoplasm, lying in direct contact with one another, and is further ensheathed entirely by a basal lamina. Dorsal roots possess visceral axons, while ventral roots lack them. In adult lampreys the ventral and dorsal roots possess individual sheaths for larger somatic axons, each being surrounded by a single Schwann cell and the basal lamina and separated from one another by a considerable amount of connective tissue. Visceral fibers are present in both the dorsal and ventral roots of adult lampreys. They aggregate to form fascicles that lie among somatic axons, being separated from them. Two layers of the meningeal tissue invaginate to form a root sheath around the distal portion of individual dorsal and ventral roots of 21‐mm larval lampreys. In adult lampreys the sheath is similarly formed but extends over most of the dorsal and ventral roots. The perineurium is not developed in 21‐mm larval lampreys, but is present and ensheaths only the proximal portion of spinal nerve trunks outside the meninges in adult lampreys: it is completely absent along most of the length of peripheral nerves. In both larval and adult lampreys, the outer cell layer of the root sheath is open‐ended near the middle of nerve roots with respect to the extramedullary connective tissue space. Similar loosening of the cellular barrier is seen along blood vessels. Thus, the outer meningeal fibrous layer is directly continuous with the extra medullary connective tissue space by way of the inner fibrous layer of the root sheath.