Development of innervation within syngeneic thymus tissue transplanted under the kidney capsule of the nude mouse: a light and ultrastructural microscope study.

The developing thymus, prior to the onset of its functional and structural organization, is innervated by the autonomic nervous system (ANS). The present study extends these earlier findings by analyzing at the light and ultrastructural microscope level the distribution of ANS nerves within normal 18-day-old embryo thymus, adult thymus, and thymic tissue transplanted under the kidney capsule of syngeneic nude mice. The results of this study showed that prior to birth the murine thymus is innervated by AChE-positive fibers that are distributed at the corticomedullary boundaries and throughout the adjacent cortex, as well as to cells beneath the thymic capsule. This pattern of distribution remains constant during adult life. The examination of the various thymic tissues by ultrastructural analysis demonstrated that myelinated and nonmyelinated fiber bundles penetrate the thymic capsule and the interface between the kidney and the thymus transplant. The myelinated fibers measured 2 micron or less in diameter; the nonmyelinated fibers were 1 micron in diameter. Myelination did not accompany the nerves into the parenchyma of the normal gland but was observed in the intralobular trabeculae of the transplanted thymus. Subcapsular nerves form a network that terminates among the thymocytes, whereas intrathymic nerves enter the parenchyma in bundles along the vasculature and interlobular septa before penetrating into the deeper layers of the thymic cortex. Some larger nerve fibers terminate in the corticomedullary boundaries and in the interlobular septa. Smaller fibers form en passant boutons near parenchymal cells. The innervation of both the normal thymus and the transplanted thymus prior to the onset of thymic immune function supports a role for ANS innervation in the development of thymic competency.