Innervation of the rat thymus gland

Current views from different laboratories on the innervation of the thymus gland are reviewed with particular reference to the rat. Noradrenergic nerve profiles of the sympathetic nervous system have been demonstrated in the subcapsular cortex, at the corticomedullary junction and in the cortex itself, and extremely sparsely in the medulla. By following beta-adrenergic receptor development in postnatal rats, it has been shown that there is a marked increase in density and morphological organization of the receptor in the medulla with the maturation of thymocyte function (monitored by measuring the proliferation response to concanavalin A) and a sexual dimorphism during the ontogeny of the receptor. Chemical sympathectomy of adult rats with 6-hydroxydopamine (6-OHDA) or guanethidine resulted in a loss of thymus weight, decreased cellularity, and increased apoptosis but a rise in the numbers of proliferating cells in the cortex. By contrast, proliferation of peripheral T cells was reduced after the use of 6-OHDA. Chemical sympathectomy also demonstrated that there were at least three nerve nets in the gland: noradrenergic neural profiles that were destroyed with both 6-OHDA and guanethidine, vasoactive intestinal polypeptide (VIP)-positive profiles that persisted, and AChE- and CGRP-positive profiles and cells that also persisted but had a different distribution to VIP-positive fibers. Some functional correlates of thymic innervation are discussed although the subject now needs to be further researched.