Innervation of the thymus gland by brain stem and spinal cord in mouse and rat.

Central nervous system (CNS) projections to the thymus were studied in the mouse and rat using the horseradish peroxidase (HRP)-retrograde transport method. With discrete HRP injections localized to the thymus, labeled neurons are evident in both medulla and spinal cord. In the medulla the largest population of labeled neurons is present in the retrofacial nucleus. Within this cytoarchitectonically distinct nucleus, the majority of neurons are labeled with large HRP injections in the thymus. In addition to retrofacial nucleus, scattered labeled neurons are found throughout the rostrocaudal extent of the nucleus ambiguus and in the dorsal medullary tegmentum adjacent to the dorsal motor vagus nucleus. With HRP injections restricted to thymus parenchyma, no labeled neurons are evident in the dorsal motor vagus nucleus. Three groups of spinal cord neurons are labeled. In segments C2-C4, neurons localized to the ventral horn are labeled in two distinct columns, one located lying laterally in the ventral horn and the other located medially. Labeling of neurons in these segments is distinct from that of large motor neurons located medially in the ventral horn extending from the level of the decussation of the pyramids through the C1 segment. The location and sizes of neurons labeled in these areas following HRP injection in the thymus are identical in the mouse and rat. These observations provide evidence for previously unknown projections from spinal cord and brain stem to the thymus which may play an important role in the regulation of thymic function.