A comparative study of the autonomic nervous system innervation of the thymus in the mouse and chicken

It is well-established that the thymus of rats and mice are directly innervated prenatally by neurons of the central nervous system (CNS). Evidence now suggests that this innervation is involved in determining the neuroendocrine-immune functions of the gland. In this study an analysis of CNS innervation of the chicken thymus was carried out using the horseradish peroxidase (HRP) retrograde transport method to establish the phylogenetic integrity of CNS-thymic innervation and to gain insight into its function. Day-old chicks received HRP injections into upper of lower thymic lobes. Lower injections resulted in HRP-labeled neurons in the ventral horn of the cervical spinal cord and in the dorsal and ventral brainstem complex of the nucleus of X. Injections into upper thymic lobes resulted in similar spinal cord labeling and in labeling of neurons in the nuclei of both IX and X. These injections produced more labeled neurons in the ventral complex of X than did injections into lower thymic lobes. Unilateral HRP injections into upper or lower thymic lobes produced predominantly ipsilateral labeling in the appropriate areas. The results of this study indicate that chick thymuses are innervated by the CNS during embryogenesis. Furthermore, there appears to be a topographical distribution of nerve fibers suggesting discrete functions for individual lobes.