Lymphoid organs (spleen, thyrsus, assorted lymph nodes, tonsil, and gut-associated lymphoid tissue) from belugas, Delphinapterus leucas, ranging in age from less than one to 16 years, were harvested during sanctioned hunts and processed for light and electron microscopy to investigate overall morphology and innervation. Overall morphology of beluga lymphoid organs was similar to descriptions of other inarni-nalian lymphoid organs with a few differences. Moreover, some differences in lymphoid organ morphology between individual whales could possibly be attributed to age differences.

Catecholamine fluorescence histochemistry and tyrosine hydroxylase (TH) (the rate limiting enzyme in norepinephrine synthesis) and neuropeptide-Y (NPY) (a peptide often found co localized with norepinephrine) immunocytochemistry was carried out to study innervation of beluga lymphoid organs. Fluorescent nerve fibers and TH+ nerve fibers were present in parenchymal lymphoid compartments, where they established contact with cells of the immune system, in addition to vascular and trabecular compartments. NPY-containing nerve fibers were distributed in a similar fashion to TH+ fibers in the lymphoid organs examined, suggesting co localization. In lymphoid zones, TH+ and NPY+ nerve fibers were observed in the periarteriolar lymphatic sheath and marginal zone of the spleen; in the outermost portion of the cortex, the corticomedullary zone, and medulla of the lymph nodes; in the parafollicular zones, and diffuse lymphocyte layer below the epithelium of the tonsil; in the outermost portion of some thymic lobules; and in the lamina propria of the gut.

This study demonstrates an anatomical link between the nervous and immune systems, which may permit central nervous system phenomena to influence autonomic outflow to lymphoid organs, and thereby influence immunologic reactivity in the beluga. This anatomical study provides a framework for functional investigation of the immune system and neural-immune system communication currently in progress.