Dual‐label retrograde transport: CNS innervation of the mouse thymus distinct from other mediastinum viscera

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Abstract
Two different retrogradely transported tracers were identified and used to distinguish between the innervation of the thymus gland and the innervation of adjacent tissues residing within the mouse mediastinum. To determine the best procedure for peripheral dual-labeled retrograde transport, several different tracers, vehicles, and injection protocols were developed and/or tested. In the periphery, it proved ineffective to use Rhodamine latex microspheres (Luma-Fluor) as a retrograde transport agent. Horseradish peroxidase (HRP) suspended in the water-soluble carrier KY Jelly and Fluro-gold (FG) in an aqueous solution was the most effective combination of retrograde transport tracers. Equivalent doses of each of these tracers were determined, and injections of HRP or FG into the thymus and the esophagus produced distinct and separate patterns of labeled neurons in the spinal cord or brain stem. Throughout these CNS areas, some neurons contained both tracers suggesting that (1) these areas are innervated by neurons that give rise to collaterals to these tissues, (2) that the retrograde tracers are transported by nerves that innervate the lymphatics of the mediastinum, and/or (3) that some diffusion of the tracers occurs from the test organs and tissues to other organs and tissues of the mediastinum.