Nitrergic, peptidergic and substance P innervation of the chick thymus.

Nitrergic and peptidergic innervation of the chick thymus was studied using histochemical and immunohistochemical methods. Nicotinamide adenine dinucleotide hydrogen phosphate-diaphorase (NADPH-d) histochemistry and anti-nitric oxide synthase (NOS) antibodies stained both nerve fibres and 'neuron-like' cells located in the septal connective tissue. NADPH-d and NOS were partially colocalised. Staining of NADPH-d positive neuron-like cells with the neuronal marker, neuron specific enolase, confirmed the neuronal nature of these cells. Antibodies against vasoactive intestinal peptide (VIP), neuropeptide tyrosine (NPY), substance P (SP) and calcitonin gene related peptide (CGRP) were used to map the peptidergic innervation of the chick thymus. The distribution of nerve fibres staining for the various neuroactive chemicals in specific thymic compartments was non-uniform. Out of all the peptides, VIP-containing nerves appeared to be the most abundant. In addition, double-labeling of the thymic sections revealed that VIP and NADPH-d were colocalised in the neuronal structures. Immunostaining of the chick embryos demonstrated that VIP, NPY, SP and CGRP were first expressed in the chick thymus during late ontogeny. The significance of these novel findings was discussed.