Calcitonin gene-related peptide in the developing and aging thymus. An immunocytochemical study

Calcitonin gene-related peptide (CGRP) is known to block Con A and PHA induced T cell proliferation. As a first step in determining the role of this peptide in T cell education and function we have studied the distribution of CGRP within the developing mouse thymus using immunocytochemistry. CGRP-like immunoreactivity (CGRP-IR) was found in the thymic nerves in close proximity to blood vessels in the 17-day-old embryonic mouse thymus. A discrete population of small cells at the cortico-medullary junction also stained intensely for CGRP. As the mouse thymus reached maturity (three to eight weeks) CGRP innervation became more dense, with fibers running along the vasculature at the cortico-medullary boundary, then branching into the cortical and medullary regions. Some fibers were invested in the blood vessels while a large portion formed varicosities among the cells of the thymus. In the mature thymus, the small CGRP-IR cortico-medullary cells were more numerous, and CGRP-IR was also found in subcapsular and trabecular mast cells. The pattern of innervation remained the same in the aging mouse thymus (six months), but there appeared to be somewhat fewer cortico-medullary cells and an increase in mast cell number. In the aged (eighteen months) thymus, the small CGRP-IR cortico-medullary cells were rarely seen, but mast cells were more numerous, most of which stained positively for CGRP, in the connective tissue. Nerves containing CGRP-IR generally had the same distribution as in the younger mice but appeared somewhat truncated. The distribution of CGRP-IR nerves in the mouse thymus at different stages of development was similar to that reported for cholinergic (AChE-positive) nerves. Since the brain-stem vagal nuclei have been shown by retrograde transport studies to project to the thymus as well as to contain CGRP-IR neurons, our findings suggest that CGRP-IR thymic nerves may be derived from the vagus complex.