Sympathetic innervation of fetal mouse thymus

This study was carried out to investigate the presence of sympathetic nerve fibers in the developing thymus. Thymic rudiments from 13 days of gestation up to birth and adult thymuses were examined by histofluorescence microscopy to detect the presence of these nerve fibers during ontogeny. Sympathetic nerve fibers were first visible around day 17 of gestation and increased in density and distribution by the time of birth and in the adult thymus. The appearance of immunoreactive cells about the same time as the presence of nerve fibers in the thymic rudiments has been discussed in light of a regulatory role for the sympathetic neuronal input on the maturation of thymocytes during ontogeny.