Parasympathetic innervation of the rat thymus during first life period: histochemical and biochemical study.

In the present study we analyzed development of the rat thymus parasympathetic innervation using histochemical determination of distribution and density of acetylcholinesterase (AChE) positive nerve profiles, as well as biochemical measuring of the activity of this enzyme. Rat thymuses from late embryonal to adult periods of life were analyzed. The AChE-positive nerve profiles were found, for the first time, on day 18 of fetal life in capsule and interlobulary septae, but also in the subcapsulary and cortico-medullary areas. The density of these profiles increased during the thymic development. The AChE positive nerve profiles in subcapsulary region were observed mainly in close proximity to the thymic epithelial cells, while in the cortico-medullary region they were found in apposition to the thymocytes. The biochemical analysis showed that the specific AChE activity in rat thymus was high on day 19 of gestation. A significant increase in the activity of this enzyme was measured by the third day of postnatal development, and its activity remained approximately at the same level up to the day 90. The present results suggest that thymus receives parasympathetic innervation relatively early in ontogeny; in addition, these nerve fibers could be involved in the regulation of the organ activity, at least, through action upon the thymocytes and/or by modulation of the thymic epithelial cell activity.