Neuropeptide Y in the human thyroid gland

Autonomic nerve fibers immunoreactive to Neuropeptide Y (ir-NPY) were counted in tissue samples from human thyroid glands. Samples were taken from the non-pathological parts of gland material removed surgically from nine female patients suffering from struma nodosa or solitary adenoma. The ir-NPY fiber population was subdivided into fibers associated with blood vessels (perivascular innervation) and fibers either attached to the wall epithelium of the follicles or coursing within the gland's connective tissue (extravascular innervation). Most perivascular fibers were found in arterioles (20-40 microns diam.). Capillaries (< 10 microns diam.) were not innervated. These fibers surrounded the vessels. They formed a characteristic reticular or coil-like pattern close to the tunica adventitia. Sometimes a second, smaller fiber layer was found between the tunica adventitia and media. Ir-NPY fibers were unmyelinated and less than 2 microns in diameter. Based on counts of the gland's arterial supply, it was calculated that about 23% of all arterial vessels (< 80 microns diameter) were innervated. On average, one innervated arteriole was found per 0.2 mm2 of thyroid tissue. Extravascular fibers were less frequent. In ca. 3 mm2 of thyroid tissue at least two ir-NPY fibers were found outside the vessels close to the follicular epithelium or within the connective tissue. No follicles with a dense ir-NPY innervation were found. Extravascular fibers were less arborized. They appear to be either smaller or less intensively stained than perivascular fibers, although some extravascular fibers were collaterals of the perivascular fibers.