Merkel nerve endings in sinus hairs of young and aged rats

Whiskers are highly sensitive touch organs in all mammals apart from man. A blood sinus encloses the hair follicles completely or almost completely. One sinus hair has several thousand sensory nerve endings. Most of them are located in the epithelial thickening below the sebaceous gland. A large number of Merkel cells with associated nerve endings can be found in that region. The absolute number depends on the species and size of the hair. In cat and rat, large whiskers can have up to 2000 Merkel cell-nerve endings innervated by approximately 100 myelinated axons. Within the epithelial thickening, Merkel cells are located in the basal epithelial layer of the follicle and arranged like scales in a pine cone. The discoid nerve terminals are located on that side of the Merkel cell opposite to the basal membrane. Synaptic contacts between Merkel cells and nerve terminals can be seen and the cytoplasm towards the nerve terminals contains numerous osmiophilic granules. In the literature there are studies on structure, development, de- and regeneration of sinus hairs; however, we are not aware of any systematic study on age-related changes. This study was carried out on rats aged between 2 days and 30 months, which is a high age in rats. All types of nerve endings were found both in young and old rats. The number of Merkel nerve endings depends on the size of the hair follicles. In young animals (2 days), they are still growing and mitoses are often seen in the thickening of the hair bulb (Merkel cell cuff). At this age, not all Merkel cells already have contact with nerve terminals. The number of dense core granules in Merkel cells increases during innervation. At the age of 19 days and later, almost all Merkel cells are innervated. Absolute numbers of Merkel nerve endings cannot be determined using electron microscopy. However, their ultrastructural appearance did not show any significant change with age.