Structure and innervation of the sensory organs on the snout of the star-nosed mole.

The star-nosed mole possesses a conspicuous specialization of its snout in the form of 22 fleshy appendages that fan out from around the nostrils. These appendages are used by the mole to explore its underground environment and are repeatedly brought into contact with objects of interest to the mole. This report describes the structure, innervation, and distribution of the sensory organs on the star of the star-nosed mole and briefly describes the behavioral use of the star. Each of the 22 appendages of the star is covered with a continuous array of Eimer's organs. These sensory receptors are modifications of the epidermal surface that take the form of bulbous papillae. Each Eimer's organ contains a column or stack of epidermal cells accompanied by nerve processes that originate from myelinated fibers in the underlying dermis. These neural processes travel through the cell column and form terminal swellings just below the outer layer of keratinized epidermis. Each Eimer's organ also contains a single Merkel cell-neurite complex within the cell column and a single lamellated corpuscle immediately below the cell column in the connective tissue of the dermis. There are approximately 30,000 Eimer's organs on the snout of this mammal, making this structure perhaps the most sensitive tactile organ yet discovered for its size. The segregation of these organs to individual appendages, not unlike the fingers of primates, affords an intriguing model for the study of somatosensory systems in mammals.