Ultrastructure of the Eimer's organ of the star-nosed mole

Abstract

The nose of the star-nosed mole consists of 22 fleshy appendages that fan out from around the nostrils and are covered with specialized epidermal sensory receptors called Eimer's organs. The Eimer's organs of the star-nosed mole are domed epidermal papillae approximately 40 to 50 microns in diameter. The center of each papilla contains a column of stacked circular epidermal cells closely associated with five to ten neural processes that originate from three myelinated fibers in the underlying dermis. At middle and lower levels in the cell column, a single nerve fiber is located in the center of the column, enclosed by the epidermal cells which wrap around the fiber and form desmosomes between their own adjacent plasma membranes. An additional five to ten fibers travel up the sides of the column ensheathed in the margins of the epidermal cells. At the top of the cell column, the nerve fibers produce a repeated series of terminal swellings. These terminal swellings converge in the center of the column, where a single epidermal cell completely encapsulates each circular arrangement of nerve terminals. No synapses or other cellular junctions were observed between the nerve terminals where they converge within the cell column. There is a single Merkel-like ending at the base of the cell column, and a single encapsulated corpuscle beneath the cell column, in the connective tissue of the dermis. The structure of Eimer's organs is consistent with a mechanoreceptive function.

Neuroanatomical evidence for segregation of nerve fibers conveying light touch and pain sensation in Eimer’s organ of the mole

“…As a result, several features of the star-nosed mole Eimer's organ are unique. For example, the central cell column in star-nosed moles is only one cell in diameter, forming a single stack of keratinocytes (20). This configuration leaves less room for free nerve endings associated with the column, and, as might be expected, there are fewer satellite fibers in star-nosed mole Eimer's organs.…”

“…In this study we used AM1-43, the fixable analog of the styryl pyridinium dye FM1-43 (19), to identify and examine the presumptive mechanosensory receptors in the rhinarium of the mole. We were particularly interested in the intraepidermal free nerve endings supplied by myelinated fibers (20), because they may play a part in the transduction of high-resolution tactile signals. Here we show robust labeling of each sensory receptor component of Eimer's organ by AM1-43.…”

“…Both the push-rod and Eimer's organ include a similar arrangement of intraepidermal free nerve endings (vesicle chain receptors in push-rods) showing the bead-on-a-string appearance (28)(29)(30). In both organs the fiber terminals form swellings below an epithelial dome at the surface of the skin, and in both organs the terminal nerve swellings are anchored by a basket-like complex of tonofibrils that may play a role in sensory transduction (20,28). Each organ also has Merkel cell-neurite complexes in the base of the epidermal compartment and lamellated corpuscles in the dermis below.…”

Fine structure of Eimer's organ in the coast mole (Scapanus orarius)

“…A larger structure in the monotremes called a push rod has a similar configuration of epithelial cells and sensory receptors (Andres and von During, 1984;Andres et al, 1991;Manger and Hughes, 1992;Iggo et al, 1996;Manger and Pettigrew, 1996). In star-nosed moles, Eimer's organ is smaller, with a central cell column only one cell in diameter and free nerve endings more closely associated with one another (for details, see Catania, 1996). The mole, platypus, and echidna all rely on touch to an important extent as they forage for small prey in environments where there is little visual information.…”

“…This result suggested exceptionally superficial nerve terminals might be transducing sensory information in Eimer's organ. This would be surprising, as we are unaware of active sensory terminals in this skin layer in other species, and because ultrastructural evidence from the starnosed mole indicates that the central column free nerve ending terminals are degraded and nonfunctional in the stratum corneum (Catania, 1996). In this study, we focused considerable attention on the state of the central column free nerve ending terminals within the stratum corneum of the coast mole to determine if they might be functional from a morphological perspective.…”