Epidermal Sensory Organs of Moles, Shrew Moles, and Desmans: A Study of the Family Talpidae with Comments on the Function and Evolution of Eimer’s Organ

Abstract

The epidermal sensory organs of members of the family Talpidae (moles, shrew-moles, and desmans) were investigated and compared to determine the range of sensory specializations and better understand how they evolved. Small domed mechanosensory organs called ‘Eimer’s organs’ were present on the rhinarium of nearly all species of talpids, but not among the sister group of shrews (Soricidae) or other insectivore families. This suggests that the common ancestor to the talpids possessed Eimer’s organs. Two species of moles from the driest habitats did not exhibit Eimer’s organs – suggesting that their sensory organs degenerated in response to harsh, abrasive soil conditions. The semi-aquatic desmans uniquely possessed tiny sensory hairs interspersed with their Eimer’s organs; these may act to sense water currents. Some species exhibited a subdivided, star-like, rhinarium – resembling an early embryonic stage of the star-nosed mole and providing clues to the evolution of the star. A single genera (Uropsilus) that branched off early in the evolution of the talpids had Eimer’s organ-like structures but lacked some typical components. These findings fill a major gap in our knowledge of talpid sensory biology and suggest (1) how Eimer’s organs evolved, (2) how the unusual appendages of the star-nosed mole evolved, (3) that the evolution of Eimer’s organ is convergent with the mechanosensory push-rod of monotremes. The results also demonstrate the features that distinguish Eimer’s organ from similar configurations of sensory receptors in other mammalian skin surfaces. Finally, a mechanism for Eimer’s organ function in detecting object and prey specific surface features is proposed.

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“…2B). Such mechanosensory organs are found on the noses of most moles (18)(19)(20) and are anatomically similar to small, domed push rods found on the snout of distantly related monotremes (21)(22)(23)(24). In star-nosed moles, each organ is about 40-60 μm in diameter and has a small (15-20 μm) central disk on the outer surface.…”

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

“…The position of the central column at the center of the organ and extending to the surface of the epithelial dome may provide a means for the entire structure to respond to very slight indentations, thus allowing for a high degree of tactile acuity. The central column free nerve endings in particular, located in a circular arrangement along the outside edge of the central column, are in an ideal position to register the mechanical stresses to the deflection of the column in different directions (Catania, 2000). This directional sensitivity has also been suggested for the vesicle chain receptors of the push rod (Andres and von During, 1984;Andres et al, 1991).…”

“…Most other mole species share this particular adaptation, and it appears to play a major role in tactile acuity (Catania, 2000). 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).…”

“…It has been proposed that Eimer's organ of the mole may function to bend and conform to objects possibly relating fine textural details (Catania, 2000). The position of the central column at the center of the organ and extending to the surface of the epithelial dome may provide a means for the entire structure to respond to very slight indentations, thus allowing for a high degree of tactile acuity.…”

“…A close look at the nose reveals a series of small bumps, or papillae, covering the surface of the glabrous rhinarium. Each papilla is an Eimer's organ (after Eimer, 1871) and nearly every species of the talpid mole is endowed with these structures (Catania, 2000). Microelectrode recordings from somatosensory cortex indicate that Eimer's organ is acutely sensitive to mechanosensory stimuli and have small receptive fields (Catania and Kaas, 1995;Catania, 2002a, 2002b;Marasco and Catania, 2007).…”