Structure and innervation of the vibrissal follicle-sinus complex in the Australian water rat, Hydromys chrysogaster.

Light and electron microscopic techniques were used to examine the structure and innervation of the mystacial vibrissal follicle-sinus complex (F-SC) in the Australian water rat. The F-SCs of this semiaquatic rodent show the same morphologic elements described in terrestrial rats but differ in size, structure, and innervation. Most striking is the size of the water rat's caudal F-SCs, measuring 6.3 mm in length and 2.4 mm in diameter. The sinus system is divisible into a ring sinus and a cavernous sinus and shows a distinct asymmetry. At the highest level of the cavernous sinus, the outer root sheath forms a ridge in the direction of the trabeculae, which bind the ridge to the capsule. A ringwulst is present only in small and medium-sized F-SCs. The mean number of myelinated axons counted in the deep vibrissal nerve (DVN) of most caudal F-SCs was 537, indicating an innervation density of the water rat's vibrissal system at least 2.5 times as high as that of terrestrial rats. The total number of nerve fibers of the small superficial nerves was less than 10% of that of the DVN. These fibers innervate almost exclusively the area of the inner conical body. Structural specializations of the water rat F-SC are discussed as an analogous development in mammals adapted to the aquatic environment, primarily in terms of thermoregulation, whereas its high degree of innervation is assessed to lend support to the hypothesis that the vibrissal system is of special significance in aquatic mammals. Copyright 1999 Wiley-Liss, Inc.