THE ANATOMICAL RECORD 214:210-225 (1986) The Sensory Innervation of the Rat Rhinarium RICHARD T. SILVERMAN, BRYCE L. MUNGER, AND ZDENEK HALATA Departments of Anatomy, The Milton S. Hershey Medical Center, The Pennsylvania State University, Hershey, Pennsylvania 17033 and Anatomisches Znstitut der Universitat Hamburg, Abteilung fur Funktionelle Anatomie, 0-2000 Hamburg 20 Federal Republic of Germany ABSTRACT The present study documents the characteristics of innervation of the rhinarium or hairless rat snout skin by light and electron microscopy. The outer glabrous surface is covered with a stratified squamous epithelium that forms both rete pegs and rete ridges, the latter on the inferior border near the philtrum. The glabrous skin contains numerous presumptive epidermal and dermal free nerve endings (FNE’s), Merkel terminals at the base of the rete ridges and pegs, and simple, nonencapsulated corpuscles. A second region of dense innervation, found on an elevation of the inner wall of the vestibule, contains similar components of innervation, with the exception that no Merkel terminals were identified. Since no Merkel terminals were present in this area of the vestibule, intraepidermal as well as dermal FNE’s could be identified with certainty. This skin is covered by a thin squamous epithelium overlying dense connective tissue. The simple corpuscles are similar to those in the rhinarium, as well as resembling those described in other species. FNE’s were frequently observed intimately associated with simple corpuscles. Several examples of large FNE’s with two to three layers of cytoplasmic lamellae were found, suggestive of transitional forms between FNE’s and simple corpuscles. Thus, the pattern of sensory innervation in the glabrous rat snout skin is similar to that found in other furred species described to date, but in addition, the sensory innervation of ridged skin in the rat also resembles that of epidermis organized into rete pegs. This dense sensory innervation may be correlated with whisking behavior of the predominately nocturnal rat. The snout of most furred animals is highly innervated with sensory nerve endings. The mystacial pad (or whisker pad, mystak in Doric Greek is moustache) contains sinus hairs, each associated with a variety of sensory receptors (Tretjakoff, 1911; Andres, 1966; Gottschaldt et al., 1973; Halata and Munger, 1980; Renehan and Munger, 1986a,b). The glabrous skin of the snout, properly referred to as the rhinarium, is also usually highly innervated in a species-specific manner. The sensory receptors described to date in glabrous skin include intraepidermal and dermal free nerve endings, Merkel terminals, and various corpuscular receptors including simple, Meissner, Pacinian, and Ruffini corpuscles (Munger, 1965; Halata, 1975). Species studied to date in the ongoing collaborative studies in the authors’ laboratories include the cat (Halata, 1970), mole (Halata, 1972a,b), opossum (Munger, 19651, rhesus monkey (Halata and Munger, 1980),bandicoot (Loo and Halata, 19861, and raccoon (Munger et al., 1971; Munger and Pubols, 1972). Several of these studies have documented differences between hairy and glabrous skin. The sensory innervation of the rat rhinarial skin has not been described, yet one would expect to find this region richly innervated because of the relative dependence of the rat on tactile sensory perception secondary to decreased visual acuity and nocturnal behavior as part of what is 0 1986 ALAN R. LISS, INC referred to as “whisking” behavior (Welker, 1964). The present study was designed as a baseline for subsequent studies of degeneration and regeneration, especially of free nerve endings and simple corpuscles, and describes the variety and distribution of sensory terminals in normal rat rhinarial skin. Our findings show a pattern of innervation in some ways similar, but in other ways different from other animals described to date, especially in terms of the distribution of free nerve endings. MATERIALS AND METHODS Albino rats were anesthetized with Nembutal (360 m g k g of body weight) and perfused with 6% glutaraldehyde solution buffered with 0.05 M phosphate buffer with a pH of 7.2. Following perfusion, the snout skin with its underlying muscle and cartilage was removed, cut into smaller cubes, and postfixed in 1%osmium tetroxide in 0.1 M phosphate buffer with 1%sucrose. The tissue was then dehydrated in increasing concentrations of alcohol and embedded in Epon 812 (Luft, 1961). Semithin sections were cut and stained for light microscopy employing a technique modified from that described by It0 and Winchester (1963) with three parts of 1% Toluidine blue in 1% borax to two parts 1%pyronin. Received April 18, 1985; accepted August 25,1985. 211 RAT SNOUT SKIN INNERVATION Thin sections were mounted on copper grids and stained with uranyl acetate (20 min) and lead citrate (5 min) for electron microscopic examination in a Philips 300 electron microscope. In addition, a n entire snout, prepared according to this protocol, was serially semithin sectioned and stained employing the modified It0 and Winchester technique described above. In a second protocol, rats were anesthetized with Nembutal (50 mgikg), perfused in the manner described by Forssmann (1976), and fixed with a 2% paraformaldehyde 2% glutaraldehyde solution. The material was then postfixed in 10% neutral buffered formalin and embedded in paraffin. The tissue was then serially sectioned at 6-8 pm and stained with a modified ammoniacal silver technique (Sevier and Munger, 1965) for light microscopy. For photomicrography of silver-stained sections we used Kodak Technical Pan film developed with POTA developer as described in Kodak Technical Data Sheet #P-255. The procedures to be followed for processing must be followed very carefully. When done correctly the negatives result in considerable contrast enhancement of blackened (silver-positive) objects such as axons. The final negative can be magnified considerably without encountering grain permitting photography a t lower magnification and lower N.A., yet with greater depth of the focus in the specimen (Rice et al., and Rice and Munger, 1986). Fig.?.This photograph of a rat snout shows the regions described by the light and electron micrographs that follow. The area referred to as the rhinarium or planum nasale is designated by the asterisk (*), and RESULTS Two regions of glabrous skin can be identified in the rat snout by light microscopy. The outer surface, the rhinarium or planum nasale (Fig. l), is similar to the glabrous skin of other animals that have been investigated, and it is characterized by free nerve (FNE’s), Merkel terminals, and simple, nonencapsulated corpuscles as defined by Halata and Munger (1983).The second densely innervated region is a small elevation found on the inner wall of the vestibule (vestibulum nasi) (Fig. 11, that is characterized by a thin epithelium consisting of four to six layers of squamous cells. This region has a dense sensory innervation containing a variety of FNE’s and scattered corpuscular endings, but no Merkel terminals. Rhinarium The rhinarium of the rat is covered with a stratified squamous epithelium that forms both rete pegs and rete ridges, apparent by light microscopy. The greatest part of the snout (Fig. 1)is characterized by pegged skin (Fig. 21, which is more typical of glabrous snout skin described in other species (Munger, 1965; Halata, 1975). On the most ventral portion of the snout, just above the philtrum, however, the epidermis is organized into rete ridges (Figs. 3,4), much like primate digital glabrous skin. The entire rhinarium is densely innervated based the prominent bulge into the vestibulum nasi is indicated by the white arrow. X 12.8. 2 12 R.T. SILVERMAN, B.L. MUNGER, AND Z. HALATA Fig. 2. These light micrographs represent typical areas of the rhinarium as visualized in silver-stained paraffin sections. The areas of the boxes in 2a are illustrated at higher magnification in 2b (representing the boxed area to the right) and 2d is the box to the left. The epidermis is relatively thick, consisting of many layers of stratified squamous epithelium. The underlying dermis contains densely packed collagen fibers, and a superficial loose reticular dermis is not present. The intraepithelial F N E s in the boxes can be seen to better advantage in 2b and 2d. 2c has a presumptive Merkel terminal within the circle, and numerous intraepithelial FNE’s indicated by the arrows. Since the intraepithelial F N E s are derived from axons high in the dermal papillae, they are probably not axons from Merkel terminals that are typically present in the basal portions of the rete ridges and rete pegs. These intraepithelial FNE’s in some cases ascend to the stratum corneum, and the examples of 2b and 2c are near the limits of resolution of the light microscope. Other axons can be seen that terminate near the epidermal-dermal junction and can only be presumptively identified as dermal FNE’s (compare with Figs. 5,7, and 8). 2a x 100 2b-2d, x400. on our evaluation of silver-stained paraffin sections (Fig. 2). Small bundles of nerve fibers course in the superficial dermis and contain numerous small diameter myelinated (presumptive A delta) and unmyelinated fibers. The latter can be identified on the basis of the presence of numerous fine axons in a single Schwann cell as described by Richardson (1960). The epidermis is also heavily innervated. Some axons branch in the region of the dermoepidermal junction and terminate in the midst of the basal cells of the eDidermis as DresumDtive Mer- kel terminals (Fig. 4).Other axons ascend in the epidermis and course towards the stratum corneum. These axons are presumptively identified as intraepidermal FNE’s (Fig. 4).Some axons appear to end blindly in the connective tissue as dermal FNE’s or simple corpuscles surrounded by lamellar cells as noted subsequently in electron micrographs. By electron microscopy, Merkel nerve endings are found throughout the rhinarium, where they are located a t the base of the rete pegs or epidermal ridges (Figs. 2, Fig. 3. This tangential section through the ventral portion of the planum nasale illustrates epidermal ridges resembling primate digital glabrous skin. This shows nerve bundles deeper in the dermis branch as they proceed superficially to supply sensory terminals higher in the ridges. The dermal papillae between the rete ridges are narrower in this area of the rhinarium as compared to the areas with rete pegs where the dermal papillae are broader, as in the center of Figure 2a or in this figure to the upper left. x 100. Fig. 4. This photomicrograph was taken from a cross-sectionthrough the ventral portion of the planum nasale showing the epidermal ridges illustrated in Figure 3 in tangential section. The stratum corneum in this section differs from that in Figure 2, which represents epidermis organized into rete pegs, in that here the ridges are reflected on the surface (arrowheads). Afferent nerve bundles proceed through the dermis and branch to supply sensory receptors in each of the ridges and their associated dermal papillae. A presumptive Merkel terminal similar to that of Figure 2c is indicated by the circle, and scattered profiles of intraepithelial FNE’s are indicated by the arrows to the right. X200. 214 R.T. SILVERMAN, B.L. MUNGER, AND Z. HALATA 4-6). Groups of approximately three to five Merkel cells and associated axons have the same structure as Merkel terminals in Tastscheiben or touch discs of other animals. The Merkel cells (Fig. 6 ) exhibit a lobulated nucleus and contain numerous electron opaque granules polarized toward the associated nerve disc, which contains mitochondria, vesicles, neurotubules, and neurofilaments. The identification of FNE’s in the rhinarial skin is presumptive as noted in Figure 7 in the absence of serial section reconstructions. However, we can be more confident of the identification of FNE’s high in dermal papillae (Fig. 8) as Merkel cells were only present a t the base of rete ridges and rete pegs. In those areas, we also could identify presumptive FNE’s, but the possibility for confusing a FNE terminal with a Merkel terminal was a constant problem. High in the dermal papillae as in Fig. 8 we encountered numerous examples of FNE’s of C fibers that fulfilled the criteria established by Cauna (1973, 1980) in serial section reconstructions from hairy and glabrous skin. Other dermal papillae contained FNE’s resembling those described by Kruger et al., (1981) as the polymodal nociceptor FNE’s of small diameter myelinated A delta fibers. In both cases, axonal varicosities contained vesicular material and scant neurofilaments or neurotubules, and the axon frequently directly abutted the basal lamina. In some cases the Schwann cell and associated axons established a n intimate contact with the contiguous basal lamina of the epidermis (Figs. 7,8). The FNE terminals of A delta fibers tended to have a 1:l ratio of axon profile to enveloping Schwann cell as noted by Kruger et al., (1981)and illustrated in Figure 8a. In all other aspects the FNE’s of A delta fibers were similar to those of C fibers. Axons were also identified within the epidermis as presumptive intraepidermal FNE’s (Fig. 7). In this skin, absolute identification of intraepidermal FNE’s is only presumptive as they could be axons derived from Merkel terminals (compare Munger, 1965; with Halata, 1975). In some cases, the Schwann cell forms one or two layers of thin cytoplasmic lamellae around the nerve terminal (Figs. 7,8a). The cytoplasmic lamellae contain numerous pinocytotic vesicles. The nerve endings lie under the dermal rete pegs or ridges, as well as high in the dermal papillae (Fig. 8). The simple nonencapsulated corpuscles (Figs. 5,9) that are present in this region are similar in structure to the simple corpuscles (Krause end bulbs) described in other species (Munger and Pubols, 1972; Halata, 1975; Halata and Munger, 1983),but they are quite small in diameter, measuring approximately 10 pm. The length of the corpuscle is variable, some measuring up to 100 pm. The corpuscles (Fig. 9) consist of an axon with a terminal swelling surrounded by cytoplasmic lamellae formed by modified Schwann cells or by lamellar cells of the inner core. An incomplete perineural capsule is present. The corpuscles are located close to the stratum basale, partially under the rete pegs and ridges of the epidermis, but also high in the dermal papillae. The axons are generally oriented perpendicular to the surface of the skin, and they display balloon-shaped terminal swellings. The axons as well as the terminal axon swelling can extend projections of various lengths between the lamellae of the inner core. These lamellae are covered by a thin basal lamina, and collagen fibers are found between lamellae. In addition, the lamellae contain numerous pinocytotic vesicles. In a few cases, the inner core branches as many a s three times conforming to the shape of the dermal papilla. The afferent nerve fiber is myelinated and has a diameter of 2-3 pm. The nerve fiber may divide at the first mode of Ranvier, and each branch can continue on into a corpuscle as observed in serial paraffin sections. The nerve terminals and axons show the same features as noted above. The number of corpuscles is greater in the ale nasi than in the transition zone to hairy skin in the dorsum nasi. In some cases axons resembling corpuscular terminals in size had scant lamellae, thus resembling free nerve endings (Fig. 7). Vestibular ridges The densely innervated ridge with its characteristic thin epithelium found on the inner wall of the vestibule (Figs. 10-12) was described briefly above. This region contains the same components of sensory innervation as the rhinarium with the exception of Merkel terminals, which are not present in this region. Free nerve endings, however, are present in abundance, including dermal as well as intraepithelial FNE’s, supplied by C-fibers as well as delta A-fibers (Fig. 11).Numerous intraepithelial FNE’s are identified by electron microscopy in this region, and these lie in the stratum basale and the stratum spinosum (Fig. 12). The terminals are balloonshaped, containing the typical features of nerve endings. FNE’s of C-fibers and A-delta fibers are identical in appearance with those described above, as are the simple corpuscles, usually found just beneath the stratum basale (Fig. 12). In addition to the thin epithelium and the absence of Merkel cells, other notable differences include more densely packed bundles of collagen and greater vascularity of the vestibular ridge. DISCUSSION The present study documents the presence of a dense sensory innervation in the glabrous skin of the rat snout or rhinarium, as well as the presence of a highly innervated ridge protruding into the nasal vestibule. The variety of sensory receptors in these two regions is similar, with differences in the pattern of these endings and distribution of Merkel terminals. Both areas contain numerous FNE’s, and the absence of Merkel terminals in the nasal vestibule removes the possible confusion of identifying intraepithelial FNE’s as contrasted to Merkel terminals. These presumptive polymodal nociceptors have only recently been conclusively identified by electron microscopy by Kruger et al., (1981) and confirmed by Munger and Halata (1983). The nature of free nerve endings has been unclear since their existence was first suggested by light microscopic evidence (see Munger, 1971; and Halata, 1975, for review). With the use of the electron microscope, these sensory receptors have been more completely examined and better characterized (Cauna, 1973, 1980). Recent authors have ventured to classify them as unassociated with any specialized epithelial receptor cells in contrast to axons associated with Merkel cells (Halata and Munger, 1983). Munger (1965) described the latter type of intraepidermal FNE’s in the opossum, in which the axon continued from the Merkel-neurite complex and proceeded through the stratum spinosum without regard to cellular boundaries. Halata later described FNE’s in RAT SNOUT SKIN INNERVATION Fig. 5. This survey electron micrograph was taken from a section cut tangentially through the rete pegs of the rhinarium and has cut through the base of several rete pegs to the upper left, lower left, and lower right. The area in the center is a dermal papiIla containing capillaries, numerous nerve bundles, and a simple corpuscle (S) to the extreme upper right. A Merkel cell (M) and its associated axon (black on white arrows) is present in the base of one rete peg to the lower left. 2 15 Numerous FNE’s are indicated by the hollow short arrows. The myelinated axons are all relatively small in diameter, the largest measuring 4 pm in diameter. Subsequent micrographs illustrate the following features at higher magnification: Merkel terminals in Figure 6; FNE’s in Figures 8 and 9, and see also Figure 12; a simple corpuscle in Figure 9. ~2,430. Fig. 6. A typical Merkel terminal is illustrated in this electron abuts a Merkel cell W)that contains numerous electron opaque granmicrograph from an area similar to the Merkel terminal in Figure 5. ules polarized toward the axon. The identity of the cell is verified by This fortuitous plane of section depicts the axon and associated the presence of desmosomes (circle) and stubby cytoplasmic spikes Schwann cell (right-hand arrow) prior to entering the epidermis. The ( S t a s ) that penetrate between contiguous epidermal keratinwytes. expanded axon terminal or disc (arrow to the left) is cup-shaped and X 11,500. thus curved in profile in this micrograph. The axon terminal closely RAT SNOUT SKIN INNERVATION Fig. 7. This electron micrograph was taken from a section at the midpoint of a dermal papilla and thus removed from any Merkel terminals. The intraepidermal axon (A) to the right is identified as an FNE. The cluster of small axons to the left (stars) in a single Schwann cell are most likely FNE’s of C fibers. The other axons in the dermis are also considered to be FNE’s, with numerous areas indicated by the 2 17 hollow short arrow where the Schwann cell investment is deficient and the axon directly abuts the basal lamina. To the upper left, the black on white arrow indicates the point of fusion of basal lamina of an axon and its associated Schwann cell with the basal lamina of the epidermis. Compare this micrograph with Figure 8 taken from the upper portion of a dermal papilla. X 17,000. 2 18 R.T. SILVERMAN, B.L. MUNGER, AND Z. HALATA Fig. 8. These two micrographs compare F N E s considered to be terminals of delta A-fibers (a) and C-fibers (b). In 8a the axons tend to have a 1:l ratio with Schwann cells, and have numerous areas where the Schwann cell investment is absent and where axons directly abut the basal lamina indicated by the hollow short arrow. The axon indicated by the upper left arrow and its associated Schwann cell directly abut the basal lamina of the epidermis (circle). 8b is photographed at much higher magnification, and the small bundle of axons in a single Schwann cell are typical small diameter C-fibers. These axon profiles are less than half the size of those in 8. Areas of direct contiguity of axon and basal lamina like that indicated by the hollow are a common feature. At the circle a strand of filamenous material extends from the basal lamina of the Schwann cell to the basal lamina of the epidermis. 8a, ~ 8 , 0 0 0Figure ; 8b, x 17,000. 220 R.T. SILVERMAN, B.L. MUNGER, AND Z. HALATA association with the Eimer organ in the mole, indicating, however, that they were present in a quantity five times greater than Merkel cells, thus concluding that not all axons could terminate on a Merkel cell. In these studies, epithelial cells were observed to invest axonal endings in the epidermis in the same manner as the Schwann cell invests axons in the dermis. Loo and Kanagasuntheram (1972) also described intraepidermal nerve endings in the snout of the tree shrew, specifying a double membrane surrounding the terminal as evidence of the extracytoplasmic location of the endings, verifying Munger’s previous conclusion (1965). In addition, they noted mitochondria1 degeneration, which they attributed to the constant shedding of the superficial epidermal layers. On the other hand, Loo and Halata (1986) described intraepidermal nerve terminals in the long-nosed bandicoot, stating that healthy looking mitochondria were present. While we could not be certain of the presence of intraepidermal FNE’s that were not associated with Merkel cells on the outer surface of the rat snout in the absence of serial section reconstructions, we are compelled to acknowledge their existence based on light microscopic evidence in silver-stained sections. In addition, when one considers the relative paucity of Merkel cells in comparison to the abundant free nerve endings, we conclude that most axons not associated with Merkel cell are most likely intraepidermal FNE’s. In the vestibule, one can say with certainty that intraepidermal axons that have been identified are unassociated with other epidermal receptor components, since no Merkel cells are present in this region. These endings are similar in character to those described above, with double membranes delineating a n area of clear cytoplasm containing normal mitochondria, vesicles, and occasionally neurotubules and neurofilaments. Some are seen closely associated with corpuscular receptors (Munger and Halata, 1983; Halata and Munger, 1983). The function of the abundance of FNE’s in the vestibule is admittedly speculative. Similar axon terminals have been identified as a “cold spot” by Hensel et al., (1974) and Hensel and Iggo (1971). The FNE’s of AG-fibers described by Kruger et al., (1981) in both physiologic and ultrastructural terms were polymodal nociceptors. Certainly the anatomical arrangement of the vestibular ridge suggests a protective function for the upper airway, and both thermal as well as mechanically sensitive axons would be expected. Dermal free nerve endings are present in abundance in the rhinarium as well as the vestibule. The morphology of these FNE’s has been described above, and these endings are similar to those described in the past. Cauna (1973), in a n electron microscopic study of C-fibers in serial sections, assigned the descriptive term “penicillate” nerve endings to those endings that appeared as small-caliber C-fiber axons invested by one Schwann cell containing up to ten axons. These FNE’s of C-fibers were supplied by a n afferent nerve that was unmyelinated. Some of the branches of these endings coursed into close proximity with the basement membrane of the epithelium, sometimes fusing basal membranes, and sometimes proceeding into the epidermis. Both mechanoreceptor function (Cauna, 1969) and thermoreceptor function (Iggo and Muir, 1969; Hensel, 1969; Hensel and Iggo, 1971; Meyer and Campbell, 1981)have been attributed to dermal free nerve endings of C-fibers. Larger free nerve endings of A-delta fibers are also present in both regions examined, and they are supplied by myelinated axons. Some of these FNE’s are surrounded by one or two layers of cytoplasmic lamellae formed by Schwann cells. These endings resemble the lanceolate terminals commonly found in hairy skin, which have been shown to be rapidly adapting mechanoreceptors (Iggo, 1974). Alternatively, they may represent a n early stage in the development of the simple corpuscles. Merkel cell-neurite complexes are present only in the rhinarium, where they are found at the base of the rete pegs and ridges in groups of three to five cells. This organization is similar to that found in Eimer’s organ of the mole, but the pattern is not as regular, nor does one find the other associated structures such as corpuscles and FNE’s in a n organized fashion as described by Halata (1972a,b). In contrast to the opossum, mole, cat, and pig, the number of Merkel terminals is small, and not all epidermal ridges and rete pegs examined have Merkel cells. The Merkel cell-neurite complex has been described above and is identical in structure to those described in other species. This receptor has been previously characterized as a type I slowly adapting mechanoreceptor (Iggo and Muir, 1969; Munger et al., 1971; Pubols et al., 1973). The significance of the presence of Merkel cells in the outer region as opposed to the absence in the vestibule is unclear. The corpuscular receptors found in the glabrous rat snout skin are similar in structure to Pacinian or simple corpuscles that have been identified in numerous other species such as the cat, monkey, pig, snout, and mole. As in other species, the corpuscle consists of an inner core surrounded by a variable number of lamellae. The corpuscles vary in size, but in general, they tend to be small and relatively simple (like those in monkey sinus hairs), lacking a perineural capsule in most instances. This is consistent with the observation that more superficial corpuscles tend to lack a capsule (Munger and Halata, 1983; Halata and Munger, 1983). Most of the corpuscles are located high in the dermis, frequently just under or surrounded by the epidermis (basal lamina of the stratum basale). The corpuscles vary in shape depending on the shape of the dermal papillae in which they are located. Axons are observed to branch just after the last node of Ranvier, inside the corpuscle, and form a branched inner core with lamellae conforming to the shape of the papilla. As many as three branches may be present. Based on the studies of Munger et al., (1971) and Munger and Pubols (1972) in the raccoon, these corpuscles are probably rapidly adapting mechanoreceptors. In comparison to raccoon simple corpuscles, those found in rat rhinarium lack a complete capsule, but the terminal axon and associated cytoplasmic lamellae are otherwise identical. Munger and Halata (1983) and Halata and Munger (1983) have proposed that the absence of a capsule has no functional significance, but is instead associated with the position in the dermis, as previously indicated. The present study has provided another clear example of one association of FNE’s with corpuscular receptors. Ide (1976) studied serial section electron micrographs and documented the presence of intraepidermal FNE’s associated with Meissner corpuscles of mouse digital skin. Halata and Munger (1983)were able to find exam- Fig. 9. A simple corpuscle is illustrated in this micrograph depicting a portion of a corpuscle such as that in Figure 5 photographed at low magnification. The central axon contains numerous mitochondria and various vesicles and electron opaque inclusions. The axoplasmic spikes (arrows) that project between the associated lamellae are a common feature of all corpuscular receptors. The cytoplasmic lamellae have numerous pinocytotic vesicles, and each lamella is enveloped by basal lamina. A portion of the nucleus of a lamellar cell (L) is also in this section. The cells to the right of the corpuscle are fibroblast-like capsular cells that partially encapsulate the receptor. The capsule is usually not complete. In some areas, basal lamina-like material associated with these capsular cells is present, but no continuous basal lamina is present in contrast to more heavily encapsulated receptors. X 17.000. 222 R.T. SILVERMAN, B.L. MUNGER, AND 2. HALATA Fig. IOa,b,c.These three light micrographs were taken from a set of serial sections, although these sections were not contiguous. The area illustrated is the ridge within the vestibule indicated by the arrow in Figure 1. A very dense plexus of axons is present at the dermoepider- ma1 junction. The underlying dermis is very vascular, and the epithelium is thin as compared with the rhinarium (see Fig. 2). Intraepithelial FNE’s do not ascend in the epithelium as in the rhinarium proper (Fig. ples of myelinated A&-fibersand their FNE terminals associated with simple corpuscles in the adult monkey lip. These dermal FNE’s associated with corpuscle may well be functionally distinct, but since no evidence to support this hypothesis is known to the authors, we tend to regard them as a special type of dermal FNE. A final minor point of interest observed in the present study is the presence of ridged skin in a nonprimate. Cummins (1964) noted the presence of ridged skin to be unique for marsupials and primates. We have not been able to confirm Cummins’ opinion that the opossum has ridged skin (unpublished observations), but the rat clearly does. The phylogenetic relevance of this observation must await further studies on other rodents. The rat, as noted above, is rather dependent on the sensory input from its snout as a result of decreased visual acuity a s we11 as nocturnal behavior. Welker (1964)examined the sniffing behavior of albino rats, and found a cycle of four movements: 1)polypnea, 2) protraction and retraction of the mystacial vibrissae, 3) head movements, and 4) movements of the tip of the nose, including the glabrous skin described above. This behavior complex, which develops early in the rat’s life, provides the rat with the ability to find food as well as maneuver in dimly lit surroundings. Rats that were deprived of somatic snout afferents were handicapped in food getting functions, and they may have showed an overall decrease in sniffing. This may serve to show the importance of the tactile sensation of this region, thereby causing one to expect dense sensory innervation. In addition, the variety of structures appearing in the glabrous snout skin contributes to the sensitivity of the region, thus analogous to the use of digital ridged glabrous skin in the reading of braille (Johnson and Phillips, 1981; Johnson and Lamb, 1981). Multiple discrete sensory terminals of varying physiologic properties are the peripheral basis of heightened tactile acuity associated with multiple parallel processing in hairy as well as glabrous primate skin (Munger, 1982; Dell and Munger, 1986). 2). X200. ACKNOWLEDGMENTS The authors wish to acknowledge the valuable technical assistance of Debbie Hinton, Marianne Blohm, and Tjandra Djasputra. Special thanks go to Doris Lineweaver for typing the manuscript. This study was supported in part by U.S. Public Health Service research contracts NIDR72-2401 and HD4-2869 and research grants HD11216 and NS 19462 to Dr. Munger, and in part by grants from the Deutsche Forschungsgemeinschaft awarded to Dr. Halata. This study was done while R.T.S. was a medical student and recipient of a NIH Short-Term Research Training Award, HL07477. RAT SNOUT SKIN INNERVATION Fig. 11. This electron micrograph was taken from a cross-section through an elevation of the epithelium of the nasal vestibule, as illustrated in Figure 10. A nerve plexus in the dermis consists of myelinated and unmyelinated fibers. A small corpuscle is also present in rectangle depicted at higher magnification in Figure 12. Note also 223 the dense packing of the collagen fibers, characteristic of vestibule, as compared to the more loosely-packed collagen that one finds in the planum nasale (see Figs. 5-7). Numerous unlabeled FNE’s are present throughout the dermis and epidermis, a few of which are illustrated at higher magnification in Figure 12. x3,800. Fig. 12. The area of the rectangle in Figure 11is illustrated at higher magnification and shows a corpuscle from the vestibule, identical in structure with that from the planum nasale seen in Figure 11. Intraepithelial F N E s are indicated by the arrows, and they are enveloped by the cytoplasm of keratinocytes in a manner analogous to a Schwann cell enveloping an axon. These intraepidermal FNE’s are similar to those depicted by Ide (1976) in his serial sections of rat glabrous digital skin where intraepithelial FNE’s associated with Meissner corpuscles were identified. Numerous dermal F N E s labeled with asterisks are present surrounding the small simple corpuscle (S). Some of these may be the parent axons to the intraepidermal FNE’s. Two bundles of Cfibers are present in the lower left. A complete capsule is not present surrounding the simple corpuscle. X 11,000. RAT SNOUT SKIN INNERVATION 225 LITERATURE CITED Andres, K.H. (1966) &er die Feinstruktur der Rezeptoren an Sinushaaren. Z. Zellforsch., 75r339-365. Andres, K.H. (1969) Zur Ultrastruktur verschiedener Mechanorezeptoren von hoheren Wirbeltieren. Anat. Am., 124:551-565. Campbell, J.N., and R.A. Meyer (1983) Sensitivity of unmyelinated nociceptive afferents in monkey varies with skin type. J. Neurophysiol., 49:98-110. Cauna, N. (1973) The free penicillate nerve endings of the human hairy skin. J. Anat., 115:277-288. Cauna, N. (1980) Fine morphological characteristics and microtopography of the free nerve endings of the human digital skin. Anat. Rec., 198:643-656. Cummins, H. (1964) Dermatoglyphics: A brief review. In: The Epidermis. W. Montagna and W. Lobitz, eds. Academic Press, New York, pp. 375-386. Dell, D., and B.L. Munger (1986) The early embryogenesis of papillary (sweat duct) ridges in primate glabrous skin: The dermatotopic map of cutaneous mechanoreceptors and fingerprints. J. Comp. Neurol. Forssmann, W.G., S. Ito, E. Weihe, A. Aoki, M. Dym, and D.W. Fawcett (1976)An improved perfusion fixation method for the testis. Anat. Rec., 188:307-314. Gottschaldt, K.M., A. Iggo, and D.W. Young (1973)Functional characteristics of mechanoreceptors in sinus hair follicles of the cat. J. Physiol., (Lond.),235:287-315. Halata, Z. (1970) Zu den Nervenendigungen (Merkelsche Endigungen) in der haarlosen Nasenhaut der Katze. Z. Zellforsch., 106:51-60. Halata, Z. (1972a) Innervation der unbehaarten Nasenhaut des Maulwurfs (Talpa europaea). I. Intraepidermale Nervenendigungen. Z. Zeiiforsch., 125t108-120. Halata, Z. (1972b)Innervation der unbehaarten Nasenhaut des Maulwurfs (Talpa europaea). 11. Innervation der Dermis (einfache eingekapselte Korperchen). Z. Zellforsch., 125t121-131. Halata, Z. (1975)The mechanoreceptors of the mammalian skin ultrastructure and morphological classification. Adv. Anat., 50:l-77. Halata, Z., and B.L. Munger (1980) Sensory nerve endings in rhesus monkey sinus hairs. J. Comp. Neurol., 192645-663. Halata, Z., and B.L. Munger (1983)The sensory innervation of primate facial skin. 11. Vermilion border and mucosa of lip. Brain Res. Rev., 5t81-107. Hensel, H. (1969) Cutane Warmerezeptoren bei Primaten. Pfluegers Arch., 313:150-152. Hensel, H., K.H. Andres, and M. von During (1974) Structure and function of cold receptors. Pfluegers Arch., 352(11r1-10. Hensel, H., and A. Iggo (1971) Analysis of cutaneous warm and cold fibers in primates. Pfluegers Arch., 329:l-8. Ide, C. (1976) The fine structure of the digital corpuscle of the mouse toe pad, with specific reference to nerve fibers. Am. J. Anat., 147:329-356. Iggo, A., and A.R. Muir (1969) The structure and function of a slowly adapting touch corpuscle in hairy skin. J. Physiol. (Lond.), 200t763~~ ~ 796. kgo, A. (1974) c h m x ~ ~ receptors. us 1x1: The Peripheral Nervous SYStem: S.J. Hubbard, ed. Pergamon, New York, pp. 397-404. Ito, S., and R.J. Winchester (1963) The fine structure of the gastric mucosa in the bat. J. Cell Biol., 16t541-578. Johnson, K., and G. Lamb (1981) Neural mechanisms of spatial tactile discrimination: Neural patterns evoked by braille-like dot patterns in the monkey. J. Physiol. (Lond.),310:117-144, 1981. Johnson, K., and J. Phillips (1981) Tactile spatial resolution. I. Twopoint discrimination gap detection, grating resolution, and letter recognition. J. Neurophysiol., 46t1177-1191, 1981. Kruger, L., E.R. Perl, and M.J. Sedivec (1981)Fine structure of myelinated mechanical nociceptor endings in cat hairy skin. J. Comp. Neurol., 198:137-154. LOO,S.K., and Z. Halata (1986) The sensory innervation of the glabrous nasal skin in the long-nosed bandicoot (Perameles nasuta). J. Anat. (in press). Loo, S.K., and R. Kanagasuntheram (1972) Innervation and structure of the snout in the tree shrew. J. Anat., 111:253-261. Luft, J.H. (1961)Improvements in epoxy resin embedding methods. J. Biophys. Biochem. Cytol., 9:409-414. Meyer, R.A., and J.N. Campbell (1981)Evidence for two distinct classes of unmyelinated nociceptive afferents in monkey. Brain Res., 224:149-152. Munger, B.L. (1965)The intraepidermal innervation of the snout skin of the opossum. A light and electron microscope study, with observations on the nature of Merkel's Tastzellen. J. Cell Biol., 26;7997. Munger, B.L. (1971) Patterns of organization of peripheral sensory receptors. In: Handbook of Sensory Physiology, Vol. 1. W.R. Lowenstein, ed. Springer-Verlag,New York, pp. 523-556. Munger, B.L. (1977) Neural-epithelial interactions in sensory receptors. J. Invest. Dermatol., 69:27-40. Munger, B.L. (1982) Multiple afferent innervation of primate facial hairs-Henry Head and Max von Frey revisited. Brain Res. Rev., 4:143. Munger, B.L., and Z. Halata (1983)The sensory innervation of primate facial skin. I. Hairy skin. Brain Res. Rev., 5:45-80. Munger, B.L., and L.M. Pubols (1972) The sensorineural organization of the digital skin of the raccoon. Brain Behav. Evol., 5:367-392. Munger, B.L., L.M. Pubols, and B.H. Pubols (1971) The Merkel rete papilla-a slowly adapting sensory receptor in mammalian glabrous skin. Brain Res., 29:47-61. Pubols, B.H., P.J. Donovick, and L.M. Pubols (1973) Opossum trigeminal afferents associated with vibrissa and rhinarial mechanoreceptors. Brain Behav. Evol., 7:360-381. Renehan, W., and B.L. Munger (1986a)Degeneration and regeneration of peripheral nerve in the rat trigeminal system: I. Identification and characterization of the multiple afferent innervation of mystacial vibrissa. J. Comp. Neurol. (in press). Renehan, W., and B.L. Munger (1986b)Degeneration and regeneration of peripheral nerve in the rat trigeminal system. 11. Response to nerve lesions. J. Comp. Neurol. (in press). Rice, F.L., A. Mance, and B.L. Munger (1986) A comparative light microscopic analysis of the sensory innervation of the mystacial pad in the hamster, mouse, rat, gerbil, rabbit, guinea pig, and cat. 11. Innervation of vibrissal follicles. J. Comp. Neurol. (in press). Rice, F.L., and B.L. Munger (1986) A comparative light microscopic analysis of the sensory innervation of the common fur between the vibrissae in the hamster, mouse, rat, gerbil, rabbit, guinea pig, and cat. J. Comp. Neurol. (in press). Richardson. K.C. (1960) Studies on the structure of autonomic nerves in the small intestine, correlating the silver-impregnated image in light microscopy with the permanganate fixed ultrastructure in electron microscopy. J. Anat., 94:457-472. &vier, A.C., and B.L. Munger (1965) A silver method applicable to parafin sections of formol-fixed tissue. J. Neuropathol. Exp. Neurol., 24r130-135. Tretjakoff, D. (1911) Die Nervenendigungen an den Sinushaaren des Rindes. Z. Wiss. Zool., 97t314-416. Welker, W.I. (1964) An analysis of sniffing behavior of the albino rat. Behavior, 22223-244.