A new type of somatosensory organ in the nasolabial skin of the dog

A new morphological type of somatosensory organ is described. It is found in the glabrous skin of the dog nose (rhinarium or planum nasale) and situated in dermis papillae. The otherwise thick epidermis forms a thin window above the organ. There are only a few layers of keratinocytes in the window and the corneocytes are much thinner than elsewhere. The organ consists of highly branching cells that wrap naked nerve endings emanating from myelinated nerve fibers originating in the outer dermal nerve plexus. The structure entirely fills the top of the dermal papilla. The intercellular spaces of the organ and its surroundings are occupied by an extended areolar basal lamina.

which are descendants of domestic dogs brought to Australia a few 1000 years ago (Oskarsson et al., 2011).
The thick epidermis of the skin on the dog rhinarium has an undulating border towards the dermis. Ridges and cones of the epidermis extend into the dermis. They are separated by papillae of dermis approaching the skin surface. A specific feature of the corneal layer of the nasolabial skin is the thickness of the corneocytes. They can be up to 5 μm thick and are spindle-shaped in cross section ( fig. 5c in Elofsson et al., 2016).
Merkel cell-neurite complexes, Vater-Pacini corpuscles, and unmyelinated nerve endings in the epidermis are the "classical" sense organs present in the dog rhinarium skin. A new type of sensory receptor was found in the dog rhinarium and is described below.

| MATERIAL AND METHODS
The investigated dogs belong to the subspecies Canis lupus dingo Meyer, 1793 and Canis lupus familiaris Linnaeus, 1758. One female (11 years) and one male (8 years) dingo, sacrificed for illness and inbreeding, were provided by Öland Zoo (Ölands djurpark) in Sweden (department protocol BI 2017, CLD M, CLD F). The dogs include specimens of golden retriever (male 11 years), and mixed breed (two males, 7 and 9 years) removed for illness and age (department protocol BI 2016, GR M, MB M 1,2). The dog specimens were supplied by the University of Veterinary Medicine, Vienna, Austria. Punch biopsies, 3 or 4 mm in diameter, were secured from the frontal part of the rhinarium three on both sides of the midline. The biopsies were divided and immersed in 2.5% or 3% glutaraldehyde (Agar Scientific, Stansted, UK) in sodium cacodylate buffer 0.15 mol L −1 (pH 7.2-7.3) (Agar Scientific) for 3 hr. After fixation, the preparations were rinsed repeatedly in buffer before further processing in 1% Os O 4 (Agar Scientific) in 0.15 mol L −1 sodium cacodylate buffer for 1 hour at room temperature (about 20 C). After rinsing in the same buffer, the preparations were dehydrated in a graded ethanol series, transferred to acetone and then Epon (Agar Scientific). The Epon was polymerized at 60 C for 48 hr.
Thin sections of specimens (2 μm) for light microscopy, previously prepared for electron microscopy and treated as above, were stained in a mixture of methylene and Azur II blue. Ultrathin continuous sections were cut with a Leica Ultracut UCT microtome (Leica Microsystems GmbH, Wetzlar, Germany). Staining on copper grids was performed with 2% uranyl acetate (Ted Pella, Redding, CA) and 1% lead citrate (Merck KGaA, Darmstadt, Germany) for 30 min. Sections were studied in a JEOL 1240 Plus (Jeol, Tokyo, Japan).

| RESULTS
The new organ is found distally in the longest dermal papillae, one or a few in each of the cushions constituting the rhinoglyphic pattern ( Figure 1). A rhinoglyphic pattern of epidermal domes is present in several mammalian orders (Hill, 1948). The sizes of these cushions or domes vary between dog breeds and sizes (own observations). In the dogs we studied, each dome was approximately 1 mm in diameter.
The inner subcapsule is also cellular and appears as a few or several layers. In the Ruffini corpuscles, the collagen fibers in the inner capsule fuse with those outside (Chouchkov, 1978;Iggo & Andres, 1982). In the Meissner corpuscles, the flattened cells of the inner capsule are arranged in horizontal stacks and the axons terminals weaves back and forth between the lamellae (Cauna & Ross, 1960;Munger & Ide, 1988).
The mechanoreceptive function of the cutaneous receptors has been established in a number of investigations (Biemesderfer, Munger, Binck, & Dubner, 1978;Chambers, Andres, Von Düring, & Iggo, 1972;Iggo & Andres, 1982;Iggo & Muir, 1969;Iggo & Ogawa, 1977;Quindien, Lai, & Barocas, 2015). For this function, it is important that the connective tissue and collagen fibers are part of the capsule and linked to the surrounding connective tissue. (Pawson et al., 2009). The function as mechanoreceptors may be complemen-  The new dermal sensory organ shares the same type of innervation with the mechanoreceptive organs. Nerve fibers from the dermal nerve plexus lose their Schwann cell clothing before the entrance into the organs. The nerve terminals of the mechanoreceptive organs may have one or more branches (Paré et al., 2001), whereas the new dermal sensory organ of dogs has widely branching terminals.
The axons from the outer dermal nerve plexus reaching to the dog organ fit with regard to axon diameter and myelination into the category of A δ (Martin-Alguacil et al., 2016). These fibers together with C-fibers, distributed on the skin surface, are found in receptors that relay painful, cold and thermal information (Alamri, Wood, Ivanusic, & Brock, 2018;McGlone & Reilly, 2010;Schepers & Ringkamp, 2010;Vriens, Nilius, & Voets, 2014). The myelinated axons of the dog organ secure a relatively fast conductance.
The organ is situated close to the skin surface in a dermal papilla and has a specific thin area of epidermis above it. The thick corneocytes was shown to be a specific feature of the epidermis of rhinaria (Elofsson et al., 2016). The thin corneocytes above the new dermal sensory organ are thus an adaptation to the presence of the organ. The position and thin barrier make the organ a candidate for a sensory organ. Since its morphology is distinctly different from known mammalian mechanosensory skin organs, a role as a receptor for tactile stimuli is less probable. The organ is absent in other carefully studied rhinaria, such as in moles and their relatives (Catania, 2000) and in lemurs (Elofsson et al., 2015). It is interesting in this context that carnivorans have lower rhinarium temperatures than other mammalian groups (Gläser & Kröger, 2017). Dogs in particular have intricate rhinarium temperature dynamics (Kröger & Goiricelaya, 2017). A temperature receptor could thus be an option for the function of the organ described here.

ACKNOWLEDGMENTS
We are grateful to Karl-Johan Nordfeldt and Öland zoo for supplying the dingoes and to Michael Leschnik at the University of Veterinary Medicine in Vienna for the opportunity to get the biopsies from dogs.
Inga Tuminaite secured the preparations from Vienna, Rita Wallén and Ola Gustafsson were of invaluable technical support. Morgan Luce checked the language. The Royal Physiographic Society in Lund financially contributed to the project.

CONFLICT OF INTEREST
No conflict of interest to declare.

DATA AVAILABILITY STATEMENT
Data sharing is not applicable to this article as no new data were created or analyzed in this study.

Rolf Elofsson
https://orcid.org/0000-0002-0562-3392 F I G U R E 4 (a) Dingo. The transition zone where an axon (star) leaves the myelin sheath (white arrow). The remaining single Schwann cell is replaced by a supporting cell extension (black arrow). Scale bar 500 nm. (b) Dingo. A section through the dog organ illustrating the intricately woven pattern inside the organ and the relation between nerve terminals (stars) and the supporting cells (arrows). Scale bar 1 μm