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. Author manuscript; available in PMC: 2014 Mar 1.
Published in final edited form as: Anat Rec (Hoboken). 2013 Mar;296(3):521–532. doi: 10.1002/ar.22660

Comparative Anatomy of Mouse and Human Nail Units

Philip Fleckman 1,*, Karin Jaeger 2,3, Kathleen A Silva 2, John P Sundberg 2
PMCID: PMC3579226  NIHMSID: NIHMS436308  PMID: 23408541

Abstract

Recent studies of mice with hair defects have resulted in major contributions to the understanding of hair disorders. To use mouse models as a tool to study nail diseases, a basic understanding of the similarities and differences between the human and mouse nail unit is required. In this study we compare the human and mouse nail unit at the macroscopic and microscopic level and use immunohistochemistry to determine the keratin expression patterns in the mouse nail unit. Both species have a proximal nail fold, cuticle, nail matrix, nail bed, nail plate, and hyponychium. Distinguishing features are the shape of the nail and the presence of an extended hyponychium in the mouse. Expression patterns of most keratins are similar. These findings indicate that the mouse nail unit shares major characteristics with the human nail unit and overall represents a very similar structure, useful for the investigation of nail diseases and nail biology.

Keywords: animal model, microscopic, claw, nail

Abnormalities of nails in humans are well known and described in association with other skin, hair, and systemic diseases (Baden, 1987; Zaias, 1990; Noronha and Zubkov, 1997; Scher, 1998; Baran et al., 2001; Barankin and Guenther, 2001; Fleckman, 2005; Scher and C R Daniel, 2005). The macroscopic and microscopic anatomy of the human nail unit are well described (Zaias, 1990; Fleckman, 2005). The nail plate is produced by the nail unit (Zaias, 1990); the nail unit includes the proximal nail fold, nail matrix, nail bed, and hyponychium (Fleckman, 2005). Although clinical and pathological descriptions as well as responses to therapy are reported, studies on the pathogenesis of nail defects in patients are difficult to perform and therefore rarely reported. The size and limited number of human nails available for study; differences in texture between soft tissue, bone, and nail that make microscopic examination difficult; concern about pain and the risk of permanent deformities that might result from biopsy; and a lack of familiarity of many investigators with the structure and function of the nail unit, including biopsy techniques all contribute to relative discomfort in studying the onychodystrophies. As a result, compared with the epidermis and the hair follicle, relatively little is known about normal development and mechanisms of disease of the human nail unit.

Nails and functionally related or essentially identical cutaneous appendages (claws, hoofs, etc.) are found in many other species; not only is development similar (Hamrick, 2001), but also many diseases that affect the nail unit in other mammals resemble or are identical to those of humans. One potentially useful animal for the study of nail disorders is the laboratory mouse. Many inbred strains exist, as do both spontaneous and induced (genetically engineered) mutations in mice. As the phenotypes of mutant mice are characterized and compared to those of various human genetic diseases, nail defects are occasionally observed that can potentially provide a manipulatable system for investigation. For example, mice with mutations in the hairless gene (hairless, gene symbol: Hr) have long, curled nails resembling onychogryphosis (Sundberg et al., 1989; Sundberg and King, 2000). Others, such as the targeted mutation for nerve growth factor receptor null (Ngfrtm1Jae) have traumatic lesions of the nail bed, suggestive of congenital insensitivity to pain (Lee et al., 1992), since they lack sensation in their digits. Nails of the nude mouse (Foxn1nu) are thin and weak, reflecting altered onycholemmal keratinization (Mecklenburg et al., 2004). Many other nail defects remain to be identified and characterized.

Because of their small size, nails in rodents are often overlooked in characterization studies of mutant mice unless the defects are prominent. A systematic method to evaluate the mouse nail unit and identify defects in representative mutant and control mice is lacking. In addition, direct anatomic comparisons between the human and mouse nail unit are required to validate the usefulness of the mouse model and to determine if development in both species is similar. Keratins are the predominant structural proteins of the nail unit and changes in keratin expression have been shown to reflect disorders. We present here several approaches for evaluation of mouse nails along with gross and histologic anatomical comparisons between the two species.

MATERIALS AND METHODS

Human Digits

Gross photographs were taken of the distal end of the third digit of the right hand of one of the authors (PF). A human neonatal digit was obtained as a result of accidental amputation with approval from the University of Washington Institutional Review Board. The digit was fixed in toto in neutral buffered 10 % formalin, embedded in butyl-methyl-methacrylate, sectioned routinely at 5 µm, and stained with Masson trichrome.

Mice

Animal studies were conducted with The Jackson Research Laboratory’s Institutional Animal Care and Use Committee approval. C57BL/6J (JR# 664; 1 female 139d, 2 females 140d, 1 female 111d, 9 females 60d, 10 males 58d, 1 female, 77d, 1 female 65d, 1 female, 69d, 1 female, 84d, 1 male, 21d) mice and FVB/NJ (JR# 1800; 1 female 135d) mice were obtained from The Jackson Laboratory (Bar Harbor, ME). Mice were maintained in a humidity-, temperature-, and light cycle (12:12) controlled vivarium under specific pathogen-free conditions (http://jaxmice.jax.org/html/health/quality_control.shtml#Animalhealth). Mice were housed in double-pen polycarbonate cages (330 cm2 floor area) at a maximum capacity of four mice per pen. Mice were allowed free access to autoclaved food (NIH 31, 6% fat; LabDiet 5K52, Purina Mills, St. Louis, MO) and acidified water (pH 2.8–3.2).

Mice were photographed while alive to visualize their stance and evaluate the relationship of the nail and foot to the surface on which they walked. They were observed while walking on a glass plate and the observations were confirmed by sequences of normal mouse gait obtained with a gait analysis device (Wooley et al., 2005). Mice were then euthanized by CO2 asphyxiation (Boggess et al., 2004). Nails of front and rear feet were examined in situ using a dissecting microscope. For the digit measurements all limbs were removed at the time of necropsy by amputation above ankles (rear feet) or wrists (front feet) and fixed with their dorsal sides on adhesive tape. The measurements were performed on pictures taken with a dissecting microscope at 7x using the imaging software MetaMorph™ (Molecular Devices Corp., Downingtown, PA).

Histology and Immunohistochemistry

Left front and rear feet were fixed in Fekete’s acid-alcohol-formalin solution, decalcified with CAL-EX decalcifying solution (Fisher Scientific, Fair Lawn, NJ) for 48 hours, processed through graded ethanols, embedded in paraffin and serially sectioned at 5 µm (Sundberg et al., 1998; Seymour et al., 2004). Individual phalanges were removed and oriented so that they were sectioned lengthwise in the dorsal/ventral midline (mid sagittal orientation). Feet were also laid flat and fixed in toto with the entire foot cut lengthwise such that all five digits plus metacarpals/metatarsals and their joints were included. Every fifth section was stained with hematoxylin and eosin (H&E). The H&E sections were screened to identify both ends of a series of optimally oriented sections that were subsequently used for immunohistochemistry. The following markers were used at concentrations as indicated on the public web page: http://tumor.informatics.jax.org/html/antibodies.html (Mikaelian et al., 2004): rabbit anti-KRT17 (McGowan and Coulombe, 1998), kindly provided by P. Coulombe, rabbit anti-mouse KRT1 (stock# PRB-165P), KRT5 (stock# PRB-160P), KRT6A (stock# PRB-169P), KRT10 (stock# PRB-159P), and KRT14 (stock# PRB-155P; Covance, Berkeley, CA) and the mouse anti-human hard keratin antibody, AE13 (hair cortex cytokeratins) (Lynch et al., 1986), kindly provided by T.T. Sun. The antibodies were also tested on tissue fixed in 4% phosphate buffered formalin (pH=7.2) to exclude differences in the labeling pattern due to the fixation method. Diamino-benzidine (Sigma, St. Louis, MO) was used as chromogen.

Scanning Electron Microscopy (SEM)

Right front and rear feet were removed at the carpal/metacarpal joints from the same mice as used for histologic evaluation. Feet were fixed overnight at 4°C in 2.5% glutaraldehyde in 0.1M phosphate buffer (pH 7.2). After rinsing the tissues several times in phosphate buffer, they were postfixed in 1% osmium tetroxide in phosphate buffer for 48 hours at 4°C and dehydrated in a graded series of ethanol (40%, 60%, 80%, 95% and 100% ethanol, each for an hour at room temperature). Critical point drying was done by gently flushing specimens four times with CO2 for 5 minutes while gradually increasing temperature in the critical point drier (Balziers, Union, FL., USA) to 41°C. Over a period of 30 to 40 minutes, pressure was released slowly to allow CO2 to evaporate while allowing the sample to return to room temperature. Feet were mounted on aluminium stubs using double-sided carbon tape and sputter-coated with a 4 nm layer of gold. Nails were examined at 20kV at a working distance of approximately 15 mm on a Hitachi S3000N VP Scanning Electron Microscope (Hitachi Science Systems, Ltd., Japan) (Bechtold, 2000; Mecklenburg et al., 2004).

Micro-computed tomography (microCT)

We used high-resolution, desktop micro-tomographic imaging systems (µCT40, Scanco Medical AG, Bassersdorf, Switzerland) to generate cross sectional and three dimensional images of the mouse digit. The MicroCT40 unit is calibrated weekly with a phantom standard provided by Scanco prior to beginning bone scans. The digit was excised from a 4 month old female C57BL/6J mouse limb stored in 95% EtOH. The excised digit was placed into a foam ring and then inserted into the holder for horizontal imaging. The digit was scanned using a 6-µm slice increment, requiring approximately 300 µCT slices. Images were reconstructed, filtered (σ = 0.8, width = 1.0), and thresholded (22% of maximum possible gray scale value) as previously described (Bouxsein et al., 2004). The images were stored in 3D arrays with an isotropic voxel size of 6 µm.

RESULTS

Stance of mice

Live mice stood on their toes with their palmar and plantar foot pads touching the surface (Fig. 1a, b). Lateral views of the nails and digits illustrate that the nails angle up, caused by a flexion in the proximal interphalangeal joints such that when the mouse walks on its footpads and toes, the nail plate will be roughly parallel to the surface. The nail arcs in a radius to provide a point of contact at the distal tip of the nail plate (Fig. 1b). However, in dead mice the nails do not reach the surface because of the missing flexion of the proximal interphalangeal joints (Fig. 1c). The mouse walks on all four limbs with the foot pads and the digital tips touching the surface. Mice are able to stand on their rear feet in order to explore the environment or use their front feet for digging or climbing. The bones of the carpus and tarsus extend in line with the humerus and femur and are further extended in line with the proximal and intermediate phalanges. The distal phalanx bends almost 90 degrees upward at the distal interphalangeal joint (Fig. 1d, e). Thus, the proximal and intermediate phalanges are larger bones than their human equivalents, acting more as extensions of the bones of the arm and leg.

Fig. 1. Foot/nail surface orientation.

Fig. 1

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The mouse walks on its feet with the plantar and palmar pads on the surface (a and b). Through extension in the proximal metacarpointerphalangeal joint and flexion in the proximal interphalangeal joint the nail tips are in contact with the surface (b). Lateral view of the nail relative to the distal phalanx postmortem reveals that the nail arcs up, then down, such that the nail does not touch the surface when all the muscles are relaxed (c). MicroCT-Scan of the middle and distal phalanx bones of the third mouse digit (d and e). The 3D reconstruction (d) and sagittal section (e) show that the dorsal angle of the distal interphalangeal joint is almost ninety degrees.

Identification of front and rear feet in mice

Front and rear feet of the mouse are different and can be identified by careful examination with the aid of a dissecting microscope (Fig. 2). Rear feet are approximately twice as large as the front feet (Table 1). Standard nomenclature for digits (Popesko et al., 1992) designates the first digit (thumb or great toe equivalent in human) as digitus I (D I). D I and V are shorter than D II, III, and IV (p<0.01). D I is shorter than D V (p<0.01) on the rear feet and extremely short on the front feet, where it is set back from the others, and may be overlooked. D I on the front foot is comparable to the short and rudimentary digit on a dog in this location (Evans and deLahunta, 1971). The palmar and plantar surfaces are only sparsely covered with short hair, and soft foot pads are visible. The volar surface has 6 pads on the rear foot and 5 pads on the front foot. 4 pads can be found interdigital near the base of D I–V. The one at the base of D V consists of two parts. The other two pads lie more proximal at the fibular and tibial side of the foot. On the palm there is no proximal pad at the radial side.

Fig. 2. Differentiating front from rear feet.

Fig. 2

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The front feet (a, b) are smaller than the rear and D I is rudimentary when viewed in either the dorsal (a) or palmar (b) surface. By contrast, the rear feet (c, d) have longer digits and a prominent D I which is shorter than D V and enables orientation. Bar = 1mm.

Table 1.

Comparison of foot and digit lengths

FEMALE feet (n=9) foot D1 D2 D3 D4 D5
left rear 17.44 2.33 4.15 4.23 4.26 3.62
SD 0.45 0.09 0.07 0.12 0.11 0.14
right rear 17.11 2.29 4.23 4.35 4.25 3.84
SD 0.47 0.09 0.09 0.11 0.09 0.17
left front 6.29 0.53 2.83 3.18 2.95 2.29
SD 0.22 0.04 0.10 0.09 0.10 0.10
right front 6.55 0.53 2.84 3.18 3.14 2.33
SD 0.20 0.04 0.08 0.11 0.17 0.08
MALE feet (n=10) foot D1 D2 D3 D4 D5
left rear 17.47 2.33 4.21 4.23 4.10 3.68
SD 0.41 0.13 0.15 0.09 0.16 0.16
right rear 17.29 2.39 4.19 4.36 4.22 3.85
SD 0.62 0.11 0.12 0.16 0.12 0.11
left front 6.98 0.58 2.89 3.20 2.98 2.21
SD 0.21 0.03 0.10 0.09 0.11 0.09
right front 6.94 0.49 2.90 3.17 3.06 2.23
SD 0.19 0.05 0.13 0.12 0.09 0.12
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Measurement of digit lengths from 10 male and 9 female, 2 month old, C57BL/6J mice illustrates that rear digits are significantly longer than the corresponding digits on the front foot and digit 1 (D1) is always shorter than digit 5 (D5) in front feet and rear feet, respectively. Lengths in mm, SD=standard deviation.

Gross anatomy

The human nail unit

The nail plate extends as a relatively flat, slightly curved, plate-like structure covering the dorsal surface of the distal digit (Fig. 3a, b). The nail plate lies on the dorsal surface of the distal digit and extends over the free edge of the digit. Thus, the nail plate acts 1) to protect the outer half of the distal digit from trauma, 2) as a pressure plate to amplify fine touch perception in underlying structures, and 3) as an extension of the distal digit. The nail plate is surrounded by and invaginates into the proximal nail fold at its base (proximal) and the lateral nail folds on either side. The cuticle, which is the stratum corneum of the tip and undersurface of the proximal nail fold, attaches to and seals the proximal nail fold to the dorsal surface of the nail plate. The lunula, the white semicircle extending from the proximal nail fold beneath the nail plate, is the distal matrix. The presence of the lunula varies between individuals and fingers, depending upon the length of the matrix and how far the proximal nail fold invaginates proximally; individual fingers may or may not have a detectable lunula. The nail bed is the red-pink area beneath the nail plate. The nail bed begins where the matrix (or lunula) ends. The pink color of the nail bed reflects blood in the vessels below this thin epithelium. An accentuated band of hyperemia in the distal nail bed marks the onychodermal band. Color change (from the white of the lunula to the red/pink of the nail bed to the accentuated red of the onychodermal band) marks change in the underlying vasculature. The hyponychium, where normal volar epidermis begins and the under surface of the nail plate separates from the nail bed, begins at the distal edge of the onychodermal band and extends to the distal groove. The free edge of the nail plate is seen as the white band around the margin of the plate; the color change reflects light that is refracted in the air beneath the unattached plate, as opposed to light that is reflected from blood in vessels lying beneath the nail bed epithelium of the attached plate.

Fig. 3. Macroanatomy of the human nail and mouse unit.

Fig. 3

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Dorsal view of the human nail unit (a) illustrates the nail plate, proximal nail fold (pnf), lateral nail folds (lnf), lunula (lu), and cuticle (cu). Lateral view (b) illustrates the slightly curved, dorsoventrally flattened plate of the human nail. Dorsal view of the mouse nail unit (c) illustrates the proximal nail fold (pnf), lunula (l), cuticle (cu), but no lateral nail fold. Lateral view (d) illustrates the more prominent curvature of the mouse nail and the complete covering of the lateral side of the distal phalanx by the nail plate. Arrowhead (d) indicates prominent vessel visible through the nail plate. (c) bar = 0.5mm; (d) bar = 1mm.

The mouse nail unit

The mouse nail extends as a semicircular, laterally flattened, circumferential plate over the distal digit (Fig. 3c, d). Rather than lying flat on the dorsal surface and curving slightly on the sides of the distal digit, the mouse nail plate partially encircles or enshrouds the dorsal surface and sides of the distal digit, leaving only the volar surface of the digit exposed (Fig. 3d, 4a–d).

Fig. 4. Scanning electron microscopy of the mouse nail unit.

Fig. 4

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SEM provides detailed 3 dimensional images of the nail unit. Note that structures beneath the nail plate, namely the lunula and vessels, are not visible by this method. (a) Side view illustrates the long hair covering the proximal nail fold. Note the longitudinal curvature and pointed tips of the nails. (b) Ventral view shows the structured undersurface of the digits with the prominent digit pads and interdigital pads. (c) Dorsal view of the nail shows the surface of the nail plate, which is built of flattened cornified cells. (d) At higher magnification the free edges of the nail plate and the hyponychium are clearly distinguishable at the undersurface of the nail plate. (e) The nail plate surface at high magnification shows flat polygonal cells. (f) At the digit pad surface cell borders are not clearly visible. Note the high density of openings of eccrine gland ducts (arrow). (a,b) bars = 2mm; (c) bar = 500µm; (d) bar=300µm,(e,f) bars = 25µm

The nail plate encircles the distal digit on three sides and both supports the underlying dorsal surface of the distal phalangeal pad and is used as a sharp extension of the phalanx for climbing, scratching, and digging food. The nail plate is surrounded by and invaginates into the proximal nail fold at its (proximal) base. The proximal nail fold extends laterally to cover the matrix, which continues to form the nail plate on the sides of the digit. The cuticle of the proximal nail fold grows out with the nail plate, attached to the dorsal and lateral surfaces of the plate, but is usually only seen by retracting the skin backwards because it is hidden under the proximal nail fold and hair that cover the digits on their dorsal aspect. The lunula is seen as a pointed, pale area at the origin of the visible nail that extends distal to the proximal nail fold (Fig. 3c). It can be seen in all nails and can occupy up to one third of the visible nail. In a tangential view the white color of the lunula is even more accentuated and opaque. One blood vessel at each side of the nail matrix in the surrounding nail bed can be readily seen through the very transparent nail plate (Fig. 3d). The nail bed itself has a pink color that reflects the underlying dermal vasculature (Fig. 3c), that is also noticeable in the histological sections. Subungual structures of the albino FVB/NJ mouse and the black C57BL/6J mice appeared similar and were unaffected by pigmentation of the nail plates; the nail plates of the C57BL/6J strain are, as the hair of their toe tips, almost unpigmented. The hyponychium, as defined by Zaias (Zaias, 1990) starts where the nail plate lifts off the nail bed which is marked by a lighter band in the nail plate (Fig. 3c), however, less obviously than in the human nail unit. The hyponychium has a very short dorsal part (4d, 5l) and extends at the ventral side of the distal phalanx down to the distal grove where the digit epidermis starts (Fig. 3d). The free edge of the nail plate extends beyond and curls over the tip of the digit and comes to a point.

Dissecting and scanning electron microscopy

While scanning electron microscopy (SEM) of human nails has been described (Forslind and Thyresson, 1975), this technique offers little advantage over simple magnification in clinical examination. By contrast, the small size of mouse nails demands higher magnification to appreciate abnormalities of the nail unit, including nail plate dystrophies. A binocular dissecting microscope was used to screen mouse nails at the time of necropsy, providing a good view of the gross structure of the mouse nail unit and showing details that might not be seen with the naked eye, such as the vasculature under the nail plate. Vessels were visible in nails immediately after euthanasia or in anesthetized mice (Figure 3d) and became more prominent within the first half hour post mortem.

Scanning electron microscopy produced more information on the mouse nail unit but was technically more difficult (Fig. 4). It was useful to show more subtle details on the surface of structures that are not discernible under the dissecting microscope. The dorsal side of the foot is covered with hair and the underlying epidermis appears wrinkled beneath the hair. At the distal end of the digits the nails emerge from under the hair (Fig. 4a). At higher magnification the surface structure becomes visible. It is made up of flattened, cornified cells that show a polygonal shape and a smooth surface (Fig. 4e). The surface of foot pad skin is clearly discernible including surface openings of the eccrine gland ducts (Fig. 4f). In contrast to the dissecting microscope, using SEM neither lunula nor blood vessels or other underlying structures is visible. In lateral view, the longitudinal curvature and the pointed tip of the mouse nail are visible (Fig. 4a). The plantar side of the foot shows several walking pads and is covered only very sparsely with short hair (Fig. 4b, d). The nail encircles the distal phalanx from three sides leaving only a tongue shaped area of epithelium, the hyponychium, uncovered (Fig. 4d).

Light microscopy

The human nail unit

The microscopic anatomy of the normal human nail unit is well characterized (Zaias, 1990; Fleckman, 2005) and is shown in figure 5. The proximal nail fold reflects at its tip and proceeds proximally and ventrally 5 to 8 mm before reflecting distally. Stratum corneum attached to the dorsal surface of the nail plate from the distal tip of the proximal nail fold and a few millimeters of the ventral (under) surface of the proximal nail fold can be seen attached to the dorsal surface of the nail plate as the cuticle (Fig. 5b). The epithelium of the ventral surface of the proximal nail fold is also known as the eponychium (Perrin et al., 2004). Just before the distal reflection of the proximal nail fold the granular layer disappears; this marks the beginning of the matrix and the onset of nail plate (onycholemmal) cornification (Perrin et al., 2004) (Fig. 5c). The thick, germinative matrix abruptly thins to a few cells at the juncture of the nail matrix and bed (Fig. 5d). The characteristic microscopic anatomy of the nail bed is seen in transverse section, revealing the regular pattern of nail bed epithelium interdigitating with papillary dermis (Fig. 6a). This regular pattern extends longitudinally (proximal-distal) the length of the nail bed, producing a pattern resembling tongue-in-groove lumber. The nail bed ends at the hyponychium with the reappearance of the granular layer, normal cornification, and the separation of the undersurface of the nail plate from the nail bed (Fig. 5e). The microanatomy of the distal nail bed and hyponychium have been described as the isthmus (Perrin, 2008). The distal groove is seen as an invagination of the epidermis and dermis just distal to the hyponychium and inferior to the free edge of the nail plate (Fig. 5a); it represents the distal aspect of the primary nail field, an area that denotes the forming nail unit in the embryo (Zaias, 1963). The dermis of the nail unit is thin, and no subcutis is present between the dermis and bone of the distal phalanx.

Fig. 5. Light microscopy of the nail unit. The human nail unit (a–f).

Fig. 5

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(a) Sagittal section through the distal part of a human newborn digit. (b–f) Higher magnification of the boxed areas in (a). (a) The nail plate (np) covers the dorsal surface of the distal digit and extends over its free edge. The nail plate invaginates proximally into the proximal nail fold (pnf). The region of nail matrix (nm), nail bed (nb) and hyponychium (hn) are marked by square brackets. The cuticle (cu) (a and b) is a layer of cornified cells that attaches to the dorsal surface of the nail plate and is produced by keratinocytes of the tip of the proximal nail fold and the eponychium (en), the epithelium covering the ventral surface of the proximal nail fold that shows a prominent granular layer but no papillae (b, insert). At the blunt, free end of the nail plate (a,f) cornified layers of the hyponychium are attached to the under surface of the nail plate. The nail matrix (c, d) is represented by a thickened epithelium at the base of the nail plate starting at the point where the eponychium with its granular layer ends (c, arrow) extending distally to the nail bed which is marked by reduction of cell layers (d, arrowhead). The hyponychium starts where the nail detaches from the nailbed, visible by the appearance of a granular layer (e, arrow). It extends to the distal grove (dg) (a). (a) bar = 1mm; (b–f) bars = 100 µm. The mouse nail unit (g–l). (g) Sagittal section through the distal part of an adult mouse nail unit. (h–l) Higher magnification of the boxed areas in (g). The region of nail matrix (nm), nail bed (nb) and hyponychium (hn) are marked by a continuous, dotted, and dashed line, respectively. Arrows in (c,e,i, and k) indicate the onset of the granular layer; (h) shows the eponychium (en) that contributes to the cuticle (cu). Insert in (i) shows a higher magnification of the boxed area with the loss of the granular layer at the transition to the apical matrix (am). Arrow tip in (i) indicates loss of granular layer (onset of onycholemmal keratinization) at the transition from nail matrix to nail bed. Arrow tip in (k) indicates reappearance of granular layer at the transition from nail bed to hyponychium. (g) bar = 500 µm; (h–l) bars = 100µm; (i, inset) bar = 50µm. Cartoons illustrate a median-sagittal section through the human and mouse nail unit, demonstrating comparative structures of both nail units.

Fig. 6. Transverse section through human and mouse nail bed.

Fig. 6

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(a) Transverse section of the human nail bed (nb) illustrates the deep interdigitations between epithelium and the underlying dermis. (Reproduced from Zaias, 1990, with permission) (b) Transverse section through the mouse nail bed shows a flatter, laterally pronounced papillary pattern of the nail bed epithelium. Arrows denote the onset of the granular layer of the hyponychium. np, nail plate. Bar = 40 µm

The mouse nail unit

The microscopic anatomy of the mouse nail unit is poorly described. The closest nonhuman analogy is with the so-called claw of the dog (Calhoun and Stinson, 1981), but the mouse nail structure resembles human nails more closely (Fig. 5g–l). The mouse nail is thickest in the center (the area of the “dorsal ridge”) and thins out laterally. A proximal nail fold and cuticle are present (Fig. 5g, h). The granular layer of the epithelium of the ventral (under) surface of the proximal nail fold (the eponychium) is lost before the reflection of the tissue (Fig. 5i).The epithelium between the loss of the granular layer and the reflection that we call apical matrix, consists of two to three cell layers and joins the ventral matrix proximally (Fig. 5i). The germinal epithelium of the ventral nail matrix is thick, but abruptly thins at the juncture with the nail bed (Fig. 5j). The keratogenous zone of the ventral nail matrix (Perrin et al., 2004) stands out through a more eosinophilic color than the lower layers of the ventral nail matrix (Fig. 5g, i). The nail bed epithelium is thin, composed of 2–3 cell layers and forms short papillae which are connected through collagen fibers with dermal structures and the distal phalanx bone (Fig. 5j, 6b). The granular layer reappears at the site of detachment of the nail plate from the nail bed (Fig. 5i, k). At this point, volar epithelium is seen that differs from that of the foot pad by decreased thickness and the relatively rare occurrence of dermal papillae. This epithelium ends at the distal groove and can be seen as analogous to the hyponychium of the human nail unit (Zaias, 1990), although it differs in its extension because the distal groove in humans is much closer to the site of detachment of the nail plate than that of the mouse. The nail plate tip continues to arc around as it extends over the tip of the digit.

Immunohistochemistry

Keratin expression of front and rear feet of 4 mice (age between 21d and 79d) was studied by immunohistochemistry. Results are illustrated in Figure 7 and summarized in Table 2 and figure 8.

Fig. 7. Keratin expression in the mouse nail unit.

Fig. 7

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Longitudinal sections through the mouse nail unit were labeled with antibodies against KRT5 (a,g), KRT14 (b,h), KRT17 (c,e,i), KRT6A (d,f,j), KRT1 (k,l), KRT10 (m,n) and acidic hair keratins detected with the monoclonal mouse antibody AE13 (o, o’). (a–e,k,m,o) show the proximal aspect of the nail unit including apical matrix (am), ventral matrix (vm), eponychium (en) and nail plate (np). (g–j,l,n) show the distal aspect of the nail unit consisting of the nail bed (nb) and the hyponychium (hn). KRT5 (a), KRT14 (b) and KRT17 (c) show a very similar pattern in the nail matrix and the eponychium. However, KRT17 is often detectable only in occasional cells in the distal matrix (c, inset) and in limited areas of the hyponychium (i). KRT6A (d) is expressed in the apical matrix but not in the ventral matrix. (e,f) show a higher magnification of the nail bed where suprabasally located KRT17 (e) and KRT6A (f) positive cells (arrow heads) were detectable. (g) shows the basal expression of KRT5 in the hyponychium, whereas K14 (h) shows a panepidermal pattern. Note the patchy pattern of KRT17 (i) in the same region. (j) KRT6A shows panepidermal labeling of the hyponychium. KRT1 labeling in the proximal (k) and distal (l) nail unit shows a strong panepidermal expression of the eponychium and suprabasal expression of the hyponychium but no labeling of nail matrix or nail bed. Note that the apical matrix also shows basal labeling (k, inset) and that in the hyponychium the labeling reaches in some areas the basal layer (l, inset). KRT10 shows a pattern similar to that of KRT1, but less extension to the basal layer (m,n, insets). AE13 labeling illustrates the expression of hard keratins in the suprabasal layers of the nail matrix (o, o’). np, nail plate. Dashed lines, border between dermis and epidermis. Bars = 50 µm

Table 2.

Keratin expression in the mouse nail unit

KRT1 KRT10 KRT14 KRT5 KRT6A KRT17 Hair K
Eponychium
basal +++ − +++ +++ − − −
suprabasal +++ +++ + + +/− +/− −
Apical matrix
basal +++ − +++ +++ + +++ −
suprabasal ++ +++ +++ +++ +++ +++ ?
Ventral matrix proximal
basal − − +++ +++ − ++ −
suprabasal − − ++ ++ − + +
keratogenous zone − − − − − − ++
Ventral matrix distal
basal − − +++ ++ − +/− −
suprabasal − − − − − − +
keratogenous zone − − − − − − ++
Nail bed
basal − − − − − − −
suprabasal − − − −(+/−1R) +/− +/−(−1F) −
Hyponychium
basal −/+ − +++ +++ ++ +/− −
suprabasal +++ ++ ++/+ + +++ +/− −
Volar digit epithelium
basal ++ + ++ +++ +/− −(+/−) −
suprabasal +++ +++ − +/− +/− − −
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+++=strong; ++=moderate; +=weak; +/−scattered positive cells. R=rear foot, F=front foot; Hair K = acidic hair keratins as detected with the antibody AE13. One front and one rear foot were studied in n=4 mice. (..) marks where one foot showed a different pattern from the other samples.

Fig. 8. Comparison of keratin expression in the human and mouse nail unit.

Fig. 8

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Human studies were summarized from: Perrin et al. (Perrin et al., 2004), McGowan et al. (McGowan and Coulombe, 2000) and De Berker et al. (De Berker et al., 2000). b, basal layer; sb, suprabasal layer(s); kgz, keratogenous zone; grey = weak or scattered expression; bold = our study; regular = published studies. *KRT16 has not been studied in the mouse nail unit.1 (Wojcik et al., 2001), 2 (Gu and Coulombe, 2007), 3 (Tong and Coulombe, 2004), 4 (Wang et al., 2003), #detected with AE13.

Epithelial Keratins

Immunolabeling of mouse keratin 5 and 14 revealed expression in the basal layer of the proximal nail fold, eponychium, hyponychium and ventral surface of the digit (Fig. 7a, b, g, h). The nail matrix showed in its proximal part labeling for KRT5/14 in a compartment consisting of the basal layer and suprabasal layers, whereas in the distal matrix the expression of KRT5/K14 was restricted to the basal layer (Fig. 7a, b). Keratin 14 was clearly expressed additionally in suprabasal cell layers of the hyponychium (Fig. 7h), where KRT5 showed only weak suprabasal labeling (Fig. 7g), though this suprabasal pattern of KRT14 did not reach the distal grove (Fig. 7h). Panepidermal KRT5/K14 expression was also detected in the apical matrix (Fig. 7a, b). We could not detect expression of KRT5/K14 in the nail bed (Fig. 7g, h).

KRT17 was detected in pattern a similar to that of KRT14 in the apical and ventral mouse nail matrix (Fig. 7c). However, strong labeling of the proximal ventral nail matrix decreased to no labeling in some regions of the distal ventral matrix, whereas in between, scattered positive cells with weak to moderate labeling were detectable in the basal layer (Fig. 7c, inset). The nail bed also showed a few positive cells that consisted of flat cells located just under the nail plate in a suprabasal location (Fig. 7e). KRT17 expression in the hyponychium also showed variability; constant moderate labeling was seen at the basal layer of the most distal part, while weak panepidermal and sometimes patchy expression was seen in the remaining portion (Fig. 7i).

KRT6A expression in the eponychium was only detected suprabasally at the transition to the apical matrix. In the apical matrix KRT6A was very strongly expressed with a suprabasal accentuation (Fig. 7d). The remaining nail matrix was negative (Fig. 7d). In the nail bed, faint labeling was a visible in scattered suprabasal cells (Fig. 7f). Panepidermal, strong labeling reappeared in the region of the hyponychium; with the exception of eccrine staining, the labeling ended shortly before or at the distal grove (Fig. 7j).

The suprabasal epidermal keratins, KRT1/10, were expressed in all regions as in the epidermis except for nail matrix and nail bed, where they were detectable only in suprabasal cells of the apical matrix (Fig. 7k–n). However, KRT1 expression often extended to the basal layer, for example in the eponychium and apical matrix (Fig. 7k, inset), weakly in some regions of the hyponychium (Fig. 7l, inset), and more strongly in the ventral digit epithelium (not shown).

Hard keratins

To study the expression of hard keratins in the mouse nail unit we used the monoclonal mouse antibody AE13, which detects acidic hair keratins in hair and nails (Lynch et al., 1986; Perrin et al., 2004). We found weak labeling of the suprabasal layers of the apical and ventral matrix that became stronger in the keratogenous zone of the proximal and distal ventral matrix (Fig. 7o). The nail plate was also slightly labeled compared to the negative stratum corneum of the surrounding epidermis. No other regions of the nail unit showed any labeling.

DISCUSSION

In this paper we compare the gross and microscopic anatomy and the pattern of keratin expression of the human nail unit with that of the mouse. While this study focused on the most commonly used inbred strain of mice, the C57BL/6J, the gross and histologic features are the same for most inbred strains of mice (Sundberg et al., 2011). The value of the mouse nail unit as a model for the homologous human appendage rests on its similarity rather than minor anatomic differences. Mouse models of human skin and hair diseases were described based on re-examination of previously described and new spontaneous and induced mutants (Sundberg and King, 2000). These observations suggested that the gross and microscopic anatomy of the nail unit of the mouse should be characterized and compared with that of humans to determine the degree of similarity and usefulness for modeling human nail disorders.

The comparative anatomy of the nail unit and the claw have been studied and argued by several authors over the years (Panzer, 1932; LeGros Clark, 1936; Spearman, 1978). These studies are rather general; they deal primarily with evolutionary concerns and suffer from lack of careful description and comparison of the gross and microscopic anatomy. Terms that imply major differences were proposed 70 years ago by LeGros Clark (LeGros Clark, 1936) where he argued that the distal phalanges of the mouse terminate in a structure more appropriately called a claw than a nail. This was based on 1) shape (sharp tip with a pronounced longitudinal curve and lateral compression, as opposed to round tip with a relatively flat longitudinal axis and slight transverse convexity), 2) sole plate (buried within the ventral groove as opposed to ending in a flat hyponychium), 3) the shape of the terminal phalanx (claw shaped as opposed to splayed), and 4) the articulation of the terminal interphalangeal joint (ideal for powerful movement of the terminal digit in the mouse). However, when invertebrates, animals that have true claws, are included in these discussions the distinction between mammals having claws or nails become more a semantic than scientific argument; the similarity between the nail unit in humans and mice becomes apparent. The anatomic and developmental similarities between mouse and man warrant using the term nail for these appendages in both species. Because the term nail is now commonly used in descriptions of phenotypes in genetically engineered mice which very closely resemble human anatomy, pathology, and molecular pathogenesis, the term nail should be used for this anatomic structure in the mouse.

To determine similarities between the mouse nail unit and the human nail unit at the protein level, we identified expression patterns of the “soft” keratins of stratified epithelia and of limited “hard” hair keratins by immunohistochemistry. An extensive survey of “hair” keratins was limited by antibody availability. Regional patterns of keratin expression in the human nail unit have been described recently by several groups (De Berker et al., 2000; McGowan and Coulombe, 2000; Perrin et al., 2004). The mouse nail unit also shows regional differentiation patterns and most, but not all of them, are similar to those found in the human nail apparatus (Figure 8).

KRT1/10 was strongly positive in the eponychium and hyponychium and negative in the nail matrix and nail bed, similar to human studies (Kitahara and Ogawa, 1997; Perrin et al., 2004). KRT1/10 expression in the human nail matrix is limited to scattered cells in the apical matrix, the most proximal part of the nail matrix at the site of reflection of the ventral aspect of the proximal nail fold to the (ventral) nail matrix. In this area KRT1/10 are expressed suprabasally together with hard keratins in an epithelium without a granular layer and is thought to represent the compartment forming the dorsal nail plate (Perrin et al., 2004). However, the formation of a dorsal nail plate remains controversial (Zaias, 1990). We observed KRT10 labeling in the ventral surface of the proximal nail fold extending more proximally towards the matrix than the granular layer. This part also shows co-expression of multiple “soft” keratins in a panepidermal pattern (KRT5, KRT6A, KRT14, KRT17) so we call it apical matrix. This apical matrix is clearly more extended in the mouse nail unit compared to humans; however, whether it contributes to the nail plate, forms the cuticle, or represents only a transition zone between interfollicular epidermis and onycholemmal keratinization remains unclear for both the mouse and human nail unit. In addition to labeling of the keratogenous zone of the matrix, AE13 immunohistochemistry shows labeling of the whole nail plate (including the uppermost layers) that is clearly distinct from the negative cuticle. However, immunolabeling of fully cornified tissue is often not reliable, and since the layers closest to the nail plate at the proximal reflection of the proximal nail fold are subject to artifact, it was not possible to tell whether hard keratin was expressed at this site. If the apical matrix contributes to the nail plate, it is limited compared to the ventral matrix; this is in accordance with the situation in the human nail unit (Perrin et al., 2004).

In the proximal nail matrix we found an extended basal cell compartment consisting of several cell layers, in agreement with findings in the human nail matrix (de Berker and Angus, 1996; Perrin et al., 2004). The distal nail matrix; however, shows only a single basal layer in the mouse. We evaluated these compartments separately in order to see changes in the keratin pattern in the superficial or deep layers, indicating a bilayered nail plate. The differences we found were very subtle. KRT17 expression alone was much weaker and only in rare cells in the distal matrix. The matrix showed consistent expression of KRT5, 14, and 17 in the basal compartment, and hard keratin expression in the keratogenous zone. These findings agree with those of Perrin et al. (Perrin et al., 2004), with the exception of KRT14 expression, which was not described by Perrin et al. KRT14 expression was described by De Berker, et al. (De Berker et al., 2000) as showing a panepidermal pattern throughout the whole nail unit. In contrast, our observations in the mouse nail unit found more limited KRT14 expression in the matrix and no labeling in the mouse nail bed.

Keratin expression in the adult mouse nail bed shows the most deviation from that of the human nail. Surprisingly we could detect no keratins with our antibodies in the basal layer, even though we found characteristic expression in the other epidermal regions and appendages of the mouse feet. This is even more remarkable, since we find clear expression of KRT5, 14, and 17 in the basal layers of the nail bed in a second study on mouse nail development in newborn mice (data not shown). Suprabasally we found expression of KRT6A and KRT17 in the uppermost layer, however positive cells were not present in a continuous band, but were scattered throughout the nail bed. This differs from observations in the human nail unit published by other groups (McGowan and Coulombe, 2000; Perrin et al., 2004), where pan-epidermal KRT17 expression was seen in the human nail bed. Coulombe, et al., also found in the mouse a K17-like protein, KRT17n, with preferential expression in the mouse nail unit that could be detected with the same antibody we used in our study (Tong and Coulombe, 2004). One possible explanation for the discrepancy is that we used paraffin embedded sections instead of frozen sections, which might have masked the epitope of KRT17n and reacted only with KRT17. This would explain also why we saw KRT17 very strongly expressed in the hair follicles. Keratin 17 expression appears to play an important role during embryonic development (McGowan and Coulombe, 1998) and belongs, as KRT6A and 16, to the inducible keratins that are upregulated during inflammation and wound healing (Leigh et al., 1995). KRT17 is constitutively present in other skin appendages, such as hair follicles and glands (McGowan and Coulombe, 2000). KRT6A, which represents a Type II binding partner of KRT17, is not constitutively expressed in normal epidermis.

The utility of a mouse model of the human nail unit has clearly shown potential in developmental studies (Godwin, 1959; Loomis et al., 1996; Plikus et al., 2007). From these observations, one might predict that the value of the mouse model will be limited to developmental and inherited anomalies. However, the possibility that the acquired human disorder onychomycosis (fungal infection of the nail unit) may involve an autosomal dominant trait (Zaias et al., 1996) suggests that mouse mutants may provide more general utility. Alternatively, inducible models can be developed using known mouse or human mycotic agents to infect the nail bed/matrix of immunodeficient laboratory mice such as nude mice (Foxn1nu) that have abnormal nails, severe combined immunodeficiency mice (Prkdcscid), or Rag1tm1Mom mice (Shultz, 1991, 1996).

ACKNOWLEDGEMENTS

The authors thank P. Hogan and L. Bechtold for assistance with the SEM, K. Seburn and C. Rosales for providing sequences of normal mouse gait, H.F. Coombs and W.G. Beamer for performing the CAT scan on the digit, and R. Underwood for help with the human nail samples. K. Jaeger thanks Prof. E. Tschachler for continuing support.

Grant sponsor: This work was supported in part by grants from The Council for Nail Disorders (PF, JPS), Endowed Research Fund of the Division of Dermatology of the University of Washington (PF), and the National Institutes of Health (AR49288, AR047204, AR054407, AR056635, OD010920, and CA34196; JPS).

Abbreviations

Hr

hairless gene symbol

D

digit number (eg, D I)

H&E

hematoxylin and eosin stain

SEM

scanning electron microscopy

LITERATURE CITED

  1. Baden HP. Diseases of the hair and nails. Chicago: Year Book Medical Publishers; 1987. [Google Scholar]
  2. Baran R, Dawber RPR, De Berker DAR, Haneke E, Tosti A, editors. Diseases of the Nails and Their Management. 3 ed. Oxford: Blackwell Science Ltd.; 2001. [Google Scholar]
  3. Barankin B, Guenther L. Dermatological manifestations of Down's syndrome. J Cutan Med Surg. 2001;5:289–293. doi: 10.1007/s102270000021. [DOI] [PubMed] [Google Scholar]
  4. Bechtold LS. Ultrastructural evaluation of mouse mutations. In: Sundberg JP, Boggess D, editors. Systematic characterization of mouse mutations. Boca Raton: CRC Press; 2000. pp. 121–129. [Google Scholar]
  5. Boggess D, Silva KA, Landel C, Mobraaten L, Sundberg JP. Approaches to handling, breeding, strain preservation, genotyping, and drug administration for mouse models of cancer. In: Holland EC, editor. Mouse models of human cancer. Hoboken, NJ: John Wiley & Sons, Inc.; 2004. pp. 3–14. [Google Scholar]
  6. Bouxsein ML, Uchiyama T, Rosen CJ, Shultz KL, Donahue LR, Turner CH, Sen S, Churchill GA, Muller R, Beamer WG. Mapping quantitative trait loci for vertebral trabecular bone volume fraction and microarchitecture in mice. J Bone Miner Res. 2004;19:587–599. doi: 10.1359/JBMR.0301255. [DOI] [PubMed] [Google Scholar]
  7. Calhoun ML, Stinson AW. Integument. In: Dellmann HD, Brown EM, editors. Textbook of veterinary histology. Philadelphia: Lea & Febiger; 1981. pp. 378–411. [Google Scholar]
  8. de Berker D, Angus B. Proliferative compartments in the normal nail unit. Br J Dermatol. 1996;135:555–559. [PubMed] [Google Scholar]
  9. De Berker D, Wojnarowska F, Sviland L, Westgate GE, Dawber RP, Leigh IM. Keratin expression in the normal nail unit: markers of regional differentiation. Br J Dermatol. 2000;142:89–96. doi: 10.1046/j.1365-2133.2000.03246.x. [DOI] [PubMed] [Google Scholar]
  10. Evans HE, deLahunta A. Miller’s Guide to the Dissection of the Dog. Philadelphia: W B Saunders Co; 1971. [Google Scholar]
  11. Fleckman P. Structure and function of the nail unit. In: Scher RK, Daniel CR, editors. Nails: diagnosis, therapy, surgery. Philadelphia: Elsevier Saunders; 2005. pp. 13–25. [Google Scholar]
  12. Forslind B, Thyresson N. On the structure of the normal nail: A scanning electron microscopic study. Arch Derm Forsch. 1975;251:199–204. doi: 10.1007/BF00561761. [DOI] [PubMed] [Google Scholar]
  13. Godwin KO. An experimental study of nail growth. J Nutrition. 1959;69:121–127. doi: 10.1093/jn/69.2.121. [DOI] [PubMed] [Google Scholar]
  14. Gu LH, Coulombe PA. Keratin expression provides novel insight into the morphogenesis and function of the companion layer in hair follicles. J Invest Dermatol. 2007;127:1061–1073. doi: 10.1038/sj.jid.5700673. [DOI] [PubMed] [Google Scholar]
  15. Hamrick MW. Development and evolution of the mammalian limb: adaptive diversification of nails, hooves, and claws. Evol Dev. 2001;3:355–363. doi: 10.1046/j.1525-142x.2001.01032.x. [DOI] [PubMed] [Google Scholar]
  16. Kitahara T, Ogawa H. Cellular features of differentiation in the nail. Microsc Res Tech. 1997;38:436–442. doi: 10.1002/(SICI)1097-0029(19970815)38:4<436::AID-JEMT11>3.0.CO;2-K. [DOI] [PubMed] [Google Scholar]
  17. Lee KF, Li E, Huber LJ, Landis SC, Sharpe AH, Chao MV, Jaenisch R. Targeted mutation of the gene encoding the low affinity NGF receptor p75 leads to deficits in the peripheral sensory nervous system. Cell. 1992;69:737–749. doi: 10.1016/0092-8674(92)90286-l. [DOI] [PubMed] [Google Scholar]
  18. LeGros Clark WE. The problem of the claw in primates. Proc Zool Soc. 1936;1:1–24. [Google Scholar]
  19. Leigh IM, Navsaria H, Purkis PE, McKay IA, Bowden PE, Riddle PN. Keratins (K16 and K17) as markers of keratinocyte hyperproliferation in psoriasis in vivo and in vitro. Br J Dermatol. 1995;133:501–511. doi: 10.1111/j.1365-2133.1995.tb02696.x. [DOI] [PubMed] [Google Scholar]
  20. Loomis CA, Harris E, Michaud J, Wurst W, Hanks M, Joyner AL. The mouse engrailed-1 gene and ventral limb patterning. Nature. 1996;382:360–363. doi: 10.1038/382360a0. [DOI] [PubMed] [Google Scholar]
  21. Lynch MH, O'Guin WM, Hardy C, Mak L, Sun TT. Acidic and basic hair/nail ("hard") keratins: their colocalization in upper cortical and cuticle cells of the human hair follicle and their relationship to"soft" keratins. J Cell Biol. 1986;103:2593–2606. doi: 10.1083/jcb.103.6.2593. [DOI] [PMC free article] [PubMed] [Google Scholar]
  22. McGowan KM, Coulombe PA. Onset of keratin 17 expression coincides with the definition of major epithelial lineages during skin development. J Cell Biol. 1998;143:469–486. doi: 10.1083/jcb.143.2.469. [DOI] [PMC free article] [PubMed] [Google Scholar]
  23. McGowan KM, Coulombe PA. Keratin 17 expression in the hard epithelial context of the hair and nail, and its relevance for the pachonychia congenita phenotype. J Invest Dermatol. 2000;114:1101–1107. doi: 10.1046/j.1523-1747.2000.00986.x. [DOI] [PubMed] [Google Scholar]
  24. Mecklenburg L, Paus R, Halata Z, Bechtold LS, Fleckman P, Sundberg JP. FOXN1 is critical for onychocyte terminal differentiation in nude (Foxn1<nu>) mice. J Invest Dermatol. 2004;123:1001–1011. doi: 10.1111/j.0022-202X.2004.23442.x. [DOI] [PubMed] [Google Scholar]
  25. Mikaelian I, Nanney LB, Parman KS, Kusewitt D, Ward JM, Naf D, Krupke DM, Eppig JT, Bult CJ, Seymour R, Ichiki T, Sundberg JP. Antibodies that label paraffin-embedded mouse tissues: a collaborative endeavor. Toxicol Pathol. 2004;32:1–11. doi: 10.1080/01926230490274335. [DOI] [PubMed] [Google Scholar]
  26. Noronha PA, Zubkov B. Nails and nail disorders in children and adults. Am Fam Physician. 1997;55:2129–2140. [PubMed] [Google Scholar]
  27. Panzer W. Beitraege zur Biologischen Anatomie des Baumkletterns der SaeugetiereIDas Nagel-Kralle-Problem. Zeitschr f d ges Anat, IAbt. 1932;98:147–198. [Google Scholar]
  28. Perrin C. The 2 clinical subbands of the distal nail unit and the nail isthmus. Anatomical explanation and new physiological observations in relation to the nail growth. Am J Dermatopathol. 2008;30:216–221. doi: 10.1097/DAD.0b013e31816a9d31. [DOI] [PubMed] [Google Scholar]
  29. Perrin C, Langbein L, Schweizer J. Expression of hair keratins in the adult nail unit: an immunohistochemical analysis of the onychogenesis in the proximal nail fold, matrix and nail bed. Br J Dermatol. 2004;151:362–371. doi: 10.1111/j.1365-2133.2004.06108.x. [DOI] [PubMed] [Google Scholar]
  30. Plikus MV, Sundberg JP, Chuong C-M. Mouse skin ectodermal organs. In: Fox JG, Davisson MT, Quimby FW, Barthold SW, Newcomer CE, Smith AL, editors. The Mouse in Biomedical Research. Amsterdam: Elsevier; 2007. pp. 691–730. [Google Scholar]
  31. Popesko P, Rajitov V, Hork J. A Color Atlas of Anatomy of Small Laboratory Animals. London: Wolfe Publishing, Ltd; 1992. [Google Scholar]
  32. Scher RK. The nails. In: H H Roenigk J, Maibach HI, editors. Psoriasis. 3rd ed. New York: Marcel Dekker, Inc.; 1998. pp. 41–43. [Google Scholar]
  33. Scher RK, C R Daniel I. Nails: diagnosis, therapy, surgery. Philadelphia: Elsevier Saunders; 2005. [Google Scholar]
  34. Seymour R, Ichiki T, Mikaelian I, Boggess D, Silva KA, Sundberg JP. Necropsy methods. In: Hedrich HJ, editor. Laboratory mouse. London: Academic Press; 2004. pp. 495–516. [Google Scholar]
  35. Shultz LD. Immunological mutants of the mouse. Am J Anat. 1991;191:303–311. doi: 10.1002/aja.1001910310. [DOI] [PubMed] [Google Scholar]
  36. Shultz LD. Single gene mouse models of immundeficiency and autoimmune diseases. In: Herzenberg LA, Weir DM, Blackwell C, editors. Weir's handbook of experimental immunology: the integrated immune system. fifth ed. Cambridge, MA: Blackwell Science; 1996. pp. 150.151–150.154. [Google Scholar]
  37. Spearman RIC. The physiology of the nail. IV. Comparative anantomy. In: Jarrett A, editor. The physiology and pathophysiology of the skin. New York: Academic Press; 1978. pp. 1827–1832. [Google Scholar]
  38. Sundberg JP, Berndt A, Sundberg BA, Silva KA, Kennedy V, Bronson R, Yuan R, Paigen B, Harrison DE, Schofield PN. The mouse as a model for understanding chronic diseases of aging: the histopathologic basis of aging in inbred mice. Pathobiology of Aging & Age-related Diseases. 2011;1:7179–7188. doi: 10.3402/pba.v1i0.7179. [DOI] [PMC free article] [PubMed] [Google Scholar]
  39. Sundberg JP, Dunstan RW, Compton JG. Hairless mouse, HRS/J hr/hr. Heidelberg: Springer-Verlag; 1989. [Google Scholar]
  40. Sundberg JP, King LE. Skin and its appendages: normal anatomy and pathology of spontaneous, transgenic and targeted mouse mutations. In: Ward JM, Mahler JF, Maronpot RR, Sundberg JP, editors. Pathology of genetically engineered mice. Ames: Iowa State University Press; 2000. pp. 181–213. [Google Scholar]
  41. Sundberg JP, Montagutelli X, Boggess D. Systematic approach to evaluation of mouse mutations with cutaneous appendage defects. In: Chuong M, editor. Cutaneous Appendages. Austin, TX: Molecular Biology Intelligence Unit I, Landes Company; 1998. pp. 421–435. [Google Scholar]
  42. Tong X, Coulombe PA. A novel mouse type I intermediate filament gene, keratin 17n (K17n), exhibits preferred expression in nail tissue. J Invest Dermatol. 2004;122:965–970. doi: 10.1111/j.0022-202X.2004.22422.x. [DOI] [PubMed] [Google Scholar]
  43. Wang Z, Wong P, Langbein L, Schweizer J, Coulombe PA. Type II epithelial keratin 6hf (K6hf) is expressed in the companion layer, matrix, and medulla in anagen-stage hair follicles. J Invest Dermatol. 2003;121:1276–1282. doi: 10.1111/j.1523-1747.2003.12644.x. [DOI] [PubMed] [Google Scholar]
  44. Wojcik SM, Longley MA, Roop DR. Discovery of a novel murine keratin 6 (K6) isoform explains the absence of hair and nail defects in mice deficient for K6a and K6b. J Cell Biol. 2001;154:619–630. doi: 10.1083/jcb.200102079. [DOI] [PMC free article] [PubMed] [Google Scholar]
  45. Wooley CM, Sher RB, Kale A, Frankel WN, Cox GA, Seburn KL. Gait analysis detects early changes in transgenic SOD1(G93A) mice. Muscle Nerve. 2005;32:43–50. doi: 10.1002/mus.20228. [DOI] [PMC free article] [PubMed] [Google Scholar]
  46. Zaias N. Embryology of the human nail. Arch Dermatol. 1963;87:77–93. doi: 10.1001/archderm.1963.01590130043010. [DOI] [PubMed] [Google Scholar]
  47. Zaias N. The nail in health and disease. 2nd ed. Norwalk, CT: Appleton & Lange; 1990. [Google Scholar]
  48. Zaias N, Tosti A, Rebell G, et al. Autosomal dominant pattern of distal subungual onychomycosis caused by Trichophyton rubrum. J Am Acad Dermatol. 1996;34:302–304. doi: 10.1016/s0190-9622(96)80142-3. [DOI] [PubMed] [Google Scholar]

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