Morphology of hair in normal and mutant laboratory mice

ARTICLE

The laboratory mouse has become the mammal of choice to study most biological and pathological systems. This choice was initially based on its small size, ease of maintenance, high fecundity, and other logistical considerations, however, these considerations have been surpassed by the large amount of genetic data available on this species and the numerous methods that have been developed to manipulate the genome of the mouse [1]. Generations of large numbers of transgenic, targeted mutagenesis (so-called "knockouts"), and conditional mutagenesis models in this species have changed our view of biomedical research and how we diagnose and treat patients.

Like most other mammals, mice are covered with hair. Many spontaneous and genetically engineered mouse mutations have abnormalities that affect the hair follicle, cycle, or fiber. Understanding the similarities and differences between the mouse hair follicle and fiber and that found in humans can aid in comparative aspects when developing models for specific human disease. More importantly, these mutant mice can be used to dissect basic physiological processes critical in hair biology.

Large numbers of mouse mutations with apparently no hair, lacking specific hair types, developmental defects, cycle defects, or structural defects of the fiber exist. Lists of such mutant mice with references to more detailed descriptions are numerous [2-4]. This paper presents an overview of the major normal hair types in the mouse and uses selected examples of mutant mice to illustrate examples of how mutations that affect specific structures within the pilosebaceous unit result in alopecia. Analogous diseases occur in humans making these mice useful manipulatable models to better under-stand the pathophysiologic process involved and to develop ways to control, prevent, or treat the corresponding diseases in humans.

Normal mouse hair follicles and fibers

The basic hair follicle structure in the mouse is similar to that found in humans (not vibrissae) and other mammals. The major differences between mice and humans are the size of the follicle and hair fiber which are much smaller, the follicle density and distribution is greater, and hair cycling patterns are in the form of a wave rather than a mosaic pattern, as seen in humans [3-5]. Traditional studies describe mouse hairs as having four basic types, a description limited to truncal or pelage hairs [6]. In fact, there are numerous distinct hair types in the mouse, as in man, based on anatomic location (Table I). Unlike humans, mice do not have whiskers. This is a lay term that refers to the long somatosensory hairs most noticable around the muzzle in most mammals called vibrissae. These particular hairs have a large blood filled sinus that is an integral part of the hair follicle with a large nerve that attaches below the sebaceous gland. Lists of mutant mice with defects or absence of specific hair follicle/fiber types have been published [2].

The truncal or pelage hairs are the most common hair types examined when mutant mice are characterized. The four major hair types are defined based on size, shape, and internal structure [2]. Hairs mounted under a coverslip with mounting media can be identified by the numbers of air cells found in the medulla: one (zigzag), two (guard), and two or more (awl and auchene). The anatomically specialized hairs can be collected and labeled so they can be easily identified. For histologic examination of tissues, location of the hair type can be determined by the accompanying structures. For example, auricular cartilage is a landmark for identifying ear and its hairs; coccygeal vertebrae and thick epidermis for tail skin and its hairs, anus for perianal hairs, and meibomian glands and mucocutaneous junctions for cilia of the eyelids [7].

Abnormalities of specific hair follicle structures or functions can provide critical information on the biological function of that structure. Examples of single gene mutations affecting specific structures are presented as an introduction into this field of study and to acknowledge the value of these types of mutant mice to hair biology research.

Sebaceous gland

The sebaceous gland presumably has numerous critical functions other than as part of the problem in acne [8]. Most of its functions can still only be guessed at. Mutations such as Tabby (Ta) and its mimics, crinkled (cr) and downless (dl) lack specialized sebaceous glands, such as the meibomian gland in the eyelid, and the entire absence of the pilosebaceous unit in the tail (Fig. 1D) [9]. These are models for human anhydrotic ectodermal dysplasia. As each mutated gene is defined the biochemical cascades for their interactive functions are becoming understood [10-17]. Alopecia in these mice is limited to the tail skin (Fig. 1A) and a small area behind each ear.

The asebia mutant mice are a group of three known alleles (different mutations in the stearoyl co A desaturase 1 gene) [18] that each result in sebaceous gland hypoplasia and marked changes in cutaneous surface lipids [19]. Longitudinal studies have found that failure of degradation of the inner root sheath in asebia mice results in retention of hair fibers, elongation of the hair follicles, prolongation of the hair follicle length into telogen, and rupture through follicles in or around the bulb. Secondary foreign body granulomas result in follicular scarring resulting in progressive scarring alopecia [19].

The bareskin (Bsk) mouse mutation has abnormalities of the sebaceous glands that are associated with development of progressive alopecia extending centrifugally from the head as the mice pass puberty. As the mice age, they develop severe scarring alopecia, again implying the importance of the sebaceous glands in this disease process (Fig. 1A-C) [9, 20].

Matrix abnormalities

Transgenic and targeted mutatagenesis mice of bone morphogenic protein 4 and Noggin develop a variety of abnormalities in and around the matrix region [21]. Another dramatic example is with the spontaneous lanceolate hair (lah) allelic mutant mice. These mice develop premature cornification of the matrix resulting in a focal weakening of the hair fiber that will then break off at the surface once it emerges. This process continues intermittently producing some hair fibers with multiple nodal swellings (Fig. 1E, F). These mice were initially thought to be models for Netherton's syndrome or monilothrix [22, 23]. Mapping and cloning of the human gene responsible for Netherton's syndrome have recently disproven this as an analog [24]. However, morphologic similarity to a novel human hair follicle disease suggests that this mutation within a cluster of adhesion molecule genes, does have a human analog (Christiano, pers. comm.). This interesting mutation provides biological evidence that repeated signals to anagen follicles may maintain the length of the hair cycle in mice and by inference, in man.

Hair cycle abnormalities

The mouse hair cycles in a wave pattern, from head to tail, in contrast to humans where this happens in a mosaic pattern [5, 25]. Mouse hair grows to a defined length and then stops. As the follicles enter the new anagen cycle, the old club hairs are pushed off to the side until such time as they are mechanically removed. The wave is not noticeable on the live mouse because of the high density of follicles in this species and its irregular pattern. As the mouse moves the hairs overlap thereby masking any evidence of the wave. The edge of a hair wave becomes evident in photographs of carefully laid out dead mice where there is no movement and the image can be magnified significantly.

Several notable mutant mice have prolongation of the hair cycle that results in localized or diffuse abnormal length to the hairs. Most notable is the angora mutant mouse (Fgf5^go) that has a block deletion in the fibroblast growth factor 5 gene [25, 26]. Others, yet to be defined at the molecular level include hairy ears (Eh) and koala (Ko). The long hairs are limited to the pinna in both mutant mice [3, 27].

The mouse mutation that epitomizes a defect in the hair cycle is the hairless (hr) mouse and its rhino alleles (hr^rh). Hairless mice are not hairless, rather they have a normal first or embryonic hair cycle that is lost beginning at two weeks of age from the head to the tail. Vibrissae remain, primarily because their hair cycle is different, explaining why these hair types are also lost with age. Histologically the dermal papilla does not reassociate correctly with the bulge or remnants of the bulge. The result is that the infundibulum dilates and fills with cornified material and sebum. The follicular remnants also develop into deep dermal cysts that may rupture causing foreign body granulomas and scarring. Large cysts form in the rhino alleles and cause the apparent thickening and folding of the skin in these mice (Fig. 1G, H). Most of the hairless alleles have now been sequenced and compared with the human and nonhuman primate analogs, papular atrichia [28-33]. A group of transgenic mice have been created that overexpress ornithine decarboxylase. These mice closely mimic the severe forms of the rhino mice (Fig. 1I) [34].

Structural abnormalities in hair fibers

Most of the structural abnormalities described for specific diseases in human hair diseases [35, 36] can be found in one or more of the mutant mice with alopecia [37]. Therefore, these defects by themselves can not be used for a diagnosis but rather are helpful in focusing on a potential analogous disease. These are by far the most common general cause of alopecia in mice. One of the most notable examples is the nude mouse, the gene symbol of which changes periodically as its function is defined (nu to Hfh11^nu, and recently changed to Foxn1^nu) [38]. Although these mice appear to be totally bald and, at the clinical level, devoid of hair follicles, they do have hair follicles that cycle. Fibers produced are weak and twist within the infundibulum breaking off at the surface (Fig. 1J) [39]. Phenotype varies with the inbred (congenic) background the mutant gene is placed on. Some, like the NU/J, do lose hair follicles by one year of age. The human analog has now been described at both the clinical and molecular level [40].

Many other mutations in laboratory mice result in weak and defective hair fibers. The nude mouse is one of the more severe examples but the general pattern is that of twisting to various degrees within the hair follicle, usually beginning around the level of the sebaceous gland, further attesting to the importance of the sebaceous gland in the follicular barrier at the level where the inner root sheath degrades. Severe twisting often results in hyperplasia of the root sheaths, especially where the defective fiber is forced into and through the wall. Once the fiber is exposed to the dermis a foreign body granuloma will ensue.

Abnormalities of the root sheaths

The inner and outer root sheaths of the hair follicle are long, undergo major developmental changes along their length, and can be very difficult to evaluate. Furthermore, the inner layer of the outer root sheath (sometimes called the companion layer) is a morphologically difficult layer to evaluate but it selectively expresses keratin 6. A number of mouse mutations directly or indirectly result in abnormalities of these layers. Many, such as the structural defects of the hair fibers discussed above, may become thickened due to trauma or perforation. Three spontaneous mutations at the balding locus and the targeted mutation for desmoglein 3 have an abnormality in this adhesion molecule. Root sheaths separate on either side of the keratin 6 positive inner layer of the outer root sheath resulting in evulsion of the follicular structure [41-43]. This potentially represents one form of pemphigus vulgaris, a disease commonly found in humans and many domestic mammals [44].

Marked hyperplasia of the inner and outer root sheaths is a feature of the harlequin ichthyosis (ichq) mutant mouse. Compact orthokeratotic hyperkeratosis, especially of the inner root sheath where it normally would undergo degredation results in marked thickening of the walls of the infundibulum and forms a dense collar around emerging hair fibers. Similar features are observed in human type II harlequin ichthyosis [45].

Autoimmune hair follicle disease

Alopecia areata is an autoimmune disease directed against anagen stage hair follicles. This disease affects humans, mice, rats, dogs, horses, and possibly many other species that have yet to be carefully defined [46]. At least 8 inbred and congenic strains of laboratory mice have been identified with an alopecia areata-like phenotype [47]. Only anagen follicles and early catagen follicles are affected. These mice develop autoantibodies directed at structures in the bulb region and the mid to lower portion of the hair follicles are infiltrated and surrounded by a mixed inflammatory cell infiltrate, primarily CD8⁺ and lesser numbers of CD4⁺ lymphocytes (Fig. 1K, L). These mice are used extensively for experimental manipulation to test new therapies or to define the disease pathogenesis [46].

A number of other mutations such as flaky skin (fsn) [48] and chronic proliferative dermatitis (cpdm) [49] have marked dermal inflammation and hair follicle abnormalities. These are less well defined as models for analogous human diseases but as their mechanisms and genes are defined they are adding critical information on how the immune system interacts with the skin in general and hair follicles in particular.

CONCLUSION

Acknowledgements

This work was supported by grants for the National Alopecia Areata Foundation (JPS, LEK), Council for Nail Disorders (JPS), the National Institutes of Health (AR43801, RR00173, CA34196; JPS), and Bureau of Veterans Affairs (LEK).

Abbreviations

Bsk: bareskin
cpdm: chronic proliferative dermatitis
cr: crinkled
dl: downless
Eh: hairy ears
Fgf5^go: fibroblast growth factor 5 ­ angora
fsn: flaky skin
hr: hairless
hr^rh: rhino allele of hairless
ichq: harlequin ichthyosis
Ko: koala
lah: lanceolate hair
nu: nude
Scd1^ab: stearoyl co A desaturase 1 ­ asebia
Ta: tabby
Tgn(K6ODCtr): ornithine decarboxylase 55Tgo transgenic mouse

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