From telogen to exogen: mechanisms underlying formation and subsequent loss of the hair club fiber

Canine noninflammatory alopecia: a comprehensive evaluation of common and distinguishing histological characteristics

BACKGROUND

Noninflammatory alopecia is a frequent problem in dogs, and the pathogenesis is still unclear.

OBJECTIVE

The objective of this study was a comparative histological description of skin biopsies from dogs with different alopecic disorders and control dogs matched for coat type, season and disease duration.

ANIMALS

Twenty-one cases of alopecia X in plush-coated dogs, 12 cases of recurrent flank alopecia, three cases of hyperestrogenism, 15 cases of hyperadrenocorticism, 12 cases of hypothyroidism and 12 cases of primary alopecic disorders of unknown cause were evaluated. The controls were biopsies from 38 dogs of different coat types.

METHODS

We evaluated five serial sections of each biopsy histologically and immunohistologically to compare the histological findings within the disease groups and with the control.

RESULTS

All the dogs with hair cycle disorders had a significant increase in the number of hairless hair follicles, which we assigned to kenogen. In addition, dogs with alopecia X had the lowest percentage of anagen follicles and the highest percentage of telogen follicles.

CONCLUSIONS

The marked increase in kenogen follicles is a strong indication that the induction of the new anagen phase is impaired in hair cycle disorders. The findings in dogs with alopecia X further suggest that premature catagen is also involved in the pathogenesis. Further work to investigate the stem cell compartment and possible initiating factors for the different cycle phases is required to elucidate the exact pathogenesis.

Physiological regeneration of skin appendages and implications for regenerative medicine

The concept of regenerative medicine is relatively new, but animals are well known to remake their hair and feathers regularly by normal regenerative physiological processes. Here, we focus on 1) how extrafollicular environments can regulate hair and feather stem cell activities and 2) how different configurations of stem cells can shape organ forms in different body regions to fulfill changing physiological needs.

The cell cycle regulator protein 14-3-3σ is essential for hair follicle integrity and epidermal homeostasis

The 14-3-3σ (Stratifin; Sfn) is a cell cycle regulator intimately involved in the program of epithelial keratinization. 14-3-3σ is unique in that it is expressed primarily in epithelial cells and is frequently silenced in epithelial cancers. Despite its well-documented role as a cell cycle regulator and as a tumor suppressor, the function of 14-3-3σ in the intricate balance of proliferation and differentiation in epithelial development is poorly understood. A mutation in 14-3-3σ was found to be responsible for the repeated epilation (Er) phenotype. It has previously been shown that Sfn(+/Er) mice are characterized by repeated hair loss and regrowth, whereas Sfn(Er/Er) mice die at birth displaying severe oral fusions and limb abnormalities as a result of defects in keratinizing epithelia. Here we show that mice heterozygous for the 14-3-3σ mutation have severe defects in hair shaft differentiation, resulting in destruction of the hair shaft during morphogenesis. Furthermore, we report that the interfollicular epidermis and sebaceous glands are hyperproliferative, coincident with expanded nuclear Yap1 (Yes-associated protein 1)--a critical modulator of epidermal stem cell proliferation. We also report that hair follicle stem cells in the bulge cycle abnormally, raising important questions as to the role of 14-3-3σ in the bulge.

Regulation of hair shedding by the type 3 IP3 receptor

Here we showed that the type 3 IP(3) receptor (IP(3)R3) is specifically expressed in hair follicles of the skin and plays an important role in the regulation of the hair cycle. We found that IP(3)R3-deficient (Itpr3(-/-)) mice had prominent alopecia, which was characterized by repeated hair loss and regrowth. The alopecic stripe runs along the body axis like a wave, suggesting disturbed hair-cycle regulation. Indeed, the hair follicles of the alopecic region were in the early anagen stage. Although the hair growth and proliferation activity of the hair matrix cells in the anagen phase were normal in Itpr3(-/-) mice, telogen club hairs in the telogen-anagen transition phase were loosely attached to the hair follicles and were easily removed in contrast to the more tightly attached club hairs of Itpr3(+/+) mice. Itpr3(-/-) keratinocytes surrounding the telogen club hairs have sparse cytokeratin filaments extending in random directions, as well as less developed desmosomes. Furthermore, nuclear factor of activated T cells c1 (NFATc1) failed to translocate into the nucleus of keratin 6-positive bulge cells in Itpr3(-/-) telogen follicles. We propose that hair shedding is actively controlled by the IP(3)R3/NFAT-dependent signaling pathway, possibly through the regulation of cytokeratin filaments in keratinocytes.

The human hair follicle, a bistable organ?

The hair cycle and its control remain today an object of debate. A number of factors, which can modulate this process, have been identified but its choreography remains elusive. For years, the hunt for the conductor has been on, but nobody ever caught him. Intuitively, the process being considered as cyclic, an automaton controlling this cycle should be looked for, by analogy with a clock. However, the putative hair follicle oscillator that would control hair cycle failed to be identified and characterized. In fact, we have revealed that human hair follicle has an autonomous behaviour and that the transitions from one phase to the next occur independently for each follicle, after time intervals given stochastically by a lognormal distribution characterized by a mean and a variance. From this analysis, one can conclude that instead of a cyclical behaviour with an intrinsic automaton, a bistable steady state controls human hair follicle behaviour, which under a stochastic way jumps from the dormant to the active steady state and vice versa.

Modulation in proteolytic activity is identified as a hallmark of exogen by transcriptional profiling of hair follicles

Exogen is the process by which the hair follicle actively sheds its club fiber from the follicle. However, little is known about signals that govern the cellular mechanisms of shedding. Here, we have identified factors that are important in regulating either the retention or release of the hair club fiber from its epithelial silo within the follicle. Using the vibrissa follicle as our model, we isolated follicle segments containing club fibers and surrounding follicle tissue at different time points before their natural release from the hair follicle. We then performed microarray analysis to identify key molecular changes as the club fiber approached final release. Among the different classes of genes that were identified, we found changes in the expression pattern of protease inhibitors and proteases, suggesting that proteolysis may mediate fiber release, either through terminal differentiation or proteolytic cleavage. We validated transcriptional changes using reverse transcription-PCR, and further immunofluorescence analysis indicated that protease inhibitors surrounding the club fiber may have an important role in regulating the process of club fiber shedding. Our findings also highlighted that molecular differentiation of the innermost layer of cells immediately surrounding the club fiber, the companion(CL), is likely to be important in hair shedding.

Normal fur development and sebum production depends on fatty acid 2-hydroxylase expression in sebaceous glands

2-Hydroxylated fatty acid (HFA)-containing sphingolipids are abundant in mammalian skin and are believed to play a role in the formation of the epidermal barrier. Fatty acid 2-hydroxylase (FA2H), required for the synthesis of 2-hydroxylated sphingolipids in various organs, is highly expressed in skin, and previous in vitro studies demonstrated its role in the synthesis of HFA sphingolipids in human keratinocytes. Unexpectedly, however, mice deficient in FA2H did not show significant changes in their epidermal HFA sphingolipids. Expression of FA2H in murine skin was restricted to the sebaceous glands, where it was required for synthesis of 2-hydroxylated glucosylceramide and a fraction of type II wax diesters. Absence of FA2H resulted in hyperproliferation of sebocytes and enlarged sebaceous glands during hair follicle morphogenesis and anagen (active growth phase) in adult mice. This was accompanied by a significant up-regulation of the epidermal growth factor receptor ligand epigen in sebocytes. Loss of FA2H significantly altered the composition and physicochemical properties of sebum, which often blocked the hair canal, apparently causing a delay in the hair fiber exit. Furthermore, mice lacking FA2H displayed a cycling alopecia with hair loss in telogen. These results underline the importance of the sebaceous glands and suggest a role of specific sebaceous gland or sebum lipids, synthesized by FA2H, in the hair follicle homeostasis.

Annotation of sheep keratin intermediate filament genes and their patterns of expression

Keratin IF (KRT) and keratin-associated protein genes encode the majority of wool and hair proteins. We have identified cDNA sequences representing nine novel sheep KRT genes, increasing the known active genes from eight to 17, a number comparable to that in the human. However, the absence of KRT37 in the type I family and the discovery of type II KRT87 in sheep exemplify species-specific compositional differences in hair KRT genes. Phylogenetic analysis of hair KRT genes within type I and type II families in the sheep, cattle and human genomes revealed a high degree of consistency in their sequence conservation and grouping. However, there were differences in the fibre compartmentalisation and keratinisation zones for the expression of six ovine KRT genes compared with their human orthologs. Transcripts of three genes (KRT40, KRT82 and KRT84) were only present in the fibre cuticle. KRT32, KRT35 and KRT85 were expressed in both the cuticle and the fibre cortex. The remaining 11 genes (KRT31, KRT33A, KRT33B, KRT34, KRT36, KRT38-39, KRT81, KRT83 and KRT86-87) were expressed only in the cortex. Species-specific differences in the expressed keratin gene sets, their relative expression levels and compartmentalisation are discussed in the context of their underlying roles in wool and hair developmental programmes and the distinctive characteristics of the fibres produced.

The canine hair cycle - a guide for the assessment of morphological and immunohistochemical criteria

The hair follicle has a lifelong capacity to cycle through recurrent phases of controlled growth (anagen), regression (catagen) and quiescence (telogen), each associated with specific morphological changes. A comprehensive classification scheme is available for mice to distinguish the cycle stages anagen I-VI, catagen I-VIII and telogen. For dogs, such a classification system does not exist, although alopecia associated with hair cycle arrest is common. We applied analogous morphological criteria and various staining techniques to subdivide the canine hair cycle stages to the same extent as has been done in mice. Of all the staining techniques applied, haematoxylin and eosin stain, Sacpic, Masson Fontana and immunohistochemistry for vimentin and laminin proved to be most useful. To evaluate the applicability of our criteria, we investigated skin biopsies from healthy beagle dogs (n=20; biopsies from shoulder and thigh) kept in controlled conditions. From each biopsy, at least 50 hair follicles were assessed. Statistical analysis revealed that 30% of the follicles were in anagen (12% early and 18% late), 8% in catagen (2% early, 5% late and 1% not determinable) and 27% in telogen. Thirty-five per cent of hair follicles could not be assigned to a specific cycle stage because not all follicles within one biopsy were oriented perfectly. In conclusion, this guide will not only be helpful for the investigation of alopecic disorders and possibly their pathogenesis, but may also serve as a basis for research projects in which the comparison of hair cycle stages is essential, e.g. comparative analysis of gene expression patterns.

Intermediate hair follicles: a new more clinically relevant model for hair growth investigations

BACKGROUND

Alopecia causes widespread psychological distress, but is relatively poorly controlled. The development of new treatments is hampered by the lack of suitable human hair follicle models. Although intermediate and vellus hair follicles are the main clinical targets for pharmacological therapy, terminal hair follicles are more frequently studied as smaller hair follicles are more difficult to obtain.

OBJECTIVES

This investigation was designed to quantify in vivo morphological and in vitro behavioural differences in organ culture between matched intermediate and terminal hair follicles, in order to develop a new clinically relevant model system.

METHODS

Microdissected terminal and intermediate hair follicles, from the same individuals, were analysed morphometrically (250 follicles; five individuals), or observed and measured over 9 days of organ culture (210 follicles; six individuals).

RESULTS

Intermediate hair follicles were less pigmented and smaller, penetrating less below the skin surface (mean +/- SEM) (2.59 +/- 0.07 vs. 3.52 +/- 0.10 mm; P = 0.02), with smaller fibre (0.03 +/- 0.002 vs. 0.07 +/- 0.002 mm), connective tissue sheath (0.24 +/- 0.01 mm vs. 0.33 +/- 0.01 mm), bulb (0.19 +/- 0.01 vs. 0.31 +/- 0.01 mm) and dermal papilla (0.06 +/- 0.002 vs. 0.12 +/- 0.01 mm) diameters (P < 0.001). Intermediate hair follicle bulbs appeared 'tubular', unlike their 'bulbous' terminal follicle counterparts. In organ culture they also grew more slowly (0.044 +/- 0.002 vs. 0.067 +/- 0.003 mm per day; P < 0.001), remained in anagen longer (84 +/- 0.03% vs. 74 +/- 0.03% at day 9; P = 0.012) and produced less hair fibre (0.36 +/- 0.02 vs. 0.50 +/- 0.03 mm; P < 0.001) than terminal follicles.

CONCLUSIONS

Smaller intermediate hair follicles showed major morphological differences from terminal follicles in vivo and retained significant, biologically relevant differences in vitro in organ culture. Therefore, intermediate hair follicles offer a novel, exciting, more clinically relevant, albeit technically difficult, model for future investigations into hair growth. This should be particularly important for developing new therapies.