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Experimental Dermatology Jul 2018The secondary hair germ (SHG)-a transitory structure in the lower portion of the mouse telogen hair follicle (HF)-is directly involved in anagen induction and eventual... (Review)
Review
The secondary hair germ (SHG)-a transitory structure in the lower portion of the mouse telogen hair follicle (HF)-is directly involved in anagen induction and eventual HF regrowth. Some crucial aspects of SHG functioning and ontogenetic relations with other HF parts, however, remain undefined. According to recent evidence (in contrast to previous bulge-centric views), the SHG is the primary target of anagen-inducing signalling and a source of both the outer root sheath (ORS) and ascending HF layers during the initial (morphogenetic) anagen subphase. The SHG is comprised of two functionally distinct cell populations. Its lower portion (originating from lower HF cells that survived catagen) forms all ascending HF layers, while the upper SHG (formed by bulge-derived cells) builds up the ORS. The predetermination of SHG cells to a specific morphogenetic fate contradicts their attribution to the "stem cell" category and supports SHG designation as a "germinative" or a "founder" cell population. The mechanisms of this predetermination driving transition of the SHG from "refractory" to the "competent" state during the telogen remain unknown. Functionally, the SHG serves as a barrier, protecting the quiescent bulge stem cell niche from the extensive follicular papilla/SHG signalling milieu. The formation of the SHG is a prerequisite for efficient "precommitment" of these cells and provides for easier sensing and a faster response to anagen-inducing signals. In general, the formation of the SHG is an evolutionary adaptation, which allowed the ancestors of modern Muridae to acquire a specific, highly synchronized pattern of hair cycling.
Topics: Animals; Biomarkers; Cell Differentiation; Hair; Hair Follicle; Humans; Mice; Models, Biological; Proteins; Signal Transduction; Stem Cells
PubMed: 29672929
DOI: 10.1111/exd.13666 -
Dermatologic Clinics Oct 2000Most hair damage occurs as a result of grooming habits and chemical exposure for cosmetic purposes. An evaluation of hair structure and biology points to the need for... (Review)
Review
Most hair damage occurs as a result of grooming habits and chemical exposure for cosmetic purposes. An evaluation of hair structure and biology points to the need for better protective mechanisms from cuticular damage and UV damage to maintain the cosmetic value of the hair. Through the cooperative efforts of dermatologists and cosmetic chemists, better hair care products can be developed. The dermatologist needs to elucidate the mechanism through which products can enhance hair shaft functioning, whereas the cosmetic chemist needs to identify substances and develop formulations to accomplish the desired end.
Topics: Chemical Phenomena; Chemistry, Physical; Detergents; Hair; Hair Follicle; Hair Preparations; Humans
PubMed: 11059373
DOI: 10.1016/s0733-8635(05)70216-3 -
Seminars in Cell & Developmental Biology Apr 2007The hair follicle attracted significant attention as a model for the investigation of diverse biological problems. Whereas its morphology and the structure of the hair... (Review)
Review
The hair follicle attracted significant attention as a model for the investigation of diverse biological problems. Whereas its morphology and the structure of the hair shaft are known in detail, the molecular biology of this miniorgan is significantly less characterised. Many efforts focussed on the development of the hair follicle and its stem cell reservoir; by contrast, the follicular product, the hair, which is interesting not only in terms of cosmetics was neglected. This review highlights our current knowledge of the control of hair structure and shape with emphasis on mouse hair follicle biology and discusses continuing problems.
Topics: Animals; Forecasting; Hair; Hair Follicle; Mice; Models, Biological
PubMed: 17324597
DOI: 10.1016/j.semcdb.2007.01.005 -
Journal of the American Academy of... Jun 2003The hair follicle is a unique composite organ, composed of epithelial and dermal compartments interacting with each other in a surprisingly autonomous way. This is a... (Review)
Review
The hair follicle is a unique composite organ, composed of epithelial and dermal compartments interacting with each other in a surprisingly autonomous way. This is a self-renewing organ that seems to be a true paradigm of epithelial and mesenchymal interactions. Each of the follicular compartments is endowed with a specific differentiation pathway under the control of an intricate network of growth factors, cytokines, and hormones. As observed for ethnic hairs, even the shape of the hair shaft is intrinsically programmed from the bulb.
Topics: Black People; Hair; Hair Follicle; Humans
PubMed: 12789164
DOI: 10.1067/mjd.2003.279 -
The Australasian Journal of Dermatology Nov 1995The light microscopic and electron microscopic appearances of the normal hair are briefly described. The major hair shaft abnormalities are reviewed in two parts,... (Review)
Review
The light microscopic and electron microscopic appearances of the normal hair are briefly described. The major hair shaft abnormalities are reviewed in two parts, discussing clinical characteristics, light and electron microscopy and, where information is available, the pathogenesis of these rare conditions. Part I describes monilethrix, pili torti and trichorrhexis nodosa.
Topics: Hair; Hair Diseases; Humans; Microscopy, Electron, Scanning
PubMed: 8593104
DOI: 10.1111/j.1440-0960.1995.tb00969.x -
The Journal of Investigative Dermatology Jun 1973
Review
Topics: Adolescent; Adult; Alopecia; Child, Preschool; Female; Genes, Recessive; Genes, Regulator; Hair; Humans; Male; Melanocytes; Morphogenesis; Phenotype; Proteins; Skin
PubMed: 4575897
DOI: 10.1111/1523-1747.ep12702942 -
Journal of the American Academy of... Jul 1991The rational evaluation of hair disorders requires familiarity with follicular anatomy. Hair structure can be easily examined by studying clipped hair shafts, entire... (Review)
Review
The rational evaluation of hair disorders requires familiarity with follicular anatomy. Hair structure can be easily examined by studying clipped hair shafts, entire hairs gently pulled or forcibly plucked from the scalp, and scalp biopsies (sectioned vertically or transversely). Anatomic features will be different depending on whether a given hair is in the anagen, catagen, or telogen phase. Follicle size will also vary, from the minute vellus hair to the long, thick terminal hair. Each follicle can be divided into distinct regions--bulb, suprabulbar zone, isthmus, and infundibulum. Activity growing (anagen) hairs are characterized by a hair matrix surrounding a dermal papilla; inner and outer root sheaths are present and well developed. A catagen hair can be identified by its markedly thickened vitreous layer and fibrous root sheath, which surrounds an epithelial column; above this column, the presumptive club forms. A telogen hair is distinguished by its fully keratinized club, which is surrounded by an epithelial sac. Below this lies the secondary hair germ and condensed dermal papilla, waiting for the mysterious signal that initiates a new life cycle.
Topics: Hair; Hair Diseases; Humans; Specimen Handling
PubMed: 1880235
DOI: 10.1016/0190-9622(91)70167-z -
Clinics in Dermatology 2001
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Journal of Drugs in Dermatology : JDD Jan 2014Eyebrow hair serves many important biologic and aesthetic functions. This article reviews the structure and function of the hair follicle, as well as hair follicle... (Review)
Review
Eyebrow hair serves many important biologic and aesthetic functions. This article reviews the structure and function of the hair follicle, as well as hair follicle morphogenesis and cycling. Eyebrow hair follicles share the same basic structure as hair follicles elsewhere on the body, but are distinguished by their shorter anagen (growing) phase. Knowledge of the hair follicle structure and cycle is important for understanding the pathophysiology of alopecia, as diseases affecting the stem cell portion of the hair follicle in the bulge region may cause permanent hair loss. Furthermore, therapeutic agents that target distinct phases and hormones involved in the hair cycle may be useful for promoting hair growth.
Topics: Eyebrows; Hair; Hair Diseases; Hair Follicle; Hormones; Humans
PubMed: 24385126
DOI: No ID Found -
Biological Reviews of the Cambridge... Aug 2014Although hair forms (straight, curly, wavy, etc.) are present in apparently infinite variations, each fibre can be reduced to a finite sequence of tandem segments of... (Review)
Review
Although hair forms (straight, curly, wavy, etc.) are present in apparently infinite variations, each fibre can be reduced to a finite sequence of tandem segments of just three types: straight, bent/curly, or twisted. Hair forms can thus be regarded as resulting from genetic pathways that induce, reverse or modulate these basic curvature modes. However, physical interconversions between twists and curls demonstrate that strict one-to-one correspondences between them and their genetic causes do not exist. Current hair-curvature theories do not distinguish between bending and twisting mechanisms. We here introduce a multiple papillary centres (MPC) model which is particularly suitable to explain twisting. The model combines previously known features of hair cross-sectional morphology with partially/completely separated dermal papillae within single follicles, and requires such papillae to induce differential growth rates of hair cortical material in their immediate neighbourhoods. The MPC model can further help to explain other, poorly understood, aspects of hair growth and morphology. Separate bending and twisting mechanisms would be preferentially affected at the major or minor ellipsoidal sides of fibres, respectively, and together they exhaust the possibilities for influencing hair-form phenotypes. As such they suggest dialectic for hair-curvature development. We define a natural-dialectic (ND) which could take advantage of speculative aspects of dialectic, but would verify its input data and results by experimental methods. We use this as a top-down approach to first define routes by which hair bending or twisting may be brought about and then review evidence in support of such routes. In particular we consider the wingless (Wnt) and mammalian target of rapamycin (mTOR) pathways as paradigm pathways for molecular hair bending and twisting mechanisms, respectively. In addition to the Wnt canonical pathway, the Wnt/Ca(2+) and planar cell polarity (PCP) pathways, and others, can explain many alternatives and specific variations of hair bending phenotypes. Mechanisms for hair papilla budding or its division by bisection or fission can explain MPC formation. Epithelial-to-mesenchymal (EMT) and mesenchymal-to-epithelial (MET) transitions, acting in collaboration with epithelial-mesenchymal communications are also considered as mechanisms affecting hair growth and its bending and twisting. These may be treated as sub-mechanisms of an overall development from neural-crest stem cell (NCSC) lineages to differentiated hair follicle (HF) cell types, thus providing a unified framework for hair growth and development.
Topics: Animals; Hair; Humans; Models, Biological
PubMed: 24617997
DOI: 10.1111/brv.12081