Horsley et al. There are many human diseases in which abnormalities in hair appearance, texture or growth are early signs of local disease of the hair follicle or systemic illness.  It signals the end of the active growth of a hair. They also found that the production of CCL2 and CCL20 chemokines by HF keratinocytes is responsible for the recruitment of LCs and pre-LCs to the epidermis, further illustrating HF keratinocytes as a potent source of proteins regulating immune cell trafficking into the skin. Later in anagen, this “traffic stop” appeared to be removed, and cells at the base of the bulb seemed to progress to become matrix and then differentiate into IRS and hair shaft cells. Of all mammals, humans have the longest growth phase of scalp hair compared to hair growth on other parts of the body. Each strand of hair is composed of three main layers: the medulla, cortex and cuticle. During this phase, newly activated bulge cells give rise to the ORS and replenish the lateral disc (Panteleyev et al., 2001). Based on the recent H2B-GFP LRC study, it would seem that a modification of the predetermination model is now warranted to explain how, possibly as a secondary step following the initial “lateral disk” activation, the H2B-GFP LRCs are stimulated to exit the bulge area, proliferate, and participate in the formation of the matrix, all within a single anagen (Tumbar et al., 2004). During catagen, cells in the follicle wall undergo apoptosis and the hair follicle regresses upward so that the dermal papilla contacts the bulge. Melanocytes are mixed amongst the matrix cells to provide the hair shaft with color. The structure and chemical composition of hair provides information about the interactions between drugs 1 that enter the hair follicle 2 and the hair itself. The pilosebaceous apparatus is composed of a hair follicle, sebaceous gland and associated arrector pili muscle. 6.3) are comprised of pockets of epithelium, which are continuous with the superficial epidermis and extend deep into the dermis. Cells expressing the transcriptional repressor Blimp1 comprise unipotent sebocyte progenitors . Hair Follicle This is a sophisticated receptacle in the scalp For example, the skin of the palms and soles does not have hair follicles whereas skin of the scalp, forearms, legs and genitalia has abundant hair follicles. A hair follicle forms a bulb around the specialized dermal cells, the dermal papillae. While in transit along the basement membrane surrounding the ORS, these cells were not able to form colonies or engraft successfully, perhaps because of their low concentration. Hair follicle (replete with its stem cell niche) is not a trivial structure in Homo sapiens. Reproduced with permission from Wheater et al., Wheater’s Functional Histology (3rd edn). The hair shaft consists of a cortex and cuticle cells, and a medulla for some types of hairs. Further studies showed that macrophage apoptosis in skin promotes premature anagen entry, possibly through release of Wnt ligands. Mammalian HFs are essential to homeostasis, regulation of body temperature, and the generation of hair shafts for protection against pathogens and physical stimuli. Well known diseases of the hair follicle include alopecia or hair loss, hirsutism or excess hair growth, and lupus erythematosus. As a result, your hair grows longer. Known as “hair follicle predetermination,” this model is based on multiple studies of hair follicle growth (Panteleyev et al., 2001), and although speculative, it offers an alternative explanation for conflicting results (Silver et al., 1969; Morris and Potten, 1999). 12 Some structural proteins are indeed asymmetrically expressed including K38, 13 K82 12 and insulin‐like growth factor binding protein 5. A seemingly difficult task of classifying and stratifying various follicular tumours can be made easier by grouping various follicular tumours according to lines of differentiation pertaining to the specific anatomical compartments within the hair follicle. The structure of the hair shaft that is present above the epidermis.  This complex interaction induces the hair follicle to produce different types of hair as seen on different parts of the body. The upper third of the outer root sheath is differentiated into several regions. Stem cells in the bulge region of the HF and the dermal papilla cells are the main regulators of the hair cycles. The mammalian hair follicle unit consists of a central follicle and a series of associated structures: sebaceous glands, arrector pili muscles, Merkel cells, and sensory nerve endings. During telogen, the lateral disc maintains contact with the DP, and, upon the next anagen, it proliferates (upward) and forms the new matrix and the inner layers of the hair follicle.  There are many structures that make up the hair follicle. The dermal papilla shrinks in volume during catagen and telogen, but it is not known if this is due to cell loss or compaction (Matsuzaki and Yoshizato, 1998). The 3 layers of hair… At the end of the anagen phase an unknown signal causes the follicle to go into the catagen phase. Both the HF bulb and bulge are considered to be immune-privileged sites, and collapse of these areas have been associated with inflammatory disorders resulting in hair loss [94,95], how and why the HF interacts with immune cells is still largely uncharted territory. As anagen progresses, stem cells in the hair germ are activated first by signals from the dermal papilla to proliferate and form a transit amplifying population that moves downward to reconstitute the matrix.  It resides in the dermal layer of the skin and is made up of 20 different cell types, each with distinct functions.  The process of hair growth occurs in distinct sequential stages. When cells exit the bulge and migrate upward, they take on the fate of a TA epidermal or SG cell. In addition to its role in defining human appearance, scalp hair also provides protection from UV sun rays and is an insulator against extremes of hot and cold temperatures. The lower most portion of a mature hair follicle (bulb) contains mitotically active germinative cells (which produce the hair shaft in a manner somewhat analogous to epidermal keratinization).14 Eccrine sweat glands main function is thermoregulation, which is accomplished through excreting sweat. A Must-Read: Toenail Clippers for Arthritic Hands. Bulge cell progeny migrate to the epidermis after different types of wounding. The key rate-limiting role of Blimp1 in sebaceous gland homeostasis was revealed following epithelial Blimp1 deletion, which led to sebaceous gland hypertrophy and oily hair coat phenotype .  found a functional link between skin-resident regulatory T cells (Tregs) and HF physiology. Ordinarily, hair follicle renewal is maintained by the stem cells associated with each follicle. Hair follicles remain in late anagen for variable lengths of time, depending on hair type and species. Figure 3.3. The follicle is the essential unit for the generation of hair.  The club hair is the final product of a hair follicle in the telogen stage, and is a dead, fully keratinized hair. , Hair follicles form the basis of the two primary methods of hair transplantation in hair restoration, Follicular Unit Transplantation (FUT) and follicular unit extraction (FUE). The architecture of this multicellular structure is highly polarized with respect to the body axes. The cells of the upper Gli 1 domain are the only bulge cells capable of contributing to regenerated epidermis after injury. During telogen, the epithelial cells are in a resting state and the hair shaft remains within the shortened follicle. The differentiated hair consists of keratinocytes that form a medullary core surrounded by a cortex. Hair formation, hair follicle or pilli development, is an excellent example of two distinct developmental processes: epithelio-mesenchymal interactions and pattern formation. This “acid mantle” facilitates the growth of the residual skin flora, hereby inhibiting pathologic microbial colonization. This is the section of the hair follicle from which the hair shaft is developed. The follicle is preparing to return to the anagen phase. Tudorita Tumbar, Elaine Fuchs, in Handbook of Stem Cells (Second Edition), 2013. Top panels, reproduced with permission from Barker et al., Tissue resident adult stem cell populations of rapidly self-renewing organs, Cell Stem Cell 7:656–670. The follicle is the essential unit for the generation of hair. Stages in the adult hair regeneration cycle, clockwise from early anagen. This phase lasts for about 2–3 weeks while the hair converts to a club hair. This suggests that macrophages play critical roles in injury response via activation of HFSCs during anagen onset as well as HF regeneration following wounding. This cap is called the matrix, and its cells are transit-amplifying cells that differentiate into the hair shaft. Then hair cells undergo apoptosis during catagen to retract the dermal papilla toward the hair germ to achieve the telogen resting phase, whereupon the cycle begins again. The hair follicle is an organ found in mammalian skin. The outer root sheath cells are continuous with the basal epidermal cells and do not come into direct contact with the dermal papillae. The hair follicle is a diagonal tube that contains the hair root. Understanding the cellular basis of skin growth, Principles of Tissue Engineering (Fifth Edition), Principles of Tissue Engineering (Fourth Edition), Nanocomposite for transdermal drug delivery, Applications of Nanocomposite Materials in Drug Delivery, Ahmad et al., 1998; Li et al., 2000, 2001, Silver et al., 1969; Morris and Potten, 1999, Watt and Hogan, 2000; Ghazizadeh and Taichman, 2001; Kopan et al., 2002; Watt, 2002, Principles of Tissue Engineering (Third Edition), The pathophysiologic basis for wound healing and cutaneous regeneration, Jan Kottner, ... Ulrike Blume-Peytavi, in, Innovations and Emerging Technologies in Wound Care. The bulge is located in the outer root sheath at the insertion point of the arrector pili muscle. observed an apoptosis-induced decrease in the number of F4/80+ CD11b+ Gr1+ skin-resident macrophages before the onset of anagen and showed that pathways regulating HFSC activation, such as β-catenin/Wnt signaling, are upregulated at this stage . Also attached to the follicle is a sebaceous gland, which produces the oily or waxy substance sebum. The function of hair in humans has long been a subject of interest and continues to be an important topic in society, developmental biology and medicine. Excisional wounding with a 4-mm (punch) trephine resulted in the migration of bulge cell progeny into the healing epidermis. The walls of the hair follicle above the matrix form the inner and outer root sheaths. (C) Contribution of stem cells to epidermal repair after injury. There are ethnic differences in several different hair characteristics. The turnover time of epidermal cells is estimated to be approximately 28 days, but it is much longer in aged skin . Apocrine glands which are found only in the axilla and anogenital regions, produce scents (known in some vertebrate species to function as pheromones.). In the first step, the hair germ cells proliferate, followed in the second step by proliferation of bulge cells, pushing the dermal papilla downward to reconstitute the hair bulb and matrix. Your hair is made of two parts - the hair follicle and the hair shaft. While the unquestionable importance of macrophages in wound healing has been known for many years, we are only beginning to elucidate their roles in HF and HFSC regeneration. With induced mechanical stress, they observed infiltration and accumulation of eGFP+ LCs near follicular structures, suggesting that HFs are important portals for the entry of LCs . Structure of the hair follicle. (2001) discovered that stem cells or their more or less committed progeny appeared to migrate along the ORS to come in contact with the DP, which then seemed to signal their pathway to differentiate. Hair follicles (Fig. The hair bulb is the base of every follicle, and it’s a cluster of cells that reproduce to make new hair cells. The hair follicle may be divided anatomically into four parts: The bulb consisting of the dermal papilla and matrix The suprabulbar area from the matrix to the insertion of the arrector pili muscle The isthmus that extends from the insertion of the arrector pili muscle to the sebaceous gland LCs are known to provide defenses against skin pathogens such as Candida albicans  and also to help remove melanin from healthy scalp follicles during catagen . Stem cells in the lower and middle bulge regenerate the hair, while stem cells in the upper bulge and isthmus renew the non-hair parts of the follicle (Brownell et al., 2011). Figure 3.2. Different hair color and follicle shape affects the timings of these phases. Mutations in a number of genes, including those encoding the transcription factors hairless and RXRα, result in a failure of the DP to be dragged upward at the end of the first postnatal hair cycle (Ahmad et al., 1998; Li et al., 2000, 2001). The hair that you see is called the hair shaft. It consists of connective tissue and a capillary loop, or tiny blood vessel. Just above the bulge, a region of the outer root sheath called the isthmus evaginates to form the sebaceous gland. The higher the density of the hair, the more sebaceous glands that are found. These follicles are extracted from donor areas of the scalp, or other parts of the body, which are typically resistant to the miniaturization effects of the hormone DHT. Hero Images / … The differentiated hair consists of keratinocytes that form a medullary core surrounded by a cortex. Hair follicles develop shafts through a continuous cyclic process. We have synthesized hair follicle-specific keratins from the companion layer (K6hf, K17) and the hair cortex (Ha1, Hb3, Hb6) in Escherichia coli. L. Guo, in Biomaterials for Treating Skin Loss, 2009. A third model has come from an integrative reassessment of the literature, with a novel interpretation of the accumulated data. Parts of the hair and skin are a follows: Papilla This is a group of specialized cells that are intertwined with capillaries. Right, explanatory diagram of structure. , A photograph of hair on a human arm emerging from follicles, "Histologic Diagnosis of Inflammatory Skin Diseases, Chapter 1. The predominant cells in the sebaceous gland, sebocytes, secrete lipid-rich products into the infundibular opening of the adjacent hair follicle. A club hair is formed during the catagen phase when the part of the hair follicle in contact with the lower portion of the hair becomes attached to the hair shaft. ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. URL: https://www.sciencedirect.com/science/article/pii/B9780123848604000034, URL: https://www.sciencedirect.com/science/article/pii/B9781845693633500067, URL: https://www.sciencedirect.com/science/article/pii/B978012818422600071X, URL: https://www.sciencedirect.com/science/article/pii/B9780123983589000756, URL: https://www.sciencedirect.com/science/article/pii/B9780128137413000169, URL: https://www.sciencedirect.com/science/article/pii/B9780128012383056245, URL: https://www.sciencedirect.com/science/article/pii/B9780123859426000640, URL: https://www.sciencedirect.com/science/article/pii/B9780123706157500792, URL: https://www.sciencedirect.com/science/article/pii/B9781845693633500043, URL: https://www.sciencedirect.com/science/article/pii/B9780128150283000110, Comprehensive Biotechnology (Second Edition), 2011, Regenerative Biology and Medicine (Second Edition). Denise Gay, ... George Cotsarelis, in Principles of Tissue Engineering (Fourth Edition), 2014. A hair follicle is a stocking-like structure that contains cells and connective tissue and surrounds the root of a hair. For a number of years, epidermal stem cells were believed to reside exclusively at the tip of rete ridges within the basal cell layer. Hair follicle consists of 3 parts : the lower portion, from the base of the follicle including hair bulb to the insertion of the arrector pili muscle or buldge ; the isthmus, from the insertion of the arrector pili to the entrance of the sebaceous duct, and the infundibulum, from … It may serve several functions, including seasonal thermal adaptation, cleansing the body surface, elimination of defective hair follicles, and protection from malignancy (Cotsarelis, 1997; Courtois et al., 1996; Paus, 1998; Randall and Ebling, 1991). When a club hair is completely formed, about a 2-week process, the hair follicle enters the telogen phase. For example, anagen lasts for only 4–7 months in eyebrow hair follicles, so these hairs grow to much shorter lengths than scalp hairs (Paus, 1998). Copyright © 2021 Elsevier B.V. or its licensors or contributors. Ali et al. Recently, an additional epithelial stem cell niche has been identified in the bulge region of the hair follicle, near the insertion site of the arrector pili muscle.15 There has been active interest in these populations, with respect to their contributions to re-epithelialization, as well as to their potential to differentiate along adnexal lines. The hair follicle regulates hair growth via a complex interaction between hormones, neuropeptides, and immune cells.  found that Blimp1+ progenitors give rise to terminally differentiated Pparγ+ sebocytes via transient amplifying progenitors, yet they do not contribute progeny toward interfollicular epidermis or hair follicles. To better understand trafficking of these dendritic cells into the skin, Nagao et al. Jan Kottner, ... Ulrike Blume-Peytavi, in Innovations and Emerging Technologies in Wound Care, 2020. There is also a shedding phase, or exogen, that is independent of anagen and telogen in which one or several hairs that might arise from a single follicle exits. Growth cycles are controlled by a chemical signal like epidermal growth factor. In this model, during mid and late anagen, SCs are stimulated to leave the bulge niche and migrate away to form the ORS but not the matrix or the rest of the hair. The telogen phase is short if not absent from vibrissae, and a new vibrissae cycle is initiated before the DP has moved upward completely to come into contact with the bulge. Attached to the follicle is a tiny bundle of muscle fiber called the arrector pili. depleted LCs in Langerin-DTR (diphtheria toxin receptor) mice by injection of diphtheria and then undertook bone marrow reconstitution with CAG-eGFP bone marrow to follow eGFP-labeled LC differentiation and skin reconstitution in vivo. Papilla The hair shaft is formed by the proliferation and differentiation of the matrix cells upward through the lumen of the follicle. Early data on hair follicle growth and stem cell function suggested that the bulb of the hair follicle, containing matrix cells, is the residence of stem cells (see Costarelis et al., 1990 and references therein). During catagen and early anagen, these bulge cell derivatives accumulated at the base of the hair follicle in a relatively undifferentiated state. , While hair transplantation dates back to the 1950s, and plucked human hair follicle cell culture in vitro to the early 1980s, it was not until 1995 when hair transplantation using individual follicular units was introduced into medical literature. A hair follicle anchors each hair into the skin. There are many structures that make up the hair follicle. When Ly6C+ macrophages were depleted in the wound region, both WIH-A and WIHN were attenuated. When matrix TA cells cease dividing, perhaps through exhaustion of their proliferative capacity, the bulb of the hair follicle undergoes apoptosis (catagen phase). Prior to the start of cycling is a phase of follicular morphogenesis (formation of the follicle). Structure Inside Follicle A. Their work raises the novel possibility that immunomodulation of Tregs may be useful in devising new therapeutics for immune-mediated skin disorders such as alopecia areata. Hair follicle associated with sebaceous gland and sweat glands such as eccrine and apocrine constituted skin appendages. When they migrate downward during early anagen, they give rise to a population of TA ORS and matrix cells that in turn further specialize to form the IRS, cortex, and medulla (Costarelis et al., 1990; Taylor et al., 2000). Another important cutaneous immune cell population are macrophages. Using the K15CrePR;R26R-transgenic mouse, bulge cells were labeled in adult mice (Ito et al., 2005). The inner root sheath cells extend to the dermal papillae, where they are mitotically active and produce cells that migrate upward. The hair cycle (adopted with changes from Panteleyev et al., 2001 8).The HF consists of a permanent (upper) portion and a cycling lower portion that undergoes cycles of destruction and restoration (see drawing for Anagen IV).The stages of HF regression (catagen) and growth (anagen) are separated by the “resting” phase—telogen. These epidermal cells must divide more frequently than bulge LRCs to account for why they were much less brightly labeled with H2B-GFP in the pulse and chase experiments outlined above (Tumbar et al., 2004). (B) Summary of stem cell domains and markers (color) and their contribution to regeneration of the follicle and interfollicular epidermis. In investigating this process, Oshima et al. They are contiguous with the basal epidermal layer but are distinct and self-propagating. The lower, cycling part represents the actual hair shaft factory, the anagen bulb (Figure 1A). 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