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. 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. 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). [33] 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. consists of several furuncles developing in adjoining hair follicles with multiple drainage sinuses. 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. The consequence is a block in bulge activation and a loss of all subsequent hair cycles. Different hair color and follicle shape affects the timings of these phases. (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 Follicle This is a sophisticated receptacle in the scalp Scalp: The time these phases last varies from person to person. Attached to the follicle is a tiny bundle of muscle fiber called the arrector pili. This is where a new hair develops. The sebaceous gland is consists of a cluster of cells — sebocytes — the sole function of which is to produce sebum. The hair is surrounded by inner and outer root sheaths. Horsley et al. One or more sebaceous glands may be open into a follicle and secrets sebum produced from cell disintegration. [18] When the body is subjected to extreme stress, as much as 70 percent of hair can prematurely enter the telogen phase and begin to fall, causing a noticeable loss of hair. For eyebrows, the cycle is completed in around 4 months, while it takes the scalp 3–4 years to finish; this is the reason eyebrow hair have a much shorter length limit compared to hair on the head. In fact, macrophages have been implicated to be essential in homeostatic hair growth. This curved region is called the infundibulum. 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 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 Hair follicles are epidermal invaginations that project into the dermis (Fig. The hair follicle is a structure which appears like a bag or sac and the hair grows from it. Mechanistically, Blimp1 represses cMyc transcription, likely limiting the input of proliferative progenitors towards the gland from the multipotent stem cell populations of the isthmus [28,31] and the bulge [13,30]. The human hair follicle (HF) is a complex structure that arises developmentally through reciprocal signaling between the dermis and the overlying epidermis. 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. 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 [102]. 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 differentiated hair follicle will eventually contain 20 or more different cell types. 2. Hair Structure and Life Cycle Hair is composed of strong structural protein called keratin. The follicle is the essential unit for the generation u0003of the hair shaft. From: Comprehensive Biotechnology (Second Edition), 2011, David L. Stocum, in Regenerative Biology and Medicine (Second Edition), 2012. Above the isthmus, the outer root sheath curves to join the IFE. The follicle is the essential unit for the generation of hair. Such contact occurs at the end of each hair cycle, as the surrounding dermal sheath shrinks during the apoptotic phase, and drags the DP upward until it comes into contact with the bulge. Hair formation, hair follicle or pilli development, is an excellent example of two distinct developmental processes: epithelio-mesenchymal interactions and pattern formation. [1] 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). Reproduced with permission from Wheater et al., Wheater’s Functional Histology (3rd edn). The 3 layers of hair… 2) […] Hair follicles (Fig. Natural moisturizing factors such as amino acids, lactate, and urea within the stratum corneum are by-products of the differentiation process and help to keep water in this layer, rendering the skin pliable and allowing enzymatic processes. Locations of various stem cell populations in the hair follicle. Fourteen percent of hair follicles are in telogen at any one time and this phase lasts for about three months for individual hair follicles. The base of the hair follicle is called the papilla. Hair follicle has a continuous growth and rest sequence named hair cycle. From drug delivery point of view, sebum may serve as a vehicle for drugs that are soluble in it. The apocrine sweat glands developed at the pilosebaceous follicle are less important as regards to the drug delivery across skin is concerned because of their restricted presence [27]. When a hair is forcibly plucked, or the follicle damaged by chemotherapy, catagen occurs prematurely and telogen is drastically shortened to bring the follicle to anagen quickly. LCs are known to provide defenses against skin pathogens such as Candida albicans [97] and also to help remove melanin from healthy scalp follicles during catagen [98]. FIGURE 64.2. The catagen phase is a short transition stage that occurs at the end of the anagen phase. The first stage is called anagen and is the active growth phase, telogen is the resting stage, catagen is the regression of the hair follicle phase, exogen is the active shedding of hair phase and lastly kenogen is the phase between the empty hair follicle and the growth of new hair.[1]. Although this contact does not appear to be the sole source of stimulation, it seems to be a necessary stimulus to activate one or more bulge cells to divide, resulting in TA cells with decreasing levels of stemness (Costarelis et al., 1990; Taylor et al., 2000). The contribution of the hair follicle to healing of the epidermis following wounding has been appreciated for decades by investigators working with mice and rabbits (Argyris, 1976). Anatomically, the triad of hair follicle, sebaceous gland and arrector pili muscle make up the pilosebaceous unit.[1]. George Cotsarelis, in Principles of Tissue Engineering (Third Edition), 2007. red hair color is pigmented by trichosidern, gray and white is from decreased melanin production and … Intracutaneous transplantation of bone marrow–derived macrophages into the dorsal skin of mice has been shown to strongly induce proliferation of epidermal basal cells and HF cells, including bulge stem cells, suggesting that macrophages can promote anagen [101]. Therefore, the new hair follicle is formed from lateral disc cells that are predetermined for this role during the previous hair cycle. Bottom: A more detailed map of stem cell marker expression domains that includes the Gli-1 expressing domains of the bulge. The predominant cells in the sebaceous gland, sebocytes, secrete lipid-rich products into the infundibular opening of the adjacent hair follicle. Bulb. When a club hair is completely formed, about a 2-week process, the hair follicle enters the telogen phase. 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. Hair follicle morphogenesis occurs only once during fetal development. 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. 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. Well known diseases of the hair follicle include alopecia or hair loss, hirsutism or excess hair growth, and lupus erythematosus. The upper third of the outer root sheath is differentiated into several regions. In this model, stem cells are activated by yet unidentified signals transmitted through direct contact with the DP. For a helpful visual, think of the hair follicle as a vase and the hair as the stem of a flower. Main components of sebum are glycerides, free fatty acids, cholesterol, cholesterol esters, and squalene. This hypothesis was hard to explain because of the cycling nature of the bulb, which undergoes extensive apoptosis in catagen, leaving only a small strand of epithelial cells connecting the DP and the bulge. Finally, some models postulate the presence of multiple populations of nonbulge skin epithelial stem cells that are long-lived and either unipotent or of interchangeable potential (hair cells can make epidermis and/or SG and vice versa) (Watt and Hogan, 2000; Ghazizadeh and Taichman, 2001; Kopan et al., 2002; Watt, 2002). Right, explanatory diagram of structure. Hair … The hair follicle (HF) has a wide range of functions including thermoregulation, physical and immunological protection against external insults, sensory perception, social interactions, and camouflage. [24], A photograph of hair on a human arm emerging from follicles, "Histologic Diagnosis of Inflammatory Skin Diseases, Chapter 1. The lower, cycling part represents the actual hair shaft factory, the anagen bulb (Figure 1A). The hair follicle consists of a complex system of multiple tissue compartments that are clearly distinguishable by their morphology and type of differentiation. Growth cycles are controlled by a chemical signal like epidermal growth factor. Hair follicles remain in late anagen for variable lengths of time, depending on hair type and species. A hair follicle forms a bulb around the specialized dermal cells, the dermal papillae. Ordinarily, hair follicle renewal is maintained by the stem cells associated with each follicle. Each strand of hair is composed of three main layers: the medulla, cortex and cuticle. Each strand of hair consists of three layers. 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). Bulge cells were also stimulated to move into the epidermis following incisional wounds and after tape stripping, indicating that bulge cell activation plays a role in replenishing lost cells from the epidermis after wounding. This is the same kind of protein that makes up the nails and the outer layer of skin. The hair shaft consists of an inner core known as the medulla. Hairs are lost and regenerated by cycling through three phases (Hardy, 1992; Messenger, 1993): (1) catagen, the cessation of hair growth and regression of the hair follicle; (2) telogen, or follicular rest; and (3) anagen, the regeneration of the follicle and a new hair (Fig. The follicle is the essential unit for the generation of hair. Bulge cell progeny migrate to the epidermis after different types of wounding. 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. At the end of the anagen phase an unknown signal causes the follicle to go into the catagen phase. Hair consists of mostly fibrous protein (65-95 %). Embryologic, Histologic, and Anatomic Aspects", "Comparative Study of Human and Animal Hair in Relation with Diameter and Medullary Index", "Dlx3 is a crucial regulator of hair follicle differentiation and cycling", "Bone morphogenetic protein-2 (BMP-2) transactivates Dlx3 through Smad1 and Smad4: Alternative mode for Dlx3 induction in mouse keratinocytes", "What Is Clarifying Shampoo & Does Your Hair Need It? Well known diseases of the hair follicle include alopecia or hair loss, hirsutism or excess hair growth and lupus erythematosus. Hair is made of a tough protein called keratin. This is the section of the hair follicle from which the hair shaft is developed. ", https://en.wikipedia.org/w/index.php?title=Hair_follicle&oldid=1001290346, Articles to be expanded from January 2018, Creative Commons Attribution-ShareAlike License, Dark brown/black haired/red haired Caucasian, A root sheath composed of an external and internal root sheath. The result of this differentiation is so-called nucleus-free corneocytes, i.e., flat platelet-like protein structures embedded in a lipid matrix forming the stratum corneum. The inner root sheath cells extend to the dermal papillae, where they are mitotically active and produce cells that migrate upward. For example, terminal hairs grow on the scalp and lanugo hairs are seen covering the bodies of fetuses in the uterus and in some newborn babies. The hair bulb forms the base of the hair follicle. It consists of the Hair matrix cells, as well as melanocytes, (pigment forming cells), which surround the Dermal Papilla. 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). This muscle is responsible for causing the follicle lissis to become more perpendicular to the surface of the skin, and causing the follicle to protrude slightly above the surrounding skin (piloerection) and a pore encased with skin oil. A hair follicle forms a bulb around the specialized dermal cells, the dermal papillae. The former is a prerequisite for wound repair and the latter is a fundamental component of authentic skin regeneration. 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. Bottom panels, reproduced with permission from Brownell et al., Nerve-derived sonic hedgehog defines a niche for hair follicle stem cells capable of becoming epidermal stem cells, Cell Stem Cell 8:552–565. 6.3) are comprised of pockets of epithelium, which are continuous with the superficial epidermis and extend deep into the dermis. Together, the dermal papilla and the matrix constitute the bulb of the hair follicle. DLX3 is a crucial regulator of hair follicle differentiation and cycling.[14][15]. [12] This damage response involves the proteolysis of type XVII collagen by neutrophil elastase in response to the DNA damage in the hair follicle stem cells. The hair follicle is an organ found in mammalian skin. Ali et al. A hair follicle anchors each hair into the skin. It consists of many epithelial cells. The turnover time of epidermal cells is estimated to be approximately 28 days, but it is much longer in aged skin [10]. As the follicle extends to its maximum length, matrix cells proliferate and differentiate into the cells of the new hair. The eccrine-sweat glands produce sweat (pH ranges from 4.0 to 6.8), the main function of which is to aid heat control. The epidermis is interrupted by hair follicles and sweat glands, except at the palms and soles where no hair follicles are present. 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. Up to 90% of the hairs on the human scalp are in anagen at any one time, and this phase can occupy anywhere from 2–7 years for a given hair. [1] The scalp hair follicle in Caucasians is elliptical in shape and, therefore, produces straight or wavy hair, whereas the scalp hair follicle of people of African descent is more curvy, resulting in the growth of tightly curled hair. D.T. Anagen phase, 2–8 years (occasionally much longer), This page was last edited on 19 January 2021, at 01:41. L. Guo, in Biomaterials for Treating Skin Loss, 2009. Interestingly, they found that LCs were almost completely absent from the epidermis of scalps from subjects with lichen planopilaris, a condition characterized by inflammation and destruction of the HF bulge, corroborating the portal role of follicles and also hinting at the potential of immunomodulation as a treatment of lichen planopilaris. Your hair structure can be divided into two parts: 1. Papilla Apocrine glands which are found only in the axilla and anogenital regions, produce scents (known in some vertebrate species to function as pheromones.). Furthermore, Castellana et al. 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). Many of the mutations identified in these genes were linked to hair diseases in humans. 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 … [2], The position and distribution of hair follicles changes over the body. Bulge stem cells are then activated to self-renew and replenish the hair germ stem cells. [1] The process of hair growth occurs in distinct sequential stages. 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). 1) An innermost layer or medulla which is only present in large thick hairs. The hair follicle regenerates by means of heterogeneous populations of adult stem cells located in distinct anatomical niches of the bulge and the isthmus of the follicle (Fuchs, 2009; Barker et al., 2010) (Fig. The walls of the hair follicle above the matrix form the inner and outer root sheaths. [1] It resides in the dermal layer of the skin and is made up of 20 different cell types, each with distinct functions. Because many enzymes involved in epidermal differentiation are pH dependent, pH also exerts direct effects on the stratum corneum cohesion, desquamation, and permeability homeostasis [12]. LRC = quiescent label-retaining cells of the bulge. Cells expressing the transcriptional repressor Blimp1 comprise unipotent sebocyte progenitors [33]. For a number of years, epidermal stem cells were believed to reside exclusively at the tip of rete ridges within the basal cell layer. Together, the dermal papilla and the matrix constitute the bulb of the hair follicle. 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