For example, hair on the head protects the skull from the sun. The hair in the nose and ears, and around the eyes eyelashes defends the body by trapping and excluding dust particles that may contain allergens and microbes. Hair of the eyebrows prevents sweat and other particles from dripping into and bothering the eyes.
Hair also has a sensory function due to sensory innervation by a hair root plexus surrounding the base of each hair follicle. Hair is extremely sensitive to air movement or other disturbances in the environment, much more so than the skin surface. This feature is also useful for the detection of the presence of insects or other potentially damaging substances on the skin surface.
This is visible in humans as goose bumps and even more obvious in animals, such as when a frightened cat raises its fur. Of course, this is much more obvious in organisms with a heavier coat than most humans, such as dogs and cats. Hair grows and is eventually shed and replaced by new hair. This occurs in three phases. The first is the anagen phase, during which cells divide rapidly at the root of the hair, pushing the hair shaft up and out.
The length of this phase is measured in years, typically from 2 to 7 years. Finally, during the telogen phase, the hair follicle is at rest and no new growth occurs. At the end of this phase, which lasts about 2 to 4 months, another anagen phase begins.
The basal cells in the hair matrix then produce a new hair follicle, which pushes the old hair out as the growth cycle repeats itself. Hair typically grows at the rate of 0. Basically, the skin is comprised of two layers that cover a third fatty layer. These three layers differ in function, thickness, and strength. The outer layer is called the epidermis ; it is a tough protective layer that contains the melanin -producing melanocytes. The inner root sheath is comprised of three parts: the Henley layer, Huxley layer, and cuticle.
The cuticle, which is the innermost part that it closest to the hair shaft, is made from dead hardened cells and give the hair shaft added protection. The hair shaft is the solitary part of the hair follicle that fully exits the surface of the skin. The hair shaft is made up of three layers: the medulla, cortex, and the cuticle. The medulla is described as an unsystematic and unstructured area located in the innermost region of the hair shaft and is not always present. The cortex , in contrast to the medulla, is highly structured and organized.
The cortex is made up of keratin and is responsible for giving hair its strength and durability, as well as its water uptake. Other immune privileged sites include the anterior chamber of the eye, testis, brain and placenta. Hair follicle IP has a unique characteristic of recurring in a cyclic pattern.
Until recently, the IP of the hair follicle is considered to be restricted to the matrix region during the anagen phase. However, evidence has accumulated that the IP of the hair follicle extends to the bulge region and is present at this site during the entire hair cycle. Since the bulge represents the hair follicle stem cell niche, sustained IP in this region may be essential for the survival of the follicle. Hair follicle IP occurs during anagen [ 30 ]. Thus hair follicle IP is limited to the proximal epithelium of anagen hair follicles.
During anagen, melanogenesis is activated in the hair bulb and suggests that hair follicle melanocyte autoantigens play a key role as potential immune targets [ 28 , 31 ]. Hair shaft pigmentation ensures multiple benefits including UV protection, thermoregulation and sexual perceptions. Furthermore, the hair pigment, melanin, is a potent free-radical scavenger. Melanin production inside the active anagen hair bulb may, therefore, help to buffer cell stress induced by reactive oxygen species.
In contrast to the continuous melanogenesis observed in epidermal melanocytes, follicular melanogenesis is a cyclic phenomenon. It is ceased in early the anagen-catagen transition, restarted with the down-regulation of key enzymes of melanogenesis, followed by hair follicle melanocyte apoptosis.
Hair follicle melanocytes and their precursors reside in the hair matrix and along the outer root sheath of anagen hair follicles. Melanin synthesis is established in lysosome-related organelles named melanosomes. In the precortical matrix, these melanosomes are transferred to the hair shaft keratinocytes and formed a pigmented hair shaft.
The hair follicle also contains melanocyte stem cells, which are located in the bulge and in the secondary hair [ 33 — 35 ]. Hair development is a continuous cyclic process and all mature follicles go through a growth cycle consisting of growth anagen , regression catagen , rest telogen and shedding exogen phases Figure 3.
The duration of the phases changes based on the location of the hair and also personal nutritional and hormonal status and age [ 15 , 33 ]. The hair cycle. The inception of anagen phase is presented by the onset of the mitotic activity in the secondary epithelial germ located between the club hair and dermal papilla in telogen hair follicle [ 5 , 16 ].
The anagen is the active growth phase in which the follicle enlarges and takes the original shape and the hair fiber is produced. Six portion of the anagen stage is demonstrated. Through the anagen I—V, hair stem cells proliferate, encloses the dermal papilla, grow downwards to the skin and begin to proliferate hair shaft and IRS, respectively.
Subsequently, hair matrix melanocytes begin to develop pigment and the form of the hair shaft begins to arise; in anagen VI, hair bulb and adjacent the dermal papilla formation is realized and the new hair shaft appears from the skin.
This phase can last up to 6—8 years in hair follicles [ 1 , 11 , 18 ]. Hair shaft synthesis and pigmentation only take place in anagen [ 11 ].
The degree of axial symmetry within the hair bulb determines the curvature of the final hair structure [ 35 ]. Fiber length is often dependent on the duration of the anagen or actively growing phase of the follicle [ 17 ]. Insulin like growth factor-1 IGF-1 , fibroblast growth factor-7 hepatic growth factor HGF , and vascular endothelial growth factor VEGF are thought to be important for anagen maintenance [ 36 ].
At the end of anagen, mitotic activity of the matrix cells is diminished and the follicle enters a highly controlled involutionary phase known as catagen. Catagen lasts approximately 2 weeks in humans, regardless of the site and follicle type [ 37 ]. During catagen the proximal of the hair shaft is keratinized and forms the club hair, whereas the distal part of the follicle is involuted by apoptosis [ 16 , 38 ].
Catagen phase is consisted of eight different stages. The first sign of catagen is the termination of melanogenesis in the hair bulb. Follicular epithelium, mesenchyme, neuroectodermal cell populations and also perifollicular vascular and neural systems demonstrates cyclic changes in differentiation and apoptosis. However, any apoptosis is occurred in dermal papilla due to the expression of suppressor bcl-2 [ 11 ].
Catagen is a process of bulbar involution. The perifollicular sheath collapses and vitreous membrane thickens. Eventually, the lower hair follicle becomes reduced to an epithelial strand, bringing the dermal papilla into close proximity of the bulge [ 36 ]. The epithelial strand begins to elongate and finally reaches to just below the insertion of pilar muscle.
After the keratinization of the presumptive club hair, the epithelial strands begin to involute and shorten progressively followed by the papilla which condenses, moves upward and locates to rest below the bulge. The column eventually reduces to a nipple and forms secondary hair germ below the club. The club hair itself is formed from cortical and cuticle cells only, and it is characterized by a lack of pigmentation [ 2 , 37 ]. The presence of hairless gene mutation contributes to the failure of dermal papilla migration toward the bulge area in catagen phase [ 3 ].
The telogen stage is defined as the duration between the completion of follicular regression and the onset of the next anagen phase. Telogen stage lasts for 2—3 months. During the telogen stage, the hair shaft is transformed to club hair and finally shed. The follicle remains in this stage until the hair germ which is responsive to anagen initiating signals from the dermal papilla, starts to show enhanced proliferative and transcriptional activity in late telogen, leading to the initiation of anagen [ 2 , 39 ].
Telogen is one of the main targets of hair cycle which is influenced by several modulatory agents like androgens, prolactin, ACTH, retinoids and thyroid hormones [ 40 ]. No unique molecular markers associated with the telogen follicle are determined yet; however, estrogen receptor expression is reported to be limited to the telogen papilla fibroblasts.
Germ cells of telogen follicles also express basonuclin and FGF-5 [ 33 ]. The bone morphogenic protein-4 BMP-4 as a growth factor plays an essential role in suppressing follicular growth and differentiation at telogen stage [ 16 ].
The macro-environment surrounding the hair follicle also takes part in regulating cycle transitions. It is not unusual for human telogen hairs to be retained from more than one follicular cycle and this suggests that anagen and exogen phases are independent.
The shedding period is believed to be an active process and independent of telogen and anagen thus this distinct shedding phase is named exogen [ 16 , 33 ]. Based on the observations: the hair follicle has no need for intact innervation, vascularization or other extrafollicular components to maintain cycling, and the basic oscillator system which controls hair cycling is located presumably in the follicle [ 42 ].
Probably, the hair cycle clock is controlled by regulating the balance of the interactions between the follicle epithelium and the surrounding mesenchyme. In this chapter, the basic anatomy and the amazing and complicated biology of the hair follicle is reviewed. Enhanced knowledge on the normal dynamics of the hair provides understanding the basis of how the follicle behaves during a disease.
However recent progress in our understanding of the biology and pathology of hair follicles should lead more effective therapies for hair disorders.
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