This results in the follicles not producing any new hairs. According to the U. S National Library of Medicine, 50 million men and 30 million women are affected by androgenetic alopecia. Alopecia areata is an autoimmune disease. The immune system mistakes the hair follicles for foreign cells and attacks them. It often causes hair to fall out in clumps. It can lead to alopecia universalis, which is a total loss of hair all over the body. No known cure exists for alopecia areata yet, but steroidal injections or topical treatments can slow down hair loss.
Folliculitis is an inflammation of the hair follicles. It can occur anywhere hair grows, including your:. Folliculitis often looks like a rash of small bumps on your skin. The bumps may be red, white, or yellow and they can contain pus.
Often, folliculitis is itchy and sore. Folliculitis is often caused by a staph infection. Folliculitis can go away without treatment, but a doctor can diagnose you and give you medication to help manage it. This can include topical treatments or oral medications to treat the cause of the infection and soothe the symptoms. Telogen effluvi um is a temporary, but common form of hair loss.
A stressful event causes hair follicles to go into the telogen phase prematurely. This causes the hair to thin and fall out. The hair often falls out in patches on the scalp, but in extreme cases, it can fall out in other places on the body, including on the legs, eyebrows, and pubic region. It shields the body against heat, light, injury, and infection. The skin also helps regulate body temperature, gathers sensory information from the environment, stores water , fat, and vitamin D , and plays a role in the immune system protecting us from disease.
The color, thickness and texture of skin vary over the body. Only few differentiated corneocytes remain and the invagination of the epidermis in this area must be considered as highly permeable for topically applied compounds [ 19 ].
Hair follicle stem cells are thought to reside in the bulge area on the isthmus close to the insertion of the arrector muscle [ 20 ]. Lineage studies have proven that bulge cells are multipotent and that their progeny generate the new lower anagen hair follicle [ 21 ]. One of the most distinguishing features of stem cells is their slow-cycling nature, presumably to conserve their proliferative potential and to minimize DNA errors that could occur during replication.
They migrate in a downward direction. On entering the hair bulb matrix, they proliferate and undergo terminal differentiation to form the hair shaft and inner root sheath. They also migrate distally to form sebaceous glands and to proliferate in response to wounding [ 16 , 20 , 22 ].
The suprabulbar region of the follicle, below the isthmus and above the hair bulb, is comprised of three layers from outermost to innermost: outer root sheath, inner root sheath and hair shaft Figure 2. Diagram of proximal hair follicle. Outer root sheath ORS extends from the epidermis at the infundibulum and continues to the hair bulb and its cells change considerably throughout the follicle.
In the infundibulum, it resembles epidermis, whereas in the isthmus level, ORS cells begin to keratinize in a trichilemmal mode. Keratinocytes in the ORS form the bulge area at the base of the isthmus. At the lower tip of the hair bulb it consists of a single layer of cuboidal cells, becoming multilayered in the region of the upper hair bulb.
In some follicles, there is a distinct single cell layer interposed between the outer and inner root sheaths, known as the companion layer [ 23 ]. Companion layer cells show numerous intercellular connections to the inner root sheath and are thought to migrated distally along with the inner root sheath to the isthmus region and to form the plane of slippage between the inner and outer root sheaths [ 1 , 3 , 14 , 16 ].
These cells take place in certain functions of the follicle such as acting as a sensory organ and serving as an immunologic sentinel for the skin [ 5 ]. The innermost layer is the cuticle of IRS whose cells interlock with those of the hair cuticle. This connection, anchoring the hair shaft to the hair follicle, is so tight.
The inner root sheath hardens before the presumptive hair within it, and so it is thought to control the definitive shape of the hair shaft. Each of the three layers of IRS undergoes abrupt keratinization. This occurs at different levels in each layer; however, the patterns of change are similar. The expanded onion-shaped portion of the lower hair follicle, including the hair matrix and the follicular papilla is known as the hair bulb which is the active reproductive portion of the hair follicle.
The hair bulb encloses folicular dermal papilla, mucopolysaccharide-rich strome, nerve fiber and capillary loop. The matrix cells are localized to the lowermost portion of the follicle and surround all sides of the follicular papilla.
The hair shaft and IRS are derived from the matrix cells. The IRS is derived from the lower and laterally located matrix cells, whereas the hair shaft is originated from upper and centrally located cells. In addition to producing the main structural components of hair, they also produce the hair keratins, and their associated proteins KAPs [ 24 ].
Melanocytes reside among matrix stem cells to produce the pigment of the hair. During their differentiation phase, matrix cells phagocytose melanin or pheomelanin from the dendritic elongations of melanocytes.
The hair assumes its color via the amount and the type of the phagocytized major pigment [ 1 , 3 , 16 , 25 ]. Follicular papilla, which is derived from a condensation of mesenchymal cells at the early stages of follicular embryogenesis, is one of the most important players during the induction and maintenance of the follicular epithelial differentiation. It is responsible for determining the follicle type.
The volume and secretory activity of follicular papilla and also the number of matrix stem cells determine the size of the anagen hair bulb, the duration of anagen phase and the diameter of the hair shaft [ 11 , 26 , 27 ].
Moreover the follicular papilla is an essential source of growth factors [ 1 , 3 , 16 , 28 ]. Keratin proteins can be divided into two major families: the type I acidic keratins and the type II basic-neutral keratins. About 54 functional keratin genes 28 type I and 26 type II keratins have been identified to date. There are 11 type I hair keratins, designated K31—K40, and 6 type II hair keratins, designated K81—K86, and the remainder are epithelial keratins [ 24 ].
The keratin-associated proteins KAP , is a large group of proteins which constitutes the matrix of the keratin. The matrix proteins are separated to three major subgroups according to their amino acid compositions [ 29 ]. Different hair and epithelial keratins are expressed in the various concentric layers of the hair follicle, with hair keratins found primarily in the cortex and hair cuticle [ 1 , 2 ].
Nerves related to the hair follicle are identical to the dermal nerve network including sensory afferents and autonomic sympathetic nerves. Smaller nerve fibers form a circular layer around the bulge area of terminal follicles and the bulb area of vellus follicles. There are several types of nerve endings associated with the hair follicle: free nerve endings, lanceolate nerve endings, Merkel cells and pilo-Ruffini corpuscles. Each nerve ending responds to distinct stimulus.
Free nerve endings transmit pain, lanceolate nerve endings detect acceleration, Merkel cells responsible of pressure sensation and pilo-Ruffini corpuscles detect tension. Perifollicular nerves related neuromediator and neuropeptides, that is, substance P, calcitonin gene-related peptide influence follicular keratinocytes and hair follicle cycling [ 1 , 3 , 16 ]. Cutaneous vascularization is provided by arterioles, which are concentrated at the lower portion of the hair follicle and compose vascular network.
During the hair cycle phases, there are some alterations in the density of perifollicular vascularization due to the upregulation of vascular endothelial growth factor expression [ 1 ]. The immunology of hair is very amazing and complicated. The hair follicle represents an immune privileged IP site, which is defined basically as a location in the body where foreign tissue grafts can survive for longer periods of time without immune rejection.
This specialized immune environment of IP is required to prevent destructive immune reactions in critical regions. 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. 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. On average, 50 hairs are lost and replaced per day. Hair loss occurs if there is more hair shed than what is replaced and can happen due to hormonal or dietary changes.
Hair loss can also result from the aging process, or the influence of hormones. Similar to the skin, hair gets its color from the pigment melanin, produced by melanocytes in the hair papilla. Different hair color results from differences in the type of melanin, which is genetically determined. Improve this page Learn More. Skip to main content. Module The Integumentary System. Search for:. Hair Learning Outcomes Describe the structure and function of hair.
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