The presence of hair is a primary differentiator of mammals as a unique class of organisms. In humans, it is a cherished and highly visible indicator of health, youth, and even class. It has a sensory function, protects from cold and UV radiation, and can have a significant psychological impact when its growth or structure is deranged. At a microscopic level, the variety in length, color, diameter, and cross-sectional shape of each hair creates the characteristic profiles seen across ethnic groups and among individuals.
Each hair is made up of two separate structures: the hair shaft, which comprises the visible part outside the skin, and the follicle which lies underneath. Above the level of the epidermis, the hair shaft is a thin, flexible cylinder of non-living, keratinized epithelial cells. Below, it is part of a living, hair follicle which enlarges at the base and forms the hair bulb. The hair bulb surrounds the dermal papilla, an important structure derived from mesenchyme. The hair shaft is made up of a cortex, surrounding cuticle cells, and sometimes a central medulla. The bulk of this hair fiber belongs to the cortical layer, which plays an important role in determining the physical and mechanical properties of the hair. It is composed predominantly of macrofibrils, which are rods of microfibrils meshed together in a matrix. The cuticle is made up of flat overlapping cells that cover the hair shaft from the root until it exits from the epidermis. The cuticle is of considerable cosmetic importance as it is responsible for lending the hair a clean and untangled appearance.
The follicle is the primary structure from which hair is able to grow. The histological arrangement of the follicle is divided into outer and inner root sheaths. The outer root sheath (ORS) has been recognized as a ready supply of multipotent stem cells which differentiate into a number of cell types including melanocytes and keratinocytes. More specifically, these stem cells are thought to reside in a distinct bulge area located between the insertion of the arrector pili muscle and the ductal opening of the sebaceous gland. The inner root sheath (IRS) consists of the Henle layer, the Huxley layer, and the previously mentioned cuticle layer which also helps affix the growing hair shaft to the follicle, a task bolstered by the production of keratins and trichohyalin by the IRS cells.
The hair bulb is the region of the follicle which actively produces the hair. It houses the dermal papilla, a rich stroma, associated nerve fibers, and a loop of the capillary that supplies nutrients. The papilla is believed to be a primary orchestrator in the hair growth process. It conducts the precise signals determining the size and color of the hair shaft via a complex mix of essential growth factors including insulin-like growth factor, stem cell factor, keratinocyte growth factor, and bone morphogenetic protein. The hair bulb itself is divided into two regions by the Auber line. Below this line, cells are still undergoing differentiation. These immature cells comprise the germination center or matrix of the follicle. Here, all cells are mitotically active and move in an upward direction where they become larger, elongate vertically, and integrate into the hair shaft.
Nerve supply to the hair follicles is similar to that of the surrounding network of dermal nerves. It is composed of both sensory afferents and autonomic sympathetic nerves. Sensory information from hair stimulation enhances tactile ability. Autonomic nervous innervation primarily provides control of the arrector pili muscle. Contraction of these tiny muscles makes the hair “stand on end.” This is likely a vestigial function related to fur; erecting the shafts served to trap air and conserve heat in cold climates as well as cut a larger silhouette in an attempt to intimidate rivals or would-be predators. Vascular supply is provided by small arterioles originating in the subcutaneous fat. Subtle hair loss on the lower extremities can sometimes hint at the underlying peripheral arterial disease.
The growth of the hair follicle is cyclical. Stages of rapid growth and elongation of the hair shaft alternate with periods of quiescence and regression driven by apoptotic signals. This cycle can be divided into three phases: anagen (growth), catagen (transition), and telogen (rest). Anagen growth is the active phase in which the hair follicle takes on its onion-like shape and works to produce the hair fiber. The anagen phase can be further broken down into proanagen and metanagen phases. Proanagen sees the follicle proliferating hair progenitor cells and begins the process of differentiation. The new hair shaft appears on the surface of the skin to mark the metanagen phase. The anagen phase as a whole can last for several years.
The catagen phase begins with the end of the anagen phase and is characterized by a transition into quiescence. During this phase, which can last a few weeks, the hair follicle undergoes apoptosis-driven regression and loses about one-sixth of its standard diameter. The formation of a club hair, an important prognostic indicator in assessing hair pathology, also occurs at this time. Next is the telogen or resting phase of the hair cycle in which the hair follicle is dormant, and growth of the hair shaft does not occur. About 10% to 15% of all hairs on the body are in this resting phase at any given time and can remain in this state for a variable amount of time depending on the location of the hair - from a few weeks for eyelashes to nearly one year in scalp hair. The exact mechanism that controls passage from one phase into the next is not fully known. The bulge activation theory posits that growth factors produced in the dermal papilla stimulate bulge stem cells to proliferate and modulate growth-phase transitions. Because these cells are transient amplifying cells, they only can go through a limited number of mitoses, thereby setting the duration of anagen and onset of catagen phases.
The hair follicle and its product are also one of the few areas of the body protected from immune surveillance in a phenomenon first described by Sir Peter Medawar in 1948 as an immune privilege (IP). IP is achieved via a number of major histocompatibility complex by the follicle, local production of immune modulators such as TGF-beta, and expression of Fas-Ligand to kill autoreactive T cells. Hair growth also is controlled by a number of hormonal signals, with androgens having the most prominent effect. The growth of hair on the face, trunk and extremities in the male and of axillary and pubic hair in both sexes is brought about by androgens, although with considerable ethnic variability: Chinese men, for example, exhibit less body hair than those from Europe.
The unwanted loss of hair, known as alopecia, is a widespread condition affecting both sexes, occurring in numerous patterns, and classified into non-scarring and scarring subtypes. The most common non-scarring type of alopecia is androgenetic or “pattern” hair loss which develops due to a combination of genetic predisposition and the action of androgen on hair follicles. While men commonly are associated with this condition, women can be affected as well and exhibit their own characteristic pattern. Alopecia can become severe to the point of eradicating all terminal scalp hairs, where it is known as alopecia totalis. When all body hairs are totally gone, it is known as alopecia universalis. Former NBA athlete Charlie Villanueva displays this pattern of alopecia.
Scarring alopecia can arise from cutaneous manifestations of lupus or a bacterial inflammatory condition known as dissecting folliculitis. These patterns of alopecia exhibit patches of shiny, bare skin and result in permanent hair loss in the affected areas.
Excess hair or hair in abnormal locations is known as hypertrichosis. It can develop as an inherited condition or a drug reaction. A proliferation of fine lanugo hairs can be a symptom of malnutrition and aid in the diagnosis of anorexia.
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