Human hair is unique and surprisingly complex. Many factors determine how hair grows, its texture, and its color. Humans are distinct from other mammals in that hair has become a largely cosmetic feature rather than a necessity. However, some areas of hair on the human body still provide protection. In addition, the body possesses different types of hair, each with different cellular construction that provides unique characteristics and functions.
What most people refer to as “hair” is two distinct structures: the hair follicle and the hair shaft. The hair follicle sits in the dermis layer of the skin and manages stem cells. These assist with regrowing hair and also healing skin after an injury. The hair shaft is the section that is normally visible. It consists of three layers: the cuticle, the cortex, and the medulla.
The hair cuticle is the exterior layer of the hair fibers, made up of dead cells that have begun to layer together and overlap. These create scales that help protect the hair shaft while simultaneously providing form and strength. The cortex is the middle layer of the hair shaft; it determines hair color and possesses a significant amount of melanin, the same group of pigments that provide skin color. Melanin distribution varies from person to person, resulting in different hair colors for different people. The innermost layer of the hair shaft is the medulla. It is nearly invisible and is by far the softest and most fragile layer.
Unlike the hair shaft, the hair follicle is an active, living structure. The base of the follicle is the papilla. It contains tiny capillaries that provide blood to the cells. Near the papilla is the structure that allows new cells to grow hairs: the germinal matrix. A bulb surrounds the papilla and the germinal matrix. It contains hormones that influence hair growth and structure. A root sheath with an inner and outer layer protects and guides the hair shaft emerging from the bulb.
There are two sheaths within the protective root sheath. The inner sheath possesses three layers, the innermost of which holds cells that point downward and slightly inward, allowing them to connect with the cells of the hair shaft cuticle. The middle and exterior layers are Huxley’s and Henle’s layers, respectively. The outer root sheath covers the inner sheath and connects to the sebaceous glands of the skin. These glands produce sebum, the oil that lubricates and waterproofs the hair and skin. The hair follicle also contains the vitreous layer, the fibrous root sheath, the suprabulb region, the isthmus, and the infundibulum.
Human hair can grow everywhere on the body, aside from mucous membranes and the special glabrous skin of the palms of the hands, soles of the feet, and lips. The hair growth cycle has three phases: anagen, catagen, and telogen. Each strand of hair performs these cycles independently, which means they're in different phases at any given time. The anagen phase is the active growth phase and usually takes around three to four years for head hair, but between one and several months for the eyelashes, eyebrows, and hair on the extremities. The catagen phase lasts for a few weeks and features a slowing growth rate. In this phase, the outer root sheath shrinks and latches onto the hair root. The telogen phase is the resting phase.
Hair on the arms differs from hair on the head, and this hair differs from that of the pubic region. There are various hair types, and hair changes at different stages of growth. When a child is born, sometimes a soft, downy hair covers their body for the first several weeks of their life. This is lanugo, the first hair that the fetal hair follicles produce. Typically, the child sheds the hair before birth, usually in the seventh or eighth month of the pregnancy. In rare cases, the child may not shed the hair until several weeks after their birth.
Once a child sheds lanugo, vellus hair replaces it. Vellus hair is short, light in color, and may not be noticeable. Unlike other types of hair, vellus hair does not connect to a sebaceous gland. It is most noticeable on children and adult women. Some people refer to vellus hair as “peach fuzz,” particularly on prepubescent males. Vellus hair provides some useful functions including thermal insulation and cooling. Sweat from the body wets the vellus hairs. The body cools when the sweat evaporates from the hair.
Terminal hairs are thicker, longer, and darker than vellus hairs. Androgenic hormones such as testosterone cause terminal hair to replace vellus hair in particular parts of the body, and professionals refer to terminal hair in these areas as androgenic hair. Examples include facial, chest, abdominal, arm, foot, leg, pubic, and armpit hair. Androgenic hair mimics the growth pattern of head hair but has a much shorter anagen phase and a longer telogen phase. The purpose of androgenic hair is to provide sensory input via the longer, stronger hairs. Much like head hair, however, androgenic hair has become primarily cosmetic.
Two types of melanin in the cortex determine hair color: eumelanin and pheomelanin. Higher levels of eumelanin typically result in darker hair. Additionally, there are brown and black eumelanin. A person with low levels of eumelanin will have blond hair, while a person with high levels of brown eumelanin will have brown hair. Pheomelanin provides a red or orange shade. Different combinations of eumelanin and pheomelanin create different hair colors. For example, pheomelanin and brown eumelanin result in auburn hair. White or gray hair is the result of a lack of pigmentation or melanin.
Human hair also varies widely in texture. Curl pattern, volume, and consistency all differ from person to person. There are various theories as to why hair texture varies so drastically. All hair consists of keratin, a fibrous structural protein. Hair with higher levels of keratin may be smoother or more glossy. However, keratin doesn’t determine curl pattern or hair volume. Curl pattern may be the result of flat hair shafts forcing hair to curl. The actual size of the hair follicle determines hair thickness. Thicker strands of hair will usually cause the hair itself to be thicker in volume.
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