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Some conditions this may occur in include, but are not limited to, hypothyroidism, hyperthyroidism, stress, vitamin deficiencies, and post-childbirth. Vascular supply is provided by small arterioles originating in the subcutaneous fat. The vessels nourish the hair follicle by delivering oxygen and nutrients, eliminating waste, and promoting growth. Subtle hair loss on the lower extremities can sometimes hint at the underlying peripheral arterial disease.
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The wall of the hair follicle is made of three concentric layers of cells. The cells of the internal root sheath surround the root of the growing hair and extend just up to the hair shaft. The external root sheath, which is an extension of the epidermis, encloses the hair root. It is made of basal cells at the base of the hair root and tends to be more keratinous in the upper regions.
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Areas of the body that appear to be hairless but have shorter, finer hairs that lack the medulla layer are called vellus hair. Neonates can be born with lanugo hair; fine hairs shed in utero or within the first weeks of life.[3] Club hair is fully keratinized hair that is fully formed in the telogen stage of the hair cycle. The only parts of the external surface of the body that are truly hairless are the palms and soles of the hands and feet, lips, labia minora, and glans penis. 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 cuticle comprises flat overlapping cells covering the hair shaft from the root until it exits from the epidermis.
How It Gets Its Shape
Hair texture (straight, curly) is determined by the shape and structure of the cortex, and to the extent that it is present, the medulla. The shape and structure of these layers are, in turn, determined by the shape of the hair follicle. Hair growth begins with the production of keratinocytes by the basal cells of the hair bulb. As new cells are deposited at the hair bulb, the hair shaft is pushed through the follicle toward the surface. Keratinization is completed as the cells are pushed to the skin surface to form the shaft of hair that is externally visible.
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This argument assumes that curly hair better impedes the passage of UV light into the body relative to straight hair (thus curly or coiled hair would be particularly advantageous for light-skinned hominids living at the equator). Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals.The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and fine vellus hair.
The arrector pili muscle, a tiny bundle of muscle fiber, is attached to the outer sheath. When the muscle contracts, it causes the hair to stand up, otherwise known as goosebumps. Tightly coiled hair in its natural state may be worn in an Afro. This hairstyle was once worn among African Americans as a symbol of racial pride.
However, while men develop longer, coarser, thicker, and darker terminal hair through sexual differentiation, women do not, leaving their vellus hair visible. Many mammals have fur and other hairs that serve different functions. Hair also has a sensory function, extending the sense of touch beyond the surface of the skin.
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But because new hairs are always growing and replacing them, this natural hair loss isn't noticeable. The hair shaft comprises a cortex, surrounding cuticle cells, and sometimes a central medulla found in thicker hair. The bulk of this hair fiber belongs to the cortical layer, which plays a vital role in determining the physical and mechanical properties of the hair, such as strength, texture, and color.
This is especially evident in men who develop a five o'clock shadow after having shaved their faces. Stubble typically appears to grow back thicker because the shaved hairs are blunted instead of tapered off at the end, although the hair never actually grows back thicker. The bulb is the rounded structure deep in the skin at the root of the hair that surrounds the papilla and germinal matrix. It has several types of stem cells, which develop into specialized cells and can renew themselves over a long period of time. At the bottom of the follicle, a piece of tissue called the papilla contains tiny blood vessels (capillaries).
The length of this phase is measured in years, typically from 2 to 7 years. The catagen phase lasts only 2 to 3 weeks, and marks a transition from the hair follicle’s active growth. Finally, during the telogen phase, the hair follicle is at rest and no new growth occurs.
Each human follicle follows this cycle independently of others, so the total amount of hair remains constant; some animals’ hair follicles have synchronous cycles, causing periodic shedding, or molts. A proliferation of fine lanugo hairs past the neonatal period can be a symptom of malnutrition and aid in diagnosing anorexia nervosa. Hair transplantation has become a therapeutic option for those who fail to have hair growth with medication. Hair has great social significance for human beings.[63][64] It can grow on most external areas of the human body, except on the palms of the hands and the soles of the feet (among other areas).
In fact, as we get older, the number of hair follicles per square inch decreases as our bodies stretch and grow. In fact, although genetic findings (Tishkoff, 2009) suggest that sub-Saharan Africans are the most genetically diverse continental group on Earth, Afro-textured hair approaches ubiquity in this region. This points to a strong, long-term selective pressure that, in stark contrast to most other regions of the genomes of sub-Saharan groups, left little room for genetic variation at the determining loci. Such a pattern, again, does not seem to support human sexual aesthetics as being the sole or primary cause of this distribution. The eyebrows provide moderate protection to the eyes from dirt, sweat and rain.
The outer root sheath (ORS) has been recognized as a ready supply of multipotent stem cells that differentiate into several 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. Hair, in mammals, the characteristic threadlike outgrowths of the outer layer of the skin (epidermis) that form an animal’s coat, or pelage.
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