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.
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The ultimate signal from this primitive dermal papilla to the epithelial placode cells indicates a rapid proliferation and differentiation. This consecutive signaling process finally leads to the production of the mature follicle. In the second stage of development, hair germ elongates into a cord of epithelial cells and forms the hair peg stages 3 and 4.
It is surrounded by mesenchymal cells that eventually transformed to the fibrous sheath. Derived from the epithelial cells of the hair peg, hair matrix cells form the hair shaft and inner root sheath IRS. Outer root sheath ORS generates two bulges along the side of the hair follicle, the proximal bulge serves as a reservoir for epithelial stem cells and the distal bulge evolves to sebaceous glands.
During the development of bullous peg stages 5—8 , the hair bulb and the main cell layers of the mature hair follicle are also formed [ 2 — 4 , 6 ]. Several molecular pathways, growth factors, proteins and genes play substantial roles for the development of the hair follicle. EDAR is necessary for placode development in primary hair follicles but not for induction of secondary hair follicles, which utilize signaling pathways that involve Noggin and SRY-box 18 SOX18 expression within the dermal papilla [ 9 , 10 ].
Human hair follicle morphogenesis occurs only once. Lanugo, vellus and terminal hairs follow the same basic architectural principles.
A second coat of fine, shorter, unpigmented lanugo hair then grows in all areas except the scalp and is shed 3—4 months after birth. Nearly whole body surface is coated with the hairs except a few areas like palms, soles and mucosal regions of lips and external genitalia. Most of these are tiny, colorless vellus hairs. The ones located in several areas like scalp, eyebrows and eyelashes are thicker, longer and pigmented and are called terminal hairs.
Humans have approximately 5 million hair follicles and , of them are located on the scalp [ 11 ] Table 2 [ 2 ]. Basically terminal hairs are found on scalp, eyebrows and eyelashes at birth while the rest of the body is covered with vellus hairs. In puberty, some vellus hairs i.
The bulb of the terminal hairs is located in the subcutaneous fat; however, the bulb of vellus hairs is in the reticular dermis. The hair is classified into three main ethnic subgroups Asian, African and European. However in a recent study, this classification is expanded to eight main subgroups by considering three parameters: curve diameter, curl index and number of waves [ 12 ]. Structural features of the hair follicle have to be considered during the classification process.
Hair shaft diameters, hair follicle density and follicular infundibulum volume are some of them. Hair shaft diameters represent little variations and hairs are found to be thicker in androgen dependent areas.
Hair follicle density is much more condense in the forehead and follicular infundibular volume is also bigger. It is important just because of the large follicular infundibular volume that is associated with more follicular reservoir ability [ 1 , 13 ].
Hair is consisted of two distinct structures: follicle—the living part located under the skin and hair shaft—fully keratinized nonliving part above the skin surface.
The arrector pili muscle, takes place between the hair bulge area and dermoepidermal junction. Above the insertion of the arrector pili muscle, sebaceous glands and, in some certain regions, apocrine glands are opened into the follicle. Hair shaft is consisted of three layers: cuticle, cortex and in certain cases medulla. Flat and square-shaped cuticle cells are adhered tightly to the cortex cells proximally.
Peripheric movements of cuticle cells make the direction of the distal free edge upward and cause extensive overlapping. These imbrications are crucial.
By interlocking with the cuticle cells of inner root sheath, they contribute to the follicular anchorage of the growing hair. These imbricated surfaces also facilitate removal of dirt and desquamated cells from the scalp. Cuticle has also important protective properties and barrier functions against physical and chemical insults [ 14 — 16 ]. During the migration of the cells from the hair bulb to compose the cortex, the shapes of them become more fusiform.
These cells coalesce tightly and are placed parallel to the axis of the shaft. The cortex comprises the bulk of the shaft and also contains melanin [ 2 , 15 , 16 ]. Medulla is located in the center of the hair shaft preferably presented in coarser fibers. The hair medulla contains structural proteins that are markedly different from other hair keratins and eosinophilic granules that are filled by an amino acid, citrulline and eventually form internal coatings within the membranes of mature cells [ 14 , 16 , 17 ].
The follicle is the essential growth structure of the hair and basically has two distinct parts: upper part consisting of infundibulum and isthmus whereas the lower part comprising of hair bulb and suprabulbar region. The upper follicle remains constant, while the lower part has continuous cycles of regeneration [ 1 , 2 , 16 , 18 ].
The infundibulum, the uppermost portion of the hair follicle extending from the opening of the sebaceous gland to the surface of the skin, is a funnel-shaped structure filled with sebum, the product of the sebaceous glands.
The isthmus is the lower portion of the upper part of hair follicle between the opening of the sebaceous gland and the insertion of arrector pili muscle. 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 inner sheath follows the hair and ends just before the opening of the oil gland, or sebaceous gland. The outer sheath continues all the way up to the gland. The sebaceous gland produces sebum , or oil, which is the body's natural conditioner. More sebum is produced during puberty, which is why acne is common during the teen years.
Sebum decreases with age, causing the skin to become dry. 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. The hair follicle is the stocking-like structure under the skin where your hair is formed. The follicle contains sheaths that mold the hair, oil glands that condition hair, and cells that produce new hair.
The hair shaft is the part of the hair that we can see. Once the hair grows beyond the skin's surface, the cells aren't alive anymore. It's made up of three layers of keratin, a hardening protein.
Those layers are:. The hair shaft is the part of hair we see above the skin. It can have three layers—the medulla, the cortex, and the cuticle. The hair on your scalp grows about a half a millimeter a day. The individual hairs are always in one of three stages of growth: anagen, catagen, and telogen. There are three stages of hair growth. In the anagen phase, hair on the scalp grows for three to four years. In the catagen phase, hair slows down its growth. In the telogen phase, hair stops growing and stays in place until a new hair pushes it out.
Some people have corkscrew curly hair, while others have thick, straight, shiny hair. This appearance is due the shape of the hair.
Straight hair has a mostly circular circumference. Strands of curly hair are flat. This is the reason why any home remedy or over-the-counter treatment for hair growth focuses on your scalp and hair follicles, not on the part of your hair that you can see. Each hair follicle has a life span. This happens as the follicles enter a phase of renewal and rest and temporarily take a break from pushing out hair strands.
Even though your hair is made of cells that are technically no longer living, you can still take care of your strands to help your hair look its best.
Here are a few quick tips:. Taking care of your hair properly can make all the difference in helping your hair strands last longer and look healthier. Hair follicles are responsible for your hair color, hair growth, hair texture, and more. These follicles have three phases of growth, and consistently…. Hair grows back at a rate of about 6 inches per year, but the rate at which it will return after hair loss depends on what caused the loss in the….
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