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The histology of your hair can vary slightly depending on your ethnicity, being influenced by race and genes. The hair follicles are tubular structures, having a base (hair bulb) that surrounds the hair papilla. During the growth phase, an extra outer layer (stratum basale) appears. It is located in the dermal papilla and is responsible for the production of hair fibers. The epithelial cells produce the hair shaft, and the melanocytes produce the hair pigment.
The Molecular Regulation of Hair Follicle Morphogenesis and Cycling
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. The glassy membrane is a thick, clear connective tissue sheath covering the hair root, connecting it to the tissue of the dermis. The perifollicular sheath collapses and vitreous membrane thickens. Eventually, the lower hair follicle becomes reduced to an epithelial strand, bringing the dermal papilla into close proximity of the bulge [36]. The epithelial strand begins to elongate and finally reaches to just below the insertion of pilar muscle.
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Moreover the follicular papilla is an essential source of growth factors [1, 3, 16, 28]. 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.
1 Anagen
Β-catenin plays a key role in the canonical WNT signaling pathway. The classical BMP signaling pathway is that ligand BMP binds to phosphorylated serine and threonine receptors and is transported into cytoplasm. In cytoplasm, BMP combines with Smad1/5/8 and phosphorylates the C terminal of Smad1/5/8. The phosphorylated Smad1/5/8 combines with Smad4 and transports to the nucleus. EDAR signaling pathway is mainly composed of EDA ligand, transmembrane receptor EDAR (including EDAA1 and EDAA2 subtypes), and intracellular binding protein EDARARR. In the presence of Hh, the binding of Ptch1 to Hh protein eliminates the inhibitory effect on Smo, and Smo transmits signals to downstream Gli transcription factors through a complex transduction process.
Additionaly, the eccrine sweat glands were converted to hair follicle-like structures in Bmpr1a conditional knockout mice (Lu et al., 2016). They are tubular and are formed from multiple layers of epithelial cells. The base of the follicle bulges, forming a hair bulb which surrounds the hair papilla. The bulb is invaginated by connective tissue known as dermal papilla. The dermal papilla contains many tiny blood vessels and nerve projections. This becomes the hair papilla once it invaginates into the hair bulb.
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These overlap creating a structure that supposedly prevents the hair from becoming matted. The hair follicles in your skin contain living cells to allow your hair to grow. The shaft—the part of the hair we see—is made up of dead cells and consists of three different layers. As a piece of hair grows, it goes through three phases before it sheds and a new one grows. The bulb is the rounded structure deep in the skin at the root of the hair that surrounds the papilla and germinal matrix.
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 bulb is a bulb-shaped, rounded structure at the bottom part of the hair follicle "stocking" that surrounds the papilla and the germinal matrix and is fed by blood vessels.
Hair may vary considerably in length, width, quantity, and distribution of follicles depending on its type and location on the human body. Terminal hairs are normally thicker and longer, with hair shaft diameters greater than 0.06 mm and hair bulbs rooted deeply in the subcutaneous tissue. On the other hand, vellus hairs are usually only 1–2 mm in length and have thinner shafts measuring less than 0.03 mm in diameter.
The growth of hair follicles and activity of these stem cells is highly regulated by various signaling pathways. Hair growth is affected by many factors such as age, climate, environment, and health status, and these factors can influence the development of hair follicle tumors, alopecia areata, and other related diseases. It begins at the surface of the epidermis and extends to the opening of the sebaceous duct.
About 30 percent of the body’s surface is covered with terminal hair in women, compared to about 90 percent in men. In this chapter, the basic anatomy and the amazing and complicated biology of the hair follicle is reviewed. Enhanced knowledge on the normal dynamics of the hair provides understanding the basis of how the follicle behaves during a disease. However recent progress in our understanding of the biology and pathology of hair follicles should lead more effective therapies for hair disorders. The hair follicle is one of the characteristic features of mammals serves as a unique miniorgan (Figure 1). In humans, hair has various functions such as protection against external factors, sebum, apocrine sweat and pheromones production and thermoregulation.
Each hair follicle is attached to a tiny muscle (arrector pili) that can make the hair stand up. These nerves sense hair movement and are sensitive to even the slightest draft. Medulla is located in the center of the hair shaft preferably presented in coarser fibers. The rest of the hair is anchored in the follicle and it lies below the surface of the skin.
The bulb holds several types of stem cells that divide every 23 to 72 hours, faster than any other cells in the body. The bulb also contains hormones that affect hair growth and structure during different stages of life, such as during puberty and during pregnancy. Your nail matrix is made of special cells whose main job is to make new fingernails or toenails. Each nail matrix produces 196 layers of cells that combine to make each of your fingernails and toenails. Each nail matrix is constantly making new nail cells, unlike your hair follicles, which go through periods of rest.
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]. The ORS of the hair follicle also contains melanocytes, Langerhans cells and Merkel cells. 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].
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