Humans have approximately 5 million hair follicles and 100,000 of them are located on the scalp. In humans whole body is covered with hairs except palm, sole, lips & genitals. Hair serves the aesthetic function of adding beauty to the human body & it’s a characteristic feature to the mammals.
Hair is a derived of the epidermis and consists of two different parts:
A muscle called arrector pili is found between the hair bulge area & dermoepidermal junction. Above the insertion of this muscle sebaceous gland & in few specific regions apocrine glands open in the hair follicle.
it is a very important growth structure of the hair consist of two main parts.
Infundibulum uppermost part of the hair follicle extending from the skin surface to the opening of sebaceous gland into the hair follicle. It is usually filled by the sebum secreted by the sebaceous gland.
Isthmus its upper part of hair follicle between sebaceous gland opening to the insertion arrector pili muscle. Here epithelium keratinization begins with the lack of granular layer named “trichilemmal keratinization”.
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
Hair bulb: it is the expanded onion shaped portion of lower part of hair follicle. It is the active reproductive means living portion of the hair follicle.bulb is connected to the papilla from below which is surrounded by matrix cells and pigment cells. Hair shaft develops from the matrix cells.
Nerve supply: Nerves related to the hair follicle are similar to the dermal nerve network including sensory afferents and autonomic sympathetic nerves. These nerve fibers form a circular layer around the bulge area of terminal follicles and the bulb area of vellus follicles. Several types of nerve endings are associated with hair follicle like Free nerve endings transmit pain, lanceolate nerve endings detect acceleration, Merkel cells responsible of pressure sensation and pilo-Ruffini corpuscles detect tension.
Vascular supply: Cutaneous vascularization is provided by arterioles, which are concentrated at the lower portion of the hair follicle and compose vascular network. During hair cycle some alteration in the perifollicular vascularisation occurs due to up & down regulation of vascular endothelial growth factor expression.
It’s basically a thin strand of hardened cells that is visible above the scalp. The thickness of a hair is varies something in-between 0.04-0.12mm. Hair shaft is consisted of three layers:
Cuticle: It’s the most outer layer of the hair and protects the inner layers from harm from the external physical & chemical insults. It has a tough and dense structure. It’s colorless in nature and allows light to shine through it. Around 10% of the hair diameter consists of the cuticle layer. It’s known to provide shine and elasticity to the hair. Cuticle cells are flat & square shapped which are adhered tightly to the cortex cells.
Cortex: It’s the thickest layer of the hair and constitutes around 80-90% of the hair’s diameter. It contains numerous hardened fibrous cells which are made up of various protein molecules. These cells are arranged together in bundles surrounded by keratins. These cells coalesce tightly and are placed parallel to the axis of the shaft. It’s also known to contain the pigment, Melanin giving the hair its natural color.
Medulla: It is located in the center of the hair shaft preferably presented in coarser fibers. . Its thickness ranges according to the thickness of the hair. Medulla’s presence in a hair determines its stiffness and thickness. It’s not at all present in fine hairs.
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