In hair development, embryonic stem cell division occurs in pre-hair germ stage and hair root is generated in the hair germ stage. The follicle, papilla, sebaceous gland and arrectorpilorummuscle are formed in thehair peg stage.
The bulb is formed in the bulbous hair peg stage, where the follicle matures. Follicles form during the embryonic stage and in humans, follicles fully form by the 22nd week of pregnancy. The follicle nucleus start developing in the eyebrow, upper lip and chin about 9 weeks after conception and develop in other areas 4 months after conception.
Hair germ cells are derived from two different cell tissues. The dermal papilla and connective sheath originate from the mesoderm and the epithelial sheath from ectoderm. These two cellular tissues go through molecular adhesion and interactions between growth factors or cytokines drive the hair growth cycle. The hair growth cycle is regulated by stem cells of the slow-growing hair bulge. New cells are generated and old cells are pushed to thelower hair matrix to form hair root and shaft.
Epidermal melanocytes(originating from pluripotential neural crest cell) are involved in the pigmentation of hair. There is an association between elements that regulate hair matrix activity and melanocytes, however, it is not yet clearly understood. The rich vascular network of dermal papilla provide nutrition to hair. The thickness ofdermal papillais proportionate to the height of hair follicle and hair shaft diameter.
Hair consists of the bulbar region and follicle. Thebulbar regioncontains dividing cells and papilla matrix. The hair sheath consists of outer cuticle, cortex(forms the shape of the shaft and contains pigments) and central core. Follicles are at 10-90oangle from the skin surface depending on the body region. Dermal papilla lies about 1-5mm in depth depending on the hair type. Dermal follicles are surrounded by sensory receptors and the erector muscle is surrounded by a nervous net. This is why hair removal is painful. However, electrolytic hair removal using micro needles is less painful and damaging to the hair sheath than laser epilation which irradiates laser beams on hair cortex.
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Lanugo, Vellus Hair and Terminal Hair
Human body hair can be categorized into lanugo, vellus hair and terminal hair. When we speak of “hair” it generally refers to terminal hair. Terminal hair is influenced by genetic factors and endocrine system. Physical properties differ depending on hair type. Vellus hair lacks pigmentation, is short with the diameter of 15-40μm. Hair cycle is about 6 months. Medium thickness hair has light pigmentation with a diameter of 40-60μm, 12 month cycle and has slightly longer follicle compared to the vellus hair.
Terminal hair has heavy pigmentation, with the follicle diameter of 60-150μm and 18 month cycle. Generally, terminal hair is not as dense as other types of hair. The depth and diameter ofindividual terminal hair follicles are almost the same in the same area. Anagen phase differs depending on the individual, season, age, race and skin type.
-To be continued
