Structure of the Hair Follicle

	Hair, Nails and Mucous Membranes

	Hair Anatomy Dermal Papilla and Matrix Histology of Hair Follicle Melanocyte Scalp Hair Hair Follicle	Structure of the Hair Follicle GENERAL ANATOMY HAIR SHAFT INNER ROOT SHEATH OUTER ROOT SHEATH CHEMICAL COMPOSITION OF HAIR GENERAL ANATOMY Elongated keratinized structures derived from invaginations of epidermal epithelium Hair is found everywhere except: palms, soles, lips, glans penis, clitoris and labia minora Infundibulum: follicular orifice to sebaceous duct entrance Isthmus: sebaceous entrance to insertion of arrector pili muscle Arrector pili muscle: consists of smooth muscle. Contraction leads to erection of hair shaft and can cause "gooseflesh". The point of insertion of the arrector pili is referred to as the bulge. This area is the location of the follicular stem cells. Lower portion: hair bulb - papilla and matrix. Melanocytes located in the matrix produce hair color. The dermal papilla consists of a specialized group of fibroblasts and has an inductive action on the epidermis promoting proliferation and differentiation. HAIR SHAFT Medulla: central portion; consists of spongy keratin with air spaces of variable sizes. Only found in terminal hair. Cortex: condensed macrofibrils/melanin; forms main bulk of the hair and contributes most to the mechanical properties of hair. Cuticle: 6-8 layers of cuticle cells, overlapping like roof tiles, free margin always pointing upward. Shafts that are thick (> 0.06 mm in diameter) are coarse terminal hairs Shafts that are thin (< 0.03 mm in diameter) are fine vellus hairs INNER ROOT SHEATH A rigid cylindrical tube. It develops before the hair within it and its prime function is to mold the hair. Disintegrates at the level of the sebaceous gland. OUTER ROOT SHEATH Derived from and is continuous with the epidermis Function unknown. It does not take part in hair formation. Surrounded by a glassy basement membrane, which in turn is surrounded by a connective tissue sheath. This connective tissue sheath has important inductive properties. CHEMICAL COMPOSITION OF HAIR Main component is protein: 65-95%. Many disulphide bonds. Very hygroscopic. Wet hair is more easily breakable. Lipids: useful as an H₂O repellent; increases at puberty and then decline with age. Trace elements: Cu, Cd, Cr, Hg, Pb, An. Come from exogenous sources (cosmetics, air pollution) or endogenous sources (matrix, papilla, sebaceous and sweat glands).