Hair cycle by aseem

1. THE HAIR CYCLE

2. Introduction Hair follicles undergo a repetitive sequence of growth and rest known as the hair cycle The timing of the phases of the hair cycle and its overall duration varies between body site, gender, species and races.

3. Stages of hair cycle ANAGEN Growth / Progression CATAGEN Involution / Regression TELOGEN Resting / Quiscent

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6. Anagen Duration of anagen determines the final length of the hair Length varies according to body site Scalp hair 02-08 Y Legs 05-07 M Arms 02-03 M Eyelashes 01-06 M Fingers 01-03 M

7. Anagen 90 to 93 percent of scalp follicles are in anagen at any given time. Growth rate : 01 cm / 28 days Anagen can be divided into seven stages: I : Mitotic Onset in Germ Cell Epithelium  Dermal Papillae (DP) growth II : Bulb Matrix Cells envelope DP  Differentiation  Descent of Bulb

8. Stage III Differentiation (Bulb Matrix Cells  Follicular Cells) Stage IV Matrix melanocytes reactivate Stage V Hair shaft emerges  Telopstosis Stage VI New hair shaft emerges from skin surface Stage VII Stable progressive growth Anagen

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10. Bulge hypothesis Franz Von Leydig Lower portion of the follicle cyclically regenerates, the upper portion stays constant The hair bulge harbours stem cells which give rise to all epithelial layers of hair follicle during anagen Loss of hair follicle stem cells in bulge leads to permanent or cicatricial alopecia

11. Catagen Onset is marked by cessation of mitotic activity of matrix cells There is well coordinated apoptosis in the cyclic portion of hair follicle Pigment production by melanocytes ceases before matrix cell proliferation, thus leading to the non pigmented proximal end of telogen club hair

12. Catagen The lower follicle retracts upwards with dermal papilla The perifollicular sheath forms a fibrous streamer comprised of fibroblasts,small blood vessels and collagen; Cuts off blood supply 1 to 2% of follicles are in catagen Duration is 2-3 weeks approx

13. Telogen Stage of rest / QUISCENT Approximately 10% of scalp hair are in T elogen Duration is 02-03 months Shedding is an active process Formation of Club Root - Fully keratinized, dead hair

14. Telogen hair

15. Exogen Stage of active hair shedding - TELOPTOSIS 1% of telogen hair are in exogen at a given time

16. Kenogen Usually whenever a telogen hair is shed, an anagen hair is already present in the follicle to replace it At times, the follicle stays empty after telogen sheds, this is known as kenogen May last from weeks to months, seen in patients of androgenetic alopecia, prepubertal children

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