THE HAIR CYCLE

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.

Stages of hair cycle

• ANAGEN–Growth / Progression

• CATAGEN– Involution / Regression

• TELOGEN–Resting / Quiscent

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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

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

• 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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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

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

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

Telogen

• Stage of rest / QUISCENT • Approximately 10% of scalp hair are in

Telogen• Duration is 02-03 months• Shedding is an active process• Formation of Club Root - Fully

keratinized, dead hair

Telogen hair

Exogen

• Stage of active hair shedding - TELOPTOSIS

• 1% of telogen hair are in exogen at a given time

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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