scalp hair:
| • | anagen hairs – growing hairs; grow for about 3 years (900 days or more) |
| • | telogen hairs – resting hairs; phase lasts ~ 3 months (100 days) |
| • | normal scalp: ~ 90% anagen; ~ 10% telogen (or catagen) |
From inside to outside the hair --> medulla, the cortex, and the cuticle (which all make up the hair shaft).
| • | The inner root sheath consists of the cuticle, Huxley’s layer, and Henle’s layer (mnemonic: hEnle’s layer is External to huxley’s): |
| • | Henle’s layer, which is only one cell layer thick, keratinizes first. The two cuticles, one in the hair shaft and one in the inner root sheath, keratinize next, followed by Huxley’s layer |
| • | inner root sheath – keratinize by means of trichohyaline granules (which unlike the keratohyaline granules seen in the epidermis, stain eosinophilic) |
some important facts:
| • | outer root sheath is stationary |
| • | hair and inner root sheath ascend together |
| • | inferior segment = follicle beneath the isthmus (temporary) |
| • | isthmus and infundibulum = permanent |
| • | color of hair = amount and distribution of melanin in the hair shaft |
| • | gray hair = decreased melanocytes therefore decreased melanosomes |
| • | the final length attained by a hair varies directly with the duration of its growing period |
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trichohyaline granules inner root sheath
trichilemmal keratinization outer root sheath
citrulline inner root sheath and medulla |