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DEVELOPMENT

  • Follicles form during third month of gestation; form first on head

  • Lining of follicle = ectodermal origin

  • Dermal papilla = mesodermal origin

  • Epidermal invaginations occur at an angle to the surface and over sites of mesenchymal cell collections

  • Eventually these epidermal cells form a column that surrounds the mesenchymal dermal papilla to form the bulb

  • The dermal papilla (along with “stem” cells in the bulge) induces hair follicle formation by the overlying epithelium

  • Additionally, 2 or 3 other collections of cells form along the follicle:

    • Upper collection becomes the mantle from which the sebaceous gland will develop

    • Lower swelling becomes the attachment for the arrector pili muscle and where follicle germinal cells reside in telogen phase

    • If a third collection of cells exists, it is found opposite and superior to the sebaceous gland and develops into the apocrine gland

STRUCTURE

  • Longitudinal structure: (superior to inferior) (Fig. 1-1)

    • Permanent portion of the hair follicle

      • – Infundibulum

      • – Area of the sebaceous gland

      • – Isthmus: begins at sebaceous gland and ends at the bulge (site of insertion of arrector pili muscle)

      • – Area of the bulge: location of follicular stem cells

    • Transient portion of the hair follicle

      • – Lower hair follicle

      • – Hair bulb: contains the matrix, melanocytes; envelopes the dermal papilla; critical line of Auber is at the widest diameter; below this line is the bulk of mitotic activity

FIGURE 1-1

Hair cycle and anatomy. The hair follicle cycle consists of stages of rest (telogen), hair growth (anagen), follicle regression (catagen), and hair shedding (exogen). The entire lower epithelial structure is formed during anagen and regresses during catagen. The transient portion of the follicle consists of matrix cells in the bulb that generate 7 different cell lineages, 3 in the hair shaft, and 4 in the inner root sheath. (Reprinted with permission from Goldsmith LA et al, Fitzpatrick's Dermatology in General Medicine, 8th Ed. New York: McGraw-Hill; 2012.)

MICROSCOPIC STRUCTURE

  • The hair follicle is arranged in concentric circles (from outer to inner) (Fig. 1-2)

    • Basement membrane (glassy membrane): PAS-positive, acellular; thin during anagen and thickens during catagen

    • Outer root sheath (ORS): present the length of the follicle; never keratinizes; stays fixed in place

    • Inner root sheath (IRS): grows toward cell surface and separates from the hair shaft at the level of the sebaceous gland

      • – Henle layer: one cell thick and first to cornify

      • – Huxley layer: two cells thick; eosinophilic-staining trichohyalin granules

      • – Cuticle

  • Hair shaft: grows toward cell surface; cornifies without trichohyalin or keratohyalin granules

    • Cuticle: shingle-like hair cells that interlock with cuticle cells of IRS

    • Cortex: arises from cells in center of hair bulb; disulfide bonds in this region give hair its tensile strength; keratinizes to form shaft; contains pigment of hair

    • Medulla: contains melanosomes; found only in terminal hairs

  • Hair cycle: human ...

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