Chapter 1: Hair Findings

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

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: ...