Hair Follicles

Classification: Integumentary
Figure 1 Bulbar region of the hair follicle

Basic structure:

Germinative zone: hair bulb, hair papilla, hair matrix

Hair shaft layers: medulla, cortex, cuticle of hair

Sheath complex: internal root/radicular sheath (IRS), external root/radicular sheath (ERS), glassy membrane, dermal sheath, perifollicular sheath

Hair regions & landmarks

For hair growth stages, see Alopecia

Germinative zone

Hair bulb: houses hair papilla and facilitates its nutrient acquisition, as well as the hair matrix which contains proliferative cells

Hair papilla: connective tissue (CT) network with capillaries that provides blood supply to growing hair follicle

Hair matrix: layer of dividing and mitotic epithelial cells (keratinocytes) that cover papilla, produce daughter cells that push the shaft up

Hair shaft layers

Medulla: large, vacuolated keratinocytes which undergo transfer of pigment; is only found in thicker hairs

Cortex: cuboidal cells that differentiate into keratin-producing cells; thickest layer and contains most melanin

Cuticle of hair: squamous cells from matrix progenitors, are anucleate keratinized cells organized in overlapping shingles pattern

Sheath complex

Internal root sheath (IRS):

  • Cuticle of IRS: deepest part of IRS; also a layer of anucleate, squamous cells organized into shingles pattern with filamentous keratin
  • Huxley’s layer: middle plate of IRS (2-4 cell layers thick); extensive tricohyalin
  • Henle’s layer: outer layer of IRS; singular cell layer that contacts ERS

External root sheath (ERS): continuation of epidermis (basale and spinosum), houses stem cells for hair regeneration

Glassy membrane: acellular hyaline basement membrane that serves as anchoring layer

Dermal sheath: contains progenitors that help rejuvenate papilla and aid in wound healing

Perifollicular sheath: derivative of dermal sheath; structural support with fibrous elements (collagen, fibroblasts)

Regions and landmarks

The hair follicle is divided by regions according to depth under the skin surface: the infundibulum (epidermis to level of opening of sebaceous gland), the isthmus (opening of sebaceous gland to insertion of arrector pili muscle), the suprabulbar (area above hair bulb, including the IRS and the ERS), and the bulbar (contains hair matrix and dermal papilla [pictured in Figure 1]) regions. The latter two regions – suprabulbar and bulbar – are known together as the inferior segment, and are only present during anagen, or hair growth.

Other landmarks

  • Follicular bulge: undifferentiated epithelial cells that contribute to follicular growth; located near arrector pili/origin of sebaceous duct
  • Adamson’s fringe: where hair shaft hardens into keratin; limits the spread of dermatophytes

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