Hair Follicle Anatomy

Follicle Structure and Zones

• The region below the bulge of a hair follicle is termed the lower follicle or transient zone, regressing during the catagen phase.
• The isthmus is the section between the bulge and sebaceous gland duct; the infundibulum extends from the isthmus to the epidermis.
• The isthmus and infundibulum together form the permanent zone of the follicle, which remains intact through all hair cycle stages.
• Follicles functionally divide into four zones based on cellular and biochemical activities:
  • Cell proliferation and differentiation zone at the bulb's base
  • Keratin gene expression zone in the upper bulb
  • Keratogenous zone where fiber hardening occurs
  • Zone of inner root sheath (IRS) degradation

Hair Bulb and Germinative Cells

• The lower bulb is the active division area, particularly below the critical level and at the dermal papilla's apex.
• Epithelial cells in this region are characterized by a high nuclear-to-cytoplasmic ratio and are termed germinative cells.
• Germinative cells organize into layers, giving rise to the follicle and hair fiber; they differ from matrix cells to avoid confusion with keratin-associated proteins during keratinization.
• Intermediate filaments in IRS cells likely contain keratin proteins, although definitive identification remains incomplete.

Outer Root Sheath (ORS)

• The ORS, the outermost follicle layer, has its own autonomous cell population.
• It consists of a single layer at the bulb base, increasing to multiple layers higher in the follicle, augmented by desmosomal connections.
• Epidermal keratins, not hair-specific intermediate filaments, populate the ORS, with stem cells giving rise to germinative cells residing in the bulge just below the sebaceous gland.
• The companion cell layer within the ORS assists in the movement of IRS and ORS during hair growth.

Dermal Papilla

• Located at the base of the follicle, the dermal papilla is derived from dermal fibroblasts and plays a critical role in hair growth and cycle regulation.
• It interacts with ectoderm during follicle development, specifying follicle properties.
• Although surrounded by germinative cells, it is separated by a basement membrane and is essential for sustaining hair growth.
• Removal of the papilla halts hair growth, but it can regenerate, leading to the potential for continued hair production despite disturbances.

Keratin and Hair Structure

• Hair keratin intermediate filament (IF) proteins consist of types I and II, with distinct molecular weights and half-cystine residue content.
• Keratin IFs integrate into microfibrils, forming coiled-coils with cross-sectional areas estimated to contain 32 protein chains.
• The tensile strength of hair stems from the cross-linking of IF and keratin-associated proteins through covalent and non-covalent bonds.
• Hair's structural integrity allows it to withstand significant force, although it can stretch when wet due to bond disruption.
• Updates in keratin protein nomenclature are necessary to rectify historical classification issues and accommodate new discoveries.

Nomenclature of Keratin Proteins

• A unified nomenclature for keratin proteins classifies Type I and Type II IF keratins under separate systems, which lack straightforward extensions.
• A proposed revised system would classify keratin-associated proteins and hair keratins uniformly, resolving existing confusion in nomenclature.

This quiz focuses on the structural components of hair follicles, highlighting regions such as the lower follicle, isthmus, and infundibulum. Understand how these areas relate to the hair growth cycle, including the catagen phase. Test your knowledge of hair follicle anatomy and their functions.