8.3 Cutaneous Appendages: Hair, Nails & Glands

8.3 Cutaneous Appendages: Hair, Nails & Glands

CIDESCO Exam Tip: Questions on appendages require precise anatomical identification (e.g., distinguishing eponychium from hyponychium, or anagen from catagen/telogen) and chemical differentiation (e.g., holocrine vs eccrine vs apocrine secretion mechanisms, hard keratin vs soft keratin).

Hair Follicle Anatomy & Physiology

Hair (pili) is a flexible, keratinized threadlike structure produced by hair follicles—tubular invaginations of the epidermis extending deep into the reticular dermis or hypodermis.

Structural Components of the Hair Unit:

• Hair Bulb: The swollen base of the hair follicle located deep in the dermis or subcutis.
• Dermal Papilla: A vascularized cone-shaped projection of connective tissue that inserts into the base of the hair bulb, supplying blood, oxygen, and growth factors essential for follicle survival.
• Hair Matrix: A germinative layer of highly mitotic epithelial cells surrounding the dermal papilla. Matrix cells divide rapidly to form the hair shaft and inner root sheath, while interspersed melanocytes supply hair pigment.
• Hair Shaft Anatomy: Consists of three concentric layers of dead keratinized cells:
  1. Medulla: Central core of loose cells and air spaces (present in thick terminal hairs; absent in fine vellus hairs).
  2. Cortex: Middle layer comprising dense, elongated cells containing hard keratin and melanin granules (gives hair its strength, elasticity, and color).
  3. Cuticle: Outermost single layer of overlapping scale-like cells (pointing upward) containing hard, highly sulfur cross-linked keratin that protects the underlying cortex.
• Follicular Sheaths: The follicle wall consists of an Inner Root Sheath (IRS) (composed of Henle's layer, Huxley's layer, and IRS cuticle) which molds the hair shaft, and an Outer Root Sheath (ORS) which is a downward continuation of the stratum basale and spinosum.
• Arrector Pili Muscle: A band of smooth muscle innervated by the sympathetic nervous system, running from the papillary dermis to the hair follicle sheath. Contraction compresses sebaceous glands and pulls the hair vertical, producing "goosebumps" (piloerection).

The Hair Growth Cycle

Hair Classification Types

1. Lanugo Hair: Extremely fine, soft, unpigmented fetal hair covering the fetus in utero; shed prior to birth (or shortly after in premature infants).
2. Vellus Hair: Short, fine, non-pigmented "peach-fuzz" hair covering most of the adult body. Lacks a medulla and has no arrector pili muscle or sebaceous unit attachment.
3. Terminal Hair: Long, coarse, pigmented hair featuring a distinct medulla and cortex. Resides on the scalp, eyebrows, eyelashes, pubic, axillary, and beard regions. Influenced heavily by circulating androgens (testosterone/dihydrotestosterone).

Structure of the Nail Unit

The nail (unguis) is a hard, protective plate of dense, clear hard keratin situated on the dorsal surface of the distal phalanges of fingers and toes.

Key Anatomical Components of the Nail:

• Nail Matrix: The active germinative tissue located beneath the nail root. Contains dividing keratinocytes that produce the nail plate. Injury to the matrix causes permanent nail dystrophy.
• Nail Root: The proximal portion of the nail embedded beneath the posterior nail fold.
• Nail Plate: The visible, hard, translucent body of the nail, composed of tightly packed, flattened anucleate corneocytes rich in hard sulfur cross-linked keratin.
• Nail Bed: The layer of vascularized epidermal tissue beneath the nail plate extending from the lunula to the hyponychium. It lacks a stratum granulosum.
• Lunula: The visible crescent-shaped white area at the base of the nail plate, representing the distal extending edge of the underlying thick nail matrix.
• Eponychium (Cuticle): The fold of stratum corneum tissue extending from the proximal nail fold onto the base of the nail plate, forming a watertight seal against microbial invasion.
• Hyponychium: The thickened stratum corneum region beneath the free edge of the nail plate, securing the nail plate to the fingertip tip and sealing the distal nail bed.
• Perionychium / Paronychium: The soft tissue folds surrounding the lateral borders of the nail plate.

Cutaneous Glands: Sebaceous vs. Sudoriferous

1. Sebaceous Glands (Oil Glands)

• Type of Secretion: Holocrine secretion—the entire glandular cell disintegrates, releasing its cytoplasm and accumulated lipid contents into the hair follicle lumen (pilosebaceous duct).
• Location: Present over the entire body except palms and soles. Highest density on face, scalp, chest, and back.
• Sebum Composition: A lipid mixture consisting of triglycerides & free fatty acids (~57%), wax esters (~26%), squalene (~12%), and cholesterol / cholesterol esters (~5%).
• Physiological Function: Lubricates hair and skin, prevents epidermal desiccation, and contributes to the protective acid mantle (pH 4.5–5.5) alongside sweat. Regulated primarily by androgens.

2. Sudoriferous Glands (Sweat Glands)

Clinical Relevance in Esthetics: Acne vulgaris involves hyperkeratinization of the pilosebaceous duct coupled with androgen-stimulated sebum overproduction and colonization by Cutibacterium acnes. In hair removal, understanding follicle depth and phase (anagen) dictates parameter selection for electro-epilation and IPL therapy.