Accessory Skin Structures: Hair, Nails, Glands, and Related Pathologies

Hair

• Accessory structures or appendages of the skin include hair, nails, and glands; they are derived from epithelium only and assist in skin function.

• Hair is a small filamentous structure that protrudes from the skin and covers most of the body except thick-skin areas (palms, soles, lips, parts of external genitalia).

• In humans, hair is too sparse to regulate body temperature as in some animals (e.g., dogs, cats), but it provides protection and sensory input.

  • Eyelashes protect the eyes from dust and particles.

  • Hair on the scalp provides protection from UV radiation and mechanical trauma; it also has a sensory role via small sensory neurons.

• Hair is composed of two main parts: the shaft and the root; both are made of stratified squamous keratinized epithelial cells.

  • Shaft: part that protrudes from the skin surface; cells are fully keratinized.

  • Root: embedded in the dermis; surrounded by a small sensory neuron; at its base is the hair papilla (a bump) with blood vessels and surrounding epithelial cells; together these form the hair bulb.

  • Surrounding the epithelial root is the dermal root sheath (connective tissue) that supports the follicle and separates it from the dermis.

  • Around the dermal root sheath, erector pili muscles attach and contract to cause hair to stand up (goosebumps).

• Hair structure (when cut in half) shows three visible regions:

  • Inner medulla (soft).

  • Middle cortex.

  • Outer cuticle (hard keratin; provides mechanical strength).

• The matrix (a small population of keratinocytes at the base) actively divides to grow hair.

• The epithelial root sheath anchors the hair follicle to the dermis; the clear portion seen when pulling hair corresponds to this region.

• The hair grows in cycles with two main phases:

  • Growth (anagen) phase: mitosis occurs; hair length increases; duration ranges from $1 ext{-}6 ext{ months to years}$ depending on hair location.

  • Resting (telogen) phase: mitosis ends; cells die; follicle shortens; hair is pushed out as new hair begins to grow.

• Hair regions and types:

  • Medulla, cortex, and cuticle.

  • Erector pili muscles can cause goosebumps when they contract.

• Hair growth rate varies:

  • Average rate is $1 ext{-}1.5\ ext{cm/month}$.

  • It is not continuous but occurs in cycles.

• Hair color and texture:

  • Determined primarily by melanin produced by melanocytes in the matrix.

  • Different levels of melanin produce different colors:

  • Blonde: little melanin.

  • Black/brown: much melanin.

  • Red: reddish pigment containing iron.

  • Melanocytes produce less melanin with age, leading to gray/white hair.

• Hair types and distribution:

  • Lanugo: thin, non-pigmented hair on babies/fetuses; typically falls out at birth.

  • Terminal hair: thick, coarse, pigmented hair found on scalp and around eyes; appears after puberty (more in males than females).

  • Vellus hair: thin, non-pigmented, softer hair found elsewhere on the body.

• Hormonal and sex differences:

  • Terminal hair becomes more prominent after puberty; more hair generally in males than females.

• Hair color connection to health and aging:

  • Melanin production is the key determinant; aging reduces melanin production in the matrix,
    leading to gray/white hair over time.

Nails

• Nails are hard accessory structures located at the ends of digits (fingers and toes).

• Composition: stratified squamous epithelium filled with hard keratin.

• Nail plate: visible component that sits on the nail bed.

• Nail bed: underlying tissue; divided into nail body (visible plate) and nail root (part under the skin where the matrix lies).

• Matrix: actively dividing cells responsible for nail growth.

• Epithelial root sheath and surrounding folds anchor and protect the nail:

  • Proximal nail fold; epichium (cuticle) is the stratified corneum region.

  • Medial and lateral nail folds help protect the nail and keep microbes out.

• Distal/free edge of the nail is attached to the nail bed by the hyponychium (often called the hyponychial region in anatomy references).

• Growth rate: nails grow at an average of $0.5\ ext{mm/week}$; toenails grow more slowly.

• Nails contain no melanocytes; they are largely translucent except at the base near the cuticle where the lunula appears as a pale crescent.

• Primary function: protection; aid in grip and manipulation of objects.

Glands of the Skin

• Skin contains two basic gland types derived from epithelial cells in the dermis: sweat glands (sudiferous) and sebaceous glands.

• Secretions are released via exocytosis (merocrine secretion) for sweat glands and holocrine secretion for sebaceous glands.

Sweat Glands (Sudiferous)

• Four types differing in structure and secretion products:

  • Eccrine (merocrine) glands: most numerous; simple coiled tubular glands in the dermis; secrete watery sweat containing water, waste products, and electrolytes like salt; ducts open onto the epidermal surface via sweat pores.

  • Apocrine glands: larger glands found in regions such as axilla, anal area, and areolar; secrete a protein-rich fluid into hair follicles; the secretion becomes odorous when metabolized by skin bacteria after puberty; activity is influenced by sex hormones.

  • Ceruminous glands: modified apocrine glands located in the ear canal; secrete cerumen (earwax) to trap particles and lubricate the tympanic membrane for hearing.

  • Mammary glands: highly specialized sweat glands that produce milk after childbirth (lactation).

Sebaceous Glands

• Sebaceous glands are branched clusters of secretory cells surrounded by ducts; they secrete sebum into hair follicles or small surface pores.

• Distribution: found almost everywhere on the body except the palms of the hands and soles of the feet; greatest density on the face and scalp.

• Secretion and regulation:

  • Sebum is a lipid-rich oily mixture produced by holocrine secretion (the whole cell ruptures to release oils) and contains cellular debris.

  • Secretion is influenced by sex hormones, especially testosterone; sebum production increases markedly after puberty.

• Functions of sebum:

  • Coats hair to form a hydrophobic barrier that reduces water loss.

  • Provides antimicrobial properties to inhibit certain bacteria on the skin.

  • Keeps skin pliable and protected.

Clinical Relevance: Acne Vulgaris (preview for Module Six)

• Acne vulgaris affects >$96\%$ of adolescents and young adults.

• Cause: accumulation of sebum and dead cells within sebaceous glands leading to a comedone (blackhead).

• Blackhead formation results from lipid oxidation turning the plug dark.

• Bacterial infection can lead to inflammation and pustule formation.

• Hormonal influence (e.g., testosterone) contributes to acne during puberty; severity may lessen by ages $20 ext{-}25$ but can persist in some individuals.

Skin Pathologies and Burns (Overview)

• Wounds: injuries to skin; include lacerations (cuts), burns, and cancers.

• Burns are classified into three degrees:

  • First-degree (superficial): epidermis damaged only; skin appears red; mild pain; no permanent damage or blistering.

  • Second-degree (partial thickness): epidermis plus part/all of dermis damaged; pain, blistering, and scarring possible.

  • Third-degree (full thickness): epidermis, dermis, hypodermis, and possibly deeper tissues (muscle or bone) damaged; typically not painful initially due to nerve destruction; extensive tissue damage and scarring; dehydration risk due to fluid loss and increased infection risk.

• Rule of Nines: used to estimate the percentage of body surface area affected by burns. Body divided into 11 areas, each representing about $9\%$ of total body area. Percentages differ in children due to different body proportions. Example provided in lecture: if the face is burned (part of head) and the torso, you add the corresponding percentages (e.g., face ≈ $4.5\%$ and torso ≈ $18\%$) to estimate the total burn area (≈ $22.5\%$ in this example).

Skin Cancers (UV-related and Other Causes)

• Cancer is a leading global disease; caused by mutations in DNA that disrupt cell cycle control and unchecked mitosis leading to tumors that can metastasize via blood vessels or lymphatics.

• UV radiation, carcinogens, and other agents increase skin cancer risk.

• Three main skin cancers:

  • Basal cell carcinoma (BCC): most common skin cancer; arises from keratinocytes in the stratum basale; typically occurs on UV-exposed skin; usually does not metastasize; treated successfully by surgical removal; appears as a nodule with a central crater.

  • Squamous cell carcinoma (SCC): second most common; arises from keratinocytes in the stratum spinosum; presents as scaly plaques that may ulcerate or bleed; more likely to metastasize than BCC; can be treated with surgery.

  • Malignant melanoma: cancer of melanocytes; highly metastatic; early detection critical due to rapid spread through dermal blood vessels into the bloodstream and lymphatics.

  • Treatments: surgical removal; may include radiation or chemotherapy depending on stage.

  • Prognosis depends on tumor size, depth of dermal invasion (e.g., Breslow depth), and presence of metastasis.

• Melanoma detection: ABCDE rule:

  • A: Asymmetry — one half does not match the other half.

  • B: Border irregularity — borders are notched or poorly defined.

  • C: Color variation — colors such as blue, black, or multiple colors indicate higher risk.

  • D: Diameter — lesions larger than $6\text{ mm}$ in diameter should be evaluated.

  • E: Evolution — changes in shape, size, or color over time require assessment.

Foundational Connections and Practical Implications

• Structure-function relationships:

  • Hair and nails are keratinized epithelia that provide protection, physical barriers, and manipulation capabilities; their growth is tied to matrix cells and surrounding dermal structures.

  • Gland types demonstrate specialization of epithelial derivatives for temperature regulation (sweat), lubrication and barrier function (sebum), and sensory/communication roles (cerumen, mammary secretions).

• Hormonal regulation and puberty:

  • Sex hormones stimulate sebum production, alter hair distribution, and influence acne prevalence.

  • Pubertal changes shift hair types (vellus to terminal) and affect sebaceous activity.

• Disease mechanisms and prevention:

  • Acne illustrates how sebaceous activity and keratinous occlusion contribute to pathology; hormones modulate risk.

  • Burns and wound management relate to barrier integrity, healing processes, and dehydration risk; rule-of-nines helps estimate fluid requirements and treatment needs.

  • Skin cancers underscore the consequences of UV exposure and the importance of early detection (ABCDE rule for melanoma).

• Metastasis and systemic implications:

  • Malignant melanoma metastasizes via dermal blood vessels and lymphatics, underscoring the need for early diagnosis and staging to guide treatment.

• Practical diagnostic cues:

  • Nail changes can reflect systemic or local issues (growth rate, lunula visibility, texture).

  • Skin lesions with changes in symmetry, border, color, diameter, and evolution warrant clinical evaluation per ABCDE criteria.

Quick Reference Formulas and Key Numbers

• Hair growth rate: $1 ext{-}1.5\ ext{cm/month}$

• Nail growth rate: $0.5\ \text{mm/week}$

• Rule of Nines (adult approximation): 11 areas × $9\%$ each; child proportions differ

• Diameter threshold for melanoma concern: >6\ \text{mm}

• Melanin/color references:

  • Little melanin → blonde hair

  • High melanin → black/brown hair

  • Iron-containing pigment → red hair

• Hair cycle timing: Anagen duration ranges from about $1\text{ month}$ to $6\text{ years}$ depending on hair location

• Secretion types:

  • Eccrine/apocrine/ceruminous/mammary glands secretion types and locations as described above

• Skin layers involved in burns and cancers:

  • Basal layer (stratum basale) → basal cell carcinoma

  • Spinous layer (stratum spinosum) → squamous cell carcinoma

  • Melanocytes (in the epidermis) → melanoma