13.2 Skin appendages (hair, nails, and glands)

The skin's appendages—hair, nails, and glands—play crucial roles in protecting our bodies and regulating temperature. These structures, though often overlooked, are essential for maintaining healthy skin and overall well-being.

Hair and nails provide physical protection, while sebaceous and sweat glands contribute to skin barrier function and temperature control. Understanding how these appendages work together helps us appreciate the complexity of our largest organ—the skin.

Hair follicle structure and growth

Hair follicle anatomy and hair shaft composition

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• Hair follicles are tube-shaped structures in the dermis and epidermis that produce and anchor hair shafts
  • Each follicle contains a hair bulb at the base, which is fed by the dermal papilla
• The hair shaft consists of three main layers: an outer cuticle, cortex, and inner medulla
  • The cuticle is formed by overlapping keratinocytes, providing protection and shine
  • The cortex contains keratin and pigment, determining the hair's strength and color
  • The medulla is a loose core of cells in the center of the hair shaft (not always present)

Hair color and growth cycle phases

• Hair color is determined by the amount and type of melanin produced by melanocytes in the hair bulb
  • Eumelanin produces dark hair colors (black and brown)
  • Pheomelanin produces lighter hair colors (blonde and red)
• Hair growth occurs in three distinct phases: anagen (active growth), catagen (transitional phase), and telogen (resting phase)
  • Anagen phase lasts 2-6 years for scalp hair, during which the hair shaft continuously grows (85-90% of hair is in this phase at any given time)
  • Catagen phase lasts 2-3 weeks, during which the follicle shrinks and detaches from the dermal papilla (1-2% of hair is in this phase)
  • Telogen phase lasts 2-4 months, during which the follicle is dormant and the hair may shed (10-15% of hair is in this phase)
    • A new anagen phase begins after telogen, restarting the growth cycle

Sebaceous and sweat glands: Composition and function

Sebaceous glands and sebum production

• Sebaceous glands are holocrine glands associated with hair follicles that secrete an oily substance called sebum
  • Sebum lubricates and waterproofs the skin and hair, helping to maintain the skin's barrier function
  • Sebum is composed of triglycerides, wax esters, squalene, and dead sebocytes (its composition varies based on diet, genetics, and hormones)
• Sebaceous glands are most abundant on the face and scalp
  • Overproduction of sebum can contribute to the development of acne
  • Sebum production is regulated by hormones, particularly androgens (testosterone)

Eccrine and apocrine sweat glands

• Sweat glands are coiled tubular glands that produce sweat to regulate body temperature and excrete waste
• Eccrine glands are abundant throughout the skin and produce a clear, odorless fluid
  • Eccrine sweat helps cool the body through evaporation and is innervated by the sympathetic nervous system
  • They are most dense on the palms, soles, and forehead
• Apocrine glands are larger and open into hair follicles in the axillary and genital regions
  • They produce a thicker, protein-rich sweat that can develop odor when metabolized by bacteria
  • Apocrine glands become active during puberty and respond to emotional stimuli (stress, anxiety)

Nail layers and growth

Nail anatomy and structure

• Nails are hardened plates of keratin that protect the tips of fingers and toes
• They consist of several parts: nail plate, nail bed, nail folds, cuticle, lunula, and matrix
  • The nail plate is the visible part of the nail composed of dead, keratinized cells (translucent, allowing the underlying blood vessels to be seen)
  • The nail bed is the skin beneath the nail plate containing blood vessels and sensory nerves (adheres tightly to the nail plate)
  • The nail folds are the skin that frames the nail on three sides (the proximal fold extends over the nail root and contains the cuticle)
  • The cuticle is a thin strip of tissue that seals the nail plate to the proximal fold (prevents infection)
  • The lunula is the white, half-moon shaped area at the base of the nail plate (part of the matrix, appears white due to its thickness)

Nail growth and health indicators

• Nail growth occurs in the matrix, a layer of rapidly dividing cells at the proximal end of the nail bed
  • As new cells are produced, older cells are pushed forward and become keratinized, forming the nail plate
  • Fingernails grow an average of 3 mm per month, while toenails grow about 1 mm per month (growth rates are affected by age, nutrition, and health status)
• Nails are an indicator of overall health
  • Changes in color, texture, or shape can signify underlying medical conditions (anemia, malnutrition, liver disease, etc.)
  • Healthy nails are smooth, uniform in color, and free of ridges or discoloration

Skin appendages: Thermoregulation and protection

Physical protection provided by hair and nails

• Hair and nails provide physical protection to the skin and underlying tissues
  • Hair shields the scalp from UV radiation and trauma (acts as a barrier)
  • Nails protect the tips of digits from injury and enhance fine motor skills
• Hair also plays a role in thermoregulation by trapping air close to the skin, providing insulation
  • Arrector pili muscles attached to hair follicles contract in response to cold or emotional stimuli, causing goosebumps and increasing insulation

Sebum and eccrine sweat in skin barrier function and thermoregulation

• Sebaceous glands contribute to the skin's barrier function by producing sebum
  • Sebum waterproofs and lubricates the skin and hair, helping prevent moisture loss and entry of pathogens
  • It also maintains the skin's slightly acidic pH, which inhibits bacterial growth
• Eccrine sweat glands are crucial for thermoregulation through evaporative cooling
  • When body temperature rises, sweat production increases (controlled by the hypothalamus in response to changes in core body temperature)
  • As sweat evaporates from the skin surface, it removes heat and cools the body
    • Sweating is less effective in high humidity because the air is already saturated with moisture, reducing evaporation
• Apocrine sweat glands respond to emotional and hormonal stimuli (stress, fear)
  • While their function in humans is not fully understood, they may play a role in pheromone production and social communication in other mammals