Skin Appendages and Functions: Hair, Nails, Glands, and Clinical Aspects

Skin Appendages: Hair/Fur

Structure and Function of Hair

Hair (or fur) consists of dead, keratinized cells and is a key appendage of the skin in mammals. It is absent from certain regions such as the palms, soles, lips, nipples, and parts of the external genitalia in humans. Hair serves several important functions:

• Warning of insects on the skin

• Protection from trauma (hair on the head)

• Insulation to reduce heat loss

• Shielding skin from sunlight

Hair is produced by hair follicles and contains hard keratin, which is tougher and more durable than the soft keratin found in the skin. The hair shaft has three regions:

• Medulla: Central core with cells and air spaces

• Cortex: Several layers of flattened cells surrounding the medulla

• Cuticle: Outermost layer of overlapping single cells

Hair color is determined by melanins produced by melanocytes in the hair follicle.

Structure of a Hair Follicle

The hair follicle extends from the epidermal surface into the dermis. Key components include:

• Hair bulb: Expanded area at the deep end of the follicle

• Hair follicle receptor (root hair plexus): Sensory nerve endings that wrap around the bulb, making hair a sensory receptor

• Wall of follicle: Composed of peripheral connective tissue sheath (from dermis), glassy membrane (thickened basal lamina), and epithelial root sheath (from epidermis)

• Hair matrix: Actively dividing area that produces hair cells

• Arrector pili: Small band of smooth muscle attached to follicle, responsible for 'goosebumps'

• Hair papilla: Dermal tissue with capillaries supplying nutrients

Skin Appendages: Nails

Structure and Function of Nails

Nails are scale-like modifications of the epidermis containing hard keratin. They protect the distal, dorsal surfaces of fingers and toes. The main parts of the nail include:

• Free edge

• Nail plate

• Nail root

• Nail bed: Epidermis beneath the nail plate

• Eponychium (cuticle): Nail fold projecting onto the nail body

• Lunule: White, crescent-shaped area due to thickened nail matrix

Skin Appendages: Sweat and Sebaceous Glands

Sweat (Sudoriferous) Glands

Sweat glands are distributed over most skin surfaces except nipples and parts of the external genitalia. There are about 3 million per person. Two main types:

• Eccrine (merocrine) sweat glands: Most numerous, abundant on palms, soles, and forehead. Ducts open to pores on the skin surface. Function in thermoregulation and secrete sweat (mostly water, salts, vitamin C, antibodies, dermicidin, metabolic wastes).

• Apocrine sweat glands: Confined to axillary and anogenital areas. Secrete viscous, milky or yellowish sweat containing fatty substances and proteins. Ducts empty into hair follicles. Begin functioning at puberty and may act as sexual scent glands. Modified apocrine glands include ceruminous (earwax) and mammary (milk) glands.

Sebaceous (Oil) Glands

Sebaceous glands are holocrine glands widely distributed except on the palms and soles. Most develop from hair follicles and secrete into them. They are relatively inactive until puberty, when they are stimulated by hormones (especially androgens). They secrete sebum (an oily, bactericidal substance that softens hair and skin).

Functions of the Skin

Protection

The skin acts as a barrier against microorganisms, abrasions, temperature extremes, and chemicals. It provides three types of barriers:

• Chemical barrier: Sweat (antimicrobial proteins), sebum, defensins, acid mantle (low pH), and melanin (UV protection)

• Physical barrier: Keratinized cells and glycolipids block water and water-soluble substances; some chemicals can penetrate (e.g., lipid-soluble substances, organic solvents, heavy metals, some drugs)

• Biological barrier: Dendritic cells in the epidermis and macrophages in the dermis activate immune responses

Body Temperature Regulation

The skin regulates body temperature through sweat production and blood vessel dilation/constriction:

• Insensible perspiration: ~500 ml/day under normal conditions

• Sensible perspiration: Up to 12 L/day when body temperature rises

• Constriction of dermal blood vessels reduces heat loss in cold environments

Cutaneous Sensations

The skin contains sensory receptors for touch, pressure, temperature, and pain:

• Exteroreceptors: Detect external stimuli (temperature, touch, pressure)

• Free nerve endings (nociceptors): Sense pain

Metabolic Functions

• Synthesizes vitamin D for calcium absorption

• Keratinocytes disarm carcinogens and activate some hormones

• Produces collagenase for collagen turnover

Blood Reservoir and Excretion

• Holds up to 5% of total blood volume

• Can constrict vessels to redirect blood to other organs

• Excretes nitrogenous wastes (ammonia, urea, uric acid) and salt/water via sweat

Burns

Classification by Depth

• 1st Degree: Epidermis only

• 2nd Degree: Epidermis and upper dermis (papillary layer)

• 3rd Degree: Epidermis and most/all of dermis

• Full Thickness (4th Degree): Epidermis, dermis, and sometimes hypodermis

Classification by Surface Area

• Rule of Nines: Divides body into regions, each accounting for 9% (or multiples) of total body surface area (BSA)

• Lund and Browder method: More precise, divides body into smaller regions

Clinical Terms Related to Skin

• Albinism: Genetic lack of melanin production

• Alopecia: Hair loss

• Boils and carbuncles: Infections of hair follicles and sebaceous glands

• Cold sores (fever blisters): Viral infections (herpes simplex)

• Contact dermatitis: Allergic reaction causing skin inflammation

• Dermatology: Study of skin and its diseases

• Eczema: Chronic inflammatory skin condition

• Psoriasis: Chronic autoimmune skin disease

• Rosacea: Chronic facial skin disorder

• Vitiligo: Loss of skin pigment in patches