Skin Appendages and Homeostatic Imbalances: Structure, Function, and Disorders

Skin and Body Membranes

Skin Appendages

The skin appendages include cutaneous glands, hair and hair follicles, and nails. Although these structures are physically located in the dermis, they are produced by cells originating in the epidermal layer and play essential roles in maintaining body homeostasis.

• Cutaneous glands: Exocrine glands releasing secretions to the skin surface via ducts; include sebaceous and sweat glands.

• Hair and hair follicles: Provide protection and sensory functions.

• Nails: Protective coverings for the fingers and toes.

Cutaneous Glands

Cutaneous glands are exocrine glands formed by cells of the stratum basale, residing mostly in the dermis. They are divided into sebaceous (oil) glands and sweat glands.

• Sebaceous (Oil) Glands: Found everywhere except palms and soles; ducts usually empty into hair follicles or directly onto skin.

• Sweat (Sudoriferous) Glands: Widely distributed; more than 2.5 million per person; two types: eccrine and apocrine.

Sebaceous (Oil) Glands

• Sebum: Oily mixture of substances and fragmented cells; lubricates skin, prevents hair brittleness, and contains antibacterial chemicals.

• Activation: Increased during adolescence due to androgen production, leading to oilier skin.

• Homeostatic Imbalances:

  • Acne: Blocked sebaceous ducts; active infection; can cause scarring.

  • Blackhead: Oxidized, dried sebum.

  • Whitehead: Non-oxidized, undried sebum.

  • Seborrhea (Cradle Cap): Overactivity; pink lesions, yellow-brown crust, oily scales.

Sweat Glands

• Eccrine Glands: Most numerous; found all over body; produce sweat (water, salts, vitamin C, metabolic wastes, lactic acid); sweat is acidic (pH 4-6).

• Function: Heat regulation; sweat evaporates, carrying body heat away; can lose up to 7 liters of water on a hot day.

• Apocrine Glands: Confined to axillary and genital areas; larger; ducts empty into hair follicles; secretion contains fatty acids and proteins, may be milky/yellowish; odorless until metabolized by skin bacteria.

• Activation: Begin functioning at puberty; minimal role in thermoregulation; activated by pain, stress, and sexual arousal.

Hair and Hair Follicles

Hair is produced by hair follicles and found almost everywhere except palms, soles, nipples, and lips. Hair serves protective functions and is influenced by hormones.

• Structure:

  • Root: Enclosed in follicle.

  • Shaft: Projects from skin surface.

  • Matrix: Growth zone at deep end of follicle; cells divide, become keratinized, and die.

  • Medulla: Central core; large cells and air spaces.

  • Cortex: Several layers of flattened cells.

  • Cuticle: Outermost; single layer of overlapping cells; heavily keratinized for strength.

• Pigmentation: Melanocytes in hair bulb produce melanin; different types and amounts create hair color.

• Shape: Oval shaft = wavy hair; flat shaft = curly/kinky; round shaft = straight/coarse.

• Hair Follicle Structure:

  • Inner epithelial root sheath: Forms the hair.

  • Outer fibrous sheath: Dermal connective tissue; supplies blood vessels.

  • Hair papilla: Blood supply to matrix.

  • Arrector pili: Smooth muscle; contracts to produce "goose bumps".

Nails

Nails are scalelike modifications of the epidermis, corresponding to hooves or claws in other animals. They protect the tips of fingers and toes.

• Structure:

  • Free edge: Distal part.

  • Body: Visible attached portion.

  • Root: Embedded in skin.

  • Nail folds: Overlapping skin borders.

  • Cuticle: Edge of proximal nail fold.

  • Nail bed: Stratum basale extends beneath nail.

  • Nail matrix: Responsible for nail growth; cells become keratinized and die.

  • Lunule: White crescent over matrix.

• Color: Nails are transparent; appear pink due to underlying blood supply; cyanotic (blue) cast indicates low oxygen.

Homeostatic Imbalances of the Skin

Infections and Allergies

Skin disorders commonly result from infections (bacteria, viruses, fungi) or allergies (abnormal immune responses).

• Athlete’s foot: Itchy, red, peeling between toes; caused by Tinea pedis fungus.

• Boils and carbuncles: Inflammation of hair follicles; carbuncles are clusters, often caused by Staphylococcus aureus.

• Cold sores: Fluid-filled blisters; caused by human herpesvirus 1; dormant in cutaneous nerves, activated by stress, fever, UV.

• Contact dermatitis: Itching, redness, swelling, blistering; caused by exposure to chemicals provoking allergic responses.

• Impetigo: Pink, fluid-filled, raised lesions with yellow crust; caused by staphylococcus or streptococcus; common in children.

• Psoriasis: Red, dry, silvery scales; chronic autoimmune disorder; rapid overproduction of skin cells; triggered by trauma, infection, hormones, or stress.

Burns

Burns are tissue damage caused by heat, electricity, UV radiation, or chemicals, leading to protein denaturation and cell death. Burns compromise skin integrity, resulting in fluid loss and increased risk of infection.

• Fluid loss: Leads to dehydration and electrolyte imbalance; can cause kidney shutdown and circulatory shock.

• Rule of Nines: Used to estimate extent of burns; divides body into 11 areas, each accounting for 9% of total surface area, plus 1% for perineum.

Body Region	% of Total Surface Area
Head and Neck	9%
Each Arm	9%
Each Leg	18%
Anterior Trunk	18%
Posterior Trunk	18%
Perineum	1%
Additional info: Table inferred from "rule of nines" description.

• Infection: Major threat after 24 hours; pathogens invade dead tissue; immune system depressed after severe burns.

• Burn Classification:

  • First-degree: Only epidermis damaged; red, swollen; heals in 2-3 days.

  • Second-degree: Epidermis and superficial dermis; red, painful, blistered; regeneration possible.

  • Third-degree: Epidermis and dermis destroyed; blanched or blackened; nerve endings destroyed; regeneration not possible; requires skin grafting.

  • Fourth-degree: Extends into deeper tissues (bone, muscle, tendons); dry, leathery; requires surgery/grafting; may require amputation.

• Critical Burns: Over 30% body with second-degree, over 10% with third/fourth-degree, burns of face/hands/feet/genitals, airway involvement, circumferential burns.

• Example: No hair regeneration in third-degree burns due to destruction of hair follicles.

Skin Cancer

Skin cancer is the most common cancer in humans. Most neoplasms are benign, but some are malignant and can metastasize. The main risk factor is overexposure to UV radiation.

• Basal Cell Carcinoma: Least malignant, most common; stratum basale cells invade dermis/subcutaneous tissue; shiny, dome-shaped nodules with "pearly" edge; slow-growing; rarely metastasizes; surgical removal cures 99%.

• Squamous Cell Carcinoma: Arises from stratum spinosum; scaly, reddened papules; forms shallow ulcers with raised borders; grows rapidly, can metastasize; induced by UV exposure; early removal offers good prognosis.

• Malignant Melanoma: Cancer of melanocytes; deadly; appears as spreading brown/black patch; metastasizes rapidly; survival about 50%; early detection critical.

Type	Origin	Appearance	Malignancy
Basal Cell Carcinoma	Stratum basale	Shiny, dome-shaped nodule	Least malignant
Squamous Cell Carcinoma	Stratum spinosum	Scaly, reddened papule	Moderate
Malignant Melanoma	Melanocytes	Spreading brown/black patch	Most malignant

• ABCDE Rule for Melanoma Detection:

  • A: Asymmetry

  • B: Border irregularity

  • C: Color variation

  • D: Diameter > 6 mm

  • E: Evolution (changes in ABCD)

• Treatment: Wide surgical excision, immunotherapy, radiation, chemotherapy.

Developmental and Aging Aspects of Skin and Appendages

Fetal and Newborn Skin

• Lanugo: Downy hair covering fetus during fifth and sixth months.

• Vernix caseosa: Oily coating produced by sebaceous glands; protects newborn skin.

• Newborn skin: Thin, visible blood vessels; thickens and accumulates subcutaneous fat with growth.

Adolescence and Adulthood

• Adolescence: Sebaceous glands activated; skin and hair become oilier; acne may appear.

• Adulthood: Acne subsides; skin optimal in twenties/thirties; then changes due to environmental exposure and aging.

Aging

• Subcutaneous tissue decreases: Increased sensitivity to cold.

• Decreased oil production: Drier, itchier skin.

• Thinning skin: More susceptible to injury and bruising.

• Loss of elasticity: Bags under eyes, sagging jowls; accelerated by smoking and sun exposure.

• Hair changes: Loss of luster, follicle decline, thinning, alopecia (baldness), graying due to decreased melanin.

Homeostatic Imbalance: Hair Loss and Graying

• Causes: Emotional stress, protein-deficient diets, chemotherapy, radiation, excessive vitamin A, fungal diseases.

• Permanent vs. Temporary: Non-genetic causes usually result in temporary hair loss.

Additional info: The rule of nines table was inferred from the description. The comparison table for skin cancers was constructed based on the text. Academic context was added to clarify the structure and function of skin appendages and their role in homeostasis.