The Integumentary System: Structure, Function, and Clinical Relevance

Section 5.1: The Skin

Objectives

• Describe the structure of the skin, including the layers of the epidermis and dermis.

• Explain the functions of the skin and its role in homeostasis.

• Understand the basis of skin pigmentation and how changes can indicate homeostatic imbalances.

Integumentary System Overview

• The integumentary system consists of: skin, hair, nails, sweat glands, and sebaceous glands.

Functions of the Skin

• Protection

  • Chemical barrier: Skin secretes chemicals such as:

    • Sweat (contains antimicrobial proteins)

    • Sebum and defensins (kill bacteria)

    • Acid mantle (low pH): stops some bacteria

    • Melanin: protects against UV radiation

  • Physical barrier:

    • Flat, dead, keratinized cells surrounded by glycolipids block most water and water-soluble substances

  • Biological barrier:

    • Cells such as dendritic cells and macrophages patrol the skin and activate the immune system

• Body Temperature Regulation

  • Insensible perspiration: Under normal conditions, sweat glands produce about 500 mL/day of unnoticeable sweat

  • Sensible perspiration: When body temperature rises, sweat gland activity increases, producing up to 12L of noticeable sweat

  • Cold environment: Dermal blood vessels constrict to conserve heat

• Cutaneous Sensations

  • Sensory receptors respond to stimuli such as temperature, touch, and pain

• Metabolic Functions

  • Skin synthesizes vitamin D (needed for calcium absorption in the intestine)

  • Requires exposure to UV light

  • Lack of vitamin D can lead to bone disorders such as rickets

• Blood Reservoir

  • Skin can hold up to 5% of the body's total blood volume

• Excretion

  • Skin excretes limited amounts of nitrogenous wastes (ammonia, urea, uric acid)

  • Sweating also causes salt and water loss

Structure of the Skin

• Epidermis: Superficial region; consists of epithelial tissue; avascular

• Dermis: Underlies epidermis; mostly fibrous connective tissue; vascular

• Hypodermis (superficial fascia): Subcutaneous layer deep to skin; mostly adipose tissue; absorbs shock and insulates; anchors skin to underlying structures (mostly muscles)

• Vascularity: Dermis and hypodermis are vascular; epidermis is avascular

Cells of the Epidermis

• Keratinocytes: Major cells of the epidermis

  • Produce keratin (protein that gives skin its protective properties)

  • Tightly connected by desmosomes

  • Millions slough off every day

• Melanocytes: Spider-shaped cells located in the deepest epidermis; produce melanin

• Dendritic (Langerhans) cells: Star-shaped macrophages that patrol deep epidermis; key activators of the immune system

• Tactile (Merkel) cells: Sensory receptors for touch

Layers of the Epidermis

• Stratum basale (basal layer): Deepest layer; single row of stem cells; 10-25% melanocytes

• Stratum spinosum (prickly layer): Several cell layers thick; contains prekeratin filaments attached to desmosomes; abundant melanosomes and dendritic cells

• Stratum granulosum (granular layer): Four to six cells thick; cells flatten, nuclei and organelles disintegrate; keratinization begins

• Stratum lucidum (clear layer): Found only in thick skin (palms, soles); thin, translucent band of dead keratinocytes

• Stratum corneum (horny layer): 20-30 rows of flat, anucleated, keratinized dead cells; accounts for three-quarters of epidermal thickness; protects deeper cells, prevents water loss, protects from abrasion and penetration

• Cell turnover: Cells change by apoptosis (controlled cell death); humans shed 50,000 cells every minute

Dermis

• Strong, flexible connective tissue

• Cells: fibroblasts, macrophages, occasionally mast cells and white blood cells

• Contains nerves, blood vessels, lymphatic vessels, hair follicles, oil glands, sweat glands

• Two layers:

  • Papillary layer: Areolar connective tissue; contains capillary loops, free nerve endings, touch receptors; dermal papillae form friction ridges (fingerprints)

  • Reticular layer: Dense irregular connective tissue; elastic fibers provide stretch-recoil properties; collagen fibers provide strength and resiliency; contains cutaneous plexus (network of blood vessels)

• Cleavage (tension) lines: Caused by collagen fibers running parallel to skin surface; important for surgical incisions

Skin Color

• Three pigments contribute to skin color:

  • Melanin: Only pigment made in skin; produced by melanocytes; packaged into melanosomes; protects DNA from UV damage; more UV exposure increases melanin production; differences in skin color due to amount and form of melanin

  • Carotene: Yellow to orange pigment; most obvious in palms and soles; from diet

  • Hemoglobin: Pinkish hue of fair skin due to oxygenated hemoglobin in blood

• Clinical relevance: Alterations in skin color can indicate disease:

  • Cyanosis: blue skin color (low oxygenation)

  • Pallor: blanching or pale color (anemia, low blood pressure, fear)

  • Erythema: redness (fever, hypertension, inflammation, allergy)

  • Jaundice: yellow cast (liver disorders)

  • Bruises: blue-black marks (blood under skin)

Section 5.2: Accessory Organs of the Integumentary System (Hair, Nails, and Glands)

Hair

• Consists of dead, keratinized cells

• Found all over the body except palms, soles, lips, nipples, and parts of external genitalia

• Functions:

  • Warn of insects on skin

  • Protect head from physical trauma

  • Protect from heat loss

  • Shield skin from sunlight

• Structure:

  • Flexible strands of dead, keratinized cells

  • Produced by hair follicles

  • Regions: shaft (keratinization complete), root (keratinization ongoing)

  • Three parts of hair shaft:

    1. Medulla: central core of large cells and air spaces

    2. Cortex: several layers of flattened cells surrounding medulla

    3. Cuticle: outer layer of overlapping single cells

  • Hair pigments made by melanocytes in hair follicles

• Hair Follicle Structure:

  • Extends from epidermal surface into dermis

  • Hair bulb: expanded area at deep end of follicle

  • Hair follicle receptor (root hair plexus): sensory nerve endings wrap around bulb

  • Hair matrix: actively dividing area of bulb that produces hair cells

  • Arrector pili: small band of smooth muscle attached to follicle; responsible for goose bumps

Nails

• Scale-like modifications of epidermis containing hard keratin

• Act as protective covers for distal, dorsal surfaces of fingers and toes

• Consist of:

  • Free edge

  • Nail plate

  • Root

  • Nail bed: epidermis underneath keratinized nail plate

• Structures:

  • Nail folds: skin folds that overlap nail borders

  • Eponychium: nail fold that projects onto nail body (cuticle)

  • Hyponychium: area under free edge of plate that accumulates dirt

  • Lunule: thickened nail matrix, appears white

  • Abnormal color/shape can indicate disease (e.g., spoon nail in iron deficiency)

Glands

• Sweat (Sudoriferous) Glands: All skin surfaces except nipples and parts of external genitalia contain sweat glands

  • Two main types:

    1. Eccrine (merocrine) sweat glands: most numerous; abundant on palms, soles, forehead; ducts connect to pores; function in thermoregulation; secretion is 99% water plus salts, vitamin C, antibodies, dermcidin, metabolic wastes

    2. Apocrine sweat glands: confined to axillary and anogenital areas; secrete viscous milky/yellowish sweat; begin functioning at puberty; ducts empty into hair follicles; modified apocrine glands include ceruminous (earwax) and mammary (milk) glands

• Sebaceous (Oil) Glands: Widely distributed except thick skin of palms and soles; most develop from hair follicles; secrete sebum (oily holocrine secretion); bactericidal properties; softens hair and skin; relatively inactive until puberty (stimulated by hormones)

Section 5.3: Homeostatic Imbalances of the Integumentary System

Skin Cancer

• Most skin tumors are benign (non-cancerous) and do not metastasize

• Risk factors: overexposure to UV radiation, frequent skin irritation

• Three major types:

  1. Basal cell carcinoma: Least malignant, most common; stratum basale cells proliferate and invade dermis/hypodermis; cured by surgical excision in 99% of cases

  2. Squamous cell carcinoma: Second most common; can metastasize; involves keratinocytes of stratum spinosum; usually a scaly reddened papule; good prognosis if treated

  3. Melanoma: Cancer of melanocytes; most dangerous; highly metastatic and resistant to chemotherapy; treated by wide surgical excision and immunotherapy; ABCD rule for detection:

     • A: Asymmetry

     • B: Border irregularity

     • C: Color (multiple colors)

     • D: Diameter (>6mm)

Burns

• Tissue damage caused by heat, electricity, radiation, or chemicals; denaturation of proteins destroys cells

• Immediate threat: dehydration and electrolyte imbalance; can lead to renal shutdown and circulatory shock

• Rule of Nines: Body divided into 11 sections, each representing 9% of body surface (except genitals, 1%); used to estimate fluid loss

• Degrees of Burns:

  • First-degree: epidermal damage only; localized redness, edema, pain

  • Second-degree: epidermal and upper dermal damage; blisters appear; partial-thickness burns

  • Third-degree: entire thickness of skin involved; skin color gray-white, cherry red, or black; no edema or pain (nerve endings destroyed); requires skin grafting

Tables

Table: Layers of the Epidermis

Layer	Location	Main Features
Stratum basale	Deepest	Single row of stem cells, mitosis, melanocytes
Stratum spinosum	Above basale	Prekeratin filaments, desmosomes, dendritic cells
Stratum granulosum	Middle	Keratinization begins, cells flatten
Stratum lucidum	Thick skin only	Translucent, dead keratinocytes
Stratum corneum	Superficial	20-30 layers, dead keratinized cells

Table: Types of Skin Cancer

Type	Origin	Features	Prognosis
Basal cell carcinoma	Stratum basale	Slow-growing, rarely metastasizes	Excellent (99% cured by excision)
Squamous cell carcinoma	Stratum spinosum	Scaly, red papule, can metastasize	Good if treated early
Melanoma	Melanocytes	Highly metastatic, resistant to therapy	Poor unless detected early

Key Equations

• Rule of Nines (Burns): $\text{Total Body Surface Area Burned} = \sum_{i=1}^{n} (\text{Area}_i \times 9\%)$ Used to estimate fluid loss in burn patients.

Additional info:

• Vitamin D synthesis in skin is initiated by UVB radiation, which converts 7-dehydrocholesterol to cholecalciferol (vitamin D3).

• Keratinocytes are the most abundant cell type in the epidermis and play a crucial role in the barrier function of skin.

• Apoptosis in epidermal cells is essential for the regular turnover and renewal of the skin surface.