Integumentary System Lecture Outline

Overview of the Integumentary System

• Definition: The integumentary system consists of the skin and accessory structures, such as hair, glands, and nails.

• Etymology: The word integument means covering.

• Diagnostic Significance: The overall physical appearance of the integumentary system can indicate physiological imbalances and underlying health conditions within the body.

• Common Skin Conditions:

  • Skin Cancer

  • Acne

  • Rosacea

  • Eczema

  • Psoriasis

  • Birthmarks

  • Cysts

  • Hair Loss

  • Rashes

  • Skin Tags

  • Warts

  • Melasma

  • Sun Exposure Damage

Functions of the Integumentary System

1. Protection

• Reduction in Body Water Loss: Prevents excessive fluid loss from internal body tissues.

• Microbial and Foreign Barrier: Acts as a physical barrier that prevents microorganisms and other foreign substances from entering the body.

• Abrasion Defense: Protects underlying structures against physical friction and mechanical damage.

• Ultraviolet Light Defense: Melanin absorbs ultraviolet (UV) light, protecting deeper structures from damaging radiation.

• Hair Protection:

  • Hair on the head functions as a heat insulator.

  • Eyebrows prevent sweat from dripping into the eyes.

  • Eyelashes shield the eyes from foreign objects.

  • Hair inside the nose and ears prevents the entry of dust and other foreign particulate matter.

• Nail Protection: Nails shield the terminal ends of the fingers and toes from mechanical damage and can be utilized in defense.

2. Sensation

• Sensory Receptors: Numerous specialized sensory receptors are embedded within the epidermis and dermis.

• Detectable Stimuli: Receptors detect sensation for pain, heat, cold, and pressure.

• Hair Movement Sensation: Although hair shafts lack a direct nerve supply, sensory nerve receptors surrounding each hair follicle detect the slightest movement of a hair.

• Goosebumps (Cutis Anserina): Goosebumps occur when tiny smooth muscle bundles in the skin, called arrector pili muscles, contract. This lifts the skin and causes hair shafts to stand erect, particularly on the arms and legs.

3. Vitamin D Production

• Synthesis Pathway:

  1. Sunlight exposure introduces UV light to the skin, causing UV light to react with an enzyme precursor known as $7\text{-dehydrocholesterol}$.

  2. This chemical reaction produces pre-vitamin D.

  3. Pre-vitamin D rearranges its molecular structure to form vitamin D.

  4. The precursor molecule is transported by the blood to the liver, where it undergoes enzymatic conversion.

  5. The enzymatically converted molecule is then transported via the bloodstream to the kidneys, where it is converted again into the active form of vitamin D.

• Physiological Function: Active vitamin D stimulates the small intestine to absorb calcium and phosphate, which are vital for multiple physiological processes.

4. Temperature Regulation

• Importance: Regulating body temperature is essential because the rates of chemical reactions in the body change with temperature. Slight temperature variations cause enzymes to function less efficiently and disrupt metabolic chemical processes.

• Thermal Stress Factors: Exercise, fever, and elevations in environmental temperature tend to raise overall body temperature.

• Heat Dissipation Mechanisms:

  • To maintain homeostasis, dermal blood vessels dilate, allowing increased blood flow to the skin so heat can dissipate through the epidermis.

  • Sweat production facilitates heat loss via evaporative cooling.

• Heat Conservation Mechanisms: When body temperature drops below normal, dermal blood vessels constrict, decreasing blood flow to the skin surface to conserve core body heat.

5. Excretion

• Minor Role: The integumentary system plays a minor overall role in removing waste products from the body.

• Sweat Composition: Sweat consists mainly of water and salts, but also contains minor concentrations of waste products such as urea, uric acid, and ammonia.

• Excretory Limit: Although the body can lose large quantities of sweat, sweat glands do not serve as a primary system for waste excretion.

6. Diagnostic Aid

• Clinical Utility: Because the skin is externally accessible, it serves as a valuable diagnostic tool.

• Cyanosis: A bluish discoloration of the skin caused by reduced blood $O_2$ content, serving as an indicator of impaired respiratory or circulatory function.

• Jaundice: A yellowish discoloration of the skin that occurs when the liver is damaged by diseases such as viral hepatitis.

• Rashes and Lesions: Skin lesions or rashes frequently present as symptoms of internal systemic health issues elsewhere in the body.

Structure of the Skin

• Major Layers: The skin is composed of two primary tissue layers:

  • Epidermis: The most superficial layer, consisting of epithelial tissue resting upon the dermis.

  • Dermis: The deep layer, consisting of dense connective tissue.

• Subcutaneous Tissue (Hypodermis): A connective tissue layer beneath the dermis upon which the skin rests. The subcutaneous tissue is explicitly not part of the skin.

Epidermis

• Tissue Classification: The epidermis, also known as the cutaneous membrane, is composed of keratinized stratified squamous epithelium.

• Primary Function: Prevents body water loss and resists mechanical abrasion.

• Stratification: Composed of distinct layers referred to as strata.

  • Stratum Corneum: The most superficial stratum, consisting of dead, flattened squamous cells filled with the protein keratin.

  • Keratin Function: Provides structural strength and durability to the stratum corneum.

  • Deep Strata: Cells within the deepest epidermal strata undergo continuous mitosis. As newly created cells form, they push older cells toward the surface, where they eventually slough or flake off.

• Epidermal Clinical Manifestations:

  • Dandruff: Excessive sloughing of dead stratum corneum cells from the surface of the scalp.

  • Callus: A localized, thickened area of the stratum corneum produced by a friction-induced increase in cell layers.

  • Corn: A specialized cone-shaped structural thickening of the stratum corneum that develops over a bony prominence.

Dermis

• Tissue Composition: Dense collagenous connective tissue containing fibroblasts, adipocytes, and macrophages.

• Structures Retained: Nerves, hair follicles, smooth muscle tissue, cutaneous glands, and lymphatic vessels extend throughout the dermis.

• Structural Fibers:

  • Collagen Fibers: Oriented in multiple directions to provide structural strength and resistance to stretch.

  • Elastic Fibers: Provide elastic recoil and skin extensibility.

• Cleavage Lines (Tension Lines):

  • Formed because collagen fibers are aligned more heavily in certain directions than others.

  • Cleavage lines offer higher resistance to stretch.

  • A surgical incision made parallel to cleavage lines gaps less and forms less scar tissue than an incision made across them.

• Stretch Marks: Occur when the skin is overstretched, producing damage within the dermis layer.

Skin Color and Pigmentation

• Determining Factors: Pigments in the skin, blood flow circulating through dermal vessels, and thickness of the stratum corneum.

• Primary Pigments:

  • Carotene: A yellow pigment derived from plant foods such as carrots and squash. Carotene accumulates in skin lipids and acts as an antioxidant protecting against damaging free radicals.

  • Melanin: A group of pigments primarily responsible for skin, hair, and eye coloration.

• Melanin Characteristics:

  • Ranges from brown to black, with some yellow or reddish variations.

  • Provides protection against harmful ultraviolet radiation from sunlight.

• Melanin Transfer:

  1. Melanin is synthesized by specialized cells called melanocytes.

  2. Melanin molecules are packaged into intracellular vesicles called melanosomes.

  3. Melanosomes migrate into long cellular processes extending from the melanocytes.

  4. Surrounding epithelial cells phagocytize the tips of the melanocyte processes, incorporating the melanosomes into their cytoplasm.

• Melanin Variations:

  • Freckles and Moles: Caused by localized accumulations of high amounts of melanin.

  • Regulation: Melanin synthesis is controlled by genetic factors, light exposure, and hormonal influences.

  • Racial Variations: All human races possess approximately the same total number of melanocytes. Differences in skin color are determined by the amount, chemical type, and physical distribution of melanin.

  • Suntan: Direct exposure to UV light stimulates melanocytes to increase melanin production.

  • Albinism: A single genetic mutation preventing melanin production despite many genes controlling skin pigmentation.

Types of Skin Cancer

• Basal Cell Carcinoma:

  • Affects stem cells in the stratum basale layer.

  • Most common form of skin cancer.

  • Typically removed successfully through surgical excision.

• Squamous Cell Carcinoma:

  • Affects keratinocytes above the stratum basale layer.

  • Second most common form of skin cancer.

  • Can metastasize and cause death.

• Malignant Melanoma:

  • Arises directly from melanocytes, often within a pre-existing mole.

  • Rare form, but represents the most serious and deadly skin cancer type.

• ABCDE Rule for Melanoma Assessment:

  • A - Asymmetry: Benign moles are symmetrical; malignant melanomas are asymmetrical (one half does not match the other).

  • B - Border: Benign moles have smooth, uniform borders; malignant lesions have irregular, notched, or scalloped borders.

  • C - Color: Benign moles display a uniform color (usually brown); malignant lesions exhibit inconsistent colors (brown, black, red, white, or blue shades).

  • D - Diameter: Benign moles are typically smaller than $6\,\text{mm}$ (pencil eraser size); malignant lesions are usually larger than $6\,\text{mm}$.

  • E - Evolving: Benign moles remain static; malignant lesions change in size, shape, color, elevation, or present new symptoms like bleeding, itching, or crusting.

Subcutaneous Tissue (Hypodermis)

• Anatomical Relations: The skin rests upon the subcutaneous tissue, which attaches skin to underlying bone and muscle while supplying it with blood vessels and nerve branches.

• Tissue Structure: Loose connective tissue containing abundant adipose tissue.

• Lipid Reserve: Contains approximately half ($50\%$) of the body's stored lipids.

• Physiological Functions: Serves as thermal insulation and mechanical padding.

• Variations: Adipose tissue quantity and physical location vary based on age, sex, and diet.

• Body Fat Assessment: Used clinically to estimate total body fat.

  • Acceptable Body Fat Range (Females): $21\%$ to $30\%$

  • Acceptable Body Fat Range (Males): $13\%$ to $25\%$

Hair Anatomy and Physiology

• Distribution: Present everywhere on human skin except the palms, soles, lips, nipples, parts of the genitalia, and distal segments of fingers and toes.

• Structural Components:

  • Hair Follicle: An tubular invagination of the epidermis extending deep into the dermis layer.

  • Hair Shaft: The portion of hair protruding above the skin surface.

  • Hair Root: The portion of hair located below the skin surface.

  • Hair Bulb: The expanded base located at the bottom of the hair root.

  • Hair Papilla: An extension of dermal connective tissue containing blood vessels that projects directly into the hair bulb.

• Histological Layers:

  • Medulla: The soft central core of the hair.

  • Cortex: The hard tissue layer surrounding the medulla.

  • Cuticle: A single outer layer of overlapping cells holding the hair securely inside the follicle.

• Growth Cycles:

  • Growth Stage: Epithelial cells within the hair bulb divide via mitosis, undergo keratinization, and elongate the hair shaft.

  • Resting Stage: Mitosis ceases, growth stops, and the hair remains anchored in the follicle.

  • Replacement: When a new growth stage begins, a new hair forms inside the hair bulb, pushing the old hair out of the follicle.

• Cycle Durations:

  • Eyelashes: Growth stage lasts approx. $30\,\text{days}$; resting stage lasts approx. $105\,\text{days}$.

  • Scalp Hair: Growth stage lasts approx. $3\,\text{years}$; resting stage lasts approx. $1\text{--}2\,\text{years}$.

• Hair Pigmentation:

  • Hair color is generated by varying types and amounts of melanin.

  • With aging, melanin production in hair follicles declines, causing hair to fade or become white when melanin is completely absent.

Glands of the Skin

Sebaceous Glands

• Anatomical Structure: Simple, branched acinar glands.

• Connections: Most sebaceous glands connect via a short duct to the superficial portion of a hair follicle.

• Secretion Mechanism: Holocrine secretion (entire cells rupture to release product).

• Secretory Product: Produces sebum, an oily white substance rich in lipids.

• Function: Sebum lubricates hair and the skin surface to prevent drying and provides antimicrobial protection against certain bacteria.

Sweat Glands

• Classifications: Divided into eccrine and apocrine sweat glands.

Eccrine Sweat Glands

• Anatomical Structure: Simple, coiled, tubular glands.

• Secretion Mechanism: Merocrine secretion.

• Distribution: Distributed across almost every region of the skin, but present in highest density on the palms and soles.

• Duct Opening: Ducts open directly onto the skin surface through sweat pores.

• Function: Functions primarily in thermal regulation by releasing a secretion that is mostly water with a few dissolved salts.

• Emotional Sweating: Triggered in palms, soles, armpits, and other areas during stress.

• Hyperhidrosis: A clinical condition characterized by excessive sweating unrelated to thermal heat or physical exercise, caused by faulty nerve signals triggering overactive eccrine sweat glands.

Apocrine Sweat Glands

• Anatomical Structure: Simple, coiled, tubular glands.

• Distribution: Open into hair follicles in the armpits and genitalia.

• Secretory Product: Produces a thick, oily secretion rich in organic substances (proteins and lipids).

• Activation: Apocrine glands become functional at puberty under hormonal control.