Skin and Body Membranes: Structure, Types, and Functions

Skin and Body Membranes

Overview

The skin and body membranes are essential components of the integumentary system, providing protection, sensation, and various physiological functions. This guide covers the types, structures, and functions of body membranes, as well as the anatomy and roles of the skin and its appendages.

Types and Functions of Body Membranes

Main Categories of Body Membranes

• Epithelial Membranes: Include cutaneous, mucous, and serous membranes.

• Connective Tissue Membranes: Include synovial membranes.

General Functions

• Covering and lining surfaces

• Lining body cavities

• Forming protective sheets around organs

Cutaneous Membrane (Skin)

• Location: External body surface

• Structure: Composed of keratinized stratified squamous epithelium and underlying dense (fibrous) connective tissue

• Function: Protective boundary against environmental hazards

Mucous Membranes (Mucosae)

• Location: Line body cavities that open to the exterior (e.g., digestive, respiratory, urinary tracts)

• Structure: Epithelium (type depends on site) over loose connective tissue (lamina propria)

• Function: Secrete protective, lubricating mucus; involved in absorption or secretion

Serous Membranes (Serosae)

• Location: Line body cavities closed to the exterior (ventral body cavity)

• Structure: Simple squamous epithelium over areolar connective tissue; two layers (visceral and parietal) separated by serous fluid

• Function: Reduce friction between organs

Specific Serous Membranes

• Peritoneum: Covers organs in the abdominal cavity

• Pleurae: Surround the lungs

• Pericardia: Surround the heart

Connective Tissue Membranes (Synovial Membranes)

• Location: Line fibrous capsules surrounding joints and tendon sheaths

• Structure: Composed of connective tissue only (no epithelial tissue)

• Function: Secrete lubricating synovial fluid to cushion organs moving against each other

The Integumentary System and Its Functions

Overview

The integumentary system consists of the skin (cutaneous membrane) and its appendages. It serves as the body's first line of defense and performs multiple vital functions.

• Protects underlying tissues

• Synthesizes vitamin D

• Excretes metabolic wastes

• Secretes and insulates

Major Functions of the Integumentary System

Function	How Accomplished
Protection from mechanical damage	Physical barrier: keratin toughens cells; fat cells cushion blows; pressure and pain receptors alert the nervous system.
Protection from chemical damage	Keratinized cells are relatively impermeable; pain receptors alert the nervous system.
Protection from microbial damage	Unbroken surface and "acid mantle" (acidic secretions inhibit microbes); phagocytes ingest foreign substances.
Protection from UV radiation	Melanin produced by melanocytes absorbs UV radiation.
Protection from desiccation (drying out)	Water-resistant glycolipid and keratin in skin cells prevent water loss.
Temperature regulation	Heat loss: sweat glands and blood flow to skin; heat retention: reduced blood flow to skin.
Excretion	Perspiration contains urea and uric acid.
Vitamin D synthesis	Modified cholesterol molecules in skin are converted to vitamin D by sunlight.

Structure of the Skin

Layers of the Skin

• Epidermis: Outermost layer; composed of keratinized stratified squamous epithelium; avascular.

• Dermis: Middle layer; composed of connective tissue; contains blood vessels, nerves, and appendages.

• Hypodermis (Subcutaneous Tissue): Deepest layer; composed of adipose tissue; anchors skin to underlying organs, insulates, and absorbs shock.

Layers of the Epidermis (from deep to superficial)

• Stratum basale: Deepest layer; actively dividing cells; contains melanocytes.

• Stratum spinosum: Several layers of keratinocytes; contains dendritic cells for immune defense.

• Stratum granulosum: Cells flatten; organelles deteriorate; cytoplasm full of granules.

• Stratum lucidum: Present only in thick, hairless skin (palms, soles); dead cells.

• Stratum corneum: Outermost layer; dead, keratin-filled cells; provides a tough, protective barrier.

Cells of the Epidermis

• Keratinocytes: Most abundant; produce keratin for strength and waterproofing.

• Melanocytes: Produce melanin pigment; protect against UV radiation.

• Dendritic (Langerhans) cells: Immune cells; antigen-presenting.

• Merkel cells: Sensory receptors for touch.

Dermis Structure

• Papillary layer: Upper dermal region; contains dermal papillae, capillary loops, pain and touch receptors; increases friction (fingerprints).

• Reticular layer: Deeper dermis; dense irregular connective tissue; contains blood vessels, sweat and oil glands, deep pressure receptors (lamellar corpuscles); provides strength and elasticity.

Hypodermis (Subcutaneous Tissue)

• Composed mainly of adipose tissue (fat)

• Anchors skin to underlying organs

• Acts as a shock absorber and insulator

• Common site for subcutaneous injections

Skin Color

• Melanin: Brown to black pigment produced by melanocytes; protects against UV damage.

• Carotene: Yellow-orange pigment from diet; accumulates in stratum corneum and fat.

• Hemoglobin: Red pigment in blood; visible in fair skin.

Skin color can be affected by physiological changes (e.g., erythema, pallor, jaundice, bruising).

Epidermal Appendages

Sebaceous (Oil) Glands

• Found all over skin except palms and soles

• Secrete sebum (oil) into hair follicles or directly onto skin

• Lubricates skin and hair; prevents brittleness; has antibacterial properties

• Activated at puberty

Sudoriferous (Sweat) Glands

• Eccrine glands: Most numerous; open via duct to skin surface; important for temperature regulation

• Apocrine glands: Found in axillary and genital areas; ducts empty into hair follicles; begin functioning at puberty; secrete sweat with fatty acids and proteins

Hair

• Produced by hair follicles; consists of hard keratinized epithelial cells

• Melanocytes in hair bulb provide pigment

• Arrector pili muscle causes hair to stand upright (goosebumps)

Nails

• Scale-like modifications of the epidermis

• Composed of hard keratin

• Nail matrix responsible for growth

Clinical Connections

• Skin cancer: Most common form of cancer; risk increased by UV exposure

• Types: Basal cell carcinoma (least malignant, most common), squamous cell carcinoma, malignant melanoma (most dangerous, metastasizes rapidly)

• Other conditions: Acne, dermatitis, alopecia (balding), and age-related changes (thinning, dryness)