Skin and Body Membranes: The Integumentary System

Overview

The integumentary system is a complex organ system that includes the skin and its associated structures. It serves as the body's primary barrier against environmental hazards and plays a vital role in protection, sensation, and homeostasis.

Body Membranes

Functions of Body Membranes

• Cover body surfaces: Membranes form the outermost layer of protection for organs and tissues.

• Line body cavities: They provide lining for internal spaces, helping to compartmentalize and protect organs.

• Form protective sheets around organs: Membranes help reduce friction and isolate organs from one another.

Body membranes are classified according to tissue type:

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

• Connective tissue membranes: Include synovial membranes.

Epithelial Membranes

Cutaneous Membrane (Skin)

The cutaneous membrane, commonly known as the skin, is a dry membrane that forms the outermost protective boundary of the body.

• Epidermis: Composed of keratinized stratified squamous epithelium, providing toughness and water resistance.

• Dermis: Mostly dense (fibrous) connective tissue, offering structural support and elasticity.

Mucous Membranes (Mucosae)

Mucous membranes are moist membranes that line all body cavities opening to the exterior, such as the digestive, respiratory, urinary, and reproductive tracts.

• Adapted for absorption or secretion: These membranes facilitate exchange of substances and protect underlying tissues.

• Construction: Epithelium type depends on location; supported by loose connective tissue called lamina propria.

Serous Membranes (Serosae)

Serous membranes line body cavities that are closed to the exterior. They occur in pairs separated by serous fluid, with a visceral layer covering organs and a parietal layer lining the cavity wall.

• Construction: Simple squamous epithelium over areolar connective tissue.

• Function: Secrete serous fluid to reduce friction between moving organs.

Specific Serous Membranes

• Peritoneum: Lines the abdominal cavity.

• Pleura: Surrounds the lungs.

• Pericardium: Surrounds the heart.

Connective Tissue Membranes

Synovial Membranes

Synovial membranes consist of loose areolar connective tissue and lack epithelial tissue. They line fibrous capsules surrounding joints, bursae, and tendon sheaths.

• Function: Secrete lubricating synovial fluid to cushion organs and reduce friction during movement.

The Integumentary System

Components

• Skin (cutaneous membrane)

• Skin appendages: Sweat glands, oil glands, hair, nails

Functions of the Integumentary System

• Protection: Shields deeper tissues from mechanical, chemical, thermal, UV, and microbial damage, as well as desiccation.

• Temperature regulation: Controls heat loss or retention via blood flow and sweat production.

• Excretion: Removes metabolic wastes such as urea and uric acid.

• Vitamin D synthesis: Initiates production of vitamin D when exposed to sunlight.

Anatomy of the Skin

Layers of the Skin

• Epidermis: Outermost layer, avascular, composed of stratified squamous epithelium. Contains keratinocytes that produce keratin.

• Dermis: Underlies the epidermis, made of connective tissue, contains blood vessels, nerves, and appendages.

• Hypodermis (subcutaneous layer): Not technically part of the integumentary system; composed mostly of adipose tissue, anchors skin to underlying organs, acts as shock absorber and insulator.

Layers of the Epidermis (from deepest to superficial)

• Stratum basale (germinativum): Deepest layer, adjacent to dermis, site of active mitosis, contains melanocytes.

• Stratum spinosum: Cells become flatter and more keratinized.

• Stratum granulosum: Cells continue to flatten and accumulate keratin.

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

• Stratum corneum: Outermost layer, consists of shingle-like dead cells filled with keratin, provides waterproof barrier.

Cells of the Epidermis

• Keratinocytes: Produce keratin, a protein that provides strength and water resistance.

• Melanocytes: Located in stratum basale, produce melanin pigment for skin color and UV protection.

• Dendritic (Langerhans) cells: Immune cells that alert and activate immune responses.

• Merkel cells: Associated with sensory nerve endings, function as touch receptors (Merkel discs).

Layers of the Dermis

• Papillary layer: Upper dermal region, contains dermal papillae that indent the epidermis, capillary loops, pain and touch receptors, and forms fingerprints.

• Reticular layer: Deepest skin layer, contains blood vessels, sweat and oil glands, and deep pressure receptors (lamellar corpuscles).

Skin Color

Factors Determining Skin Color

• Melanin: Pigment produced by melanocytes; color ranges from yellow to brown to black; amount depends on genetics and sun exposure.

• Carotene: Orange-yellow pigment from certain vegetables, accumulates in stratum corneum and subcutaneous tissue.

• Hemoglobin: Red coloring from blood cells in dermal capillaries; oxygen content affects redness.

Clinical Color Changes

• Redness (erythema): Due to embarrassment, inflammation, hypertension, fever, or allergy.

• Pallor (blanching): Due to emotional stress, anemia, low blood pressure, or impaired blood flow.

• Jaundice: Yellow cast indicating liver disorder.

• Bruises (hematomas): Black and blue marks from blood escaping vessels.

Associated Structures of the Integument

Glands

• Sebaceous (oil) glands: Produce sebum to lubricate skin and hair; most ducts empty into hair follicles; activated at puberty.

• Sweat (sudoriferous) glands: Two types:

  • Eccrine glands: Widely distributed, open via ducts to skin surface, produce acidic sweat for temperature regulation.

  • Apocrine glands: Ducts empty into hair follicles in armpit and genital areas, begin functioning at puberty, secrete sweat containing fatty acids and proteins.

Hair

• Produced by hair follicles: Root is enclosed in follicle; shaft projects from skin surface.

• Structure: Central medulla, cortex surrounds medulla, cuticle on outside (most heavily keratinized).

• Associated structures: Hair follicle (epithelial and fibrous sheath), dermal region supplies blood, arrector pili muscle causes hair to stand upright.

Nails

• Heavily keratinized: Scale-like modifications of epidermis.

• Growth: Stratum basale extends beneath nail bed; nail matrix responsible for growth.

• Anatomy: Free edge, body (visible attached portion), nail folds (skin overlaps edges), root (embedded in skin).

Skin Repair After Injury

Phases of Skin Repair

1. Hemostasis: Immediate response to injury; blood vessels constrict, platelets form a clot to stop bleeding and protect against pathogens.

2. Inflammation: Immune cells migrate to the site, clear debris and bacteria, prevent infection, and prepare for tissue repair.

3. Proliferation: Keratinocytes proliferate and migrate to cover wound; angiogenesis forms new blood vessels; fibroblasts produce collagen and extracellular matrix.

4. Maturation (Remodeling): Tissue strengthens and remodels; collagen fibers reorganize, restoring strength and elasticity.

5. Scar Formation: In deeper injuries, dense collagen forms a scar with different texture and color than normal skin.

Skin repair is a dynamic process involving multiple cell types and signaling pathways. Minor injuries heal quickly, while deeper wounds may result in noticeable scars.

Summary Table: Types of Body Membranes

Key Terms and Definitions

• Keratin: A tough, fibrous protein that provides strength and waterproofing to the skin.

• Melanin: Pigment produced by melanocytes, responsible for skin color and UV protection.

• Stratum basale: Deepest layer of the epidermis, site of cell division.

• Dermal papillae: Projections in the papillary layer of the dermis that form fingerprints.

• Arrector pili muscle: Small muscle attached to hair follicle, causes hair to stand upright.

• Hemostasis: The process of stopping bleeding after injury.

• Angiogenesis: Formation of new blood vessels during tissue repair.

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