Skin and Body Membranes

Body Membranes

Body membranes are thin layers of tissue that cover surfaces, line body cavities, and form protective sheets around organs. They are classified according to the tissue types that compose them.

• Functions of body membranes:

  • Cover body surfaces

  • Line body cavities

  • Form protective sheets around organs

• Types of body membranes:

  • Epithelial membranes

    • Cutaneous membranes (skin)

    • Mucous membranes

    • Serous membranes

  • Connective tissue membranes

    • Synovial membranes

Epithelial Membranes

Epithelial membranes are simple organs, also called covering and lining membranes. They contain both an epithelial tissue layer and a connective tissue layer.

• Cutaneous membrane (skin):

  • Dry membrane

  • Outermost protective boundary

  • Consists of two layers:

    • Epidermis: keratinized stratified squamous epithelium

    • Dermis: dense (fibrous) connective tissue

• Mucous membranes (mucosae):

  • Moist membranes

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

  • Adapted for absorption or secretion

  • Epithelium type depends on site; always rests on loose connective tissue (lamina propria)

• Serous membranes (serosae):

  • Line closed ventral body cavities

  • Occur in pairs: parietal (lines cavity) and visceral (covers organ)

  • Layers separated by serous fluid

  • Consist of simple squamous epithelium and areolar connective tissue

  • Specific serous membranes:

    • Peritoneum: abdominal cavity

    • Pleura: lungs

    • Pericardium: heart

Connective Tissue Membranes

Connective tissue membranes, such as synovial membranes, are composed entirely of loose areolar connective tissue and contain no epithelial cells.

• Line fibrous capsules surrounding joints

• Line bursae and tendon sheaths

• Secrete lubricating synovial fluid to reduce friction during movement

Integumentary System

Overview

The integumentary system consists of the skin (cutaneous membrane) and its appendages: sweat glands, oil glands, hair, and nails.

• Functions:

  • Protects deeper body tissues from mechanical, chemical, thermal, UV, and microbial damage, as well as water loss

  • Regulates heat loss (thermoregulation)

  • Acts as a mini-excretory system (sweat aids in loss of urea, salts, and water)

  • Synthesizes vitamin D

  • Secretions create a protective acid mantle

Structure of the Skin

The skin is composed of two main layers: the epidermis and the dermis. Below the dermis is the subcutaneous tissue (hypodermis), which anchors the skin to underlying organs and serves as a shock absorber and insulator.

• Epidermis: Stratified squamous epithelium, avascular, composed of five layers (strata)

• Dermis: Dense connective tissue, vascular, contains nerves and appendages

• Subcutaneous tissue (hypodermis): Not technically part of the skin, mostly adipose tissue

Layers of the Epidermis

• Stratum basale (germinativum):

  • Deepest layer, closest to dermis

  • Cells undergo mitosis

  • Wavy border anchors epidermis to dermis

• Stratum spinosum: Cells become increasingly flatter and more keratinized

• Stratum granulosum: Further keratinization

• Stratum lucidum: Only in thick, hairless skin (palms, soles)

• Stratum corneum: Outermost layer, shingle-like dead cells filled with keratin (prevents water loss)

Cells of the Epidermis

• Keratinocytes: Produce keratin, a tough, protective protein

• Melanocytes: Produce melanin pigment, mostly in stratum basale; melanin protects against UV radiation

• Epidermal dendritic cells: Alert and activate immune cells to threats

• Merkel cells: Associated with sensory nerve endings; serve as touch receptors (Merkel discs)

Dermis

• Composed of connective tissue

• Two regions:

  • Papillary layer (upper):

    • Areolar connective tissue

    • Contains dermal papillae (projections that indent the epidermis)

    • Dermal papillae contain capillary loops, pain and touch receptors

    • On hands and feet, papillae form fingerprints

  • Reticular layer (deepest):

    • Dense irregular connective tissue

    • Contains blood vessels, sweat and oil glands, deep pressure receptors (lamellar corpuscles)

• Other features: cutaneous sensory receptors, phagocytes, collagen and elastic fibers, blood vessels, nerve supply

Skin Color

Skin color is determined by three main pigments:

1. Melanin: Yellow, reddish brown, or black pigment produced by melanocytes

2. Carotene: Orange-yellow pigment (from some vegetables)

3. Hemoglobin: Red coloring from blood cells in dermal capillaries

Skin color can also be affected by physiological factors such as erythema (redness), pallor (blanching), jaundice (yellow cast), and bruises (hematomas).

Appendages of the Skin

Cutaneous Glands

• Sebaceous (oil) glands:

  • Produce sebum (oil) to lubricate skin and hair, prevent brittleness, and kill bacteria

  • Most have ducts that empty into hair follicles; some open directly onto skin surface

  • Activated at puberty by androgens

• Sweat (sudoriferous) glands:

  • Produce sweat for thermoregulation

  • Widely distributed in skin

  • Two types:

    • Eccrine glands: More numerous, open via duct to skin surface, produce watery sweat, function in temperature regulation

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

Hair and Hair Follicles

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

• Root is enclosed in follicle; shaft projects from surface

• Melanocytes provide pigment for hair color

• Hair grows in the matrix of the hair bulb in stratum basale

• Hair anatomy:

  • Central medulla

  • Cortex surrounds medulla

  • Cuticle on outside of cortex (most heavily keratinized region)

• Associated structures:

  • Hair follicle: inner epithelial root sheath and outer fibrous sheath

  • Dermal region provides blood supply to hair bulb

  • Arrector pili muscle: smooth muscle that pulls hair upright when cold or frightened

Nails

• Heavily keratinized, scalelike modifications of the epidermis

• Stratum basale extends beneath the nail bed, responsible for growth

• Parts of the nail:

  • Free edge

  • Body (visible attached portion)

  • Nail folds (skin folds overlapping nail edges)

  • Root (embedded in skin)

  • Growth occurs from nail matrix at nail bed

Homeostatic Imbalances of Skin

Infections and Allergies

• Fungal infections: Athlete's foot (Tinea pedis)

• Bacterial infections: Boils and carbuncles (inflammation of hair follicles)

• Viral infections: Cold sores (herpes simplex virus)

• Contact dermatitis: Allergic response to chemicals

• Impetigo: Pink, fluid-filled lesions caused by bacteria

• Psoriasis: Chronic, triggered by trauma, infection, hormonal changes, or stress

Burns

• Tissue damage and cell death caused by heat, electricity, UV radiation, or chemicals

• Associated dangers:

  • Protein denaturation and cell death

  • Dehydration and electrolyte imbalance

  • Circulatory shock

Summary Table: Types of Body Membranes

Additional info:

• Keratinization is the process by which cells fill with keratin and move to the surface, providing a tough, protective layer.

• Melanin production increases with exposure to sunlight, providing some protection against UV radiation.

• Burns are classified by depth (first, second, third degree), but this was not detailed in the notes.