The Integumentary System and Body Membranes: Study Guide

The Integumentary System

Functions of the Skin

The skin, or integumentary system, is the body's largest organ and serves multiple essential functions for protection and homeostasis.

• Protection: Acts as a barrier against mechanical injury, pathogens, and harmful chemicals.

• Water Loss Prevention: Prevents dehydration by reducing water loss.

• Temperature Regulation: Regulates body temperature via sweat glands and blood vessel dilation/constriction.

• Sensory Reception: Contains receptors for touch, pressure, pain, and temperature.

• Metabolic Functions: Synthesizes vitamin D when exposed to sunlight.

• Excretion: Removes waste products through sweat.

Skin Structure: Layers and Components

The skin is composed of three main layers, each with distinct structures and functions.

• Epidermis: The outermost layer, made of keratinized stratified squamous epithelium. Major cell types include:

  • Keratinocytes: Produce keratin for strength and waterproofing.

  • Melanocytes: Produce melanin pigment for UV protection.

  • Dendritic (Langerhans) cells: Immune defense.

  • Tactile (Merkel) cells: Sensory touch receptors.

• Dermis: Deep to the epidermis, composed mainly of dense irregular connective tissue. Contains blood vessels, nerves, glands, and hair follicles.

  • Papillary layer: Superficial, contains dermal papillae and capillaries.

  • Reticular layer: Deep, dense irregular connective tissue with larger blood vessels and nerves.

• Hypodermis (Subcutaneous layer): Not technically part of the skin; consists mainly of adipose tissue for insulation and energy storage.

Layers of the Epidermis

The epidermis is organized into distinct strata, each with unique characteristics.

• Stratum corneum: Outermost layer; dead, flattened keratinocytes; provides waterproofing and protection.

• Stratum lucidum: Present only in thick skin (palms, soles); clear, thin layer of dead cells.

• Stratum granulosum: Cells contain granules of keratin and begin to die.

• Stratum spinosum: Several layers of keratinocytes; cells appear spiny due to desmosomes.

• Stratum basale: Deepest layer; single row of stem cells; site of cell division; contains melanocytes.

Factors Determining Skin Color

Skin color is determined by several factors, primarily the presence and activity of melanocytes.

• Melanin: Brown-black pigment produced by melanocytes; protects against UV radiation.

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

• Hemoglobin: Red pigment in blood; gives pinkish hue to fair skin.

Function of Melanin: Absorbs UV radiation, preventing DNA damage in skin cells.

Hair, Sebaceous Glands, and Sweat Glands

These structures are distributed throughout the skin and serve various functions.

• Hair: Found almost everywhere except palms, soles, lips, and some genital areas. Functions include protection, sensation, and thermoregulation.

• Sebaceous (oil) glands: Associated with hair follicles; secrete sebum to lubricate skin and hair.

• Sweat (sudoriferous) glands: Two main types:

  • Eccrine glands: Widely distributed; produce watery sweat for thermoregulation.

  • Apocrine glands: Larger; found in axillary and anogenital areas; secrete thicker sweat, activated at puberty.

Comparison: Eccrine vs. Apocrine Sweat Glands

Feature	Eccrine Sweat Glands	Apocrine Sweat Glands
Location	All over body, especially palms, soles, forehead	Axillary and anogenital regions
Secretion	Watery, clear sweat	Thicker, milky sweat
Function	Thermoregulation	Activated at puberty; may be involved in scent
Associated with hair	No	Yes

Structure and Function: Epidermis vs. Dermis

Feature	Epidermis	Dermis
Cell Type	Keratinized stratified squamous epithelium	Dense irregular connective tissue
Blood Supply	Avascular	Richly vascularized
Function	Protection, waterproofing	Support, nourishment, houses glands and receptors

Microscopic Structures and Sensory Receptors

• Lamellar (Pacinian) corpuscle: Deep pressure receptor in dermis.

• Tactile (Meissner's) corpuscle: Light touch receptor in dermal papillae.

• Blood vessels: Present in dermis; supply nutrients and remove wastes. Epidermis relies on diffusion.

Body Membranes

Overview of Body Membranes

Body membranes cover surfaces, line cavities, and protect organs. Epithelial membranes are simple organs composed of epithelial tissue attached to connective tissue.

Types of Epithelial Membranes

• Cutaneous membrane: The skin; keratinized stratified squamous epithelium plus connective tissue. Dry, protective, exposed to air.

• Mucous membranes (mucosae): Epithelium plus lamina propria; line cavities open to exterior (respiratory, digestive, urinary, reproductive). Moist, often secrete mucus.

• Serous membranes (serosae): Simple squamous epithelium (mesothelium) plus areolar connective tissue; line closed cavities. Double-layered (parietal and visceral), secrete serous fluid to reduce friction.

Comparison of Epithelial Membranes

Membrane	Epithelium Type	Connective Tissue	Location	Function
Cutaneous	Keratinized stratified squamous	Dense irregular CT (dermis)	External body surface	Protection, water loss prevention
Mucous	Varies (columnar, squamous, etc.)	Lamina propria (loose CT)	Lines open cavities (respiratory, digestive, urinary, reproductive)	Secretion, absorption, protection
Serous	Simple squamous (mesothelium)	Areolar CT	Lines closed cavities (thoracic, abdominopelvic)	Reduces friction

Serous Membranes: Specific Examples

• Pericardium: Surrounds the heart; visceral pericardium covers heart, parietal pericardium lines sac.

• Pleura: Surrounds lungs; visceral pleura covers lungs, parietal pleura lines thoracic cavity.

• Peritoneum: Surrounds abdominal organs; visceral peritoneum covers organs, parietal peritoneum lines cavity.

Field Guide to Epithelial Tissue

Overview of Epithelial Tissue Types

Epithelial tissues are classified by cell shape and number of layers. They cover surfaces, line cavities, and form glands.

• Simple squamous epithelium: Single layer of flat cells; found in alveoli, capillaries, serous membranes; allows diffusion.

• Simple cuboidal epithelium: Single layer of cube-shaped cells; found in kidney tubules, glands; functions in secretion and absorption.

• Simple columnar epithelium: Single layer of tall cells; found in digestive tract; absorption and secretion; may contain goblet cells.

• Pseudostratified columnar epithelium: Appears layered but all cells touch basement membrane; found in respiratory tract; ciliated, contains goblet cells.

• Stratified squamous epithelium: Multiple layers of flat cells; found in skin, mouth, esophagus; protection against abrasion.

Example: Human skin slide shows keratinized stratified squamous epithelium; trachea slide shows pseudostratified ciliated columnar epithelium with goblet cells.