Lecture 4 Guide

Integumentary System Overview

Introduction

The integumentary system is the body's largest organ system, consisting of the skin and its associated structures such as hair, nails, and glands. It serves as the primary barrier between the internal environment and the external world, playing crucial roles in protection, sensation, and homeostasis.

Functions of the Integumentary System

Main Functions

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

• Temperature Maintenance: Regulates body temperature through sweat production and blood flow modulation.

• Synthesis and Storage of Nutrients: Synthesizes vitamin D3 when exposed to sunlight and stores lipids in adipose tissue.

• Sensory Reception: Contains receptors for touch, pressure, pain, and temperature, allowing the body to sense environmental changes.

• Excretion and Secretion: Removes salts, water, and organic wastes through sweat and sebaceous glands.

Structure of the Skin

Layers of the Skin

• Epidermis: The outermost layer, composed of stratified squamous epithelium. It provides a waterproof barrier and creates our skin tone.

• Dermis: Located beneath the epidermis, this layer consists of dense irregular connective tissue with blood vessels, muscle tissue, and nerve endings. It supports and nourishes the epidermis.

• Hypodermis (Subcutaneous Layer): The deepest layer, primarily made up of adipose tissue and other areolar connective tissue. It insulates the body and anchors the skin to underlying structures.

Note: Skin thickness is often measured by the thickness of the hypodermic (subcutaneous) tissue.

Cells of the Epidermis

Keratinocytes

• Primary cell type, making up about 90% of epidermal cells.

• Produce keratin, a tough, protective, and waterproof protein that resists abrasion.

• Synthesize antimicrobial molecules to protect against infection.

• Undergo keratinization: cells are produced and multiply in the deepest layers, then are pushed up and out, becoming more keratinized as they move toward the surface.

Melanocytes

• Located only in the deepest layer of the epidermis (stratum basale).

• Produce melanin, the pigment responsible for skin and hair color; amount produced is genetically determined.

• Melanin production is stimulated by UV light exposure, which can help protect against DNA damage but also increase risk for skin cancer.

• Other factors influencing melanin production include hormones, foods, and certain diseases.

Abnormalities in Melanocytes

Common Disorders

• Albinism: A genetic condition characterized by a deficiency or absence of melanin production, resulting in very light skin, hair, and eyes.

• Melanoma: A deadly form of skin cancer arising from melanocytes; often associated with excessive UV exposure.

Condition	Description	Clinical Image
Basal Cell Carcinoma	Most common skin cancer, arises from basal cells in the epidermis.	Image: Pearly, raised lesion (see slide)
Melanoma	Most dangerous skin cancer, arises from melanocytes; often dark, irregular lesion.	Image: Dark, irregular patch (see slide)

Layers of the Epidermis

From Deep to Superficial

• Stratum Basale: Deepest layer, adjacent to the dermis; contains stem cells for cell division and growth. Receives nutrients from the dermal blood supply.

• Stratum Spinosum: Adds desmosomes (protein filaments) for cell adhesion and strength.

• Stratum Granulosum: Cells accumulate granules; most keratin is formed here. Cells begin to die as their nuclei and organelles disintegrate.

• Stratum Lucidum: Found only in thick, hairless skin (palms, soles); a clear, glassy layer of dead cells.

• Stratum Corneum: Outermost layer; 15-30 cells thick, composed of dead, keratinized cells. Cells are constantly shed and replaced every 25-45 days.

Thin Skin vs. Thick Skin

Comparison

Feature	Thin Skin	Thick Skin
Location	Most of the body	Palms, soles
Stratum Lucidum	Absent	Present
Hair Follicles	Present	Absent
Thickness	Thinner	Thicker

Dermis Structure

Papillary Layer

• Contains dermal papillae, which increase surface area for exchange of gases, nutrients, and waste products.

• Includes blood vessels, lymph vessels, and sensory receptors (pain and touch).

• Touch receptors: Meissner's corpuscles detect light touch.

Reticular Layer

• Thicker, deeper layer of the dermis.

• Contains blood vessels, pressure receptors (Pacinian corpuscles), and smooth muscle (arrector pili muscle).

• Arrector pili muscle contracts to cause "goosebumps" by making hair stand up.

Glands of the Skin

Sebaceous (Oil) Glands

• Surround hair shafts; ducts empty into hair follicles.

• Secrete sebum (oil and dead cells), which lubricates skin and hair, and has antibacterial properties.

• Activity increases during adolescence due to sex hormones; overactivity can lead to acne.

Sudoriferous (Sweat) Glands

• Eccrine (Merocrine) Glands: Most numerous, found all over the body; empty directly onto skin surface. Sweat is mostly water (99%), with salts and small amounts of waste. No odor; function is body heat regulation. Active from birth.

• Apocrine Glands: Fewer in number, found in axilla, genital, and anal areas; empty into hair follicles. Sweat contains water, salts, fatty acids, and proteins, producing odor. Activate at puberty, often during stress; function is not fully understood (possibly related to sexual maturity or communication).

Feature	Eccrine Glands	Apocrine Glands
Location	All over body	Axilla, genital, anal areas
Sweat Composition	Mostly water, salts, waste	Water, salts, fatty acids, proteins
Odor	No odor	Odorous
Activation	From birth	At puberty

Hair, Hair Follicles, and Nails

Structure and Function

• Hair: Composed of dead keratinized cells; provides protection, insulation, and sensory input. Color determined by melanin produced by melanocytes; graying occurs as melanin production decreases with age.

• Hair Follicle: Small tunnel in the skin where hair is produced; contains actively growing cells below skin level.

• Nails: Composed of hard keratin; protect the dorsal surfaces of fingers and toes. The visible portion is the nail body; the root is where nail production occurs.

• Lunula: Pale, crescent-shaped area at the base of the nail, where blood vessels are obscured.

Skin Injuries and Healing

Stages of Healing

1. Inflammation: Initial response to injury; increased blood flow and immune activity.

2. Migration: Cells move to the site of injury to begin repair.

3. Proliferation: Rapid cell division to replace lost tissue.

4. Scarring: Formation of fibrous tissue to restore integrity.

Types of Scars

Type	Description	Commonality
Keloid	Firm, smooth, raised; grows beyond initial wound boundaries; darker than surrounding skin	Less common; more often in Black and Asian individuals
Hypertrophic	Slightly raised; grows within original wound boundaries	More common

Burns

Classification by Depth

Degree	Layers Affected	Features
First Degree	Surface of epidermis	Redness, mild pain; most sunburns
Second Degree	Epidermis and part of dermis	Blisters, pain; hair follicles and glands may be affected
Third Degree	Entire epidermis and dermis, may extend to hypodermis	Charred, numb (nerves destroyed); requires medical intervention

Effects of Aging on Skin

Age-Related Changes

• Increased risk of injuries and infections

• Greater sensitivity to sun exposure

• Skin becomes dry and scaly

• Loss of pigment and changes in skin color

• Wrinkling due to loss of elasticity

• Slower repair and healing processes

Additional info: The integumentary system is essential for maintaining homeostasis and protecting the body from environmental hazards. Disorders of the skin can have significant impacts on overall health and quality of life.