The Integumentary System: Structure and Function

The Integumentary System

Overview

The integumentary system is the body's largest organ system, providing protection and several vital physiological functions. It includes the skin and its associated structures.

• Components: Skin, sudoriferous (sweat) glands, sebaceous (oil) glands, hairs, nails, and subcutaneous tissue.

• Functions: Protection, sensation, thermoregulation, metabolic functions, blood reservoir, and excretion.

Skin Structure

Main Layers of the Skin

The skin consists of two primary layers, with an additional underlying tissue that supports its function.

• Epidermis: Outermost protective shield, composed of epithelial tissue; avascular.

• Dermis: Underlies the epidermis, composed mainly of dense connective tissue; vascular and provides structural strength.

• Subcutaneous Tissue (Hypodermis or Superficial Fascia): Not technically part of the skin, but shares some functions. Composed mostly of adipose tissue with some areolar connective tissue. Functions include anchoring the skin to underlying structures (mainly muscles), shock absorption, and insulation.

Skin Structure Diagram

The skin is organized into distinct layers, each with specialized structures such as hair follicles, sweat glands, sebaceous glands, nerve endings, and blood vessels.

The Epidermis

Cell Types of the Epidermis

The epidermis is a keratinized stratified squamous epithelium composed of four main cell types:

• Keratinocytes: Produce keratin, a protein that provides protective properties. These are the most abundant cells in the epidermis.

• Melanocytes: Produce melanin pigment, which protects against ultraviolet (UV) radiation.

• Dendritic (Langerhans) Cells: Star-shaped immune cells that patrol the epidermis and activate immune responses.

• Tactile (Merkel) Cells: Sensory receptors for touch, located at the epidermal-dermal junction.

Layers of the Epidermis

The epidermis is organized into four or five distinct layers (strata), depending on skin thickness:

• Stratum basale: Deepest layer; a single row of mitotically active stem cells. Also called the stratum germinativum.

• Stratum spinosum: Several layers thick; contains keratinocytes with pre-keratin filaments and abundant dendritic cells.

• Stratum granulosum: Four to six layers of flattened cells; keratinization begins here, and cells accumulate keratohyaline and lamellar granules.

• Stratum lucidum: Present only in thick skin (palms, soles); a thin, translucent band of dead keratinocytes.

• Stratum corneum: Outermost layer; 20-30 rows of dead, keratinized cells providing protection and preventing water loss.

Thick skin (palms, fingertips, soles) contains all five layers, while thin skin (most of the body) lacks the stratum lucidum.

Summary Table: Layers of the Epidermis

Layer	Main Features	Location
Stratum basale	Single row of stem cells, mitotically active, contains melanocytes	Deepest layer
Stratum spinosum	Several layers, spiny keratinocytes, dendritic cells	Above basale
Stratum granulosum	4-6 layers, keratinization begins, lamellar granules	Middle layer
Stratum lucidum	2-3 layers, clear, dead keratinocytes	Only in thick skin
Stratum corneum	20-30 layers, dead, keratinized cells	Surface layer

Example:

On the palms of your hands, all five layers are present, providing extra protection and resistance to abrasion.

Additional info: The epidermis is avascular, relying on diffusion from the underlying dermis for nutrient supply.