The Integumentary System: Structure and Function

The Integumentary System

Overview of the Integumentary System

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

• Skin: The primary organ, serving as a barrier and interface with the environment.

• Sudoriferous (sweat) glands: Involved in thermoregulation and excretion.

• Sebaceous (oil) glands: Secrete sebum to lubricate and protect the skin.

• Hairs: Provide protection and sensory input.

• Nails: Protect the distal phalanges and enhance fine touch.

• Subcutaneous tissue: Also known as the hypodermis, it anchors the skin and stores fat.

Skin Structure

General Organization

The skin is composed of two main layers: the epidermis and the dermis, with the subcutaneous tissue (hypodermis) lying beneath.

• Epidermis: The outermost layer, composed of keratinized stratified squamous epithelium.

• Dermis: A deeper, connective tissue layer containing nerves, blood vessels, and appendages.

• Subcutaneous tissue (hypodermis): Not technically part of the skin, but supports it structurally and functionally.

The Epidermis

Cell Types of the Epidermis

The epidermis is primarily made up of four cell types, each with specialized functions:

• Keratinocytes: Produce keratin, a protein that provides strength and waterproofing. These are the most abundant cells, constantly renewed every 25–45 days.

• Melanocytes: Located in the deepest layer, they produce melanin pigment, which protects against ultraviolet (UV) radiation by transferring melanosomes to keratinocytes.

• Dendritic cells (Langerhans cells): Star-shaped immune cells that patrol the epidermis, ingesting foreign substances and activating immune responses.

• Tactile epithelial cells (Merkel cells): Sensory receptors for touch, found 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: The deepest, mitotically active layer, also called the stratum germinativum. Contains stem cells and melanocytes.

• Stratum spinosum: Several layers thick, with cells connected by desmosomes. Contains keratinocytes, melanosomes, and dendritic cells.

• Stratum granulosum: Four to six layers of flattened cells where keratinization begins. Cells accumulate keratohyaline and lamellar granules, contributing to waterproofing.

• Stratum lucidum (only in thick skin): A thin, clear layer of dead keratinocytes, found in areas like palms and soles.

• Stratum corneum: The outermost, thickest layer (20–30 rows) of dead, keratinized cells providing protection from abrasion, water loss, and pathogens.

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

Functions of Epidermal Cells

• Keratinocytes: Form the bulk of the epidermis, producing keratin for protection.

• Melanocytes: Synthesize melanin to shield DNA from UV damage.

• Dendritic (Langerhans) cells: Act as immune sentinels, ingesting pathogens and presenting antigens to T cells.

• Merkel (tactile) cells: Detect light touch and relay sensory information to the nervous system.

Summary Table: Epidermal Layers and Key Features

Layer	Main Features	Location
Stratum basale	Single row of stem cells, mitotically active, contains melanocytes	Deepest layer
Stratum spinosum	Several layers, spiny appearance, contains keratinocytes and dendritic cells	Above basale
Stratum granulosum	Flattened cells, keratinization begins, lamellar granules	Middle layer
Stratum lucidum	Clear, dead keratinocytes, only in thick skin	Between granulosum and corneum (thick skin only)
Stratum corneum	20–30 layers of dead, keratinized cells, protective barrier	Most superficial