Integumentary System: Structure and Function

Integumentary System

Overview and Primary Functions

The integumentary system consists of the skin, hair, nails, and glands (sweat and sebaceous). Its primary function is protection, but it also plays roles in thermoregulation, sensation, vitamin D synthesis, and excretion.

• Thermoregulation: Regulates body temperature via sweat and blood flow.

• Sensation: Detects environmental stimuli through nerve endings in the skin.

• Vitamin D Synthesis: Produces vitamin D when exposed to UV light.

• Excretion: Removes waste products through sweat.

• Water Resistance: Prevents dehydration by limiting water loss.

Skin Structure

Superficial Epidermis and Deep Dermis

The skin is divided into two main layers: the epidermis (superficial) and the dermis (deep).

• Epidermis: The outermost layer, made mostly of epithelial cells. It provides a protective barrier and is avascular (lacks blood vessels).

• Dermis: The deeper layer, mainly composed of connective tissue. It contains blood vessels, nerves, hair follicles, and glands, and provides structural support and nourishment to the epidermis.

Major Cell Types in the Epidermis

There are four major cell types in the epidermis, each with distinct functions:

• Keratinoctyes: Produce keratin, a tough, fibrous protein that provides strength and waterproofing for the skin.

• Melanocytes: Produce melanin, a pigment that gives skin its color and protects underlying cells from ultraviolet (UV) radiation damage.

• Langerhans Cells: Act as immune cells by detecting pathogens and initiating immune responses to protect the skin from infection.

• Merkel Cells: Function as sensory receptors involved in the sensation of touch, especially in areas of high tactile sensitivity.

Layers of the Epidermis

Five Layers of the Epidermis (Thick Skin)

The epidermis is organized into five distinct layers in thick skin (such as the palms and soles), and four layers in thin skin.

1. Stratum corneum: Outermost layer, made of dead, flattened keratinized cells.

2. Stratum lucidum: Thin, clear layer found only in thick skin.

3. Stratum granulosum: Where cells start to die and accumulate keratohyalin granules.

4. Stratum spinosum: Layer providing strength and flexibility.

5. Stratum basale (basal layer): Deepest layer, where new keratinocytes are formed from stem cells.

The stratum lucidum is absent in thin skin.

Stratum Basale and Stem Cells

The stratum basale contains stem cells that continuously divide to produce new keratinocytes. These cells move upward through the layers of the epidermis to replace older cells that are shed from the skin surface.

Thick vs. Thin Skin

• Thick skin: Found on soles of feet and palms of hands; contains all five layers.

• Thin skin: Covers most of the body; lacks the stratum lucidum.

Subcutaneous Layer (Hypodermis)

Composition and Function

The subcutaneous layer (also called the hypodermis) lies beneath the dermis and is composed of:

• Adipose tissue (fat): Cushions the body, insulates against heat loss, and stores energy.

• Areolar connective tissue: Provides elasticity and support.

Skin Color

Determinants of Skin Color

Skin color is determined by the presence of hemoglobin, carotene, and melanin.

• Hemoglobin: Red-colored pigment found in red blood cells.

• Carotene: Found in orange and yellow vegetables and fruits (e.g., carrots, sweet potatoes, pumpkins); stored in the skin.

• Melanin: Produced by melanocytes and varies from orange to dark brown/black. Melanin is stored in melanosomes and transferred to keratinocytes. Its main function is to protect DNA in stem cells from ultraviolet (UV) radiation.

Dermis Structure

Papillary and Reticular Layers

The dermis contains two layers:

• Papillary layer: Areolar connective tissue; forms the superficial layer.

• Reticular layer: Dense irregular connective tissue; forms the deeper layer.

Functions of Hair

Protection, Sensation, and Thermoregulation

• Protection: Hair protects the scalp from UV radiation and physical injury. Eyelashes and eyebrows protect the eyes from dust and sweat.

• Sensation: Hair follicles are surrounded by nerve endings that help detect light touch or movement.

• Thermoregulation: Hair helps retain heat by trapping a layer of air close to the skin.

Glands of the Skin

Sebaceous and Sweat Glands

Sebaceous glands and sweat glands are both types of exocrine glands, which use ducts to release their contents.

• Sebaceous glands: Produce sebum, which lubricates and softens the skin and hair, helps prevent water loss, and has antibacterial properties.

• Sweat glands: Two major categories:

  • Apocrine glands: Found mainly in the armpits (axillary region), around the nipples (areolar region), and in the groin and anal regions.

  • Eccrine glands: Produce sweat (primarily water and salts) for thermoregulation (cooling the body by evaporation).

Nails

Structure and Function

Nails are located at the ends of fingers and toes. They are composed of keratinized dead cells (a tough protein called keratin).

• Protect the distal tips of fingers and toes from injury.

• Enhance fine touch and manipulation by providing support and counterpressure to the fingertips.

• Assist in scratching and picking up small objects.

Summary Table: Layers of the Epidermis

Layer	Location	Main Features
Stratum corneum	Outermost	Dead, keratinized cells; protection
Stratum lucidum	Palms, soles (thick skin only)	Clear, thin layer; extra protection
Stratum granulosum	Middle	Keratohyalin granules; cells start to die
Stratum spinosum	Above basal layer	Strength, flexibility; some cell division
Stratum basale	Deepest	Stem cells; cell division; melanocytes

Key Equations

• Vitamin D Synthesis: $\text{7-dehydrocholesterol} + UV \rightarrow \text{cholecalciferol (Vitamin D}_3)$