Chapter 5: The Integumentary System – Skin and Hypodermis

Skin and the Hypodermis

Overview of the Integumentary System

The integumentary system consists primarily of the skin, which is the largest organ of the human body. It plays a vital role in protection, regulation, and sensation.

• Skin accounts for approximately 7% of body weight.

• Thickness varies from 1.5 to 4.4 mm depending on location.

• Divided into two distinct layers:

  • Epidermis: The superficial layer composed of epithelial tissue.

  • Dermis: The deeper layer composed of connective tissue.

• Hypodermis (subcutaneous layer): Lies deep to the dermis, composed of areolar and adipose tissues. Although not technically part of the integumentary system, it shares some properties with skin and anchors it to underlying structures.

Main Functions of Skin and Hypodermis

• Protection: Cushions organs and protects against physical trauma, chemicals, water loss, and ultraviolet (UV) radiation.

• Body Temperature Regulation: Capillary networks and sweat glands help regulate heat loss.

• Excretion: Sweat removes urea, salts, and water from the body.

• Production of Vitamin D: Epidermal cells use UV radiation to synthesize vitamin D, essential for calcium absorption.

• Sensory Reception: Contains sensory receptors associated with nerve endings for touch, pain, and temperature.

Structure of the Skin

Gross Anatomy

The skin is organized into layers, each with distinct structures and functions. The hypodermis lies beneath the dermis and is primarily involved in fat storage and insulation.

• Epidermis: Outermost layer, provides a waterproof barrier and creates skin tone.

• Dermis: Contains tough connective tissue, hair follicles, and sweat glands.

• Hypodermis: Made of fat and connective tissue.

Skin Structure Diagram

The following diagram (Figure 5.1) illustrates the main components of the skin:

• Hair shaft and hair follicle

• Epidermis (with layers: papillary and reticular dermis)

• Dermis (with vascular and nervous structures)

• Hypodermis (adipose tissue)

• Appendages of skin: sweat glands, sebaceous (oil) glands, arrector pili muscle

• Nervous structures: sensory nerve fibers, lamellar corpuscle, hair root sensory nerve ending

Table: Layers and Components of the Skin

Layer	Main Components	Functions
Epidermis	Keratinocytes, melanocytes, tactile cells, dendritic cells	Protection, waterproofing, pigmentation, immune defense
Dermis	Connective tissue, blood vessels, nerves, hair follicles, glands	Support, nourishment, sensation, thermoregulation
Hypodermis	Adipose tissue, areolar tissue	Insulation, energy storage, anchoring skin

Key Terms

• Organ: A group of tissues working together to perform specific functions.

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

• Dermis: The middle layer of skin, composed of connective tissue.

• Hypodermis: The deepest layer, primarily composed of fat and connective tissue.

• Adipose tissue: Tissue that stores fat.

• Areolar tissue: Loose connective tissue that holds organs in place and attaches epithelial tissue to underlying tissues.

Example: Clinical Application

• Damage to the epidermis (e.g., first-degree burns) affects only the outer protective layer, while deeper injuries may involve the dermis and hypodermis, impacting sensation, thermoregulation, and healing.

Additional info: The skin is a dynamic organ that not only serves as a barrier but also plays a role in immune defense, metabolic processes, and communication with the nervous system.