Chapter 5: Integumentary System

Overview of the Integumentary System

The Integumentary System is composed of the skin and its accessory structures, including hair, nails, and glands. The skin, also known as the cutaneous membrane, is the largest organ in the body, accounting for 10-15% of total body weight. It consists of two main layers: the superficial epidermis and the deep dermis. Below the dermis lies the hypodermis (superficial fascia or subcutaneous tissue), which is not considered part of the skin.

• Functions: Protection, sensation, thermoregulation, excretion, and synthesis of vitamin D.

• Accessory Structures: Hair, nails, and various glands.

• Skin Anatomy: Includes sensory neurons, receptors, and arrector pili muscles.

Basic Anatomy of the Skin

The skin is organized into distinct layers and contains various structures essential for its function.

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

• Dermis: Middle layer, contains connective tissue, hair follicles, and sweat glands.

• Hypodermis: Subcutaneous layer, stores fat and anchors skin to underlying tissues (not part of the skin proper).

• Accessory Structures: Hair shafts, sweat glands, sebaceous glands, blood vessels, and nerves.

Cellulite

Definition and Characteristics

Cellulite refers to the dimpled, 'orange peel' appearance of the skin, commonly found on the thighs, hips, and gluteal area. It results from collagen bands forming around adipose tissue in the hypodermis.

• Influenced by genetics, sex, age, and distribution of adipose tissue.

• Now considered a normal condition rather than a disease.

• Diet and exercise may minimize cellulite, but do not eliminate it entirely.

• There is little evidence that topical products can 'cure' cellulite.

Module 5.1: Overview of The Integumentary System

Functions of the Integumentary System

• Protection: Shields underlying tissues from mechanical damage, pathogens, and environmental threats.

• Sensation: Sensory receptors detect stimuli such as touch, pain, and temperature.

• Thermoregulation: Maintains stable internal body temperature through mechanisms like sweating and blood flow regulation.

• Excretion: Eliminates metabolic waste products, including lactic acid and urea, via sweat.

• Synthesis of Vitamin D: Skin produces vitamin D, essential for calcium absorption in the small intestine.

Thermoregulation

The skin plays a crucial role in maintaining homeostasis by regulating body temperature through negative feedback loops.

• When body temperature rises, sweat glands are activated and blood vessels dilate to dissipate heat.

• When body temperature falls, blood vessels constrict and shivering may occur to conserve heat.

Example: Figures 5.2 and 5.3 illustrate the negative feedback mechanisms for rising and falling body temperature, respectively.

Vitamin D Synthesis

Vitamin D is essential for calcium absorption and various physiological processes.

• Precursor compound is found in deep epidermal cells.

• UV radiation converts this precursor to cholecalciferol (vitamin D3), which enters the bloodstream.

• Cholecalciferol is modified by the liver and kidneys to form calcitriol, the active form of vitamin D.

• Calcitriol increases calcium absorption in the small intestine.

Module 5.2: The Epidermis

Structure and Cell Types

The epidermis is the superficial layer of the skin, composed of several cell types and up to five layers (strata).

• Keratinocytes: Make up about 95% of epidermal cells; produce keratin, a protein that provides strength and resistance to mechanical stress.

• Keratinocytes are linked by desmosomes, which hold cells together.

Layers of the Epidermis

The epidermis is organized into distinct layers, each with specialized functions.

• Stratum basale (deepest)

• Stratum spinosum

• Stratum granulosum

• Stratum lucidum (only in thick skin)

• Stratum corneum (most superficial)

Figure 5.4 illustrates the structure of the epidermis and its layers.

Topical Medications

The lipid-based coating of the epidermis affects the absorption of medications and toxins.

• Some medications are safe when applied to the skin but toxic if swallowed.

• Hydrophobic or nonpolar substances can cross the epidermis, making transdermal drug delivery possible (e.g., hormone patches).

• Nonpolar toxins may also cross the epidermis.

Keratinocyte Life Cycle

Keratinocytes are continually produced and shed in a regulated cycle.

• Cells in the stratum corneum are shed and replaced.

• Keratinocytes in the stratum basale and spinosum divide and move toward the surface, differentiating as they go.

• The process from production to shedding takes 40-50 days.

Other Cells of the Epidermis

• Dendritic (Langerhans) Cells: Immune cells in the stratum spinosum; phagocytose pathogens.

• Tactile (Merkel) Cells: Sensory receptors in the stratum basale; detect light touch and texture.

• Melanocytes: Found in the stratum basale; produce melanin, a pigment responsible for skin color.

Thick and Thin Skin

The skin varies in thickness and structure depending on its location and function.

• Thick Skin: Found on palms, soles, and palmar surfaces of fingers and toes; contains all five strata and a thick stratum corneum; lacks hair follicles but has many sweat glands.

• Thin Skin: Covers most of the body; lacks stratum lucidum and has a thinner stratum corneum; contains hair follicles, sweat, and sebaceous glands.

• Callus: Additional layers of stratum corneum formed in response to repeated pressure.

Figure 5.5 compares thick and thin skin histologically and functionally.