Integumentary System

Main Structural Features of the Epidermis

The integumentary system consists of the skin (cutaneous membrane) and its accessory structures. The skin is the largest organ of the body and serves as a protective barrier.

• Cutaneous Membrane: Composed of two main components:

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

  • Dermis: The deeper connective tissue layer beneath the epidermis.

• Accessory Structures: Include hair, hair follicles, exocrine glands (sebaceous and sweat glands), and nails. These are embedded in the dermis and may project up to or above the surface of the epidermis.

• Glands: Sebaceous glands (oil) and sweat glands are found within the skin and play roles in lubrication and temperature regulation.

• Vascular and Neural Supply: Blood vessels branch throughout the dermis, and nerve fiber endings and sensory receptors monitor touch, pressure, temperature, and pain.

Main Structures of the Epidermis

The epidermis is a multilayered structure that provides protection and regulates exchange with the environment.

• Layers of the Epidermis:

  • Superficial Layer: Composed of dead cells (stratum corneum).

  • Deepest Layer: Called the Stratum Germinativum (basal layer), forms epidermal ridges (fingerprints).

  • Cell Types:

    • Melanocytes: Produce melanin, which determines skin tone.

    • Keratinocytes: Main cell type, produce keratin for strength and waterproofing.

    • Merkel Cells: Sensory cells for touch.

  • Thin Skin: Has 4 layers, covers most of the body surface.

  • Thick Skin: Has 5 layers, found on palms of hands and soles of feet.

Main Structures of the Dermis

The dermis lies beneath the epidermis and provides structural support, nourishment, and sensory input.

• Location: Between the epidermis and the subcutaneous tissue.

• Contents: Contains blood vessels, nerves, and connective tissue.

• Plexuses:

  • Subpapillary Plexus: Network of blood vessels and nerves deep to the papillary layer.

  • Cutaneous Plexus: Network of blood vessels and nerves deep to the reticular layer.

• Layers of the Dermis:

  • Papillary Layer: Contains capillaries and sensory nerve fibers.

  • Reticular Layer: Contains dense connective tissue (collagen and elastic fibers), providing strength and flexibility.

    • The "network" of collagen and elastin is better in younger skin; with age, these fibers break down, causing less elastic skin.

• Innervation:

  • Merkel's Discs: Detect light touch and pressure.

  • Meissner's Corpuscles: Detect light touches.

  • Pacinian Corpuscles: Detect pressure and vibration.

Structures of the Subcutaneous Layer (Hypodermis)

The subcutaneous layer, also known as the hypodermis, lies beneath the dermis and connects the skin to underlying tissues.

• Adipose Tissue: Main component, stores fat.

• Functions:

  • Energy reservoir

  • Thermal insulation

  • Elasticity

  • Stabilizes position of skin while permitting movement

Various Kinds of Glands in the Skin

The skin contains several types of glands that contribute to its protective and regulatory functions.

• Sebaceous Glands: Also known as oil glands, secrete oily lipid (sebum) into hair follicles. Sebum contains triglycerides, cholesterol, proteins, and electrolytes.

• Sweat Glands:

  • Apocrine Sweat Glands: Secrete thick, odorous fluid into hair follicles.

  • Eccrine Sweat Glands: Reduce body temperature, excrete water and electrolytes, and provide protection from environmental hazards. Discharge water fluid onto the skin.

Accessory Structures of the Skin

Accessory structures include hair, nails, and glands, which originate from the epidermis but are located within the dermis.

• Hair:

  • Follicle is an oblique tube within the skin.

  • Bulb is swelling in the base; papilla provides nutrients.

  • Hair Color: Determined by melanocytes.

  • Structure of Hair Follicle:

    • Epithelial root

    • Connective tissue

    • Hair receptors

    • Piloerector muscle

Skin Renewal

The epidermis is continually renewed by the production of new cells in the basal layer.

• In babies, skin cell renewal takes around 14 days.

• For teenagers, it is about 28 days.

• In middle age, it takes about 38 to 42 days.

Key Terms and Definitions

• Hyper-: Prefix meaning "more".

• Kerat/o: Root meaning "skin".

• Epidermolytic Hyperkeratosis: A genetic skin disorder characterized by thickening of the outer layer of the skin.

Table: Comparison of Epidermal Layers in Thin vs. Thick Skin

Example: Role of Sweat Glands in Thermoregulation

• When body temperature rises, eccrine sweat glands secrete sweat onto the skin surface. As the sweat evaporates, it cools the body, helping maintain homeostasis.

Additional info:

• Merkel's discs, Meissner's corpuscles, and Pacinian corpuscles are specialized sensory receptors found in the dermis, each detecting different types of stimuli.

• Collagen and elastin fibers in the dermis provide both strength and flexibility to the skin.