The Integumentary System: Structure, Function, and Clinical Correlates

Integumentary System

Overview

The integumentary system is the body's largest organ system, providing protection, sensation, and regulation. It consists of the skin and its derivatives, as well as associated structures.

• Components:

  • Skin

  • Hair

  • Nails

  • Glands (sebaceous, sudoriferous, ceruminous, mammary)

  • Muscles (arrector pili)

  • Sense receptors (nervous tissue)

  • Hypodermis (subcutaneous layer)

Skin Structure

Layers of the Skin

The skin is composed of two main layers, each with distinct structures and functions:

• Epidermis: The superficial, outermost layer; composed of stratified squamous epithelium; avascular.

• Dermis: The deeper layer; composed of connective tissue; contains blood vessels, nerves, glands, and hair follicles.

Epidermis: Structure and Cell Types

• Stratified squamous epithelium (multiple layers of flat cells)

• Avascular (lacks blood vessels)

• Divided into 4-5 strata (sub-layers):

  • Stratum Basale: Single row of cells; contains:

    • Keratinocytes (90%): Undergo mitosis; produce keratin (a tough, protective protein); new cells are pushed toward the surface.

    • Melanocytes (10%): Produce melanin (pigment) for UV protection; responsible for skin color variation due to differing melanin production, not cell number.

  • Stratum Spinosum: Several layers; low levels of mitosis.

  • Stratum Granulosum: Contains granules for keratin production and glycolipids for waterproofing; last layer of living cells; no mitosis.

  • Stratum Lucidum: Present only in thick skin; flat, dead cells; no mitosis.

  • Stratum Corneum: Many layers of flat, dead, keratin-filled cells; glycolipids between cells create a waterproof barrier; cells are shed and replaced from below.

• Skin color: Determined by melanin (from melanocytes) and hemoglobin (in dermal blood vessels, giving a pinkish tinge in fair skin).

Dermis: Structure and Sub-layers

• Contains: Blood vessels, nerves, glands, hair follicles, arrector pili muscles.

• Sub-layers:

  • Papillary Layer: Areolar connective tissue; vascular; forms dermal papillae (projections into epidermis); in thick skin, forms epidermal ridges (fingerprints) for grip.

  • Reticular Layer: Dense irregular connective tissue; forms most of the dermis; provides strength and elasticity.

Thin vs. Thick Skin

The terms "thin" and "thick" skin refer to the thickness of the epidermis, not the dermis.

• Thin Skin: Covers most of the body; lacks stratum lucidum; contains hair follicles, sebaceous glands, and arrector pili muscles.

• Thick Skin: Found on palms and soles; has stratum lucidum; lacks hair follicles, sebaceous glands, and arrector pili muscles.

Epidermal Derivatives

Hair

• Composed of dead, keratinized cells.

• Parts:

  • Root: Embedded within the skin.

  • Shaft: Visible part above the skin surface.

  • Hair follicle: Surrounds the root; includes:

    • Epithelial root sheath: Epidermal layers extending into the dermis.

    • Bulb: Expanded region at base of root.

    • Matrix: Single layer of basale-derived cells; site of hair growth and melanin production for hair color.

  • Associated structures:

    • Outer connective tissue sheath (from dermis): Holds follicle in place.

    • Hair papilla (from dermis): Contains blood supply for growing hair.

    • Root hair plexus: Free nerve endings for touch sensation.

    • Sebaceous (oil) gland: Opens into follicle.

    • Arrector pili muscle (smooth muscle): Causes "goosebumps" by contracting.

Nails

• Heavily keratinized epidermal cells.

• Parts: Nail root (buried in skin), body (visible portion), free edge.

Skin Exocrine Glands

• Sebaceous glands: Connected mainly to hair follicles; secrete sebum (oily mixture of fats, salts, proteins); softens and lubricates hair and skin; prevents drying; has antibiotic properties.

• Sudoriferous (sweat) glands: Secretory portions in dermis; ducts open onto skin surface; functions include temperature regulation (cooling), antibiotic action (acidic sweat), and waste removal (e.g., urea).

• Ceruminous glands: Modified sweat glands in ear canal; produce ear wax (cerumen) to prevent entry of insects and debris.

• Mammary glands: Modified sweat glands; produce milk.

Cutaneous Sense Receptors

These are specialized sensory neurons that respond to various stimuli, providing the sense of touch, pressure, temperature, and pain.

• Touch Receptors:

  • Free nerve endings (in epidermis)

  • Root hair plexuses

  • Tactile (Meissner’s) corpuscles (in dermal papillae; encapsulated nerve endings for light touch)

• Pressure Receptors:

  • Free nerve endings (in dermis)

  • Lamellar (Pacinian) corpuscles (deep in dermis or hypodermis; detect deep pressure and vibration)

• Thermoreceptors: Free nerve endings for temperature detection.

• Nociceptors: Free nerve endings for pain detection; destroyed in third-degree burns, leading to loss of pain sensation in affected areas.

Hypodermis (Subcutaneous Layer)

• Not part of the skin but closely associated.

• Also called superficial fascia.

• Composed of adipose connective tissue; stores about half of the body's fat (insulation and energy storage).

• Fascia: Connective tissue layers that surround and support organs.

Related Medical Conditions

• Albinism: Genetic condition characterized by lack of melanin production by melanocytes, resulting in very light skin, hair, and eyes.

• Psoriasis: Autoimmune disorder causing accelerated mitosis of keratinocytes in the stratum basale; leads to accumulation of immature keratinocytes in the epidermis and failure of the stratum corneum to shed, resulting in thick, scaly plaques on the skin surface.