Chapter 5: The Integumentary System

5.1 Skin Structure

The Integumentary System consists of the skin and its accessory structures, including hair, nails, and glands. The skin is the largest organ in the body, accounting for 10-15% of total body weight, and is composed of two main layers: the superficial epidermis and the deeper dermis. Below the dermis lies the hypodermis, which is not technically part of the skin.

• Epidermis: Keratinized stratified squamous epithelium; avascular; rests on a basement membrane.

• Dermis: Loose and dense irregular connective tissue; vascularized; contains accessory structures.

• Accessory Structures: Nails, hair, sweat glands, sebaceous glands, sensory neurons, and arrector pili muscles.

• Hypodermis (Superficial Fascia): Loose connective and adipose tissue; highly vascularized; not part of the skin.

5.1 Functions of the Integumentary System

The integumentary system is vital for maintaining homeostasis and serves several key functions:

• Protection: Shields underlying tissues from mechanical trauma, pathogens, and environmental threats. The skin's acid mantle (slightly acidic pH) deters pathogen growth.

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

• Thermoregulation: Maintains stable internal temperature via negative feedback loops involving sweat glands and blood vessels.

• Excretion: Eliminates small amounts of metabolic waste (e.g., lactic acid, urea) in sweat.

• Synthesis of Vitamin D: UV radiation converts a precursor in the epidermis to cholecalciferol, which is modified by the liver and kidneys to form calcitriol, essential for calcium absorption.

Thermoregulation Example:

• When body temperature rises: Thermoreceptors signal the hypothalamus, which triggers sweating and vasodilation to release heat.

• When body temperature falls: Thermoreceptors signal the hypothalamus, which triggers vasoconstriction to conserve heat.

5.2 The Epidermis

The epidermis is the superficial layer of the skin, composed mainly of keratinocytes and organized into up to five strata (layers).

• Keratinocytes: 95% of epidermal cells; produce keratin for strength and resilience; connected by desmosomes.

Layers (Strata) of the Epidermis (Deep to Superficial):

• Stratum Basale (Germinativum): Single layer of stem cells; mitotically active; produces keratin and vitamin D precursors.

• Stratum Spinosum: Thickest layer; spiky-shaped cells; metabolically and mitotically active; produces keratin and vitamin D.

• Stratum Granulosum: 3-5 rows; contains granules (keratin bundles and membrane-coating granules); forms water barrier.

• Stratum Lucidum: Clear, narrow layer of dead keratinocytes; found only in thick skin.

• Stratum Corneum: Multiple layers of dead, flattened keratinocytes; cells are shed (sloughed) daily.

Mnemonic: Brilliant Studying Gives Loads of Confidence (Basale, Spinosum, Granulosum, Lucidum, Corneum)

Keratinocyte Life Cycle

• Cells in the stratum basale and spinosum divide and push older cells upward through the strata.

• Cells are eventually shed from the stratum corneum; the process takes 40-50 days.

Other Cells of the Epidermis

• Dendritic (Langerhans) Cells: Immune phagocytes in the stratum spinosum.

• Tactile (Merkel) Cells: Sensory receptors for light touch; found in the stratum basale.

• Melanocytes: Produce melanin pigment; located in the stratum basale.

Thick and Thin Skin

• Thick Skin: Palms, soles, fingers, toes; all five strata; thick stratum corneum; no hair follicles; many sweat glands.

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

• Callus: Localized thickening of the stratum corneum due to repeated pressure.

5.3 The Dermis

The dermis is the deep layer of the skin, providing structural support and housing blood vessels, nerves, and accessory structures. It is divided into two layers:

• Papillary Layer: Superficial 20%; loose connective tissue; contains dermal papillae (projections) with tactile (Meissner) corpuscles for light touch.

• Reticular Layer: Deep 80%; dense irregular connective tissue; contains blood vessels, sweat and sebaceous glands, hair follicles, and lamellated (Pacinian) corpuscles for deep pressure and vibration.

Skin Markings

• Dermal Ridges: Prominent in thick skin; form epidermal ridges (fingerprints).

• Tension (Cleavage) Lines: Gaps between collagen bundles; important for surgical incisions.

• Flexure Lines: Deep creases at joints (e.g., palms).

5.4 Melanin and Other Pigments

Melanin is a pigment produced by melanocytes in the stratum basale and stored in melanosomes. It protects DNA from UV radiation and determines skin color.

• Melanin is transferred to keratinocytes and degrades after a few days; synthesis increases with UV exposure.

• Melanin also regulates vitamin D synthesis by controlling UV penetration.

• Variations: Freckles (localized melanin), Moles (melanocyte proliferation), Albinism (genetic lack of melanin).

Other Pigments:

• Carotene: Yellow-orange pigment from diet; accumulates in stratum corneum.

• Hemoglobin: Red pigment in blood; gives skin a pinkish hue in fair-skinned individuals.

Skin Color as a Diagnostic Tool

5.5 Accessory Structures of the Integument

Accessory structures include hair, nails, and glands, all derived from the epithelium and assisting in the skin's functions.

Hair

• Structure: Shaft (above skin), root (in dermis), follicle (epithelial and dermal sheaths), bulb (base with papilla and matrix).

• Function: Protection (e.g., eyelashes, scalp), sensation (sensory neurons).

• Growth: Alternates between growth and resting stages; average growth is 1-1.5 cm/month.

• Types: Lanugo (fetal), terminal (scalp, eyebrows), vellus (body hair).

• Pigment: Determined by melanin; red hair contains iron pigment; gray/white hair results from decreased melanin with age.

Nails

• Structure: Nail plate (body and root), nail bed, nail matrix (growth area), proximal/lateral nail folds, eponychium (cuticle), hyponychium (distal attachment).

• Function: Protects fingertips, aids in manipulation.

Glands

• Sweat (Sudoriferous) Glands:

  • Eccrine: Most common; watery sweat for thermoregulation; ducts open to skin surface.

  • Apocrine: Axillae, anal, areolae; thick, protein-rich sweat; ducts open to hair follicles; odor develops when metabolized by bacteria.

  • Ceruminous: Modified apocrine glands in ear; produce cerumen (ear wax).

  • Mammary: Specialized for milk production.

• Sebaceous Glands: Secrete sebum (oily, hydrophobic barrier) via holocrine secretion; associated with hair follicles; secretion increases at puberty.

5.6 Wounds, Burns, and Skin Cancer

Disruptions in skin integrity include wounds, burns, and cancers, each with distinct characteristics and clinical implications.

Wounds

• Lacerations: Cuts or tears; may require sutures.

• Burns: Classified by depth and extent of tissue damage.

Burns

Rule of Nines: Used to estimate the percentage of body surface area affected by burns.

Skin Cancer

• Basal Cell Carcinoma: Most common; arises from stratum basale keratinocytes; rarely metastasizes.

• Squamous Cell Carcinoma: Second most common; arises from stratum spinosum keratinocytes; may metastasize.

• Malignant Melanoma: Least common, most dangerous; arises from melanocytes; high metastatic potential.

ABCDE Rule for Melanoma:

• Asymmetry

• Border irregularity

• Color variation

• Diameter > 6 mm

• Evolving shape/size