The Integumentary System

Overview

The integumentary system is composed of the skin and its accessory structures. It serves as the body's first line of defense, provides sensory information, and plays a role in homeostasis.

• Cutaneous membrane (skin): Consists of two main components:

  • Epidermis: Superficial layer; keratinized stratified squamous epithelium; avascular (no blood vessels).

  • Dermis: Deep to epidermis; loose connective tissue and dense irregular connective tissue.

• Accessory structures: Embedded in cutaneous membrane; include sweat glands, sebaceous glands, hair, and nails.

• Hypodermis (subcutaneous fat): Deep to dermis; anchors skin to deeper structures like muscle and bone; made of loose connective tissue and adipose tissue; abundant blood supply.

Functions of the Integumentary System

• Protection: From mechanical trauma (impact/abrasion), pathogens, radiation, and chemical attack.

• Cutaneous sensations: Enables nervous system to perceive changes in body’s internal or external surroundings; critical to homeostasis.

• Body Temperature Regulation: Insulation, evaporative cooling via sweating.

• Vitamin D Synthesis: Steroid converted to calcitriol, important for calcium metabolism.

• Storage of Lipids: Lipid storage in dermis (adipocytes) and adipose tissue in subcutaneous layer (insulation and protection).

Layers of the Epidermis

Strata of the Epidermis

The epidermis is organized into distinct layers (strata), each with unique cell types and functions. From deepest to most superficial:

Mnemonic for Epidermal Layers

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

Cell Types in the Epidermis

• Keratinocyte: Most dominant epithelial cell; produces keratin, a tough protein. Migration from stratum basale to corneum takes 40-50 days.

• Langerhans (Dendritic) cell: In stratum spinosum; phagocytes of immune system, protect skin and deeper tissues from pathogens.

• Merkel cell: Scattered in stratum basale; sensory receptors for light touch and texture discrimination. Numerous in fingertips, lips, and base of hairs.

• Melanocyte: In stratum basale; produces melanin (pigment ranging from orange-red to brown-black).

Thick vs. Thin Skin

• Thick skin: Found on palms and soles; contains stratum lucidum; more layers of keratinocytes; no hair or sebaceous glands.

• Thin skin: Covers most of body; fewer layers; contains hair follicles and sebaceous glands.

The Dermis

Structure and Function

The dermis is a highly vascular layer deep to the epidermis. It provides support, anchors skin, contains sensory receptors, and stores lipids.

• Papillary layer: Superficial; loose connective tissue; contains dermal papillae (projections into epidermis), capillaries, and Meissner/tactile corpuscles (touch receptors).

• Reticular layer: Deep; dense irregular connective tissue; contains collagen fibers, elastic fibers, and Pacinian/lamellated corpuscles (vibration receptors).

Dermal Features

• Blisters: Fluid-filled pockets between epidermis and dermis.

• Striae (stretch marks): Formed when skin stretches too far; elastic fibers start to tear.

• Epidermal ridges: Small visible lines in epidermis; best seen in thick skin.

• Fingerprints: Created by epidermal ridges and dermal papillae.

• Cleavage/tension lines: Indentations in epidermis from gaps between collagen bundles.

• Flexure lines: Deep creases in areas of body where reticular layer is tightly anchored.

Skin Pigmentation

Melanin

Melanin is the key pigment responsible for skin color. It is produced by melanocytes in the stratum basale and transferred to keratinocytes.

• Differences in melanin production: Vary by genetics and number of melanocytes.

• Effect of UV light: Melanin shields keratinocyte nuclei from UV radiation; more melanin = darker skin, better protection.

• Vitamin D synthesis: Less melanin allows more UV penetration, increasing vitamin D production.

• Freckles and moles: Patches of accumulated melanin.

• Albinism: Lack of melanin production (not lack of melanocytes).

Other Skin Pigments

• Carotene: Yellow-orange pigment from food (egg yolks, orange vegetables); imparts yellow-orange color, especially in stratum corneum of thick skin.

• Hemoglobin: Iron-containing protein in red blood cells; binds and transports oxygen; imparts pinkish hue to skin, especially in light-skinned individuals.

Clinical Color Changes

• Pallor: Decreased blood flow in dermis; results in pale skin.

• Cyanosis: Low oxygen; skin turns bluish.

• Jaundice: Yellowish skin due to bilirubin accumulation (not in textbook).

Accessory Structures

Hair

• Function: Thermoregulation, protection/cushion, sensation.

• Structures:

  • Shaft: Visible part; dead keratinized cells.

  • Root: Anchors hair into dermis.

  • Matrix: Layer of dividing cells; hair growth.

  • Follicle: Outer portion; anchors hair into dermis.

  • Arrector pili muscle: Smooth muscle attached to follicle; contracts to make hair stand up (goosebumps).

• Pigment and Texture:

  • Lanugo: Thin, nonpigmented hair covering fetus; replaced by vellus or terminal hair.

  • Terminal hair: Thick, coarse, pigmented; scalp, eyebrows, eyelashes.

  • Vellus hair: Thinner, nonpigmented; body hair.

  • Color: Determined by melanin.

Nails

• Function: Hard because of keratin; protects tips of fingers from trauma.

• Structures:

  • Body: Visible part.

  • Root: Lies deep under skin; cells divide and grow outward.

  • Matrix: Group of dividing cells; nail growth.

  • Nail folds: Hold nail in position at end of finger.

  • Eponychium: Cuticle.

Glands

• Sudoriferous (sweat) glands:

  • Eccrine/merocrine: Most common; found on palms, soles, forehead; secrete watery fluid for thermoregulation.

  • Apocrine: Secrete thick, odorous fluid into hair follicles; found in axillary and genital regions; active at puberty; influenced by hormones.

• Sebaceous glands: Secrete oily sebum into hair follicles; lubricates and waterproofs skin and hair.

Key Equations and Scientific Terms

• Keratin: Tough, fibrous protein produced by keratinocytes; provides strength and waterproofing.

• Melanin: Pigment produced by melanocytes; protects against UV radiation.

• Vitamin D Synthesis:

• Hemoglobin: (oxygen transport)

Regeneration and Repair

• Regeneration: Damaged cells replaced with new cells of same type; restores function.

• Fibrosis: Damaged cells replaced with collagen and scar tissue; function not fully restored.

• Capacity for repair varies by tissue:

  • Epithelial tissue: High regenerative capacity.

  • Connective tissue: Moderate capacity (except cartilage).

  • Muscle tissue: Limited capacity.

  • Nervous tissue: Neurons have very limited capacity; neuroglia can divide.

Summary Table: Epidermal Layers

Additional info: Some explanations and terminology have been expanded for clarity and completeness, including scientific terms, equations, and clinical relevance.