Chapter 5: The Integumentary System – High-Yield Study Guide

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The Integumentary System

Overview

The integumentary system, primarily composed of the skin, is the largest organ system in the body. It serves as a protective barrier, regulates temperature, and houses sensory receptors. The skin consists of three main layers: the epidermis, dermis, and hypodermis (subcutaneous tissue).

Structure of the skin showing epidermis, dermis, and hypodermis with accessory structures

Epidermal Structure & Cells

The Five Layers of the Epidermis

The epidermis is avascular and relies on diffusion from the underlying dermis. It is composed of stratified squamous epithelial tissue and organized into five distinct layers (from deep to superficial):

Stratum Basale: Single row of actively dividing stem cells; closest to the blood supply.
Stratum Spinosum: Thickest living layer; cells appear spiny due to desmosomes.
Stratum Granulosum: Cells release hydrophobic lipids, forming a water barrier and leading to apoptosis.
Stratum Lucidum: Clear, dead layer found only in thick skin (palms, soles).
Stratum Corneum: Multiple layers of dead, flattened, keratinized cells that are shed daily.

Structure of the epidermis with labeled layers and cell types Diagram showing cell movement through the epidermal strata Comparison of thick and thin skin under the microscope

Core Cell Types in the Epidermis

Keratinocytes: Make up 95% of epidermal cells; produce keratin for strength and are connected by desmosomes.
Melanocytes: Located in the stratum basale; produce melanin in melanosomes, which protect DNA from UV radiation.
Tactile (Merkel) Cells: Also in the stratum basale; function as light touch receptors.
Dendritic (Langerhans) Cells: Found in the stratum spinosum; act as immune phagocytes.

Melanocytes and melanin function in the epidermis

Dermal Layers & Skin Markings

Dermal Structure

The dermis is highly vascular and composed of two layers:

Papillary Layer: Areolar connective tissue; contains dermal papillae (wavy folds with capillary loops) and Meissner corpuscles (light touch receptors). Forms friction ridges (fingerprints).
Reticular Layer: Dense irregular connective tissue; contains collagen and elastic fibers, Pacinian corpuscles (deep pressure/vibration receptors), flexure lines, and tension lines (cleavage lines).

Structure of the dermis with papillary and reticular layers Epidermal ridges and fingerprint patterns Tension lines and their importance for surgical incisions

Clinical Note: Blisters and Stretch Marks

Blisters: Fluid-filled separation between epidermis and dermis due to friction or burns.
Striae (Stretch Marks): Tearing of dermal elastic fibers from rapid stretching.

Skin Pigmentation & Clinical Clues

Determinants of Skin Color

Skin color is determined by the interaction of three main pigments:

Melanin: Orange-red to black pigment produced by melanocytes; everyone has the same number of melanocytes, but production varies.
Carotene: Yellow-orange pigment from diet; accumulates in the stratum corneum.
Hemoglobin: Bright red blood protein; gives a pink hue to fair skin.

Pathological Discoloration Cheat Sheet

Cyanosis: Bluish tint from oxygen-starved hemoglobin; indicates respiratory distress or shock.
Erythema: Redness from increased blood flow due to exercise, fever, or inflammation.
Pallor: Pale appearance from decreased blood flow, revealing white dermal collagen.
Jaundice: Yellowish tint from bilirubin accumulation, indicating liver dysfunction.

Accessory Structures: Hair, Nails, and Glands

Hair Structure and Function

Hair Layers: Medulla (core) → Cortex (middle) → Cuticle (outer layer).
Hair Matrix: Growth zone at the base of the follicle; site of rapid mitosis.

Hair structure with labeled layers and follicle anatomy Frontal section of hair root and follicle

Nail Structure and Function

Nail Matrix: Hidden growth zone under the root; produces the nail plate.
Lunula: Opaque, white crescent at the nail base.

Nail structure with labeled parts Sagittal section of a fingertip showing nail matrix and bed

Cutaneous Glands

Type	Secretion	Location	Function
Eccrine Sweat	Watery sweat	Most skin	Thermoregulation, antimicrobial protection
Apocrine Sweat	Protein-rich sweat	Axillae, groin	Body odor (bacterial action), activated at puberty
Sebaceous	Sebum (oil)	Face, scalp (not palms/soles)	Moisture seal, antibacterial, lubricates hair

Sweat glands and sebaceous glands structure

Pathology: Burns & Skin Cancer

Burn Depth Classification

1st-Degree: Epidermis only; redness and pain, no blisters.
2nd-Degree: Epidermis + part of dermis; blisters and intense pain.
3rd-Degree: Epidermis, dermis, and hypodermis; painless (nerve destruction), severe scarring.

The three classes of burns with diagrams and clinical images

Rule of Nines

Used to estimate the percentage of body surface area burned for fluid resuscitation:

Rule of nines for estimating burn extent

Skin Cancer Profiles

Basal Cell Carcinoma: Most common; arises from stratum basale keratinocytes; rarely metastasizes.
Squamous Cell Carcinoma: From stratum spinosum keratinocytes; may ulcerate and bleed.
Malignant Melanoma: Most dangerous; arises from melanocytes; highly metastatic.

ABCDE Framework for Melanoma Detection

Asymmetry: Two halves do not match.
Border: Irregular, ragged, or blurred edges.
Color: Uneven shades or unusual colors.
Diameter: Greater than 6 mm.
Evolving: Changes in size, shape, or color.

Additional info:

The hypodermis (subcutaneous layer) is not technically part of the skin but anchors it to underlying tissues and stores fat.
Incisions made parallel to tension lines heal with less scarring due to the orientation of collagen fibers.