Layers and Structure of the Skin

Overview

The skin, or integumentary system, is the largest organ of the human body. It consists of multiple layers, each with specialized functions that contribute to protection, sensation, and homeostasis.

• Epidermis: Outermost layer; composed of keratinized stratified squamous epithelium.

• Dermis: Middle layer; made of connective tissue, containing nerves, blood vessels, and glands.

• Hypodermis (Subcutaneous layer): Not technically part of the skin; mostly adipose tissue providing insulation and cushioning.

Layers of the Epidermis (Deep to Superficial)

• Stratum basale: Site of cell division (mitosis).

• Stratum spinosum: Contains desmosomes; provides strength.

• Stratum granulosum: Keratinization begins here.

• Stratum lucidum: Only in thick skin (palms/soles).

• Stratum corneum: Dead, keratinized cells; provides protection.

Layers of the Dermis

• Papillary layer: Areolar connective tissue; contains capillaries and sensory receptors.

• Reticular layer: Dense irregular connective tissue; contains collagen and elastic fibers.

• Cleavage lines: Regions where the skin heals best if cut parallel to these lines.

Hypodermis (Subcutaneous Tissue)

• Anchors skin to muscles.

• Provides insulation and cushioning.

Skin Color

Determinants of Skin Color

• Melanin: Pigment from melanocytes; protects DNA from UV radiation.

• Carotene: Yellow/orange pigment from diet (vitamin A source).

• Hemoglobin: Red pigment in blood; visible in fair skin.

Changes in skin color can indicate clinical conditions (e.g., cyanosis for hypoxia, jaundice for liver dysfunction).

Accessory Structures of the Skin

Hair

• Made of dead keratinized cells.

• Functions: Protection, sensory input, warmth.

• Parts: Shaft, root, follicle.

• Grows from the hair matrix; attached to arrector pili muscle (causes "goosebumps").

Nails

• Modified epidermis for protection.

• Nail matrix: Site of growth.

• Cuticle (eponychium) and lunule are visible features.

Glands

• Eccrine (merocrine) glands: Sweat for cooling (water + salt); found throughout the body.

• Apocrine glands: Found in axillary/genital areas; produce thicker sweat.

• Ceruminous glands: Produce earwax (cerumen) for protection.

• Sebaceous glands: Produce sebum (oil); lubricates and waterproofs skin.

Functions of the Skin

• Protection: Physical, chemical, and biological barriers.

• Temperature Regulation: Sweat and blood flow control.

• Sensation: Touch, pressure, pain, and temperature receptors.

• Metabolic Function: Vitamin D synthesis.

• Blood Reservoir: Holds approximately 5% of blood volume.

• Excretion: Salt, water, and waste via sweat.

Skin Disorders and Clinical Relevance

Skin Cancers

• Basal Cell Carcinoma: Least malignant, most common.

• Squamous Cell Carcinoma: Grows quickly, may metastasize.

• Melanoma: Most dangerous; cancer of melanocytes.

ABCDE Rule for Melanoma: Asymmetry, Border irregularity, Color variation, Diameter >6mm.

Burns

• First-degree: Epidermis only (redness, pain, sunburn).

• Second-degree: Epidermis + upper dermis (blisters).

• Third-degree: Full thickness (charred, no pain, nerve loss).

Rule of Nines: Used to estimate fluid loss in burn patients.

Development and Aging of the Skin

• Skin develops from ectoderm (epidermis) and mesoderm (dermis).

• Infancy: Thicker skin, more fat.

• Adolescence: Oiler skin.

• Aging: Thinner skin, fewer collagen/elastic fibers, wrinkles, dry skin.

Summary Table: Layers of the Skin

Key Equations and Rules

• Rule of Nines (for burns): Body surface area estimation for fluid loss.

ABCDE Rule for Melanoma: Asymmetry, Border, Color, Diameter (>6mm), Evolving.

Additional info:

• Skin color can be affected by disease states such as anemia (pallor), cyanosis (bluish tint), jaundice (yellowing), and erythema (redness).

• Accessory structures (hair, nails, glands) are derived from the epidermis but reside in the dermis.