Anatomy & Physiology Study Guide: The Integumentary System (Skin)

The Integumentary System

Overview of Skin Structure

The skin is the largest organ of the human body and serves as a protective barrier. It consists of three main layers: the epidermis, dermis, and hypodermis (subcutaneous layer). Each layer has distinct functions and cellular compositions.

• Epidermis: The outermost layer, composed primarily of keratinized stratified squamous epithelium. Responsible for protection and water resistance.

• Dermis: The middle layer, made of connective tissue, containing blood vessels, nerves, hair follicles, and glands. Divided into the papillary and reticular layers.

• Hypodermis (Subcutaneous Layer): The deepest layer, consisting mainly of adipose tissue, providing insulation and cushioning.

Example: In the provided figure, the layers are labeled as follows: A = Epidermis, B = Papillary layer of dermis, C = Reticular layer of dermis, D = Hypodermis.

Functions of the Skin

The skin performs several vital functions essential for homeostasis and protection.

• Protection: Acts as a barrier against mechanical injury, pathogens, and harmful substances.

• Thermoregulation: Regulates body temperature through sweat production and blood vessel dilation/constriction.

• Sensation: Contains sensory receptors for touch, pain, temperature, and pressure.

• Vitamin D Synthesis: Initiates the synthesis of vitamin D when exposed to UV radiation.

• Excretion: Removes waste products through sweat.

Layers of the Skin

Epidermal Layers

The epidermis is composed of several layers, each with specific roles in protection and regeneration.

• Stratum basale: Deepest layer; site of cell division and melanocyte location.

• Stratum spinosum: Provides strength and flexibility; contains keratinocytes.

• Stratum granulosum: Cells begin to die and accumulate keratin.

• Stratum lucidum: Present only in thick skin (palms, soles); provides extra protection.

• Stratum corneum: Outermost layer; consists of dead, keratinized cells.

Example: A splinter penetrating the skin of the sole of the foot would pass through the stratum corneum, lucidum, granulosum, spinosum, and basale.

Dermal Layers

The dermis is divided into two layers:

• Papillary layer: Superficial; contains capillaries and sensory receptors.

• Reticular layer: Deep; dense connective tissue, provides strength and elasticity.

Example: The reticular layer constitutes about 80% of the dermis and is responsible for tension lines in the skin.

Accessory Structures of the Skin

Hair and Hair Follicles

Hair is produced by follicles located in the dermis. It serves protective and sensory functions.

• Root hair plexus: Nerve endings around hair follicles that detect hair movement.

• Growth factors: Age, hormones, and genetics influence hair growth.

Nails

Nails are protective coverings on the dorsal surface of fingers and toes, composed of hard keratin.

• Nail bed: Appears pink due to underlying capillaries.

• Lunula: The crescent-shaped area at the base of the nail.

Glands of the Skin

The skin contains several types of glands:

• Sebaceous glands: Secrete sebum (oil) to lubricate skin and hair; activity increases during puberty.

• Sudoriferous (sweat) glands: Two types:

  • Eccrine glands: Widely distributed; regulate temperature by secreting watery sweat.

  • Apocrine glands: Located in axillary and genital areas; secrete a thicker fluid, active at puberty.

Example: Apocrine glands are found in the axillae and anogenital areas and are influenced by hormones.

Skin Color and Pigmentation

Determinants of Skin Color

Skin color is determined by the presence of three pigments:

• Melanin: Produced by melanocytes; provides brown to black color and protects against UV radiation.

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

• Hemoglobin: Red pigment in blood; gives a pinkish hue to fair skin.

Example: The pinkish hue in fair-skinned individuals is due to hemoglobin in dermal capillaries.

Clinical Considerations

Burns

Burns are classified by the depth of tissue damage:

• First-degree burn: Damages only the epidermis; redness and mild pain.

• Second-degree burn: Damages epidermis and upper dermis; blisters and severe pain.

• Third-degree burn: Destroys epidermis and dermis; may affect underlying tissues; requires medical intervention.

Example: A burn that damages the epidermis and upper dermis is a second-degree burn.

Skin Cancer

Skin cancer arises from uncontrolled cell growth in the skin. The main types are:

• Basal cell carcinoma: Most common, least malignant.

• Squamous cell carcinoma: Arises from keratinocytes; can metastasize.

• Melanoma: Most dangerous; arises from melanocytes; high metastatic potential.

Example: Melanoma appears as a scaly, reddened patch and tends to grow rapidly.

Other Clinical Terms

• Cyanosis: Bluish skin due to lack of oxygen; treated with supplemental oxygen.

• Jaundice: Yellow skin due to liver dysfunction.

• Albinism: Lack of melanin production.

Review Table: Layers and Functions of the Skin

Layer	Main Components	Functions
Epidermis	Keratinocytes, melanocytes	Protection, water resistance, UV defense
Papillary Dermis	Loose connective tissue, capillaries	Nourishes epidermis, sensory reception
Reticular Dermis	Dense connective tissue, glands, hair follicles	Strength, elasticity, houses accessory structures
Hypodermis	Adipose tissue	Insulation, energy storage, cushioning

Key Equations and Scientific Concepts

• Vitamin D Synthesis: UV radiation converts 7-dehydrocholesterol in the skin to vitamin D3 (cholecalciferol).

• Burn Classification:

  • First-degree: $\text{Damage to epidermis only}$

  • Second-degree: $\text{Damage to epidermis and part of dermis}$

  • Third-degree: $\text{Damage to epidermis, dermis, and possibly deeper tissues}$

Summary of Key Terms

• Keratin: Protein that provides strength and waterproofing to skin cells.

• Melanocyte: Cell that produces melanin pigment.

• Sudoriferous gland: Sweat gland; includes eccrine and apocrine types.

• Sebaceous gland: Oil gland associated with hair follicles.

• Stratum corneum: Outermost layer of the epidermis.

• Reticular layer: Deep layer of the dermis, provides strength.

Additional info: Some explanations and examples have been expanded for clarity and completeness, based on standard Anatomy & Physiology curriculum.