The Skin: Structure and Layers

The Skin Consists of Two Layers: Epidermis and Dermis

The skin is the largest organ of the body and is composed of two main layers: the epidermis and the dermis. Beneath these lies the subcutaneous tissue (hypodermis), which is not technically part of the skin but is closely associated with its function.

• Epidermis: The outermost layer, made of stratified squamous epithelium, provides a protective barrier.

• Dermis: The inner layer, composed of connective tissue, supports and nourishes the epidermis.

• Subcutaneous tissue (Hypodermis): A layer of loose connective tissue and fat beneath the dermis, providing insulation and cushioning.

The Epidermis: Keratinized Stratified Squamous Epithelium

Structure and Function of Epidermal Layers

The epidermis is a multilayered epithelium that protects underlying tissues. Most cells are keratinocytes, which produce the protein keratin.

• Layers of the Epidermis (from deep to superficial):

  1. Stratum basale (germinativum): Deepest layer; site of cell division and contains melanocytes and Merkel cells.

  2. Stratum spinosum: Several layers of keratinocytes; contains Langerhans cells (immune function).

  3. Stratum granulosum: Keratinocytes begin to die and accumulate keratin.

  4. Stratum lucidum: Present only in thick skin (palms, soles); thin, clear layer of dead cells.

  5. Stratum corneum: Outermost layer; many layers of dead, keratinized cells providing a tough, protective barrier.

• Cell Types:

  • Keratinocytes: Produce keratin for waterproofing and protection.

  • Melanocytes: Synthesize melanin pigment, protecting against UV radiation.

  • Langerhans cells: Immune cells that detect pathogens.

  • Merkel cells: Sensory receptors for touch.

The Dermis: Papillary and Reticular Layers

Structure and Function of the Dermis

The dermis is a strong, flexible connective tissue layer that supports the epidermis and houses blood vessels, nerves, and appendages.

• Papillary layer: Thin, superficial layer with loose connective tissue; forms dermal papillae that interlock with the epidermis, increasing surface area for exchange of gases, nutrients, and waste.

• Reticular layer: Thicker, deeper layer with dense irregular connective tissue; contains collagen and elastin fibers for strength and elasticity.

• Functions: Provides structural support, houses sensory receptors, and contains blood vessels for thermoregulation.

Skin Color: Melanin, Carotene, and Hemoglobin

Determinants of Skin Color

Skin color is determined by the presence and relative amounts of three pigments: melanin, carotene, and hemoglobin.

• Melanin: Produced by melanocytes; ranges from yellow to brown to black. Protects DNA from UV damage.

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

• Hemoglobin: Oxygen-carrying pigment in red blood cells; gives skin a pinkish hue when blood vessels are close to the surface.

Hair: Structure and Function

Hair Consists of Dead, Keratinized Cells

Hair is produced by hair follicles and serves protective and sensory functions.

• Structure of Hair:

  • Shaft: Part above the skin surface; composed of dead, keratinized cells.

  • Root: Part below the skin surface; embedded in the follicle.

  • Hair follicle: Invagination of the epidermis into the dermis; contains the hair bulb and matrix (site of growth).

  • Associated structures: Arrector pili muscle (causes hair to stand), sebaceous glands (secrete sebum).

• Types of Hair: Vellus (fine, pale), terminal (coarse, pigmented).

• Hair color: Determined by type and amount of melanin produced by melanocytes in the hair bulb.

• Growth cycle: Hair grows in cycles of active growth and rest; rate varies by body region, age, and sex.

Sweat and Sebaceous Glands

Sweat Glands Help Control Body Temperature; Sebaceous Glands Secrete Sebum

The skin contains two main types of exocrine glands: sweat (sudoriferous) glands and sebaceous (oil) glands.

• Sweat Glands:

  • Eccrine glands: Widely distributed; secrete watery sweat for thermoregulation.

  • Apocrine glands: Found in axillary and genital areas; secrete a thicker, milky sweat; become active at puberty.

  • Functions: Regulate body temperature, excrete wastes, and provide a minor antibacterial effect.

• Sebaceous Glands:

  • Secrete sebum, an oily substance that lubricates and waterproofs skin and hair.

  • Most open into hair follicles; some open directly onto skin surface.

  • Activity increases at puberty; overactivity can contribute to acne.

Functions of the Skin

First and Foremost, the Skin is a Barrier

The skin performs several vital functions to maintain homeostasis and protect the body.

• Protection: Physical, chemical, and biological barriers prevent entry of pathogens and minimize water loss.

• Body temperature regulation: Sweat glands and blood vessels help dissipate or conserve heat.

• Cutaneous sensation: Sensory receptors detect touch, pressure, pain, and temperature.

• Metabolic functions: Synthesis of vitamin D precursor in response to UV light.

• Excretion: Small amounts of nitrogenous wastes eliminated in sweat.

Skin Cancer and Burns: Major Clinical Challenges

Skin Cancer

Skin cancer is the most common type of cancer. There are three major types, classified by the cell of origin and malignancy.

• Basal cell carcinoma: Originates in stratum basale; least malignant, most common.

• Squamous cell carcinoma: Originates in keratinocytes of stratum spinosum; can metastasize.

• Melanoma: Originates in melanocytes; most dangerous due to rapid metastasis and resistance to therapy.

• ABCDE Rule: Used to identify melanoma: Asymmetry, Border irregularity, Color, Diameter, Evolving.

Burns

Burns are classified by depth and severity. They can cause dehydration, infection, and loss of function.

• First-degree burns: Affect only the epidermis; redness and pain.

• Second-degree burns: Affect epidermis and part of dermis; blisters, pain, and swelling.

• Third-degree burns: Destroy entire epidermis and dermis; may extend into subcutaneous tissue; skin may appear white, red, or blackened; often painless due to nerve destruction.

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

Additional info:

• The rule of nines divides the body into regions, each accounting for 9% (or multiples) of total body surface area, to quickly estimate fluid loss and treatment needs in burn patients.