Chapter 5 epidermis and dermis

Functions of the Skin

  • Protection

    • Acts as a physical barrier between the body and the environment.

    • Keeps dirt, bacteria, and viruses out while retaining body moisture.

    • Secretions from the skin help lubricate it and regulate microbial growth.

  • Body Temperature Regulation

    • Blood flow to the skin can be adjusted to manage heat dissipation.

    • Vasodilation increases blood flow, causing the skin to darken and releasing heat.

    • Vasoconstriction decreases blood flow, making the skin lighter and retaining heat.

  • Sensation

    • Contains sensory receptors that detect touch (fine and rough), temperature, and pain.

    • Different layers of skin house various types of sensory receptors.

  • Metabolic Functions

    • Converts inactive vitamin D to its active form.

    • Skin serves as a reservoir for blood.

Structure of the Skin

  • The skin is composed of two primary layers:

    • Epidermis (outer layer): Thin, stratified squamous epithelium with no blood vessels.

    • Dermis (inner layer): Thicker, primarily connective tissue with blood vessels.

    • Hypodermis: Located beneath the dermis, containing subcutaneous fat, not considered part of the skin.

Cell Types in the Epidermis

  • Keratinocytes

    • Most abundant cell type, producing keratin, which provides toughness and resilience.

    • Connected by desmosomes and undergo mitosis in the basal layer.

    • Turnover rate varies but can be approximately 25-30 days.

  • Langerhans Cells

    • Immune cells that behave as sentinels, alerting the immune system to potential threats.

    • Secrete inflammatory chemicals in response to skin breaches.

  • Merkel Cells

    • Sensory cells responsible for fine touch perception, connected to sensory neurons.

    • Located in the basal layer of the epidermis.

  • Melanocytes

    • Produce melanin, which determines skin color and protects DNA from UV damage.

    • Melanin production is influenced by sun exposure.

Layers of the Epidermis

  • Stratum Basale

    • Deepest layer, single layer of mitotically active keratinocytes, melanocytes, and Merkel cells.

    • Nutrients diffused from the dermal blood vessels.

  • Stratum Spinosum

    • Contains cuboidal-shaped keratinocytes and Langerhans cells.

    • Cells produce keratin and start differentiation.

  • Stratum Granulosum

    • Thin layer where keratinocytes begin to die, breaking down organelles.

    • Keratin bundles form, making cells tougher.

  • Stratum Corneum

    • Outer protective layer consisting of dead, flattened keratinized cells (bags of keratin).

    • Acts as a waterproof barrier.

  • Stratum Lucidum (only in thick skin)

    • Clear layer found primarily in palms and soles of feet, providing added protection.

Structure of the Dermis

  • Papillary Layer

    • Upper layer characterized by loose areolar connective tissue with projections (dermal papillae) extending into the epidermis.

    • Increases surface area and contributes to fingerprints and sensations.

  • Reticular Layer

    • Deeper, thicker layer made up of dense irregular connective tissue.

    • Contains structures such as hair follicles, blood vessels, sweat glands, and nerve endings.

Cleavage and Flexure Lines

  • Cleavage Lines

    • Indicate the orientation of collagen fibers within the dermis which impacts healing and scarring.

    • Surgical cuts along these lines minimize scar visibility.

  • Flexure Lines

    • Permanent lines found in specific areas (e.g., palms, soles) related to tissue attachment to deeper structures, facilitating movement.

Conclusion

  • The skin consists of complex layers and functions, playing essential roles in protection, sensation, and metabolic regulation. Understanding its anatomy provides insights into its numerous functions and importance to overall health.