AE

2C notes (integument, epidermis)

Integumentary System

Overview

  • The integumentary system forms a protective barrier for the body.

  • Composed of three main layers:

    • Epidermis (stratified squamous epithelium)

    • Dermis (dense irregular connective tissue)

    • Hypodermis- subcutaneous layer (areolar and adipose connective tissue)

Layers of the Skin

Epidermis

  • Composition: Stratified squamous epithelium, primarily made of keratinocytes.

  • Key Functions: Protects against pathogens, regulates water loss.

Dermis

  • Location: Deep to the epidermis.

  • Composition: Dense irregular connective tissue.

  • Functions: Provides strength and flexibility.

Hypodermis (Subcutaneous Layer)

  • Location: Lies deep to the dermis.

  • Composition: Areolar and adipose connective tissue.

  • Functions: Anchors skin to underlying structures, provides insulation.

Cellular Components of the Epidermis

Keratinocytes

  • Most abundant cell type in the epidermis.

  • Shape changes as they migrate upward through layers.

Stratum Basale

  • Characteristics: Deepest layer, single layer of cuboidal/columnar cells.

  • Cell Types:

    • Living Keratinocytes (stem cells generating new cells)

    • Melanocytes (produce melanin for UV protection)- packaged in vesicles and transferred to keratinocytes

      • Types of Melanin:

        • Pheomelanin (yellow-red)

        • Eumelanin (brown-black)

    • Tactile cells (specialized for touch sensation)- when compressed, they release chemicals that stimulate nearby sensory nerve endings

Stratum Spinosum

  • Consists of several layers of keratinocytes; they start producing keratin filaments.

  • Desmosomes create the 'spiny' appearance, linking keratinocytes for mechanical strength.

  • Epidermal Dendritic (Langerhans) Cells: Mobile immune cells that detect and respond to pathogens.

Stratum Granulosum

  • Contains 3–5 layers of flattened keratinocytes.

  • Keratohyalin Granules aid in keratin aggregation.

  • Lamellar Bodies release lipids that contribute to water resistance.

  • Cells flatten and organelles degenerate as they move upward.

Stratum Lucidum

  • Found only in thick skin (palms and soles).

  • Thin, translucent layer of 2–3 layers of dead, keratin-filled cells.

Stratum Corneum

  • Outermost layer with 20–30 layers of dead, interlocking, keratinized cells.

  • Acts as a barrier against infection, abrasion, and water loss.

Types of Skin

  • Thick Skin: Found on palms and soles; contains all 5 strata (including stratum lucidum).

  • Thin Skin: Covers most of the body; lacks the stratum lucidum, 4 strata

Keratinization Process

  • Involves keratinocytes filling with keratin, dying, and forming protective layers.

  • Functions: Provides protection, water resistance, abrasion resistance, and prevents pathogen entry.

  • Through keratinization, the epidermis forms a tough, protective, and water-resistant barrier, with each layer contributing increasingly to its structural strength.

Variability in Epidermis

  • Variations: Thickness, skin markings, friction ridges, and color.

  • Skin Markings:

    • Hemangiomas: Blood vessel overgrowth.

    • +Freckles: Localized melanocyte activity.

    • Nevi: Moles.

  • Friction Ridges: Unique to each individual, create fingerprints.

Influences on Skin Color

  • Factors:

    • Hemoglobin (red with oxygen)

    • Melanin (amount and activity)

    • Carotene (yellow-orange from diet)

Vitamin D Synthesis

  • Precursor: 7-dehydrocholesterol (lipid) in skin converts to previtamin D under UVB rays and then to vitamin D3 (cholecalciferol).

  • Impact of UV Exposure: More UV → increased activity of melanocytes for DNA protection, but melanin reduces vitamin D synthesis.

    • Example: Moving from low sunlight to high UV → increased melanocyte activity, decreased vitamin D levels if diet remains unchanged.