Skin and Hypodermis Notes

The Skin and the Hypodermis

  • The skin is the largest organ, accounting for 7% of body weight.
  • Skin thickness varies from 1.5 to 4.4 mm.
  • The skin is divided into two distinct layers:
    • Epidermis
    • Dermis
  • Hypodermis lies deep to the dermis.
    • Composed of areolar and adipose tissues.
    • Not part of the integumentary system but shares some of the skin’s properties.

Skin Structure (Figure 5.1)

  • Illustrates the various components of the skin:
    • Epidermis
    • Dermis
      • Papillary dermis
      • Reticular dermis
    • Hypodermis
    • Nervous structures: Sensory nerve fiber with free nerve endings, Lamellar corpuscle, Hair follicle receptor (root hair plexus)
    • Adipose tissue
    • Dermal papillae
    • Subpapillary vascular plexus
    • Sweat pore
    • Appendages of skin: Eccrine sweat gland, Arrector pili muscle, Sebaceous (oil) gland, Hair follicle, Hair root
    • Dermal vascular plexus

Gross Structure of Skin and Underlying Tissues (Figure 5.2)

  • Displays the relationship between skin layers and underlying tissues:
    • Epidermis
    • Dermis
    • Hypodermis
    • Deep fascia
    • Muscle

Functions of the Skin and Hypodermis

  • Five important functions:
    1. Protection
      • Cushions organs and protects from bumps, chemicals, water loss, and ultraviolet (UV) radiation.
    2. Body temperature regulation
      • Capillary network and sweat glands regulate heat loss.
    3. Excretion
      • Urea, salts, and water lost through sweat.
    4. Production of Vitamin D
      • Epidermal cells use UV radiation to synthesize vitamin D.
    5. Sensory reception
      • Contains sense organs associated with nerve endings.

Epidermis

  • Composed of keratinized stratified squamous epithelium.
  • Contains four main cell types:
    • Keratinocytes
      • Location: stratum spinosum
      • Produce keratin, a fibrous protein.
    • Melanocytes
      • Location: basal layer
      • Manufacture and secrete pigment.
    • Tactile epithelial cells
      • Location: basal layer
      • Attached to sensory nerve endings.
    • Dendritic cells
      • Location: stratum spinosum
      • Part of the immune system; macrophage-like.

Keratinocytes

  • Most abundant cell type in the epidermis.
  • Arise from the deepest layer of the epidermis.
  • Produce keratin, a tough fibrous protein.
  • Produce antibiotics and enzymes.
  • Are dead at the skin’s surface.

Layers of the Epidermis

  • Stratum basale (stratum germinativum)
  • Stratum spinosum
  • Stratum granulosum
  • Stratum lucidum (only in thick skin)
  • Stratum corneum
  • Mnemonic: "Come, Let's Get Sun Burnt"

Epidermal Cells and Layers (Figure 5.3a and 5.3b)

  • Stratum corneum:
    • Most superficial layer; 20-30 layers of dead cells represented only by flat membranous sacs filled with keratin.
    • Glycolipids in extracellular space.
  • Stratum granulosum:
    • One to five layers of flattened cells, organelles deteriorating.
    • Cytoplasm full of lamellar granules (release lipids) and keratohyaline granules.
  • Stratum spinosum:
    • Several layers of keratinocytes unified by desmosomes.
    • Cells contain thick bundles of intermediate filaments made of pre-keratin.
  • Stratum basale:
    • Deepest epidermal layer; one row of actively mitotic stem cells.
    • Some newly formed cells become part of the more superficial layers.
    • Contains occasional melanocytes and dendritic cells.

Stratum Basale

  • Deepest layer of the epidermis.
  • Attached to the underlying dermis.
  • Cells actively divide.
  • Contains:
    • Tactile epithelial cells—associated with sensory nerve endings.
    • Melanocytes—secrete the pigment melanin.

Thick Skin (Figure 5.4)

  • Illustrates the layers of the epidermis in thick skin:
    • Stratum corneum
    • Stratum lucidum
    • Stratum granulosum
    • Stratum spinosum
    • Stratum basale
    • Dermal papilla
    • Collagen fibers
    • Papillary dermis
    • Reticular dermis

Stratum Spinosum

  • “Spiny” appearance caused by artifacts of histological preparation.
  • Contains thick bundles of intermediate filaments (tonofilaments).
    • Resist tension and contain the protein pre-keratin.
  • Among the keratinocytes are star-shaped dendritic cells.
    • Are a type of macrophage.
    • Function in the immune system.

Stratum Granulosum

  • Consists of a few layers of keratinocytes.
  • Keratinocytes contain:
    • Keratohyaline granules—help form keratin.
    • Lamellar granules—contain waterproofing glycolipid.
  • Cells above this layer are too far from dermal capillaries to receive nourishment.

Stratum Lucidum

  • Occurs only in thick skin.
    • Locations: palms and soles.
  • Composed of a few rows of flat, dead keratinocytes.

Stratum Corneum

  • Thick layer of dead keratinocytes and thickened plasma membranes.
  • Protects skin against abrasion and penetration.

Dermis

  • Second major region of the skin.
  • Strong, flexible connective tissue.
  • Richly supplied with blood vessels and nerves.
  • Has two layers:
    • Papillary dermis—includes dermal papillae.
    • Reticular dermis
      • Deeper layer—80% of thickness of dermis.

Papillary Dermis

  • Includes dermal papillae.
    • Increase surface area for exchange of gases, nutrients, and wastes.
  • Dermal papillae lie on dermal ridges.
    • Epidermal ridges (friction ridges)—elevations of dermal ridges.
      • Are fingerprints.
      • Increase gripping ability of hands and feet.