zoo 106 lab integ system

• Epidermis of the skin

  • 4 cell types
  • Keratinocytes are the most dominant cells
• Keratinocytes


  1. Divide

  2. Grow

  3. Migrate up

  4. Undergo keratinization

  5. Form protective epidermal and surface layer
• Epidermis


  Composed of stratified keratinized squamous epithelium
• Other cell types in the epidermis

  • Melanocytes
  • Langerhans cells
  • Merkel cells
• In thick skin, five distinct and recognizable cell layers can be identified
• Stratum Basale (Germinativum) - Deepest layer


  • Single layer of columnar to cuboidal cells
  • Cells attached by desmosomes and to basement membrane by hemidesmosomes
  • Serve as stem cells for epidermis
  • High mitotic activity
  • Produce and contain intermediate keratin filaments
• Stratum Spinosum - Second layer

  • 4-6 rows of cells
  • Cells shrink, forming cytoplasmic extensions (spines)
  • Spines anchor desmosomes and tonofilaments
  • Synthesis and assembly of keratin filaments into tonofilaments
• Stratum Granulosum - Third layer


  • 3-5 layers of flattened cells
  • Accumulate keratohyalin granules
  • Granules contain filaggrin that associates with and cross-links keratin tonofilaments
  • Cytoplasm contains lamellar granules that are discharged into intercellular spaces as lipid barrier
• Stratum Lucidum - Fourth layer (in thick skin only)


  • Translucent, barely visible
  • Tightly packed cells lacking nuclei and organelles
  • Contain densely packed keratin filaments
• Stratum Corneum - Fifth layer

  • Flattened, dead cells filled with soft keratin filaments
  • Cells continually shed and replaced by new cells from stratum basale
  • Hydrolytic enzymes disrupt nucleus and organelles during keratinization