Epithelium

• Tissue


  A group of cells with similar structure and function, plus the extracellular substance surrounding them
• Histology


  The study of tissues
• Types of tissues in the human body

  • Epithelial
  • Connective
  • Muscle
  • Nervous
• Epithelial tissue


  • Covers and protects surfaces, both outside and inside the body
  • Includes exocrine and endocrine glands
• Characteristics of epithelial tissue

  • Mostly composed of cells
  • Covers body surfaces
  • Distinct cell surfaces
  • Cell and matrix connections
  • Nonvascular
  • Capable of regeneration
• Functions of epithelial tissues

  • Protects underlying structures
  • Acts as a barrier
  • Permits passage of substances
  • Secretes substances
  • Absorption of substances
• Classification of epithelia based on cell layers

  • Simple
  • Stratified
  • Pseudostratified
• Classification of epithelia based on cell shape

  • Squamous
  • Cuboidal
  • Columnar
  • Transitional
• Simple epithelium


  A single layer of cells, with each cell extending from the basement membrane to the free surface
• Stratified epithelium


  More than one layer of cells, but only the basal layer attaches the deepest layer to the basement membrane
• Pseudostratified columnar epithelium


  A special type of simple epithelium that appears to be falsely stratified, consisting of one layer of cells all attached to the basement membrane
• Types of epithelial cells based on shape

  • Squamous - flat or scalelike
  • Cuboidal - cube-shaped
  • Columnar - taller than wide
• Simple squamous epithelium


  A single layer of thin, flat cells that allow some substances to easily pass through but not others
• Functions of simple squamous epithelium

  • Lungs - gas exchange
  • Kidneys - filter wastes from blood while keeping blood cells inside
• Simple cuboidal epithelium


  A single layer of cube-like cells that carry out active transport, facilitated diffusion, or secretion
• Simple columnar epithelium


  A single layer of tall, thin cells that can perform complex functions like secretion
• Functions of simple columnar epithelium

  • Small intestine - produces and secretes mucus and digestive enzymes
• Pseudostratified columnar epithelium


  Secretes mucus which is moved by cilia toward the throat to be swallowed
• Types of stratified squamous epithelium

  • Keratinized
  • Nonkeratinized
• Keratinized stratified squamous epithelium

  The outer layer of skin, reduces water loss from the body
• Nonkeratinized stratified squamous epithelium

  Found in the mouth, provides protection against abrasion and allows more water movement than keratinized
• Stratified cuboidal epithelium


  More than one layer of cuboidal epithelial cells, found in sweat gland ducts, ovarian follicular cells, and salivary glands
• Stratified columnar epithelium


  More than one layer of cells, with the surface cells columnar and the deeper cells irregular or cuboidal, found in mammary gland ducts, larynx, and male urethra
• Transitional epithelium


  A special type of stratified epithelium that can be greatly stretched, lining cavities that can expand greatly like the urinary bladder
• Most epithelia have a free surface that is not in contact with other cells and faces away from underlying tissues
• Characteristics of the free surface

  • Smooth
  • Lined with microvilli
  • Lined with cilia
• Cell connections in epithelia

  • Desmosomes - mechanical links
  • Hemidesmosomes - anchor cells to basement membrane
  • Tight junctions - prevent passage of materials between cells
  • Gap junctions - allow small molecules/ions to pass between cells
• Glands


  Secretory organs that secrete substances onto a surface, into a cavity, or into the bloodstream
• Types of glands


  • Exocrine - with ducts
  • Endocrine - ductless
• Structures of exocrine glands

  • Simple - single, non-branched duct
  • Compound - multiple, branched ducts
  • Tubular - secretory regions shaped as small tubes
  • Acinar/alveolar - secretory regions shaped as saclike structures
• Types of exocrine secretion

  • Merocrine - release by exocytosis
  • Apocrine - release as pinched-off fragments
  • Holocrine - shedding of entire cells
• Connective tissue


  A diverse primary tissue type that makes up part of every organ in the body, consisting of cells separated by abundant extracellular matrix
• Functions of connective tissue

  • Enclose and separate other tissues
  • Connect tissues to one another
  • Support and move parts of the body
  • Store compounds
  • Cushion and insulate
  • Transport
  • Protect
• Connective tissue cells


  • Osteoblasts, osteocytes, osteoclasts
  • Fibroblasts, fibrocytes
  • Chondroblasts, chondrocytes
  • Macrophages
  • Mast cells
• Components of the extracellular matrix

  • Protein fibers - collagen, reticular, elastic
  • Ground substance - non-fibrous proteins and molecules
  • Fluid
• Collagen fibers


  Flexible but resist stretching, resemble microscopic ropes
• Reticular fibers


  Very fine, short collagen fibers that branch to form a supporting network
• Elastic fibers


  Have the ability to return to original shape after being stretched or compressed, giving tissue an elastic quality
• Ground substance


  Consists of non-fibrous molecules and is shapeless, includes proteoglycans
• Major components of connective tissue

  • Protein fibers
  • Ground substance
  • Fluid