Animal Tissues

• Tissues are collections of similar cells and the extracellular matrix surrounding them
• Histology is the study of tissues
• The four primary tissue types are:

  • Epithelial tissue
  • Connective tissue
  • Muscle tissue
  • Nervous tissue
• Primary tissue types are derived from the embryonic germ layers:

  • Endoderm forms the lining of the digestive tract and its derivatives
  • Mesoderm forms tissues such as muscle, bone, and blood vessels
  • Ectoderm forms the outermost layer of skin and the nervous system
• Epithelial tissue covers surfaces, has little extracellular material, usually has a basement membrane, and has no blood vessels
• Epithelial tissue functions include:

  1. Protecting underlying structures (e.g., outer layer of skin and oral mucosa)

  2. Acting as barriers (e.g., outer layer of skin)

  3. Permitting the passage of substances (e.g., epithelium in the lungs)

  4. Secreting substances (e.g., mucous glands and sweat glands)

  5. Absorbing substances (e.g., epithelial cells of the intestine)
• Epithelial tissue is classified based on the number of cell layers and the shape of the cells:

  • Simple epithelium has one layer of cells
  • Stratified epithelium has more than one cell layer
  • Pseudostratified epithelium has one layer but appears to have two or more layers
  • Transitional epithelium is stratified epithelium that can be greatly stretched
  • Cell shapes include squamous (flat and thin), cuboidal (cubelike), and columnar (tall and thin)
• A gland is a single cell or a multicellular structure that secretes:

  • Endocrine glands do not have ducts and secrete hormones directly into the blood
  • Exocrine glands have ducts and release secretions onto a surface or into a cavity (e.g., sweat glands and mammary glands)
• Connective tissue consists of cells separated by abundant extracellular matrix and functions include:

  1. Enclosing and separating

  2. Connecting tissues to one another (e.g., ligaments and tendons)

  3. Supporting and moving (e.g., bones and cartilage)

  4. Storing (e.g., adipose tissue and bones)

  5. Cushioning and insulating (e.g., adipose tissue)

  6. Transporting (e.g., blood)

  7. Protecting (e.g., blood and bones)
• Connective tissue cells produce the extracellular matrix and end in suffixes that identify cell functions:

  • Blast cells form the matrix
  • Cyte cells maintain it
  • Clast cells break it down
• The extracellular matrix of connective tissue contains protein fibers such as collagen fibers, reticular fibers, and elastic fibers, as well as ground substance with proteoglycans that hold water
• Connective tissue classification includes:

  • Loose, or areolar, connective tissue
  • Adipose tissue
  • Dense connective tissue
  • Cartilage (hyaline, fibrocartilage, elastic)
  • Bone (compact and cancellous)
  • Blood
• Muscle tissue is specialized to contract, making movement possible, and includes three types:

  • Skeletal (voluntary, multinucleated, striated)
  • Cardiac (involuntary, striated)
  • Smooth (involuntary, not striated)
• Nervous tissue conducts action potentials and includes neurons and neuroglia cells
• Glial cells in the nervous system include:

  • Ependymal cells
  • Oligodendrocytes
  • Satellite glial cells
  • Astrocytes
  • Microglia
  • Schwann cells
• Membranes in the body include:

  • Mucous membranes (line cavities that open to the outside of the body)
  • Serous membranes (line trunk cavities that do not open to the outside of the body)
  • Synovial membranes (line freely movable joints)