Tissues (four)

Skeletal Muscle

  • Comprised of long cylindrical fibers (cells).
  • Nuclei located on the periphery.
  • Striations are present.
  • Vascular tissue.
  • Classic appearance in cross-section: peripherally located nuclei.

Cardiac Muscle

  • Striated muscle.
  • Smaller, branching cells.
  • Centrally placed nucleus.
  • Intercalated discs: dense pink bands unique to cardiac muscle.
  • Intercalated discs have mechanical (desmosomes) and functional roles (gap junctions).
  • Functionally a syncytium due to gap junctions.
  • Mitochondria constitute about 25% of its weight.

Smooth Muscle

  • Involuntary and non-striated.
  • Found in internal organs (viscera).
  • Typically layered.
  • Autorhythmic and can contract independently of nervous stimuli but is affected by nervous stimuli and other factors.
  • Specialized for slow, long contractions.
  • Cells are fusiform or spindle-shaped.
  • Has same contractile elements as skeletal and cardiac muscle, but not arranged in regular repeating units.
  • Dense bodies anchor contractile elements, functioning like Z lines.
  • Contraction results in a "corkscrew" appearance of the nucleus.
  • Communication via gap junctions, acting like a syncytium.
  • Can produce connective tissue matrix (collagen, elastin, or reticular fibers).
  • Capable of division for maintenance or growth.

Comparing Muscle Types

  • Similarities (Cardiac and Smooth): Single centrally placed nucleus, same contractile elements, both produce gap junctions, short relative to skeletal muscles.
  • Contrasts (Cardiac): Intercalated discs, striated, different cell shapes.
  • Contrasts (Smooth): Dense bodies, not striated, different cell shapes.

Tissue Recognition in Organs

  • Start by identifying free surfaces (lumen).
  • Look for epithelium; identify type based on layers and cell shapes.
  • Epithelium sits on connective tissue and is associated with a basement membrane.
  • Look for muscle layers (smooth muscle).
  • Outside muscle layers, identify connective tissue.
Classification of Epithelium
  • Formula: How many layers and what are the shapes of the cells.
  • Exceptions: Pseudostratified columnar epithelium and transitional epithelium.
Connective Tissue
  • Made up of: cells + matrix (more matrix than cells)
  • Matrix determines the properties of the connective tissue (liquid, jelly, or very hard).
  • Presence of fibers in the matrix (not always visible).

Muscle Identification Tips

  • Ask yes/no questions:
    • Are striations visible?
    • What is the cell shape (cylindrical, branching, fusiform)?
    • Are cells long or short?
    • How many nuclei are in each cell?
    • Where are the nuclei located (periphery or center)?
    • Are intercalated discs present?

Muscle Growth

  • Skeletal and cardiac muscle increase in size via hypertrophy.
  • Smooth muscle increases in size via both hypertrophy and hyperplasia (cell division).

Organ Structure

  • Often composed of layers or tunics (coats).

    • Tunica intima (innermost layer).
    • Tunica media (muscle layer).
    • Tunica adventitia or tunica externa (outermost layer).

  • Gut Tube:

    • Epithelium (lining the lumen).
    • Mucosa (epithelium + underlying connective tissue).
    • Muscularis mucosa (smooth muscle layer in some regions).
    • Submucosa (connective tissue layer with glands).
    • Circular muscle layer.
    • Longitudinal muscle layer.
    • Serosa (outermost layer of simple squamous epithelium).