Epithelial Tissue

• Cell tissues are collections of similar cells performing specific functions in the body
• They are organized into four basic types: epithelial, connective, muscle, and nervous tissue
• The study of tissues is called histology

Epithelial Tissue

• Covers body surfaces, lines body cavities and organs, and forms glands
• Functions in protection, absorption, filtration, excretion, secretion, and sensory reception
• Key Characteristics:
  • Cellularity: composed of closely packed cells with minimal extracellular matrix
  • Special contacts: cells are connected by tight junctions and desmosomes
  • Polarity: apical (upper, free) and basal (lower, attached) surfaces
  • Support: supported by connective tissue
  • Avascularity: lacks blood vessels; nourished by diffusion
  • Regeneration: high regenerative capacity
• Classification:
  • Based on the number of cell layers: simple (single layer) or stratified (multiple layers)
  • Based on cell shape: squamous (flattened), cuboidal (cube-shaped), or columnar (column-like)
• Simple Epithelia:
  • Simple squamous epithelium: single layer of flattened cells and allows for diffusion and filtration
  • Simple cuboidal epithelium: single layer of cube-shaped cells involved in secretion and absorption
  • Simple columnar epithelium: single layer of column-shaped cells involved in absorption and secretion and may have microvilli or cilia
  • Pseudostratified columnar epithelium: single layer of cells of varying heights and all cells attach to the basement membrane, but not all reach the apical surface, involved in secretion and propulsion of mucus
• Stratified Epithelia:
  • Stratified squamous epithelium: multiple layers of flattened cells and protects against abrasion
  • Stratified cuboidal epithelium: typically two layers of cube-shaped cells and found in some sweat and mammary glands
  • Stratified columnar epithelium: multiple layers of column-shaped cells and found in the male urethra and some glandular ducts
  • Transitional epithelium: able to change shape from cuboidal to squamous and found in the urinary bladder
• Glandular Epithelium:
  • Endocrine glands: ductless glands that secrete hormones into the bloodstream
  • Exocrine glands: secrete products onto body surfaces or into body cavities via ducts
  • Exocrine glands can be unicellular or multicellular
  • Modes of secretion: merocrine (secretion by exocytosis), apocrine (accumulation of products at the apical surface which then pinches off), and holocrine (accumulation of products until the cell ruptures)

Connective Tissue

• Supports, connects, and separates different types of tissues and organs in the body
• Characterized by an extracellular matrix composed of ground substance and fibers
• Key Characteristics:
  • Extracellular matrix: nonliving material that separates the living cells
  • Ground substance: unstructured material that fills the space between cells and contains fibers
  • Fibers: collagen, elastic, or reticular fibers
  • Common origin: all arise from mesenchyme
  • Varying degrees of vascularity
• Types:
  • Connective tissue proper: loose and dense connective tissues
  • Cartilage: hyaline, elastic, and fibrocartilage
  • Bone tissue
  • Blood
• Connective Tissue Proper:
  • Loose connective tissues:
    • Areolar connective tissue: supports and binds other tissues and is universal packing material
    • Adipose tissue: stores fat and insulates and cushions organs
    • Reticular connective tissue: forms a soft internal skeleton (stroma) that supports other cell types
  • Dense connective tissues:
    • Dense regular connective tissue: primarily parallel collagen fibers and resists strong tension when applied in one direction; forms tendons and ligaments
    • Dense irregular connective tissue: irregularly arranged collagen fibers and withstands tension exerted in many directions; found in the dermis of the skin
    • Elastic connective tissue: contains a high proportion of elastic fibers and allows tissue to recoil after stretching
• Cartilage:
  • Chondrocytes are the main cell type; located in lacunae
  • Hyaline cartilage: supports and reinforces; resilient cushioning properties; covers ends of long bones
  • Elastic cartilage: maintains the shape of a structure while allowing great flexibility; supports the external ear
  • Fibrocartilage: tensile strength allows it to absorb compressive shock; found in intervertebral discs
• Bone Tissue:
  • Osteocytes are the main cell type; located in lacunae
  • Supports and protects; provides levers for muscles to act on; stores calcium and other minerals
• Blood:
  • Red blood cells, white blood cells, and platelets; surrounded by a fluid matrix (plasma)
  • Functions in transport of respiratory gases, nutrients, wastes, etc

Muscle Tissue

• Responsible for movement
• Three types: skeletal, cardiac, and smooth
• Key Characteristics:
  • Highly vascularized
  • Responsible for movement
• Types:
  • Skeletal muscle:
    • Attached to bones and responsible for voluntary movement
    • Cells are long, cylindrical, multinucleate, and striated
  • Cardiac muscle:
    • Forms the walls of the heart and is responsible for involuntary movement
    • Cells are branched, uninucleate, striated, and connected by intercalated discs
  • Smooth muscle:
    • Found in the walls of hollow organs and responsible for involuntary movement
    • Cells are spindle-shaped, uninucleate, and lack striations

Nervous Tissue

• Responsible for communication and control in the body
• Consists of neurons and supporting cells called neuroglia
• Key Characteristics:
  • Neurons: generate and conduct nerve impulses
  • Neuroglia: support, insulate, and protect neurons
• Types:
  • Brain
  • Spinal cord
  • Nerves