Tissue Level of Organization - Epithelial Tissue

Tissue Level of Organization

• Tissues are composed of cells and extracellular matrix
• The four basic tissue types are epithelial, connective, muscular, and nervous
• Reading material for further study: Tortora, 16th ed., Ch. 4

Epithelial Tissue

• Composed of sheets of cells forming protective linings for the body, including hollow organs, body cavities, and ducts.
• Acts as a barrier between internal and external environments.
• Can function as glands for secretion.
• Continuous sheets of cells with tight junctions limiting solute and fluid diffusion around epithelial cells.
• Exhibits varied shapes and forms, influencing its function.
• Contains its own nerve supply.
• Nutrient supply and waste removal rely on adjacent connective tissue through diffusion; avascular (no capillary blood supply).

Classification of Epithelial Tissue

• Two main categories based on function:

  • Covering and lining epithelium:
    • Forms single or multiple layers
    • Different shapes (squamous, cuboidal, columnar)
    • Additional structures (cilia, microvilli, goblet cells) enhance function.
  • Glandular epithelium:
    • Secreting cells within the lining structure
    • Secretion glands (ex. Intestines, upper airways)

• Specific classifications based on cell arrangement and shape (see further details below).

Classification of Epithelial Tissue - Arrangement

• Simple epithelium: Single layer of cells, facilitating transport of solvents and solutes by diffusion and osmosis; often associated with absorption and secretion.
• Pseudostratified epithelium: Appears stratified but is a single layer of cells, with nuclei at different levels, potentially with microvilli and cilia (e.g., upper airway).
• Stratified epithelium: Two or more layers of cells, providing protection from wear and tear (e.g., skin).

Classification of Epithelial Tissue - Shape

• Squamous cells: Thin and flat, allowing easy passage of substances.
• Cuboidal cells: Wide and cube-shaped, often containing microvilli and involved in secretion or absorption.
• Columnar cells: Tall and column-shaped, potentially with microvilli or cilia; involved in secretion and absorption.
• Transitional cells: Stratified epithelium; capable of changing shape depending on physiological conditions; characteristic for the lining of the bladder.

Connective Tissue

• Most abundant and widely distributed tissue type in the body with varied forms and functions.
• Composed of cells and extracellular matrix.
• Extracellular matrix made up of fibers (collagen, elastic, reticular) and ground substance.
• Common cell types include fibroblasts, macrophages, mast cells, plasma cells, adipocytes, eosinophils, neutrophils, leukocytes, osteocytes, and chondrocytes.

Types of Connective Tissue

• Embryonic connective tissue:

  • Mesenchyme: Found under the skin associated with the developing bones of embryos.
  • Mucous connective tissues: Found in the umbilical cord.

• Mature connective tissue:

  • Loose connective tissue (areolar): Primarily for binding and support.
  • Dense connective tissue: Made up of high concentrations of collagen fibers; forms a supportive framework
  • Cartilage: Dense, collagenous and elastic fibers with a supportive function
  • Bone: Supports and protects

• Functions: Protection, support, connection and binding, transportation, and immunity.

Various Cell Types in Connective Tissue

• Fibroblasts: Present in various connective tissues that secrete intracellular fibers and ground substance for the matrix.
• Macrophages: Phagocytes that develop from monocytes, involved in phagocytosis at infection or inflammation sites.
• Plasma cells: Produce antibodies, part of the immune system.
• Mast cells: Involved in inflammatory response, able to bind to, ingest, and kill bacteria.
• Adipocytes: Fat cells; prominent in fat tissues that store energy.
• Leukocytes: A type of white blood cell that contribute to the functions of the immune system.
• Osteocytes: Cells in bone involved in bone formation; have a dense extracellular matrix.
• Chondrocytes: Cells that produce and maintain the cartilage matrix.

Types of Fibers in Connective Tissue

• Collagen fibers: Provide strength and resist pulling.
• Elastic fibers: Form a network within the tissue and return to their original shape after being stretched (elasticity).
• Reticular fibers: Form a meshwork; provide support to tissues, like spleen, lymph, and liver.

Ground Substance

• Extracellular material between the cells and fibers.
• Composed of water and large organic molecules, such as glycosaminoglycans (GAGs).
• Provides medium for exchange between blood and cells

Common Features of Connective Tissues

• Composition: Cells and extracellular matrix
• Matrix variation: Soft (adipose) to hard (bone)
• Components: Fibers and ground substance
• Vascularity: Generally vascular except cartilage and tendons
• Presence of nerves: Usually with nerves except in the case of cartilage.