Histology: Tissue Types and Epithelial Tissue

• Histology is the study of the microscopic structure of tissues.
• It is an essential tool in biology and medicine, allowing detailed examination of tissue organization, cellular structure, and the presence of any abnormalities.
• Histology involves tissue preparation, sectioning, staining, and microscopic examination.

Tissue Types

• There are four basic tissue types in the human body: Epithelial, Connective, Muscle, and Nervous tissue.

Epithelial Tissue

• Epithelial tissue covers body surfaces, lines body cavities and forms glands.
• It serves functions such as protection, absorption, secretion, excretion and filtration.
• Epithelial tissue is characterized by tightly packed cells with minimal extracellular matrix.
• It exhibits specialized contacts like tight junctions, adherens junctions, desmosomes, and gap junctions.
• Epithelial tissue has an apical (free) surface and a basal surface attached to a basement membrane.
• It is avascular (lacks blood vessels) but innervated (supplied by nerves).
• Epithelial tissue has a high regenerative capacity.
• Epithelial tissue is classified based on cell shape and number of layers.
  • Squamous cells are flattened and scale-like.
  • Cuboidal cells are cube-shaped.
  • Columnar cells are column-shaped.
  • Simple epithelium has a single layer of cells.
  • Stratified epithelium has multiple layers of cells.
• Specific types of Epithelial Tissue:
  • Simple squamous epithelium allows for diffusion and filtration. It is found in the air sacs of lungs (alveoli) and lining blood vessels (endothelium).
  • Simple cuboidal epithelium is involved in secretion and absorption. It is found in kidney tubules and glands.
  • Simple columnar epithelium is involved in absorption and secretion. It lines the gastrointestinal tract.
    • It may have microvilli (for absorption) or goblet cells (secreting mucus).
  • Pseudostratified columnar epithelium appears stratified but is a single layer of cells.
    • It is found lining the trachea and upper respiratory tract, often with cilia and goblet cells.
  • Stratified squamous epithelium protects against abrasion.
    • It forms the epidermis of the skin (keratinized) and lines the mouth, esophagus, and vagina (non-keratinized).
  • Transitional epithelium is capable of stretching.
    • It lines the urinary bladder, ureters, and parts of the urethra.
  • Glandular epithelium forms glands, which produce and secrete various substances.
    • Endocrine glands secrete hormones directly into the bloodstream.
    • Exocrine glands secrete products onto body surfaces or into ducts.
      • Examples include sweat glands, salivary glands, and mammary glands.

Connective Tissue

• Connective tissue supports, connects, and separates different tissues and organs in the body.
• Major functions include support, protection, insulation, and transportation.
• Connective tissue has abundant extracellular matrix, consisting of ground substance and fibers.
• The cells are widely scattered within the matrix.
• Connective tissue originates from mesenchyme.
• Types of Connective Tissue:
  • Connective Tissue Proper:
    • Loose connective tissue: including areolar, adipose, and reticular.
      • Areolar connective tissue supports and binds other tissues. It is found under epithelia and around organs.
      • Adipose tissue stores fat for energy and insulation. It is found under the skin and around organs.
      • Reticular connective tissue forms a supportive framework for lymphoid organs.
    • Dense connective tissue: including dense regular, dense irregular, and elastic.
      • Dense regular connective tissue has parallel collagen fibers and provides strength in one direction. Found in tendons and ligaments.
      • Dense irregular connective tissue has irregularly arranged collagen fibers and provides strength in multiple directions. Found in the dermis of the skin.
      • Elastic connective tissue contains elastic fibers, allowing for recoil after stretching. It is found in the walls of large arteries.
  • Cartilage:
    • Cartilage provides support and flexibility.
    • It is avascular and lacks nerves.
    • Types of cartilage:
      • Hyaline cartilage: provides support and cushioning. Found in articular surfaces of bones, the nose, and trachea.
      • Elastic cartilage: flexible and maintains shape and structure. Found in the external ear (auricle) and epiglottis.
      • Fibrocartilage: resists compression and tension. Found in intervertebral discs and the meniscus of the knee.
  • Bone (Osseous Tissue):
    • Bone supports and protects the body, stores minerals, and provides attachment sites for muscles.
    • It has a hard, calcified matrix.
    • Compact bone is dense and organized.
    • Spongy bone is trabecular and contains red marrow (for blood cell formation).
  • Blood:
    • Blood transports oxygen, carbon dioxide, nutrients, and waste products.
    • It consists of cells (red blood cells, white blood cells, and platelets) suspended in a fluid matrix (plasma).

Muscle Tissue

• Muscle tissue is specialized for contraction, enabling movement.
• There are three types of muscle tissue: skeletal, cardiac, and smooth.
  • Skeletal muscle:
    • Attached to bones and produces voluntary movement.
    • It is striated (has alternating light and dark bands).
    • Cells are multinucleated.
  • Cardiac muscle:
    • Found in the heart and responsible for pumping blood.
    • It is striated.
    • Cells are branched and connected by intercalated discs.
    • Involuntary control.
  • Smooth muscle:
    • Found in the walls of hollow organs (e.g., blood vessels, digestive tract).
    • It lacks striations.
    • Cells are spindle-shaped with a single nucleus.
    • Involuntary control.

Nervous Tissue

• Nervous tissue is specialized for conducting electrical impulses.
• It is found in the brain, spinal cord, and nerves.
• Nervous tissue consists of two main cell types:
  • Neurons: generate and transmit electrical signals (action potentials).
    • Composed of a cell body (soma), dendrites (receive signals), and an axon (transmits signals).
  • Neuroglia (glial cells): support, insulate, and protect neurons.
    • Types of neuroglia include astrocytes, oligodendrocytes, microglia, and ependymal cells.

Cell Structure

• The cell is the basic structural and functional unit of living organisms.
• Major cell structures (organelles) and their functions include:
  • Plasma Membrane:
    • Outer boundary of the cell.
    • Controls the movement of substances in and out of the cell.
    • Composed of a phospholipid bilayer with embedded proteins.
  • Nucleus:
    • Contains the cell's genetic material (DNA).
    • Controls cellular activities.
    • Surrounded by a nuclear envelope with nuclear pores.
  • Nucleolus:
    • Found within the nucleus.
    • Site of ribosome synthesis.
  • Ribosomes:
    • Site of protein synthesis.
    • Can be free in the cytoplasm or attached to the endoplasmic reticulum.
  • Endoplasmic Reticulum (ER):
    • Network of interconnected membranes.
    • Rough ER (RER) has ribosomes attached and is involved in protein synthesis and modification.
    • Smooth ER (SER) lacks ribosomes and is involved in lipid synthesis, detoxification, and calcium storage.
  • Golgi Apparatus:
    • Modifies, sorts, and packages proteins and lipids.
    • Forms vesicles that transport molecules to other locations.
  • Lysosomes:
    • Contain enzymes for intracellular digestion.
    • Break down cellular waste and debris.
  • Mitochondria:
    • Generate ATP (energy) through cellular respiration.
    • Have a double membrane structure.
  • Cytoskeleton:
    • Network of protein filaments that provides structural support and facilitates movement.
    • Includes microfilaments, intermediate filaments, and microtubules.
  • Centrioles:
    • Involved in cell division.
    • Organize the mitotic spindle.
  • Cilia and Flagella:
    • Cilia are short, hair-like appendages that move substances across cell surfaces.
    • Flagella are long, whip-like appendages that propel cells (e.g., sperm).