Tissues

I. Overview

• Tissue - groups of cells with similar structure and function
• Four principle human tissue types: Diagram
  • epithelium (covering)
  • connective (support)
  • muscle (movement)
  • nervous (control)
• Fertilization produces a zygote that undergoes repeated cell divisions forming a blastocycst (blastula) that contains three primary germ (cell) layers
• Each germ layer participate in the formation of one or more tissue types: Ectoderm (neural tissue), Mesoderm (connective & muscular), and Endoderm (epithelium).

II. Epithelium

• Types.... Diagram
  • Covering and lining
  • Glandular
• Functions: Protection, absorption, filtration, excretion, secretion
• Characteristics:
  1. Cellularity - composed entirely of cells
  2. Specialized contacts - fit close together to form continuous sheets of cells
  3. Polarity - epithelium always has one free surface (apical) and one bound surface (basal)
  4. Avascularity - no blood vessels associated with epithelium, nutrients diffuse into cells
  5. Basement membrane - basal surface of epithelium rests on thin supporting basal lamina (glycoproteins); connective tissue under basal lamina secrete collagenous fibers that make a layer called reticular lamina (basal + reticular lamina - basement membrane)
  6. Regeneration - epithelium undergo mitosis
• Epithelia are classified according to their shape and arrangement. Diagram
  • Shape: squamal cuboidal, columnar, pseudostratified
  • Arrangement: simple or stratified
• Types of epithelium....
  • Simple squamal - single layer of flattened cells; located in air sacs of lungs, capillaries, and kidney glomerulus; functions in the passage of material (filtration and diffusion) and secretion. Diagram
  • Simple cuboidal - single layer of cube-shaped cells; located in kidney tubules and ovary surface; functions in secretion and absorption. Diagram
  • Simple columnar - single layer of column-shaped cells non-ciliated line the digestive tract and gall bladder, ciliated line small bronchi of lungs and line uterine tubes; function in absorption and secretion (of mucus and enzymes). Diagram
  • Pseudostratified (columnar) - single layer of cells of differing height; non- ciliated lines male urethra, ciliated lines the trachea and most of the upper respiratory tract; functions in the secretion of mucus.
  • Stratified squamal - many layers of flattened cells; non-keratinized lines esophagus, mouth, and vagina, keratinized lines epidermis; functions in protection. Diagram 
  • Stratified cuboidal - many layers of cube-shaped cells; found in ducts of sweat, mammary, and salivary glands; functions in protection. Diagram
  • Stratified columnar - many layers of column-shaped cells; located in large passageways of respiratory tract and also found in the male urethra; functions in protection. Diagram
  • Transitional - many layers of cells with various shapes; lines the ureters and bladder; functions in the distension of organs. Diagram
• Glandular epithelia Diagram
  • gland - one or more cells that make and secrete a particular product
  • Glandular epithelium is classified as being unicellular or multicellular as well as exocrine or endocrine.
    • Endocrine are ductless glands, secreting chemical messengers (hormones) directly into an
          extracellular space then diffuse into the blood or lymph.
    • Exocrine are glands that secret products into a duct. They are more numerous than endocrine
          glands. Examples: sweat and oil, salivary, liver, pancreas, mammary, and mucous. Exocrine
          glands are further classified as being unicellular or multicellular.
    • Unicellular exocrine glands are cells interposed in and between epithelium. These glands do
          not have ducts, yet are single cells that produce mucin (glycoprotein) which becomes mucus
          when dissolved in water. Example: goblet cells between intestinal columnar epithelium.
    • Multicellular exocrine glands are composed of two structural elements: an epithelium-derived
          duct and a secretory unit (secretory cells).
      • Types of multicellular exocrine glands:
        • Merocrine glands - secrete products (as soon as product are made) by exocytosis.
                                          examples....pancreas, sweat, and salivary.
        • Halocrine glands - accumulate products until cell ruptures. example.... sebaceous glands.
        • Apocrine glands - accumulation of products, then apex of cell pinches off. example... mammary glands.

III. Connective Tissue

• Functions: binding and support, protection, insulation, transportation
• Characteristics:
  1. Common origin: originate from mesenchyme (embryonic tissue)
  2. Degrees of vascularity: extent of blood supply
  3. Extracellular matrix: deposits of glycoproteins, fibrous proteins (elastic collagen, and reticular fibers), as well as minerals.
• Connective tissue cells are referred to as being a "blast" or "cyte" depending on their age/function. Blast cells are active cells that secrete both fibers and ground substance (matrix). Cyte cells are mature cells that maintain (repair or regenerate) matrix. Examples of connective tissue cell types are as follows:
  • Connective tissue proper - fibroblasts/fibrocytes
  • Cartilage - chondroblast/chondrocyte
  • Blood - hemocytoblast/hemocyte
  • Bone - osteoblast/osteocyte
• Connective tissue classes:
  • Embryonic Tissue
    • Mesenchyme
  • Connective Tissue Proper
    • Loose Connective
      • Areolar - matrix contains all three fiber types; widely distributed under epithelium; forms
            lamina propria and wraps and cushions organs. Diagram
      • Adipose - matrix contains sparse amount of all three tissue types; adipocytes have a flattened
            nucleus as a result of fat droplets contained inside of cell; used as fuel resource. Diagram
      • Reticular - reticular fibers w/ reticular cells in loose ground substance; found in lymph
            nodes, bone marrow, and spleen; fibers form internal skeleton that supports other cell types. Diagram
    • Dense Connective Diagram
      • Dense regular - fibroblasts with collagen fibers; forms ligaments (connecting bone to bone),
            tendons (connecting bone to muscle), and aponeurosis (connecting muscle to muscle).
      • Dense irregular - irregularly arranged collagen fibers with fibroblast; found in the dermis of
            the skin and fibrous joint capsules; withstands tension and provides strength.
      • Elastic connective - fibroblasts with elastic fibers; found in the walls of the aorta and vocal
            cords; provides durability and strength.
    • Cartilage Diagram
      • Hyaline - chondroblasts produce matrix with collagen fibers, chrondrocytes located within lacunae (spaces); found in ribs, nose, trachea, and the larynx; supports and reinforces.
      • Elastic - Chondroblasts produce matrix with elastic fibers; supports external ear; maintains shape and structure.
      • Fibrocartilage - matrix similar to hyaline but less firm and also contains more and thicker collagen fibers; component of intervertebral discs; gives tensile strength.
    • Other
      • Bone (osseous) - hard, calcified matrix containing many collagen fibers; osteocytes lie in lacunae; high vascularized; forms skeletal system; provides support and protection as well as blood production. Diagram
      • Blood - Red and white blood cells; located in blood vessels; transports respiratory gases, wastes, nutrients, etc... Diagram

V. Muscular Tissue Diagram Diagram

• Highly vascularized muscular tissue is comprised of elongated cells (called fibers) containing myofilaments (actin and myosin proteins). There three types of muscular tissue:
  • Skeletal - long, cylindrical, multinucleate cells with striations (specific arrangement of actin and myosin); attached to skeleton and skin; under voluntary control; provides movement.
  • Cardiac - branching uninucleate striated cells with junctions (intercalated discs); located in the walls of the heart; is under involuntary control; propels blood (circulation).
  • Smooth - spindle-shaped uninucleate cells without striations; found in the walls of hollow organs; under involuntary control; propels substances.

VI. Nervous Tissue

• Nervous tissue (nervous system) conducts electrical impulses and has integrative functions. Diagram
• Nervous tissue is composed of neurons which are branching cells located in the brain, spinal cord, and nerves. Neurons transmit electrochemical signals from sensory receptors to effectors
• Neurons are composed of dendrites, a cell body, and an axon.