Tissues A Powerpoint : Epithelial Tissues

Introduction to Tissues

  • Tissues are defined as groups of cells that are similar in both structure and function.

  • There are four primary types of tissues in the human body:

    • Epithelial tissue.

    • Connective tissue.

    • Muscle tissue.

    • Nerve tissue.

General Characteristics of Epithelial Tissue

  • Cellularity: This tissue is composed almost entirely of cells with very little extracellular matrix.

  • Special contacts: Epithelial cells form continuous sheets. These sheets are held together by specialized structural protein junctions, including:

    • Tight junctions.

    • Desmosomes.

  • Polarity: Epithelial tissues exhibit polarity, meaning they have two distinct sides:

    • Apical surface: The upper, free surface exposed to the body exterior or the cavity of an internal organ.

    • Basal surface: The lower, attached surface.

  • Supported by connective tissue: All epithelial sheets rest upon and are supported by connective tissue. This support system includes:

    • Basal laminae.

    • Reticular laminae.

  • Avascular but innervated: Epithelial tissue contains no blood vessels (it is avascular) but is supplied by nerve fibers (it is innervated).

  • Regenerative: Epithelia have a high regenerative capacity and can rapidly replace lost or damaged cells through cell division.

Classification of Epithelial Tissue

  • Epithelia are classified based on two criteria:

    • Number of cell layers:

      • Simple: Consists of a single cell layer.

      • Stratified: Consists of two or more cell layers stacked on top of each other.

    • Shape of cells:

      • Squamous: Flattened and scale-like cells.

      • Cuboidal: Boxlike cells, approximately as tall as they are wide.

      • Columnar: Tall and column-shaped cells.

Simple Epithelia

  • Simple Squamous Epithelium:

    • Description: A single layer of flattened cells featuring disc-shaped central nuclei and sparse cytoplasm. It is considered the simplest of the epithelia.

    • Function: Facilitates the passage of materials through diffusion and filtration in areas where protection is not a primary concern. It also secretes lubricating substances in the serosae.

    • Location: Found in the kidney glomeruli, air sacs of the lungs (alveolar walls), lining of the heart, blood vessels, lymphatic vessels, and the lining of the ventral body cavity (serosae).

    • Visual Documentation: Typically viewed at 400×400\times magnification in photomicrographs of lung tissue.

  • Simple Cuboidal Epithelium:

    • Description: A single layer of cube-like cells with large, spherical central nuclei.

    • Function: Primarily involved in secretion and absorption.

    • Location: Located in kidney tubules, ducts and secretory portions of small glands, and on the surface of the ovary.

  • Simple Columnar Epithelium:

    • Description: A single layer of tall cells with round to oval nuclei. Some varieties bear cilia on their apical surface. This layer may also contain goblet cells, which are unicellular glands that secrete mucus.

    • Function: Specialized for absorption and the secretion of mucus, enzymes, and other substances. Ciliated types are responsible for propelling mucus or reproductive cells melalui ciliary action.

    • Location (Nonciliated): Lines most of the digestive tract (from the stomach to the anal canal), gallbladder, and excretory ducts of some glands.

    • Location (Ciliated): Lines small bronchi, uterine tubes, and specific regions of the uterus.

    • Visual Documentation: Often viewed at 1300×1300\times magnification in photomicrographs of the stomach mucosa.

  • Pseudostratified Columnar Epithelium:

    • Description: A single layer of cells with differing heights; because some cells do not reach the free surface and nuclei are positioned at different levels, it gives the false impression of being stratified.

    • Function: Involved in secretion (particularly of mucus) and the propulsion of mucus by ciliary action.

    • Location (Nonciliated): Found in male sperm-carrying ducts.

    • Location (Ciliated): Lines the trachea and most of the upper respiratory tract.

Stratified Epithelia

  • Stratified Squamous Epithelium:

    • Description: A thick membrane composed of several cell layers. The basal cells are typically cuboidal or columnar and are metabolically active, while the surface cells are flattened (squamous).

    • Function: Protects underlying tissues in areas subjected to high abrasion.

    • Location (Keratinized): Forms the external part of the skin’s epidermis. The surface cells are full of keratin and are dead.

    • Location (Nonkeratinized): Forms the moist linings of the esophagus, mouth, and vagina.

  • Stratified Cuboidal Epithelium:

    • Frequency: Quite rare in the human body.

    • Physicality: Typically consists of 22 cell layers.

    • Location: Found in some larger glands, such as sweat glands and mammary glands.

  • Stratified Columnar Epithelium:

    • Frequency: Also has a limited distribution in the body.

    • Location: Found in the pharynx, the male urethra, and lining some glandular ducts. It also occurs at transition areas between two other types of epithelia.

  • Transitional Epithelium:

    • Description: Consists of several cell layers. The basal cells are cuboidal or columnar, while the apical (surface) cells vary in appearance (often dome-shaped) depending on the degree of organ stretch.

    • Function: Stretches readily and permits distension of the urinary organs by contained urine.

    • Location: Lines the ureters, urinary bladder, and part of the urethra.

Glandular Epithelia

  • A gland is defined as one or more cells that produce and secrete an aqueous fluid.

  • Classification of Glands:

    • Site of product release:

      • Endocrine: Ductless glands.

      • Exocrine: Glands that retain ducts.

    • Relative number of cells:

      • Unicellular (e.g., goblet cells).

      • Multicellular.

  • Endocrine Glands:

    • These are ductless glands that produce hormones, which they release directly into the extracellular space to enter the blood or lymphatic fluid.

    • Secretions include amino acids, proteins, glycoproteins, and steroids.

  • Exocrine Glands:

    • More numerous than endocrine glands.

    • These glands secrete their products onto body surfaces (skin) or into body cavities.

    • Examples include mucous, sweat, oil (sebaceous), and salivary glands.

    • The goblet cell is the only important unicellular exocrine gland in humans.

    • Multicellular exocrine glands consist of two parts: a duct and a secretory unit.

Structural Classification of Multicellular Exocrine Glands

  • Classified by Duct Structure:

    • Simple: The duct does not branch.

    • Compound: The duct branches.

  • Classified by Secretory Unit Structure:

    • Tubular: Secretory cells form tubes.

    • Alveolar (Acinar): Secretory cells form small, flask-like sacs.

    • Tubuloalveolar: Contains both tubular and alveolar secretory units.

  • Specific Examples of Multicellular Exocrine Glands:

    • Simple tubular: Intestinal glands.

    • Simple branched tubular: Stomach (gastric) glands.

    • Simple alveolar: No important examples identified in humans.

    • Simple branched alveolar: Sebaceous (oil) glands.

    • Compound tubular: Brunner's glands of the small intestine.

    • Compound alveolar: Mammary glands.

    • Compound tubuloalveolar: Salivary glands.