Epithelia

• Multicellular organisms function more efficiently if cells become specialized for specific functions.

  ❖Tissue = group of cells with similar structure and function

  ❖A tissue is composed of cells that function together in a specialized activity.

  ❖They are classified according to the shape of the cell, size, intracellular matrix.

  ❖Histology is the study of tissues.
• • Connective tissue a diverse primary tissue type that makes up part of every organ in the body
  • Epithelial – a covering or lining tissue
  • Muscle tissue - a tissue that contracts or shortens, making movement possible
  • Nervous tissue - responsible for coordinating and controlling many body activities

  
• 1. Covering and lining epithelium ( outer layer of the skin and some organs)
  2. Glandular epithelium ( constitute the secreting portion of glands)

  
• Functions of epi

  The epithelium covers or lines all of the internal and external body surfaces (i.e., skin, nasal cavity, gut, etc.)

  • The epithelium acts as a barrier, controlling:

  ▪ Protects underlying structures

  ▪ Acts as a barrier

  ▪ Permits passage of substances

  ▪ Secreting substances

  ▪ Absorption of substances
• Classification of Covering Epithelium

  Epithelium is classified based on the number of layers:

  • simple (one layer of cells); or

  • stratified (two or more layers of cells)

  
• Classification of Covering Epithelium Classification based on cell shape as either:

  • A. SQUAMOUS: contains flat cells (width is much greater than the height). This facilitates transport and rapid diffusion across the epithelium.

  • B. CUBOIDAL: square/cuboidal cell shape. These cells usually active in excretion, secretion or absorption, and the Golgi and organelles lie between the nucleus and the apical surface.

  • C. COLUMNAR: height is greater than width. These cells are highly active in secretion

  
• Simple Squamous Epithelium lines the lungs, and all blood vessels (where it is called the endothelium ), and forms the mesothelial lining of all the body cavities.

  
• • Structure single layre of flat, often hexagonal cells; the nuclei appear as bumps when viewed in cross section because cells are so flat
  • Function diffusion, filtration, some secretion and some protection against friction
  • Location lining of blood vessels and the heart lymphatic vessels, alveoli of the lungs portions of the kidney tubules, lining of serous membranes of body caveties (pleural, pericardial, peritoneal)

  
• Simple Cuboidal Epithelium It lines secretory regions of some glands, and tubules in the kidney.

  
• Simple Cuboidal Epithelium

  Structure: • Single layer of cube-shaped cells; some cells have microvilli (kidney tubules) or cilia (terminal bronchioles of the lungs) •

  Function: • Secretion and absorption by cells of the kidney tubules; secretion by cells of glands and choroid plexuses; movement of particles embedded in mucus out of the terminal bronchioles by ciliated cells • Location: • Kidney tubules, glands and their ducts, choroid plexuses of the brain, lining of terminal bronchioles of the lungs and surfaces of the ovaries.

  
• Simple Columnar Epithelium It lines the stomach (and the gall bladder).

  
• Simple Columnar E

  Structure:

  • Single layer of tall, narrow cells
  • Some cells have cilia (found in bronchioles of lungs, auditory tubes, uterine tubes, and uterus) or microvilli (found in intestines)
• Simple Columnar E

  Function:

  • Movement of particles out of the bronchioles of the lungs by ciliated cells
  • Partially responsible for the movement of oocytes through the uterine tubes by ciliated cells
  • Secretion by cells of the glands, the stomach, and the intestines
  • Absorption by cells of the intestines
• Simple Columnar E

  Location:

  • Found in glands and some ducts
  • Also found in bronchioles of lungs, auditory tubes, uterus, uterine tubes, stomach, intestines, gallbladder, bile ducts, and ventricles of the brain
• Simple Columnar Epithelium

  
• Pseudostratified epithelium

  This is a simple epithelium that looks stratified because the nuclei of the cells that make up this type of epithelium are found at different levels, giving it a stratified appearance.

  • Ciliated pseudostratified columnar epithelium cells are found in places such as the trachea and upper respiratory tract, where their cilia and mucous secretions help collect foreign materials so you can cough or sneeze them out.

  
• Structure of Pseudostratified Columnar Epithelium:

  • Single layer of cells
  • Some cells are tall and thin and reach the free surface, while others do not
  • Nuclei of these cells are at different levels and appear stratified
  • Cells are almost always ciliated
  • Associated with goblet cells that secrete mucus onto the free surface
• Function of Pseudostratified Columnar Epithelium:

  • Synthesize and secrete mucus onto the free surface
  • Move mucus (or fluid) that contains foreign particles over the surface of the free surface and from passages
• Location of Pseudostratified Columnar Epithelium:

  • Lining of nasal cavity, nasal sinuses, auditory tubes, pharynx, trachea, and bronchi of lung
• Pseudostratified Column Epithelium

  
• Stratified Squamous Epithelium It protects against abrasion. Examples include the epithelium of skin and the oesophagus (gastro-intestinal tract).

  
• Stratified Squamous Epithelium--

  non-keratinized (mouth cavity, vocal cords, vagina, anus)

  keratinized (epidermis of the skin) – the cells are released continuously from the surface

  
• Transitional epithelium - change their appearance, appearing cuboidal in relaxed epithelium and squamous when the epithelium is stretched

  
• Transitional epithelium

  • Structure: • Stratified cells that appear cuboidal when the organ or tube is not stretched and squamous when the organ or tube is stretched by fluid

  • Function: • Accommodates fluctuations in the volume of fluid in an organ or a tube; protects against the caustic effects of urine

  • Location: • Lining of urinary bladder, ureters, and superior urethra •
• Stratified Cuboidal Epithelium It lines the excretory regions of glands, e.g., the sweat glands of skin.

  
• Stratified Columnar Epithelium

  A rare type of epithelial tissue composed of column shaped cells arranged in multiple layers.

  • It can be found in the conjunctiva of the eye, in parts of the pharynx, anus, the uterus, and the male urethra and vas deferens

  
• Specialization of Epithelia

  Microvilli:

  a small thin protrusions on the apical surface of cells, which contain bundles of actin filaments, and increase the surface area of the cell for absorption.

  Numerous microvilli together act as a structure called as “Brush Borders”

  Abundant in small intestine to facilitate absorption

  
• Cilia: long fine projections on the apical surface that contain a core of microtubules. Motile cilia beat rhythmically, moving mucus on the apical surface of cells

  
• Goblet cells: secrete mucus (glygoproteins and proteoglycans) onto the apical surface of the epithelium. These are single ‘glandular ’ cells. When the airways are exposed to irritants, goblet cells and submucosal glands produce extra mucus to clear the airway