CLASSIFICATION OF EPITHELIA

Classification by Cell Shape

There are four categories of epithelia based on cell shape:

  1. Squamous – Cells are flat or shield-like.
  2. Cuboidal – The cells have the same height as width. They appear box-like. The nucleus is spherical and found in the center of the cell.
  3. Columnar – The cells are taller than wide. The nucleus is oval and tends to be found near the bottom of the cell. The apical surface of columnar epithelial cells often shows the presence of microvilli and cilia.
  4. Transitional epithelium – The appearance of transitional epithelial cells will vary with distension of the urinary organ which they line. When stretching is minimal, the cells appear full or balloon-like. When the organ is distended, the epithelium becomes flattened.

 

Classification by number of layers or strata of cells

1.  Simple – An epithelium that is one cell thick.

2.  Stratified epithelium - the identity of the epithelium is based on the appearance of the uppermost cells.

3.  Pseudostratified - a columnar epithelium that appears stratified but is not.  The misleading appearance of the epithelium is due to the crowding of adjacent cells.

Description and Appearance of the Epithelia

Simple Squamous

Description:

  1. The cells are flat plates joined to make a simple sheet.
  2. In a cross sectional view, the cells bulge due to the presence of the nucleus.
  3. The cell borders are serrated and interlocking.

Occurrence:

  1. Lines the body cavities as the mesothelium of serrous membranes.
  2. Lines blood vessels and lymphatics (endothelium).
  3. Form the walls of respiratory alveoli.
  4. Comprise the parietal layer of Bowman’s capsule of the nephron.

Simple Cuboidal

Description:

  1. Cells have a block-like appearance. In a surface view, the cell may appear hexagonal. When cut in cross section the shape of the cells can range from cube to trapezoid to pyramidal.
  2. The nucleus is very round and centrally located within the cell.

Occurrence:

  1. Found in many glands both exocrine and endocrine, for example, lining thyroid follicles and ducts of sweat glands.
  2. They are especially common in kidney tissue.
  3. Ciliated simple cuboidal cells line the passages of the smaller bronchioles.
  4. Found in the germinal epithelium of the ovary.

Simple Columnar

Description:

  1. The cells are actually shaped like elongated prisms resembling the cells in a honeycomb.
  2. A vertical cross section shows a row of rectangular boxes with the nuclei at one level below the middle of the cell.
  3. Crowding due to folds or curves in the epithelium displaces nuclei into staggered positions. The cells become pyramidal in shape. This commonly occurs in tubules, the alveoli of glands and at the base of villi in the small intestine.

Occurrence:

  1. Lines the alimentary canal from the stomach to the rectum.
  2. Lines the gall bladder.
  3. Lines the epididymus and seminal vesicles.
  4. Found in the oviduct (ampulla).

Pseudostratified columnar epithelium

Description:

  1. Consist of basal, fusiform and columnar cells all of which rest on a basement membrane.
  2. Only the columnar cells reach the free surface.
  3. The free surfaces are often invested with cilia.
  4. Mucus-producing goblet cells are are often observed.

Occurrence:

  1. Line a great portion of the nasal cavity.
  2. Commonly seen in the nasopharynx, the trachea and bronchi.
  3. Line portions of the seminal vesicles and most of the vas deferens.

Stratified Squamous

Description:

  1. In a vertical cross section, the deepest layer rests on a basement membrane and is cuboidal in shape.
  2. Cells in an intermediate position appear polygonal in outline.
  3. The keratinized stratified squamous of the skin exhibits certain unique characteristics:
  1. The cells in the middle and upper layers of the epidermis are joined to each other by desmosomes. As the cells dehydrate during processing, they take on a stellate appearance.
  2. Cells of the middle and upper portions of the epidermis show numerous intercellular spaces between themselves.
  3. At the surface of the epidermis, the cells have become filled with keratin. They die and become part of the dead stratum corneum

     4.  In the nonkeratinized stratified squamous of the esophagus, the shape of the cells gradually changes
     from the bottom to the top of the epithelium (from cuboidal to a typical squamous shape).

     5.  Non keratinized squamous epithelia does not produce a dead stratum corneum.

Occurrence:

Stratified squamous is found where ever there is exposure to friction, mechanical insult or drying. This would include:

1.  Surface of the skin.
2.  Lining of the mouth, esophagus and anus.
3.  It is found in the vagina and urethra (near the openiong).

Stratified Cuboidal

Description:

  1. Vertical section shows a rather random arrangement of polyhedral cells.
  2. The superficial cells appear more cuboidal.

Occurrence:

  1. Located in the wall of an ovarian follicle.
  2. Found in the ducts of sweat and sebaceous glands.
  3. Lines the conjunctiva of the eye.

Stratified Columnar

Description:

  1. The surface cells are columnar and sometimes ciliated.
  2. Deeper cells are irregular polyhedrons.
  3. It is often confused with pseudostratified columnar and with simple columnar that has been sectioned tangentially.

Occurrence:

  1. Its occurrence in humans is localized and limited.
  2. It is found in portions of the pharynx and larynx.
  3. It lines a portion of the urethra.
  4. It can be found in the ducts of salivary and mammary glands.
  5. Generally, it occurs where columnar or pseudostratified epithelium meets stratified squamous epithelium (e.g. in the larynx)

Transitional epithelium

Description:

  1. The appearamnce of the epithelium varies considerably with stretching.
  2. In a relaxed state,
    a.  The cells are arranged six deep.
    b.  Basal cells are small and polyhedral.
    c.  Cells in the middle are club or pear-shaped.
    d.  Surface cells are bloated and cuboidal with a bulging free surface.

      3.  With stretching

a.  The cells flatten out and apparently slide past one another.
b.  This is actually accomplished by a smoothing out of the interdigitations between adjoining cells, as seen in electron micrographs.

Occurrence:

     Transitional epithelium continuously lines the urinary tract from the pelvis of the kidney to the beginning of       the urethra.