Epithial Cells - Overview

Epithelial tissue cells are the most common cells found in the human body. They are one of four primary animal tissues: epithelial, mesenchymal, nervous and lymphoid tissue. Many epithelial tissues include some mesenchymal tissue (also referred to as connective tissue).

Epithelial cells mostly line surfaces: skin, mouth, sinuses, inner linings e.g., GI tract, gallbladder, urinary bladder, collecting tubules of kidney, and bile ducts.

This collection seeks to answer the following questions: Where are epithelial cells found? What functions do they perform? What are the different types? What are the different structures? What do they look like?

Where are Epithelial Cells Found?

  • Epithelial cells line the major cavities of the body.
  • Epithelia form the structure of the lung, including the alveoli or air sacs where gas exhange occurs.
  • Cells line most organs, such as the stomach and small intestine, kidney, and pancreas. They also line the esophagus.
  • Cells are also found in ducts and glands, like the bile duct and sailvary glands.
  • Epithelia can specialize to act as sensory receptors. They form taste buds, line the nose, and are in the ear. They are also found in the eye.
  • Female reproductive organs are lined with ciliated epithelial cells.
  • The skin is made of epithelial cells. Its striated layers demonstrate the extensive morphology of epithelia.
  • Capillary beds are made of epithelium.
  • Epithelia is the first type of cell to differentiate in the embryo. This occurs during the eight-cell stage.

Types of Epithelial Tissue

Epithelial tissue can be divided into three groups depending on the number of layers of which it composes.

  • Simple: Epithelial tissue which is only one cell thick on a basement membrane is known as simple epithelium. Gastrointestinal tract is an example.
  • Stratified: Usually three or more cells layered one on the other. Skin is stratified squamous, urinary bladder is stratified transitional. Note that transitional is always stratified.

  • Pseudo-stratified: Usually two cells appearing stratified, although both cells are at the basement membrane, there is not enough room for both nuclei low in the cell and one nucleus is displaced slightly toward the lumen, imparting a two layer look. Respiratory epithelium is an example.

Patterns of Epithelial Cells

There are three patterns of Epithelial cells (Squamous, Columnar and Transitional). These three patterns appear with the three types of Epithelial cells (Simple, Stratified and Pseudo-Stratified).

Epithelial Cells - Squamous Pattern

Flat cells will have inter-cellular bridges that can be seen with usual preparations.

Simple Squamous: Simple squamous epithelial cells function as mediators of filtration and diffusion and are found in locations where rapid diffusion or filtration take place. In the kidney, simple squamous epithelium lines the Bowman’s capsule and the glomerulus. Simple squamous epithelium also makes up the alveoli in the lungs. simple squamous epithelium lines the cavities, which arise from the coelom of the mesoderm (i.e. the peritoneal, pleural and pericardial cavities); simple squamous epithelium is referred to as “mesothelium” in these cases. “Endothelium,” which lines the interior surfaces of the entire circulatory system is another example of a specific form of simple squamous epithelium.

Stratified Squamous: consists of squamous (flattened) epithelial cells arranged in layers upon a basal membrane. Only one layer is in contact with the basement membrane; the other layers adhere to one another to maintain structural integrity. These can be found as Keratinized (epidermis of the palm of the hand and sole of the foot, and the masticatory mucosa.) and Non-keratinized (cornea, lining mucosa of oral cavity, esophagus, anal canal, foreskin, vagina, and the internal portion of the lips).

Epithelial Cells - Columnar Pattern

Columnar cells are called columnar if their height is at least twice their width, and Cuboidal if not so tall.

Simple Columnar: simple columnar epithelial cells function to protect. For example, the epithelium in the stomach and digestive tract provides an impermeable barrier against any bacteria that could be ingested but is permeable to any necessary ions.

Stratified Columnar: forms cell layers of the ocular conjunctiva and the linings of the pharynx, anus, uterus, urethra, and vas deferens.

Simple Cuboidal: lines the surface of small excretory ducts in various organs and glands in the body, compose some of the kidney tubules in the cortex of the kidney, create the covering of the ovary, and line the wall of the respiratory bronchiole.

Stratified Cuboidal: forms surface of the epidermis and the lining of mouth.

Pseudo-Stratified (Cilated) Columnar: ciliated columnar epithelial cells are rectangular in shape and have between 200 to 300 hair-like protrusions called cilia. Found mainly in the tracheal and bronchial regions of the pulmonary system and also in the fallopian tubes of the female reproductive system.

Epithelial Cells - Transitional Pattern

Transitional cells are intermediate between Squamous and Columnar, always stratified, have less fixed cell-cell connections and change their shape as the organ lumen fills or empties. Urinary bladder is typical of transitional pattern. Gallbladder also fills and empties but generally slowly and is lined by simple columnar.

Transitional stratified: This tissue structure type is found in urothelium, including that of the urinary bladder, the ureters, and the superior urethra and gland ducts of the prostate.

Epithelium Summary

  • Epithelium lines organs, cavities and surfaces.
  • Epithelial cells are polarized.
  • Functions of the epithelium include barrier, sensory, secretory, ion transportation and adsorption, transcytosis, movement and innate/adaptive immunity.
  • Squamous epithelium = flat.
  • Cuboidal epithelium = cube-shaped.
  • Columnar epithelium = taller than wide.
  • Simple epithelium = one layer thick.
  • Stratified epithelium = multi-layered.
  • Pseudostratified epithelium = appears multi-layered, but is one layer.
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