Epithelial tissue

The term “epithelium” was introduced by Frederic Ruysch in 1703.

Epithelium tissue is made up of one or more layers of cells that are compactly arranged with a little intracellular matrix.

Important features of Epithelial tissue:

  • The Epithelial tissues line the external surfaces of organs and blood vessels throughout the body, as well as the inner surfaces of cavities in many internal organs.
  • It is also known as covering tissue.
  • All glands are made up of epithelial cells.
  • Functions of epithelial cells include secretion, selective absorption, protection, Transcellular transport, and Sensing.
  • Epithelial layers are without blood vessels, so they receive nourishment via diffusion of substances from the underlying connective tissue, through the basement membrane.
  • The Part of the basement membrane which lies close to epithelial cells is called the basal lamina.
  • Cell junction – The structure which provides a structural and functional link between the adjacent epithelial cells is called a cell junction. There are three types of cell junctions as follows:
    • Tight junctions (Zona occludes), Checks the flow of materials between cells, for example- the brain.
    • Gap Junction – it Facilitates the communication between the cells by connecting the cytoplasm for rapid transfer of ions, small molecules, and sometimes big molecules.
    • Adhering Junction– it performs the function of cementing to keep neighboring cells together.

Types of epithelial tissue:

Type of epithelial tissue on the basis of shape: There are three principal shapes associated with epithelial cells squamous, cuboidal, and columnar.

  • The squamous epithelium has cells that are wider than their height (flat and scale-like). This is found in the lining of the mouth, esophagus, and blood vessels, and in the alveoli of the lungs.
  • Cuboidal epithelium has cells whose height and width are approximately the same (cube-shaped).
  • The columnar epithelium has cells taller than they are wide (column-shaped). Columnar epithelium can be further classified into the ciliated columnar epithelium and glandular columnar epithelium.

Epithelium tissues are divided into two types:

  1. Simple epithelium
  2. Compound epithelium

Simple epithelium

The simple epithelium tissue is composed of a single layer of cells and it functions as a lining for body cavities, ducts, and tubes. On the basis of structural modification of cells, the simple epithelium is divided into three types:

    • Simple Squamous epithelium
    • Simple cuboidal epithelium
    • Simple Columnar epithelium
    • Pseudostratified epithelium

Simple Squamous epithelium:  it is made of a single thin layer of flattened cells with irregular boundaries. They are found in the walls of blood vessels and air sacs of the lungs and are involved in functions like forming a diffusion boundary.

Simple Cuboidal epithelium: It is composed of a single layer of cube-like cells.

  • This type of tissue is commonly found in ducts of glands and tubular parts of nephrons in kidneys, pancreas, and salivary gland
  • Its main functions are secretion and absorption.

Simple Columnar epithelium: The columnar epithelium is composed of a single layer of tall and slender cells. The nuclei of columnar cells are located at the base.

  • They are found inlining of the stomach and intestine which helps in secretion and absorption. They also found gall bladder and bile duct from the gastric/ intestinal/ pancreatic gland.
    • Ciliated Epithelium: If the columnar or cuboidal cells bear cilia on their free surface, called the ciliated epithelium. It may be of ciliated columnar tissue and ciliated cuboidal tissue. (in Goblet cells)
      • Ciliated columnar lines the nasal passage, oviduct (Fallopian tubes), the central canal of the spinal cord, and the tympanic cavity of the middle ear.
      • Ciliated cuboidal occurs in certain parts of the nephron (Kidney)
    • The ciliated epithelium functions as protection and movement of mucus, urine, eggs, and cerebrospinal fluid in a particular direction.
Pseudostratified epithelium

The pseudostratified epithelium is columnar with cilia or without cilia. The cells are unequal, rounded nuclei, and goblet cells are found in between the epithelium.

  • The epithelium is singe layered but appears multicellular due to nuclei lying at different levels.
  • The locations of pseudostratified epithelium-
    • Pseudostratified columnar epithelium: lines large ducts of glands e.g. parotid salivary gland, the urethra of a male (human), in the olfactory mucosa
    • pseudostratified ciliated columnar lines of the trachea and large bronchi
  • The pseudostratified epithelium function in the protection and movement of secretion from glands, urine and semen from the urethra, and mucus laden with a duct.

Compound epithelium

The compound epithelium consists of two or more (multicellular) cell layers and it performs a protective function. The compound epithelium has a limited role in secretion and absorption as they are multi-layered.

It is therefore found that body linings have to withstand mechanical or chemical insult such that layers can be abraded and lost without exposing subepithelial layers. It covers the dry skin surface of the skin, the moist surface of the buccal cavity, pharynx, the inner lining of ducts of salivary glands, and pancreatic ducts.

The compound epithelium is classified as:

  1. Stratified compound epithelium
  2. Transitional Compound Epithelium
  • Stratified compound epithelium: It consists of many layers of cells on the basis of the shape of cells present in the superficial layers, the stratified epithelium of the following types:
    1. stratified squamous epithelium: It possesses an outer layer of squamous cells and an inner layer of columnar cells, that undergo mitosis, so it is also called a germinative layer. It lines the esophagus, Vocal cord, and also lines vagina. It protects against abrasion.
    2. Stratified Cuboidal epithelium: consisting of the outer layer of cuboidal cells and basal layer of columnar cells. Found in sweat gland, salivary gland, mammary gland, and pancreatic gland. It performs the function of protection.
    3. Stratified columnar epithelium: It lines epiglottis, mammary glands, and part of the male urethra.
    4. Stratified ciliated epithelium: it lines the larynx and upper part of the soft palate.
  • Transitional Compound Epithelium: It also consists of more than one layer but it is thinner and more stretchable tissue compared to the stratified epithelium. It lines the inner surface of the urinary bladder, ureter, and renal pelvis. It functions as it allows the urinary organs to stretch and expand.

Multicellular Glandular Epithelium

Some groups of columnar or cuboidal cells get specialized for secretion and form Glandular epithelium.

  • The Glandular epithelium is mainly of two types:
    • Unicellular Glandular, consisting of isolated glandular cells (goblet cells of the alimentary canal).
    • Multicellular Glandular, consisting of a cluster of cells (salivary gland).
  • On the basis of the mode of pouring their secretions, glands are divided into two categories as follows:
    • Exocrine gland The exocrine glands secrete mucus, saliva, earwax, oil, milk, digestive enzymes, and other cell product which are released through ducts or tubes.
    • Endocrine glands These glands do not possess ducts. They produce hormones and secrets directly into the fluid bathing the gland.

Different types of epithelium tissue summary in the given figure:

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