Animal Tissues

All complex animals consist of only four types of cells. These cell types vary in structure according to their function. The basic classification of animal tissues is as follows:

EPITHELIAL TISSUE

  • Cells of epithelial tissue form a layer on free surfaces which either faces a body fluid or faces the external environment.
  • Hence, it either acts as an inner lining of various body parts such as visceral organs, blood vessels or body cavity.
  • The cells are compactly arranged with very thin layer of extracellular material in between them.
  • Adjacent cells are connected by means of cell junctions.
  • There are two types of epithelial tissues – Simple Epithelium and Compound Epithelium.

Simple Epithelium

  • It is usually composed of a single layer of cells.
  • They are found in the lining of body cavities, ducts and tubes.
  • On the basis of structural modifications, simple epithelium is divided into:
    • Squamous epithelium
    • Cuboidal epithelium
    • Columnar epithelium
    • Pseudo-stratified epithelium

Squamous Epithelium

Simple-squamous-epithelium-300x187.jpg

  • It is made of a single layer of flattened cells with irregular boundaries.
  • They are found on the inner surfaces of blood vessels and air sacs (alveoli) of lungs.
  • Function: Forms a diffusion boundary for gases and molecules.

 

Cuboidal Epithelium

  • It is composed of a single layer of cube like cells.
  • They are usually found on the inner surfaces of the ducts of glands, and tubules of nephrons.
  • Function: Secretion and absorption
  • In nephrons, the epithelium in PCT (proximal convoluted tubule) has finger-like extensions called microvilli.

 

Columnar Epithelium

  • It is made of a single layer of tall and slender cells with their nuclei at the base.
  • Sometimes, they may have microvilli.
  • It is usually found in the lining of stomach and intestines.
  • Function: Secretion and absorption

If the columnar or cuboidal cells bear cilia on their free surfaces, they are called ciliated epithelium. It helps to move particles or mucus in a specified direction. Such surfaces are found in the lining of hollow organs like fallopian tube and bronchioles.

 

Pseudostratified Epithelium

  • It is made of cells that reach the basement membrane.
  • But, they appear to be stratified because the nuclei of the cells are seen at different levels.
  • Hence, they are called pseudostratified.

 

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