Tissue : A group of similar cells along with intercellular substances which perform a specific function.
ANIMAL TISSUES :
Animal tissues are classified into four types:
- Epithelial tissues.
- Connective tissues.
- Muscular tissues.
- Neural tissues.
Epithelial tissues :
- These tissues commonly called epithelium.
- Has free surface, which faces either a body fluid or the outside environment.
- Cells are compactly packed with little intercellular matrix.
- There are two types of epithelial tissues:
- Simple epithelium.
- Compound or stratified epithelium.
- Simple epithelium is composed of a single layer of cells and functions as a lining for body cavities, ducts and tubes.
- The compound epithelium consists of two or more cell layers and has protective function as it does in our skin.
Simple epithelium, on the basis of modification, divided into three types:
- Squamous
- Cuboidal
- Columnar.
Squamous epithelium :
- Made of single thin layer of flattened cells with irregular boundaries.
- Found in the wall of blood vessels and air sacs of lungs.
- Functions as diffusion boundary.
Cuboidal epithelium :
- Composed of a single layer cube – like cells.
- Found in ducts of glands and tubular part of nephrons in kidneys.
- Main function is secretion and absorption.
Columnar epithelium :
- Composed of a single layer of tall and slender cells.
- Their nuclei located at the base.
- Free surface may have microvilli.
- Found in the lining of stomach and intestine
- Helps in absorption and secretion.
Ciliated epithelium :
- It is a modified Cuboidal or columnar epithelium.
- Cell bears cilia on their free surfaces.
- Found in the inner surface of hollow organs like bronchioles and fallopian tubes.
- Their function is to move particles or mucus in a specific direction over the epithelium.
Glandular epithelium :
- It is a modified Cuboidal or columnar epithelium.
- Get specialized for secretion.
- Simple glands having unicellular and isolated cells of alimentary canal.
- Multicellular glands: consisting of cluster of cells, as in salivary gland.
- Exocrine gland: the secretory product transported to a point by means of a duct. These glands secrete mucus, saliva, ear wax, milk, digestive enzymes etc
- Endocrine gland: commonly called as ductless gland, because the secretory products directly poured into blood. The glands secretes hormone.
Compound epithelium :
- Made of more than one layer of cells.
- Limited role in absorption and secretion.
- Main function is to provide protection against chemical and mechanical stresses.
- They cover the dry surface of skin, moist surface of buccal cavity, pharynx, inner lining of duct of salivary gland and pancreatic duct.
