Wednesday, July 29, 2020

Structural Organisation In Animals - Notes | Class 11 | Part 1: Animal Tissues | Epithelial Tissues



A group of cells having same origin, structure and function are called the tissues. 
Animal tissues are 4 types:
  1. Epithelial 
  2. Connective 
  3. Muscular 
  4. Neural


·   It has a free surface that faces body fluid or outside environment.

·   Covers or lines body or body parts.

·   Compactly packed cells with little intercellular matrix.

·   Epithelial tissues are 2 types: Simple and Compound.

1. Simple epithelium

It is composed of a single layer of cells. It lines body cavities, ducts and tubes. Based on structural modification of cells, simple epithelium is 3 types:

o   Squamous epithelium:

-    Thin layer of flattened cells with irregular boundaries.

-    Found in the walls of blood vessels and lung alveoli.

-    Functions: Form a diffusion boundary.

o   Cuboidal (cubical) epithelium:

-    Composed of cube-like cells.

-    Found in ducts of glands and tubular parts of nephrons.

-    Functions: Secretion and absorption.

-    The epithelium of proximal convoluted tubule (PCT) of nephron in the kidney has microvilli.

o   Columnar epithelium:

-    Composed of tall and slender cells.

-    Their nuclei are located at the base.

-    Free surface may have microvilli.

-    Found in the lining of stomach and intestine.

-    Functions: Secretion and absorption.

Modification of columnar or cuboidal cells

·   Ciliated epithelium:

-    Cells bearing cilia on their free surface.

-    Present in the inner surface of hollow organs like bronchioles and fallopian tubes.

-    Functions: To move particles or mucus in a specific direction over the epithelium.

·   Glandular epithelium: For secretion. They are 2 types:

§ Unicellular: Consists of isolated glandular cells.

E.g. Goblet cells of the alimentary canal.

§ Multicellular: Contains cluster of cells. E.g. salivary glands.

Based on mode of pouring of secretions, glands are 2 types:

§ Exocrine glands: Here, secretions are released through ducts (tubes). Exocrine glands secrete mucus, saliva, earwax, oil, milk, digestive enzymes etc.

§ Endocrine glands: Ductless. They produce hormones.

2.  Compound epithelium

·   Made up of more than one layer (multi-layered) of cells.

·   Limited role in secretion & absorption.

·   They cover dry surface of skin, moist surface of buccal cavity, pharynx, inner lining of ducts of salivary glands and pancreatic ducts.

·   Function: Protect against chemical & mechanical stresses.

Cell junctions

The junctions that provide structural and functional links between adjacent cells.
They are found in epithelium and other tissues.
They are 3 types:
  1. Tight junctions: Help to stop substances from leaking across a tissue.
  2. Adhering junctions: Perform cementing to keep neighbouring cells together.
  3. Gap junctions: Facilitate communication b/w adjoining cells by connecting the cytoplasm for rapid transfer of ions, small molecules and sometimes big molecules.

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