Class 11th Biology notes, Study notes of Biology

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Animal Tissue

Tissue is a group of cells having same embryonic origin, structure and function.  (^) Various tissues combine together in an orderly manner to form large functional unit called organs. Organs combine together and form organ-system. The cells are of two types, somatic cells and germ cells. The word somatic is derived from the Greek word 'soma' means 'body'. All body cells of an organism except sperm and ova are somatic cells. The sperm and ova are germ cells. They belong to reproductive system.  (^) Cells → Tissues → Organs → Organ systems →Body

Epithelial tissue is classified into two types : Simple epithelium and Compound epithelium. Simple epithelium is made up of single layer of cells. Compound epithelium is made up of two or more layers of cells. Lowermost layer lies on basement membrane. A. Simple epithelial tissue :

1. Squamous epithelial tissue : Cells of this tissue are flat, thin, polygonal with serrated margin. Cells fit together like tiles of footpath. Hence it is called pavement epithelium.  (^) Prominent spherical or oval nucleus is present at the centre of the cell. Function : Protection, absorption, transport, filtration, secretion. It is found in blood vessels, alveoli, coelom, etc

3. Columnar epithelium : Columnar epithelial cells are tall, pillar like. Inner ends of the cells are narrow while free ends are broad and flat. Free surface shows large number of microvilli. Nucleus is oval and is present in the lower half of the cell. Function : Secretion, absorption. It is found in inner lining of intestine, gall bladder, gastric glands, intestinal glands, etc

4. Ciliated epithelium :  (^) Cells of this tissue are cuboidal or columnar.  (^) Free ends of cells are broad while narrow ends rest on a basement membrane.  (^) Free ends show hair like cilia.  (^) Nucleus is oval and placed at basal end of cell.  (^) Function : To create a movement of materials in contact in a specific direction and thus able to prevent entry of foreign particles in the trachea, push the ovum through oviduct.  (^) It is found in inner lining of buccal cavity of frog, nasal cavity, trachea, oviduct of vertebrates, etc.

6. Sensory epithelial tissue : It is composed of modified form of columnar cells and elongated neurosensory cells. Sensory hairs are present at the free end of the cell.  (^) Function : It perceive external as well as internal stimuli. These are found in nose (Olfactory) Ear (Auditory hair cells) Eye (photoreceptors).

B. Compound epithelial tissue : a. Stratified epithelium : Nucleus is present in stratum germinativum. Cells at free surface become flat and lack nucleus called stratum corneum. Function : Protection Ex. : Epidermis of skin, oesophagus cornea, vagina, rectum

b. Transitional epithelium: Structure of transitional epithelium is same like stratified epithelium. The cells can undergo a change in their shape and structure depending on degree of stretch. Function : Distension of organ Ex. : Urinary bladder

Hemidesmosomes (HDs) : Allow the cells to strongly adhere to the underlying basement membrane. These maintain tissue homeostasis by signaling

Gap Junctions (GJs) : This intercellular connection allows passage of ions and small molecules between cells as well as exchange of chemical messages between cells.

Desmosomes (Ds) : These provide mechanical strength to epithelial tissue, cardiac muscles and meninges.