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A group of one or more types of physically linked cells having a similar origin and specialized for a specific function along with the intercellular material if any.


The microscopic study of the tissues and organs in relation to their function is known as histology.

3. Types of tissues:

In an triploblastic animal, tissues develop from all the three basic layers, viz. ectoderm, mesoderm and endoderm. They are classified into epithelial, connective, muscular and nervous tissue.

4. Epithelium:

An epithelium is a tissue composed of one or more layers of cells covering the external surface of the body and viscera, or lining the cavities in the body and viscera. Epithelial tissue are of following types squamous, columnar, cuboidal, ciliated, glandular, sensort and stratified.

5. Simple epithelium:

It is plate or scale like. Cells are flattened and fitted together by means of junctions so that it looks like the tile of a mosaic.

6. Stratified epithelium:

It is the main protective epithelium of the body and found in the regions of friction. It is two layered or multilayered.

7. Squamous epithelium:

This tissue is composed of a single layer of thin and flat, plate-like cells. The cells fit closely, like the bricks in a wall, to form a smooth membrane. It is found in the outer layer of the skin, and covers internal cavities and ducts. It secretes coelomic fluid and is protective and excretory in function.

8. Cuboidal epithelium:

This is composed of cube-like cells that fit closely. The functions of this tissue are gamete formation, secretion and absorption, in addition to providing support.

9. Columnar epithelium:

This tissue is generally composed of a single layer of column like cells. A conspicuous striated border of microvilli at the free surface end of each cell increases the surface area of the cell for absorption and secretion. Found in the inner lining of the alimentary canal, the functions of this tissue include secretion and absorption. It also provides mechanical support.

10. Ciliated epithelium:

In some parts of the body, columnar epithelium develops protoplasmic outgrowth called cilia. The constant lashing movements of the cilia help move substances. Ciliated epithelium helps the movement of ova in the fallopian tubes and the movement of mucus in the respiratory tract.

11. Exocrine glands:

The exocrine glands send their secretion by way of ducts to the site of action, e.g., digestive glands.

12. Endocrine glands:

The endocrine glands lack ducts. They discharge their secretions, called hormones, into the blood that carries them to the target organs, e.g., pituitary gland.

13. Heterocrine glands:

The heterocrine glands have exocrine part as well as endocrine part e.g., pancreas,

14. Merocrine glands:

The secretion leaves the cells through the cell membrane and the cell     content remains intact e.g., goblet cells.

15. Apocrine glands:

The secretion accumulates as secretory granules in the distal part of the cell.  This part later breaks down and leaves as a secretion. e.g., mammary glands.

16. Holocrine glands:

The whole cell fills with a secretion and later breaks down and leaves as a secretion. e.g., sebaceous glands.

17. Connective tissue:

This tissue binds together the various tissues in an organ to support different parts of the body and form packaging around different organs. It has both cellular and extracellular components. Connective tissue consists of areolar tissue adipose tissue, tendon, ligament cartilage and bone. Blood and lymph are fluid connective tissues adapted for the transportation of materials in the body.

18. Cells of the connective tissue:

1. Fibroblasts:

They are most numerous and flattened having irregular cells with branching processes. The old inactive fibro blasts are called fibrocytes. They produce collagen fibres and are particularly active during wound repair.

2. Macrophages:
These are also called histocyte or clasmatocyte. They may be fixed or nomadic and phagocytic in nature.

3. Plasma cells:
They are derived from B-lymphocytes when needed. They produce antibodies.

4. Mast cells or mastocyte:
They are located in blood vessel. They produce heparin- an anticoagulant, histamine- a vasodilator, serotonin- a vasoconstrictor.

5. Fat cells or lipocytes or adipocytes:

They are filled with large globule of lipid. They produce and store fat.

6. Pigment cells.

They are also called melanocytes or chromatophores. They produce melanin.

19. Areolar tissue:

This tissue is found between the skin and muscles, around blood vessels and nerves, and in the bone marrow. It supports internal organs and helps in the repair of tissues.

20. Adipose tissue:

This tissue is found below the skin, between internal organs and in the yellow bone marrow. It stores fat and acts as an insulator.

21. White fibrous tissue:

This tissue forms cords called tendons which connect muscles with bones. It has great strength.

22. Yellow elastic tissue:

This tissue, contain very little matrix and a few cells, has considerable strength and remarkable elasticity. It forms cords called ligaments, which join bones together.

23. Ligament:

Fibrous connective tissue that connects bones at joints.

24. Reticular tissue:

It consists of star-shaped cells. This tissue is present in the  spleen, liver, bone marrow, etc.

25. Skeletal tissue:

Skeletal tissue is of two types- cartilage and bone.

26. Cartilage:

A non-porous connective tissue consisting of rounded cells (Chondroblasts) lying in a matrix, with numerous collagen fibres.

27. Osteocyte:

A bone cell. Bone cells are embedded in a hard matrix, calcium and phosphorus salts. The matrix is deposited in the form of concentric layers of lamellae formed around a central canal.

28. Blood:

Fluid connective tissue that circulates in the vascular system, carries oxygen and nutrients throughout the body, consists of fluid part (plasma) in which three types of blood cells, viz. red blood cells (erythrocytes), white blood cells (erythrocytes), white blood cells (leucocytes) and platelets (thrombocytes) are present.

29. Functions of blood:

The two major functions of blood are transport and temperature regulation. The blood transports gases, digested flood, hormones and waste materials to and form various organs. It also helps maintain uniform temperature.

30. Plasma:

The liquid part of blood, contains blood cells.

31. Red blood cells or corpuscles (Erythrocytes):

Cells in the blood containing haemoglobin, carry oxygen as oxyhaemoglobin.

32. White blood cells (Leucocytes):

Colourless uncleated blood cells or irregular shape, help to destroy microbes and the toxins they produce, also called ‘leucocytes’.

33. Platelets (Thrombocytes):

Non- nucleated, disclike bodies. Platelets contain an enzyme that helps in the clotting of blood.

34. Muscle:

Tissue consisting of cells which are highly contractile, responsible for movement.

35. Muscular tissue:

Muscular tissue consists of striated (Voluntary), unstriated (involuntary) and cardiac muscles.

36. Striated muscle:

Striped or voluntary muscle: contractile tissue consisting of bundles of elongated fibres with many nuclei and with conspicuous stripes at right angles to the fibres. Such muscles are capable of rapid contraction under the influence of nervous stimuli and are concerned with voluntary movements such as those of locomotion.

37. Involuntary muscle:

Muscular tissue consisting of narrow spindle –shaped cells each with a centrally placed nucleus and with no transverse striations. Such muscles have the power of slow contraction carried out automatically and unconsciously; the movements of the alimentary canal is an example.

38. Cardiac muscle:

Muscular tissue of a type found only in the wall of the heart. Its fibres somewhat resemble those of striated muscle but, unlike them, branch rejoin to form a network. Its chief property is that of rhythmic contraction even when isolated from the body.

39. Sarcoplasm:

Cytoplasm of striated muscle fibres.

40. Sarcolemma:

Sheath surrounding a striated muscle fibre.

41. Myofibrils:

Contractile fibrils present in the cytoplasm of muscle fibres. These are composed of myosin, action and tropomyosin.

42. Nervous system:

The organ system which coordinates the activities of the body.

43. Nervous tissue:

Nervous system consists of neurons (nerve cells) which are joined end to end forming long nerve fibres. The nerve fibres conduct messages from one part of the body to the other.

44. Neuron:

A nerve cell, the functional unit of nervous system, composed of a cell body (Cyton), an axon, dendrites and dendrons. It conducts impulses.

45. Nerve fibre:

A nerve fibre is an axon with its investments. The tube formed by myelin sheath is filled with myelin. The myelin sheath is interrupted by ‘Nodes of Ranvier’. The junctions between the neurons are called ‘synapses’.

46. Neurolemma:

The outer sheath or covering of a nerve fibre.

47. Dendrite:
The fine branch of a Dendron in a nerve cell.

48. Dendron:

Short process arising from the cell body of a nerve cell, conducts impulses towards cell body.

49. Neuroglia:

It is non-excitable supporting component of nervous tissues. The glial cells are protective and supporting in function.

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