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The cells in the body of the multicellular animals vary in structure and function. It is seen that one variety of cell performs one kind of work and constitutes one type of tissue.
Tissue may be defined as an aggregate of same type of cells combined by subserving the same general function independently and united by varying amounts of intercellular substance (e.g., blood, bones, cartilage, muscle, nervous tissues, etc.).
An organ is a group of more commonly two or more tissues, which basically function independently in some instances, in particular patterns to form larger functional units (e.g., blood vessels, kidney, skin, glands, etc.).
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When several organs function interrelatedly they form organ system (e.g., respiratory system composing the nose, larynx, trachea and lungs; urinary system comprising kidney, urethra and ureters). This arrangement based upon the principle of division of labour and is an essential requirement for coordinated living. One group of cells is set apart for one function and will specialise for that job.
During the initial stages of embryological development the cells are all similar. As development proceeds, one group of cells takes up one particular work, and, in order to perform the work in the best possible way undergoes the necessary change in structure and mode of life. This process of adaptation of a cell, for the purpose of doing a particular function, is called differentiation or specialisation.
The fertilised ovum (zygote) divides at first into two cells, these again into two more, and so on until a large mass of embryonic cells is formed. The first few cells are believed to be totipotent, which means that, each one of them has got the potency of creating a total individual. The first evidence of differentiation is found in this cellular mass, where the cells become arranged in three distinct layers, known as the ectoderm, the mesoderm and the entoderm.
The following tissues, in general, are derived from the three germ layers, described below:
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i. Ectoderm:
Epithelium of mouth, nose, anterior surface of cornea, external auditory canal, skin, hair, urethra, etc., and also some glands like sweat (or sudoriferous), sebaceous (or oil) and mammary glands. The endocrine system, pituitary, adrenal medulla and pineal gland, brain and cranial motor nerves.
ii. Mesoderm:
Connective tissue including blood and bone marrow cells, the three different kinds of muscle in general, lymphatic organs, endothelium of blood vessels, epithelium of urinogenital tracts (e.g., urethra) and the adrenal cortex.
iii. Entoderm:
Epithelium of the digestive and respiratory tract, bladder, thyroid, parathyroid and thymus. To understand the systems, one must know the tissues with which they are made of.
