Progress in science always depended on the invention of new instruments and novel experimental techniques. The invention of the telescope by Galileo in 1609 revolutionized the whole study of planets. Similarly the advancement in biological sciences was initiated with the invention of the first simple microscope by Antony Van Leewenhoek in the early part of the 17th century. The first compound microscope was made by Robert Hooke at the same time. These pioneers in the field of microscopy laid the foundation for the science of the study of cells, otherwise known as cytology or cell biology.
All living organisms from the Amoeba to Man, or algae to big trees are composed of individual units called cells. Unicellular organisms such as the Amoeba or algae consist of only a single cell, whereas multicellular organisms possess many cells. The multicellular body of man or an elephant or a giant tree is composed of millions of cells.
A unicellular organism performs all bodily functions such as respiration, excretion, locomotion, etc., by the same single cell; the various functions are distributed to various compartments of the cell known as the cell organelles. However, in multicellular organisms this division of labour is done by the development of tissues, organs and organ systems. Tissues are group of cells having similar structure and function, e.g. the epithelial tissue (forms the covering of the body as well as surfaces of internal organs), the muscular tissue (contracts and helps in movement), the nervous tissue (conducts nerve impulses), the skeletal tissue (supports the body), etc. Many types of tissues gather together and form organs; organs are, in other words, groups of tissues. A number of organs, when interconnected functionally, form organ systems such as the respiratory system, the digestive system, the reproductive system, etc. All the organs in the organ system consists of many tissues like the epithelial, muscular, glandular and connective tissues and every tissue consists of individual, microscopic, living units called cells.