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Cytology is the branch of biology that studies cells, the building blocks of life. The name for this science is translated from kytos, the Greek term for "cavity." Cytology's roots travel back to 1665, when British botanist Robert Hooke, examining a cross-section of cork, gave the spaces the name "cells," meaning "little rooms" or "cavities."

Cytology's beginnings as a science occurred in 1839 with the first accurately conceived cell theory. This theory maintains that all organisms plants and animals alike are comprised of one or more like units called cells. Each of these units individually contain all the properties of life, and are the cornerstone of virtually all living organisms. Further, cell theory states that hereditary traits are passed on from generation to generation via cell division. Cell division generally has a regular, timed cyclical period during which the cell grows, divides, or dies. Virtually all cells perform biochemical functions, generating and transmitting energy, and storing genetic data carried down to further generations of cells. Cytology differs from its cousin, pathology, in that cytology concentrates on the structure, function and biochemistry of normal and abnormal living cells. Pathology pursues changes in cells caused by decay and death.

Cells can vary dramatically in size and shape from organism to organism. While plant and animals cell diameters generally average between 10–30 micrometers (0.00036–0.00108 inches), sizes can range from a few thousand atomic diameters for single-celled microorganisms, all the way up to 20–in (50–cm) diameters for the monocellular ostrich egg. Cell structures also differ between advanced single-celled and multicellular organisms (plants and animals) and more primitive prokaryotic cells (e.g., bacteria). Plant cells are the most representative of a prototypical cell, as they have a nucleus, cell membrane and cell wall. Animal cells, on the other hand, lack a formalized cell wall, although they contain the former two. Prokaryote cells (e.g., bacteria) are unique in that they lack a nucleus and possess no membrane-enclosed organelles. Exceptions to the cell theory include syncytial organisms (e.g., certain slime molds and microscopic flatworms) without cellular partitions; however, they are derived secondarily from organisms with cells via the breakdown of cellular membranes. Finally, the number of cells within an organism can range from one for organisms like an amoeba, to 100 trillion cells for a human being.

Cytology has greatly benefitted from the electron microscope, which reveals internal and external cell dynamics too small to be monitored by traditional optical microscopes. Also, fluorescence or contrast microscopy with more traditional visual observation equipment enables the cell substance to be revealed when a specific cell material is stained with a chemical compound to illuminate specific structures within the cells. For example, basic dyes (e.g., hematoxylin) illuminates the nucleus, while acidic dyes (e.g., eosin) stain the cytoplasm (the cellular material within the membrane (excluding the nucleus). Finally, newer techniques including radioactive isotopes and high-speed centrifuges have helped advance cytology.

Cytological techniques are beneficial in identifying the characteristics of certain hereditary human diseases, as well as in plant and animal breeding to help determine the chromosonal structure to help design and evaluate breeding experiments. A far more controversial discussion deals with the role of cytology as it relates to cloning.

Over time, cytology's prominence as a separate science has diminished, integrating into other disciplines to create a more comprehensive biological-chemical approach. Associated disciplines include cytogenetics (study of behavior of chromosomes and genes relating to heredity) and cytochemistry (study of chemical contents of cells and tissues).

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