Fluorescence is the ability for a substance to emit light within the visible spectrum. It is usually a a result of absorption of electromagnetic radiation which may or may not be in the visible range.
Fluorescence microscopy uses the ability of certain molecules to fluoresce when an electromagnetic radiation source, usually ultraviolet light, is absorbed by them. There are few naturally occurring substances which behave in this way e.g. vitamin A. The major use of fluorescence microscopy has come in applying extrinsic compounds to cells which consist of a molecule with fluorescent properties bound to a molecule specific for a given cell component. The latter is usually an antibody.
Alternatively, lipophilic molecules with fluorescent properties can be trapped within cells by metabolic reactions. An example is fura-2, an ester that is converted to an acid within the cell; this molecule changes its fluorescence spectrum in the presence of calcium. Hence, it has been used to monitor free calcium flux during events such as secondary messenger signalling.
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