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In biology, tissue is a cellular organizational level between cells and a complete organism. A tissue is an ensemble of similar cells from the same origin that together carry out a specific function. Organs are then formed by tissues grouped together. The study of tissues is known as histology or, in connection with disease, histopathology.
The classical tools for studying tissues are the paraffin block in which tissue samples are embedded and then sectioned, stained, and mounted on slides.