General histological woes: Definition and classification of tissues.

The modern view that the human body is composed of tissues and body fluids, and that there are four basic tissue types, may be a more significant departure from Artistotle's homoeomeres, and from Bichat's membranes and tissues, than commonly appreciated. The older concepts described these body parts as structural and functional parts of organs, whereas it is now commonplace to consider a tissue to be a grouping of similar cells with a variable amount of extracellular matrix. The development of the microscope as a useful tool in human anatomy shifted focus from tissues to cells and led to changes in the definition of tissue and the classification of tissues. Not all of these changes have been consistent with observable facts: many tissues contain diverse cell types, not all "connective tissues" are proper connective tissues, and some specialized tissues are not easily classified as subtypes of one of the four basic types. Here we propose corrective measures, including re-recognition of compound tissues, cataloging all adult human tissue types, and increasing the emphasis on function during the construction of a complete taxonomy of human adult tissues. Specific problems in the current scheme and a preliminary reclassification of human adult tissues are discussed.