Acronym: HPP
The term hippocampus refers to a histologically and embryologically defined structure, the archicortex (ARC). The ARC is located in the limbic lobe of the human, macaque, rat and mouse. The term has also come to be used for so many substructures of the ARC that it has lost utility for a standard nomenclature. A standard term for use in quantitative neuroanatomy must represent one and only one structure. Thus, in NeuroNames, 'hippocampus' is categorized as a synonym of all names of structures to which it is commonly applied ( Bowden-1997 Bowden-2012 ).
      The ARC is a C-shaped structure most of which is located in the lower limb of the limbic lobe ( Stephan-1975 ). There, in the primate, the horizontal hippocampal complex (HPC) narrows at its caudal end to become the fasciolar gyrus (FSG). The FSG curves around the splenium of the corpus callosum and narrows further to become the supracallosal gyrus (SCG). The SCG continues on the dorsal surface of the body of the corpus callosum and around the genu of the corpus callosum to become the paraterminal gyrus (PTG).
      In the rodent, the archicortex is the same except that the HPC equivalent, the hippocampal region, does not contain the subiculum. Also the equivalents of FSG ( fasciola cinerea ) and of SCG and PTG ( indusium griseum ) are not separate structures but are embedded in the:same cortical fold as the CA fields ( fasciola cinerea ) and in the retrosplenial area, anterior cingulate area, and mesial cerebral cortex abutting the septal nuclei ( indusium griseum ) ( Swanson-2004 ).
      During the second half of the twentieth century, the term 'hippocampus' was used by equally qualified groups of neuroscientists for three overlapping combinations and permutations of the primary structures of archicortex. Histologists and physiologists usually meant the CA fields (CAF) ( Nomina-1983 ), which designated precisely the location of cells of interest in their disciplines. Neurosurgeons, neuroimagers and psychologists tended to mean the HPC, which includes the cellular structures CAF, dentate gyrus and subiculum ( Crosby-1962 ); a combination of structures operating at the level of anatomical resolution that correlates best with many behavioral and psychological functions. Embryologists, evolutionary biologists, and comparative anatomists used the term as a synonym for the entire ARC, which, in primates includes the HPC, FSG, SCG and PTG, because those structures develop as a structural and presumably functional unit from the same part of the embryonic brain ( Stephan-1975 ). Structural differences associated with the same physiological and behavioral functions in the rodent gave rise to related terms such as ' retrohippocampal region ' ( Swanson-2004 ).
      The biggest contributor to increasing terminological confusion was introduction in the early 1980s of the misnomer 'medial temporal lobe' ('MTL') for the limbic lobe's HPC and hippocampal formation (HPF). In1985, the use frequency of 'hippocampal formation' was 9 times greater than that of 'medial temporal lobe', but by 2019, its frequency fell to half that of 'medial temporal lobe' ( Ngram-2019 ). Between 2000 and 2019, annual use of 'hippocampus'. compared to the second most frequently used term, 'hippocampal formation' increased from 12 times to 30 times greater ( Ngram-2019 ).
      The increase in use frequency of 'hippocampus' compared to other more specific terms was associated with growing ambiguity of the term. Authors from diverse disciplines who needed to refer to a part of archicortex without having to investigate the latest precise definitions of all other terms in the evolving nomenclature presumably defaulted to the more ambiguous term.
      Fundamental rules in the development of a standard nomenclature for use in computerized quantitative applications are that a unique name, or 'tag', must be assigned to each item in a set and that no tag may signify more than a single item in the set ( Bowden-1997 NeuroNames-2012 ). Thus, in the standard nomenclature of NeuroNames, hiippocampus' is categorized not as a standard name but as a synonym of several names of structures to which it is applied. Updated 16 May 2024.

Also known as: No other name for this structure has appeared in PubMed.NeuroNames ID : 3157

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