Comparative anatomy of the claustrum in selected species: A morphometric analysis

The morphology of the claustrum was studied by stereological methods in rep resentatives of five mammalian orders (Insectivora, Rodentia, Lagomorpha, C arnivora and Primates). In each species under study, a dorsal and a ventral part of the nucleus can be distinguished. Based on differences in shape an d separation from surrounding structures, five morphological types of the c laustrum occur. The claustrum of Insectivora and some rodents represents th e least complicated morphological type. The nucleus is very poorly separate d from the surrounding structures. The human claustrum is morphologically t he most complicated, although the two above-mentioned principal divisions a re apparent. The ventrally situated paraamygdalar part of the human claustr um may correspond to the endopiriform nucleus or ventral part of the claust rum of other mammals, because of its morphological characteristics and conn ections with the limbic system. In guinea pigs, traditionally classified as members of the Rodentia, a characteristic morphological type of the claust rum is present. This observation may support arguments questioning the curr ent position of this species in mammalian classification. Based on stereolo gical studies, the increase of the claustral volume that occurs with increa se of the hemispheric volume is significantly smaller than the increase of the isocortical volume and larger than the increase of the allocortical vol ume. The increase of the volume of the dorsal and ventral parts of the clau strum does not differ significantly in the species under study. Neurons of the claustrum represent differentiated morphology. The numerical density of neurons in the dorsal part of the claustrum is significantly higher than i n the ventral one. Differences in the morphology and cellular structure of the two parts of the claustrum may suggest differences in function of the t wo parts of the nucleus, most probably concerned with transfer of informati on among various cortical regions. Changes in the claustrum, a cortico-rela ted structure, that occur with increased brain volume, may suggest that its development is less dynamic than that of the isocortex.