Cyto- and chemoarchitecture of the cerebral cortex of the Australian echidna (Tachyglossus aculeatus). I. Areal organization.

We have examined the topography of the cerebral cortex of the Australian echidna (Tachyglossus aculeatus), using Nissl and myelin staining, immunoreactivity for parvalbumin, calbindin, and nonphosphorylated neurofilament protein (SMI-32 antibody), and histochemistry for acetylcholinesterase (AChE) and NADPH diaphorase. Myelinated fibers terminating in layer IV of the cortex were abundant in the primary sensory cortical areas (areas S1, R, and PV of somatosensory cortex; primary visual cortex) as well as the frontal cortex. Parvalbumin immunoreactivity was particularly intense in the neuropil and somata of somatosensory regions (S1, R, and PV areas) but was poor in motor cortex. Immunoreactivity with the SMI-32 antibody was largely confined to a single sublayer of layer V pyramidal neurons in discrete subregions of the somatosensory, visual, and auditory cortices, as well as a large field in the frontal cortex (Fr1). Surprisingly, SMI-32 neurons were absent from the motor cortex. In AChE preparations, S1, R, V1, and A regions displayed intense reactivity in supragranular layers. Our findings indicate that there is substantial regional differentiation in the expanded frontal cortex of this monotreme. Although we agree with many of the boundaries identified by previous authors in this unusual mammal (Abbie [1940] J. Comp. Neurol. 72:429-467), we present an updated nomenclature for cortical areas that more accurately reflects findings from functional and chemoarchitectural studies.