Neuronal classes in the isocortex of a monotreme, the Australian echidna (Tachyglossus aculeatus)

We have used Valverde-Golgi and Golgi-Colonnier techniques to analyze cortical neuronal morphology in four regions (frontal cortex, primary motor cortex, primary somatosensory cortex, primary visual cortex) of the isocortex of the echidna (Tachyglossus aculeatus). Eight classes of neurons could be identified--pyramidal, spinous bipolar, aspinous bipolar, spinous bitufted, aspinous bitufted, spinous multipolar, aspinous multipolar and neurogliaform. All except the pyramidal neurons were morphologically similar to neuronal classes seen in eutherian and metatherian isocortex. Pyramidal neurons made up a small proportion of all cortical neurons encountered in our preparations of echidna cortex (34% in visual cortex, 35% in somatosensory cortex, 41% in frontal cortex and 49% in motor cortex) compared to both reported values in eutherian cortex and values we found in rat cortex impregnations prepared in an identical fashion to the echidna material (75% in rat motor and 78% in rat somatosensory cortex). Many pyramidal neurons in the echidna isocortex were atypical (30-42% depending on region) with inverted somata, short or branching apical dendrites and/or few basal dendrites, very different from the usual pyramidal neuron morphology in eutherian cortex. Dendritic spine density on apical and basal dendrites of echidna pyramidal neurons in somatosensory cortex and apical dendrites of motor cortex pyramidal neurons was also lower than that found in the rat. The present findings are consistent with both pyramidal neurons and the many diverse types of non-pyramidal neurons having already emerged as discrete morphological entities very early in mammalian cortical evolution, at the time of divergence of the therian and prototherian lineage.