Number estimates of neuronal phenotypes in layer II of the medial entorhinal cortex of rat and mouse.

Modelling entorhinal function or evaluating the consequences of neuronal losses which accompany neurodegenerative disorders requires detailed information on the quantitative cellular composition of the normal entorhinal cortex. Using design-based stereological methods, we estimated the numbers, proportions, densities and sectional areas of layer II cells in the medial entorhinal area (MEA), and its constituent caudal entorhinal (CE) and medial entorhinal (ME) fields, in the rat and mouse. We estimated layer II of the MEA to contain approximately 58,000 neurons in the rat and approximately 24,000 neurons in the mouse. Field CE accounted for more than three-quarters of the total neuron population in both species. In the rat, layer II of the MEA is comprised of 38% ovoid stellate cells, 29% polygonal stellate cells and 17% pyramidal cells. The remainder is comprised of much smaller populations of horizontal bipolar, tripolar, oblique pyramidal and small round cells. In the mouse, MEA layer II is comprised of 52% ovoid stellate cells, 22% polygonal stellate cells and 14% pyramidal cells. Significant species differences in the proportions of ovoid and polygonal stellate cells suggest differences in physiological and functional properties. The majority of MEA layer II cells contribute to the entorhinal-hippocampal pathways. The degree of divergence from MEA layer II cells to the dentate granule cells was similar in the rat and mouse. In both rat and mouse, the only dorsoventral difference we observed is a gradient in polygonal stellate cell sectional area, which may relate to the dorsoventral increase in the size and spacing of individual neuronal firing fields. In summary, we found species-specific cellular compositions of MEA layer II, while, within a species, quantitative parameters other than cell size are stable along the dorsoventral and mediolateral axis of the MEA.