A cyto-architectonic study of the cortex of the tammar wallaby, Macropus eugenii

This study gives a detailed account of the cyto-architectonic features of the cortex of a tammar wallaby, a member of the Macropodidae, a family of Australian metatherian mammals. With the aid of the physiological data and data from pathway studies, six different cortical regions - frontal, parietal, temporal, occipital, insular and cingulate - were identified based on their eutherian homologues. Each region is distinguished by its individual cyto-architectonic properties. The frontal region consists of six layers throughout. Layer I is extremely thick, and an agranular region was not seen. The parietal region contains six layers of which layer IV is very prominent due to its great thickness and high cell density. Layer V is also thick and is present throughout this region. The temporal region consists of two areas: a dorsal area, physiologically defined as auditory, and a ventral temporal area. The cyto-architectonic features of the dorsal temporal area are similar to those of the parietal region, but the thickness of the neocortical mantle is narrower than that in the parietal region. In the ventral temporal area, layer VI has a speckled appearance and the cells are arranged in horizontal rows. In the occipital region, layers III and IV are very thick and well differentiated, giving a striated appearance to this region. The insular region was identified ventral to the frontal region and consists of both agranular and granular areas. The cingulate region, mostly situated in the medial part of the cortex, lacks a layer IV. Midway along its rostrocaudal extent, cells in layer III stain deeply and are tightly packed, forming a band separating layers I and II from layers V and VI. Layer VI has a speckled appearance and its cells are oriented in horizontal rows. Most of the cortical cyto-architectonic features in wallabies are comparable to those in eutherian mammals. The cortex of the tammar apparently lacks the agranular region described within the motor cortex of a number of eutherian mammals. Nonetheless, the cyto-architectonic features of the tammar are otherwise comparable to those of contemporary eutherian mammals.