Architectonics of the thalamus in the echidna (Tachyglossus aculeatus): search for the mediodorsal nucleus.

Architectural characteristics of the thalamus in echidnas and rats were compared in sections stained to reveal cell bodies, myelin, acetylcholinesterase, succinate dehydrogenase and cytochrome oxidase. Numerous species differences were noticed: in general, the thalamus is architecturally more homogeneous in echidnas than in rats, especially anteriorly. In this report we emphasize the presence of a relatively large structure localized in the anteromediodorsal part of the thalamus in echidnas. This structure, previously shown to project to the frontal cortex, contains very small amounts of acetylcholinesterase and the oxidative enzymes; in this respect it resembles the mediodorsal nucleus of rats. The same properties make this formation different from the anterodorsal and anteroventral nuclei in rats, the equivalents of which could not be identified in echidnas. The anteromediodorsal region of the thalamus in echidnas consists chiefly of two cytoarchitecturally different regions: the medial, 'polymorphic' part contains relatively small, densely packed, multiform perikarya, whereas the lateral, 'monomorphic' part is characterised by larger, sparse neurons with little cytoplasm and round, large, empty-looking nuclei in which the nucleolus is clearly seen. We conclude tentatively that this brain structure of echidnas corresponds to the mediodorsal nucleus in placental species. Further studies of connections and chemical properties will be essential to determine the degree of correspondence of the presumed 'frontal lobe system' in echidnas to that in other mammals.