The inferior olivary complex of marsupials

The inferior olivary complex (IOC) of marsupials was investigated to determine if correspondence exists between the IOC of marsupials and the IOC of eutherian mammals. We examined serial sections of the IOC of members of the eight major marsupial families: Didelphidae ( Didelphis virginana , Philander opossum , Marmosa murina , and Caluromys sp .), Caenolestidae ( Caenolestes obscurus and Lestoros inca ), Dasyuridae ( Antechinus flavipes , Dasyurus viverrinus and Sarcophilus harrisii ), Peramelidae ( Isodon obesulus and Perameles nasuta ), Phalangeridae ( Trichosurus vulpecula and Pseudocheirus peregrinus ), Petauridae ( Schoinobates volans and Petaurus breviceps ), Vombatidae ( Vombatus ursinus ) and Macropodidae ( Potorous tridactylus , Setonix brachyurus , Macropus eugenii and Macropus rufus ). Evidence is presented that the principal nucleus (PIO) and dorsal nucleus (DIO) of the marsupial IOC are directly comparable to nuclei in eutherians. Nauta degeneration studies on the pattern of termination of spinal and rubral afferents in Trichosurus vulpecula support arguments for homology between the PIO and DIO of marsupials and of eutherians. However the probable homologue of the eutherian medial nucleus (MIO) has certain features that are unique to marsupials; while it can be divided into the same subnuclei found in eutherians (subnuclei a, b, c and the cap of Kooy) most of the cells of the marsupial medial nucleus homologue are located lateral to the PIO. The few cells of the MIO homologue that lie medial to the PIO are connected in some marsupials to the main lateral portion by a sheet of cells that lies ventral to the PIO. For this reason we propose that the medial nucleus homologue be named the ventral nucleus (VIO) in marsupials. Because of the ventral position occupied by medial nucleus cells during development in both eutherian and marsupial mammals, the term VIO could also be properly applied to the eutherian MIO as well.