THE MORPHOLOGICAL DEVELOPMENT OF THE INFERIOR COLLICULUS IN A MARSUPIAL, THE NORTHERN QUOLL (DASYURUS-HALLUCATUS)

As a part of a project concerning the development of hearing, some fea tures of the morphological development of the inferior colliculus were studied in a marsupial, the Northern quoll or native cat (Dasyurus ha llucatus). Marsupials are of particular interest in developmental stud ies because much embryonic development occurs outside the uterus, in t he pouch. Nissl-stained material was prepared from pouch-young at vari ous ages between 11 and 81 days, and for a number of adults. Four pouc h-young were injected with tritiated thymidine and killed later during pouch life. The inferior colliculus is first recognizable in pouch-yo ung aged 23 days, when it is bordered by a cell-sparse ring of tissue. By this time, the labelling patterns following injections of tritiate d thymidine made on days 7-9 suggest that migration of cells to the in ferior colliculus from the ventricular germinal zone has been largely completed. At 81 days, close to the time when the young move out of th e pouch, the adult cytoarchitecture-a central nucleus flanked by dorsa l and lateral cortical regions-is clear. Cell areas expand monotonical ly as a function of age. The period of days 45-50 is associated with a large expansion of cell volume and a concomitant decrease in packing density. It is likely that functional connections are forming during t his period, which may herald the onset of hearing in the quoll. Total cell numbers increase to a peak at day 36, fall to a minimum at day 50 , and rise again to the adult value. The second increase is likely to be a phase of glial proliferation, in part associated with the onset o f myelination. This increase correlates with departure of the young fr om the pouch. (C) 1994 Wiley-Liss, Inc.