DEVELOPMENT OF THE CHIASM OF A MARSUPIAL, THE QUOKKA WALLABY

We have previously shown that the mature optic chiasm of a marsupial i s divided morphologically into three regions, two lateral regions in w hich ipsilaterally projecting axons are confined and a central region containing only contralaterally projecting axons. By contrast, in the chiasms of eutherian (placental) mammals studied to date, there is no tripartite configuration. Ipsilaterally and contralaterally projecting axons from each eye are mixed in the caudal nerve and in each hemichi asm and encounter axons from the opposite eye near the midline of the chiasm. Here, we show that, unlike eutherians, marsupials have astrocy tic processes in high concentrations in lateral regions of the nerve a nd rostral chiasm. Early in development, during the period when optic axons are growing through the chiasm, many intrachiasmatic cells are s een with densities five to eight times higher in lateral than in centr al chiasmatic regions. Such cells continue to be added to all chiasmat ic regions; later in development, considerably more are added centrall y, as the chiasm increases in volume. In the mature chiasm, cell densi ties are similar in all regions. By contrast to the marsupial, cell ad dition in the chiasm of a placental mammal, the ferret, is almost enti rely restricted to later developmental stages, after axons have grown through the chiasm, and there are no obvious spatial variations in the distribution of cells during the period examined. During development, similar to the adult marsupial, ipsilaterally projecting axons do not approach the chiasmatic midline but remain confined laterally. We pro pose that the cells generated early and seen in high densities in the lateral chiasmatic regions of the marsupial may play a role in guiding retinal axons through this region of pathway selection. These data su ggest that there is not a common pattern of developmental mechanisms t hat control the path of axons through the chiasm of different mammals. (C) 1995 Wiley-Liss, Inc.