FIRST EVIDENCE OF DIVERSITY IN EUTHERIAN CHIASMATIC ARCHITECTURE - TREE SHREWS, LIKE MARSUPIALS, HAVE SPATIALLY SEGREGATED CROSSED AND UNCROSSED CHIASMATIC PATHWAYS

In the optic chiasm of mammals, axons either cross the midline to the opposite side of the brain or remain uncrossed. In the eutherian speci es studied to date, uncrossed axons in the caudal nerve are found in a ll regions. In the chiasm, they are dispersed through the hemichiasm, with many axons approaching the midline and then turning back, to ente r the same side of the brain as the originating eye. In marsupials, by contrast, uncrossed axons never approach the midline; instead, they r emain grouped in the lateral nerve and chiasm. The impression gained f rom these data is that there is a major difference in chiasmatic archi tecture between eutherian and marsupial mammals. Therefore, the mechan isms by which axons choose their route through the chiasm was also tho ught to differ between the two major groups of mammals. However, the p resent study shows that the chiasm of a highly visual eutherian mammal , the tree shrew, is similar to that found in marsupials, with uncross ed axons confined to lateral regions and not approaching the midline. However, unlike marsupials, in the tree shrew, optic fascicles in the chiasm are often separated by thick collagen bundles. It is probable t hat the chiasmatic structure described to date for eutherian mammals i s not ubiquitous, as was previously thought, and theories explaining t he mechanisms by which axons chose their route through the chiasm duri ng development will have to be expanded. (C) 1998 Wiley-Liss, Inc.