THE CHRONOTOPIC REORDERING OF OPTIC AXONS

Retinal ganglion cell axons become reordered as they pass through the chiasmatic region of the optic pathway. Studies in carnivores and rode nts show that the fiber order established in the optic tract is a chro nological index of their arrival time during development and that the cause of the reordering may relate to the changing glial environment, as well as to the spatial and temporal distribution of proteoglycans w ithin the developing pathway. Primate optic axons become similarly reo rdered, allowing one to predict a developmental sequence of ganglion c ell genesis from fiber position within the mature optic tract. Fiber p osition within the tract also anticipates the pattern of geniculate in nervation, but a prominent exception to this rule is found in the pros imian Galago. The chronotopic reordering is found in every mammalian s pecies that has been examined, including eutherians and metatherians, suggesting that the mechanism producing it is evolutionarily conserved .