DEVELOPMENT OF PRIMARY VISUAL PROJECTIONS OCCURS ENTIRELY POSTNATALLYIN THE FAT-TAILED DUNNART, A MARSUPIAL MOUSE, SMINTHOPSIS-CRASSICAUDATA

We have examined the development of retinal projections in a diminutiv e polyprotodont marsupial, the fat-tailed dunnart, Sminthopsis crassic audata. Here. we document the most immature mammalian visual system at birth described to date. At postnatal day (Pi 0, the retinal ganglion cell layer has yet to form, and axons have not entered the optic stal k. By P4, the retinal ganglion cell layer could be distinguished at th e posterior pole, and the front of growing axons extended one-third th e length of the optic stalk, a distance of approximately 150 Fun; a fe m pioneer growth cones had grown beyond the main axon group but had st ill to reach the midline. Axons had decussated at the optic chiasm by P10 to penetrate the base of the contralateral optic tract and, by P15 , had reached the dorsal lateral geniculate nucleus (dLGN), superior c olliculus (SC), and accessory optic system (AOS); ipsilaterally projec ting axons matured slightly later: From P20, axons had reached the cau dal SC both contralaterally and ipsilaterally and terminated throughou t the depth of the retinorecipient layers. After P30, the projections gradually re fined. Within the rostral dLGN, segregation into four con tralateral and four ipsilateral bands occurred by P50, approximately 5 days after Eye opening. The projection to the ipsilateral SC underwen t refinement by P50, becoming restricted to its rostral pole, and pres ented as discrete patches within the stratum opticum. At birth, tile d unnart visual system is comparable to early to midembryonic stages [em bryonic day (E) 12, E14, E19, E24, and E30, respectively] in the mouse , rat, ferret, cat, and monkey The extreme immaturity of the neonatal dunnart together with the observation that the entire development of t he primary optic pathway occurs postnatally over a protracted period m ake this marsupial especially valuable far investigating factors that control pathway formation in the early developing mammalian primary vi sual system. (C) 1995 Wiley-Liss, Inc.