Exp Brain Res (1991) 87:371-382 Experimental Brain Research 9 Springer-Verlag1991 Initial stages of retinofugal axon development in the hamster: evidence for two distinct modes of growth S. Jhaveri, M.A. Edwards*, and G.E. Schneider Massachusetts Institute of Technology,Department of Brain and Cognitive Sciences, Cambridge, MA 02139, USA Received March 15, 1991 / Accepted July 3, 1991 Summary. In order to characterize differences in growth patterns of axons as they elongate toward their targets and during the initial stages of terminal arbor formation within the targets, we examined the primary visual sys- tem of fetal and newborn hamsters using three mor- phological methods: the Cajal-deCastro reduced silver method, the rapid Golgi technique, and anterograde transport of HRP. Axons emerge from the retina be- tween the 10th and 1lth embryonic days (E10-E11). The front of retinal axons crosses the chiasm, extends over the primitive dorsal nucleus of the lateral geniculate body (LGBd) by El3, and advances to the back of the superior colliculus (SC) by E13.5-E14. The rate of axon growth during this advance is nearly 2 mm/day. Collateral sprouts appear on axons around E15.5. In the LGBd and SC, these sprouts arise from multiple sites along the parent axons. Only one or a few of the sprouts continue to grow and branch, while others are eliminated. The net rate of axon collateral advance in this second phase is an order of magnitude slower than during the stage of axon elongation. Thus, formation of CNS projections may involve two qualitatively distinct modes of axon growth. The arborization mode contrasts with the elongation mode by the presence of branching, a lack of fasciculation and a slower average rate of extension. The stereotypic direct advance of axons during elongation also differs from the remodelling which occurs during arborization. The delay between axon arrival at targets and onset of arborization could be a reflection of axons "waiting" for a matura- tional change to occur in the retina or in targets. Ar- borization in the LGBd and SC is initiated around the same time, implicating the former possibility. However, a slower differentiation of retinal arbors in the SC, in addition to morphological differences of arbors in the two structures, suggests that alterations in substrate fac- tors also play a critical role in triggering the early stages of arbor formation. * Present address." Dept. Developmental Neurobiology, Eunice Kennedy Shriver Center, 200 Trapelo Rd., Waltham, MA 02154, USA Offprint requests to: S. Jhaveri Key words: Visual system Optic tract - Axon elonga- tion - Axon arborization - Growth rates Introduction In recent years, the timing of appearance of retinal axons in central terminal areas and the role of interactions between axons from the two eyes in regulating the pat- tern of retinofugal projections in central visual targets have been extensively investigated in the rat (Lund and Bunt 1975; Bunt et al. 1983; Manford et al. 1984), ham- ster (So et al. 1978; Frost et al. 1979; Frost 1984; So et al. 1984; Insausti et al. 1985; Woo et al. 1985), mouse (Godement et al. 1984; Edwards et al. 1986a), cat (Wil- liams and Chalupa 1982; Shatz 1983), and in other mam- mals (Cavalcante and Rocha-Miranda 1978; Rakic 1977; Cucchiaro and Guillery 1984). In many of these studies, a delay was reported between the arrival of optic axons in their targets and initiation of putative terminal formation within target fields. This transitional period is consistent with the so-called "waiting period" prior to axon arborization (Rakic 1977; Wise and Jones 1978; Distel and HoU/inder 1980; Schreyer and Jones 1982; Shatz and Luskin 1986; Ramirez et al. 1990). In the present study on the primary visual system of the ham- ster, we address several unanswered questions about morphological transformations of optic axons before, during, and immediately after this transition period, fo- cussing more closely than in previous studies on the onset of the arborization process. Silver impregnation of nor- mal fibers was used to reveal the distribution of early axon fascicles, the Golgi method for characterization of developmental changes in morphology of individual ax- ons, and anterograde tracing with horseradish peroxidase (HRP) for sensitive labeling of the entire population of optic fibers. Other studies (see above; Cowan et al. 1984; Collelo and Guillery 1990; Godement et al. 1990; Simon and O'Leary 1990; Sretavan 1990; Bhide and Frost 1991) have emphasised different aspects of the developing retinal projection in rodents.