The catalogue of data presented here form many systems demonstrates that multiple mechanisms are involved in the formation of topographic maps. We are not yet in a position to explain why a particular mechanism appears to dominate in some situations and not in others. Certain generalizations can be made, however. First, at least some form of chemospecificity can be invoked to help explain connectivity in all of the experiments we have cited. Often, the differential identities of a population of neurons can be reflected in an orderly pattern of axon outgrowth and in the actively maintained preservation of neighbor relations as the axons grow toward their targets; such orderly arrangements are not obligatory, but, where present, they facilitate the speedy establishment of orderly maps when the axons reach their target nuclei. Within a terminal zone, chemospecific cues may dominate and constrain a given axon to terminate in a specific location, but axon-axon interactions commonly supercede chemospecific matching. At least two forms of axon-axon interaction occur, one based on some sort of biochemical properties related to the axon's embryological identity and another based on the axons' electrical activity. Tasks for the future are to identify the cellular bases of each of these mechanisms and to understand the situations in which each is manifested.

The connections between the nucleus isthmi and the tectum in the frog have been determined by several anatomical techniques: iontophoresis of horseradish peroxidase into the tectum, iontophoresis of 3H-porline into the nucleus isthmi and the tectum, and Fink-Heimer degeneration staining after lesions of the nucleus isthmi. The results show that the nucleus isthmi projects bilaterally to the tectal lobes. The ipsilateral isthmio-tectal fibers are distributed in the superficial layers of the tectum, coincident with the retionotectal terminals. The contralateral isthmio-tectal fibers travel anteriorly adjacent to the lateral optic tract and cross the midline in the supraoptic ventral decussation, where they turn dorsally and caudally; upon reaching the tectum, the fibers end in two discrete layers, layers 8 and A of Potter. The tectum projects to the ipsilateral nucleus isthmi and there is a reciprocal topographic relationship between the two structures. Thus, a retino-tecto-isthmio-tectal route exists which may contribute to the indirect ipsilateral retinotectal projection which is observed electrophysiologically. The connections between the nucleus isthmi and the tectum in the frog are strinkingly similar to the connections between the parabigeminal nucleus and the superior colliculus of mammals.

The accessory olfactory bulb (AOB) is a critical olfactory structure that has been implicated in mediating social behavior. It receives input from the vomeronasal organ and projects to targets in the amygdaloid complex. Its anterior and posterior components (aAOB and pAOB) display molecular, connectional and functional segregation in processing reproductive and defensive and aggressive behaviors, respectively. We observed a dichotomy in the development of the projection neurons of the aAOB and pAOB in mice. We found that they had distinct sites of origin and that different regulatory molecules were required for their specification and migration. aAOB neurons arose locally in the rostral telencephalon, similar to main olfactory bulb neurons. In contrast, pAOB neurons arose caudally, from the neuroepithelium of the diencephalic-telencephalic boundary, from which they migrated rostrally to reach their destination. This unusual origin and migration is conserved in Xenopus, providing an insight into the origin of a key component of this system in evolution.

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