Projections of the olfactory bulb and nervus terminalis in the silver lamprey.

The connections of the olfactory bulb were traced using horseradish peroxidase. A homologue of the medial olfactory tract in gnathostomes projects to the ipsilateral septal nucleus, preoptic area and, possibly, the rostral striatum. A homologue of the lateral olfactory tract projects to the ipsilateral lateral pallium, dorsal pallium and, possibly, the medial pallium, as well as to the posterior diencephalon. A component of the lateral olfactory tract decussates in the habenular and posterior commissures and distributes to the contralateral hemisphere and caudal diencephalon. A dorsal component of secondary olfactory fibers terminates, ipsilaterally, in a dorsomedially situated neuropil that has previously been interpreted as a single glomerulus of the olfactory bulb or as an accessory olfactory bulb, as well as in the contralateral olfactory bulb after decussation in the dorsal commissure. Afferents to the olfactory bulb arise from the ipsilateral dorsal pallium, lateral pallium, a cell-poor region adjacent to the preoptic area, and the midbrain tegmentum. The extent of the secondary olfactory projections in silver lampreys could be interpreted to support the phylogenetic hypothesis that all regions of the telencephalon received secondary olfactory projections in the earliest vertebrates, but this interpretation is not unequivocal, due to questions concerning the pallial homologues in lampreys and gnathostomes. Application of horseradish peroxidase to the olfactory epithelium revealed projections to the striatum, preoptic area, hypothalamus and posterior tuberculum that are comparable to projections of the nervus terminalis in other vertebrates.