Development and connectivity of olfactory pathways in the brain of the lobster Homarus americanus.

The main output pathways from the olfactory lobes (primary olfactory centers) and accessory lobes (higher-order integrative areas) of decapod crustaceans terminate within both of the main neuropil regions of the lateral protocerebrum: the medulla terminalis and the hemiellipsoid body. The present study examines the morphogenesis of the lateral protocerebral neuropils of the lobster, Homarus americanus, and the development of their neuronal connections with the paired olfactory and accessory lobes. The medulla terminalis was found to emerge during the initial stages of embryogenesis and to be the target neuropil of the output pathway from the olfactory lobe. In contrast, the hemiellipsoid body is first apparent during mid-embryonic development and is innervated by the output pathway from the accessory lobe. The dye injections used to elucidate these pathways also resulted in the labeling of a previously undescribed pathway linking the olfactory lobe and the ventral nerve cord. To increase our understanding of the morphology of the olfactory pathways in H. americanus we also examined the connectivity of the lateral protocerebral neuropils of embryonic lobsters. These studies identified several interneuronal populations that may be involved in the higher-order processing of olfactory inputs. In addition, we examined the neuroanatomy of ascending pathways from the antenna II and lateral antenna I neuropils (neuropils involved in the processing of chemosensory and tactile inputs). These studies showed that the ascending pathways from these neuropils innervate the same regions of the medulla terminalis and that these regions are different from those innervated by the olfactory lobe output pathway.