Dendritogenesis, axonogenesis, pathfinding, and target recognition are all affected in distinct ways when Xenopus retinal ganglion cells (RGCs) are transfected with constitutively active (ca), wild-type (wt), and dominant negative (dn) Rho-family GTPases in vivo. Dendritogenesis required Rac1 and Cdc42 activity. Moreover, ca-Rac1 caused dendrite hyperproliferation. Axonogenesis, in contrast, was inhibited by ca-Rac1. This phenotype was partially rescued by the coexpression of dn cyclindependent kinase (Cdk5), a proposed effector of Rac1, suggesting that Rac1 activity must be regulated tightly for normal axonogenesis. Growth cone morphology was particularly sensitive to dn-RhoA and wt-Cdc42 constructs. These also caused targeting errors, such as tectal bypass, suggesting that cytoskeletal rearrangements are involved in target recognition and are transduced by these pathways.

The allometric changes characterizing the growth of the deutocerebrum (midbrain) of the American lobster (Homarus americanus) are studied using computerized three-dimensional reconstructions of serial brain sections. During the embryogenesis of the midbrain, the paired accessory lobes (higher order processing areas) appear later than the paired olfactory lobes (primary olfactory centers), but the former grow faster from their emergence until metamorphosis. The accessory lobes, as they enlarge, shift progressively from a medial to a posterior position in the lateral deutocerebrum. In early juvenile stages the accessory lobes are one of the largest neuropils of the brain. However, these lobes stop growing in adult animals, whereas the brain and olfactory lobes continue to enlarge, albeit at a slow rate. The overall shape of the brain and the relative proportions and locations of the deutocerebral neuropils and associated cell clusters of various lobster ontogenetic stages are similar to those of selected adult decapods. In addition, the relation between deutocerebral organization and brain size seem parallel during lobster development and across crustacean species. Measurements of the brains of 13 species of decapods (illustrated in Sandeman et al. [1993] J. Exp. Zool. 265:112, plus Homarus) indicate the following trends: Small brains possess olfactory lobes but no accessory lobes, larger brains possess accessory lobes that are medial and small relative to the olfactory lobes, and the largest brains contain relatively voluminous posterior accessory lobes. These observations indicate that some differences in the organization of the deutocerebrum are related to absolute brain size in crustaceans and suggest that ontogenetic scaling of proportions may apply to the deutocerebral neuropils of decapods. Peramorphosis and paedomorphosis in the evolution of the decapod brain are considered.

The olfactory and accessory lobes constitute prominent histological structures within the larval and mature lobster deutocerebrum, and both are associated with a dense innervation from paired serotonergic nerve cells, the dorsal giant neurons (DGNs). During development, the cell bodies of the DGNs are the first central somata to express serotonin (5‐HT), and the onset of their 5‐HT immunoreactivity coincides with the beginning of accessory lobe formation. In contrast, the olfactory lobe anlagen emerge much earlier and grow in the apparent absence of serotonin. The role of serotonergic input for the development of these brain structures was investigated in lobster embryos after serotonin had been depleted pharmacologically with the neurotoxin 5,7‐dihydroxytryptamine. A ∼90% reduction of serotonin was confirmed in eggs using high‐performance liquid chromatography with electrochemical detection. Morphometric analyses suggested that serotonin depletion dramatically slowed the growth of olfactory and accessory lobes, although glomeruli differentiated at the normal time in both areas. The toxin exhibited a high degree of specificity for serotonergic neurons and associated target regions, and serotonin depletion persisted for at least 2 months following treatment. The goal of future experiments is to determine which of the cell types that innervate the olfactory and accessory lobes are affected by toxin treatment, thereby resulting in the retarded growth of these areas. © 1997 John Wiley & Sons, Inc. J Neurobiol 33: 357–373, 1997

It is feasible to study the anatomical, physiological, and biochemical properties of identifiable neurons in lobster embryos. To exploit fully the advantages of this preparation and to lay the foundation for single-cell studies, our recent goals have been to 1) establish a quantitative staging system for embryos, 2) document in detail the lobster's embryonic development, 3) determine when uniquely identifiable neurons first acquire their transmitter phenotypes, and 4) identify particular neurons that may serve developmental functions. Behavioral, anatomical, morphometric, and immunocytochemical studies have led to a detailed characterization of the growth and maturation of lobster embryos and to the adoption of a percent-staging system based upon the eye index of Perkins (Fish. Bull., 70:95-99, 1972). It is clear from these studies that the lobster nauplius molts at approximately 12% embryonic development (E12%) into a metanauplius, which subsequently undergoes a complete molt cycle within the egg. This molt cycle climaxes with the emergence of the first-stage larva shortly after hatching. Serotonin and proctolin, neurohormones widely distributed in the lobster nervous system, appear at different times in development. Serotonin immunoreactive neurons begin to appear at approximately E10%, with the adult complement being established by E50%. In contrast, proctolin immunoreactive neurons appear later and attain their full complement over a protracted period including larval and juvenile stages. The development of serotonergic deutocerebral neurons and their targets, the olfactory and accessory lobes in the brain, are also examined. The olfactory lobes are forming by E10% and have acquired their glomerular organization by E50%, whereas the formation of the accessory lobes is delayed; the early rudiments of the accessory lobes are seen by E50%, and glomeruli do not form until the second larval stage.

Olfactory glomeruli are columnar and radially arranged at the periphery of the primary chemosensory areas, the olfactory lobes (OLs), in the American lobster Homarus americanus. The number of olfactory glomeruli reaches nearly 100/lobe in midembryonic life, increases rapidly during larval life, and stabilizes at about 200 in juvenile and adult lobsters. The accessory lobes (ALs), higher order integration areas, are composed of cortical columns and of spherical glomeruli. Two populations of spherical glomeruli are defined, the cortical glomeruli located at the bases of the columns, and the medullary glomeruli scattered throughout the ALs. Both cortical columns and spherical glomeruli are seen for the first time in the second larval stage. There are about 1000 cortical columns and 1700 glomeruli/AL in the postlarva and these numbers remain constant during the life of the lobster. In both OLs and ALs, it is the size of the interglomerular spaces and of the glomeruli themselves that increases. Therefore, the data suggest that in both OLs and ALs the glomeruli were already generated when the lobster metamorphoses (stage III to IV) and switches from a planktonic to a benthic existence, and that the new sensory neurons that are formed at each molt in the antennulae grow into existing olfactory glomeruli. Stability of the glomerular population in the primary olfactory centers, once the full complement of glomeruli is acquired, has also been reported in insects, fish, and mammals. © 1996 John Wiley & Sons, Inc.