Adult neurogenesis, the generation of new neurons from adult precursor cells, occurs in the brains of a phylogenetically diverse array of animals. In the higher (amniotic) vertebrates, these precursor cells are glial cells that reside within specialized regions, known as neurogenic niches, the elements of which both support and regulate neurogenesis. The in vivo identity and location of the precursor cells responsible for adult neurogenesis in nonvertebrate taxa, however, remain largely unknown. Among the invertebrates, adult neurogenesis has been particularly well characterized in freshwater crayfish (Arthropoda, Crustacea), although the identity of the precursor cells sustaining continuous neuronal proliferation in these animals has yet to be established. Here we provide evidence suggesting that, as in the higher vertebrates, the precursor cells maintaining adult neurogenesis in the crayfish Procambarus clarkii are glial cells. These precursor cells reside within a specialized region, or niche, on the ventral surface of the brain, and their progeny migrate from this niche along glial fibers and then proliferate to form new neurons in the central olfactory pathway. The niche in which these precursor cells reside has many features in common with the neurogenic niches of higher vertebrates. These commonalities include: glial cells functioning as both precursor and support cells, directed migration, close association with the brain vasculature, and specialized basal laminae. The cellular machinery maintaining adult neurogenesis appears, therefore, to be shared by widely disparate taxa. These extensive structural and functional parallels suggest a common strategy for the generation of new neurons in adult brains. Indexing termsglia; migration; neurogenic niche; olfaction; vasculature New neurons continue to be added to specific regions in the brains of many animals throughout adulthood (Kempermann, 2000). In vertebrates, adult-born neurons are the progeny of precursor cells residing within specialized brain regions, termed neurogenic niches (GarciaVerdugo et al., 2002;Doetsch, 2003a;Ma et al., 2005). The cellular and extracellular elements that make up these niches not only support the precursor cells structurally but also functionally regulate their activity and the development of their progeny (Song et al., 2002;Doetsch, 2003a,b;Shen et al., 2004;Ma et al., 2005). Glial cells are key components of the neurogenic niches of adult vertebrates, acting both as the precursor cells and in the support and regulation of neurogenesis (Garcia-Verdugo et al., 2002;Song et al., 2002;Doetsch, 2003a,b;Garcia et al., 2004;Seri et al., 2004;Ma et al., 2005). These cells also guide and regulate the migration of newborn cells to the regions of the brain in which they differentiate into neurons (Lois et al., 1996;Bolteus and Bordey, 2004 Garcia-Verdugo et al., 2002;Mercier et al., 2002;Palmer, 2002;Doetsch, 2003a;Ma et al., 2005).While adult neurogenesis is known to occur in a phylogenetically diverse array of animals, neur...

1/f scaling has been observed throughout human physiology and behavior, but its origins and meaning remain a matter of debate. Some argue that it is a byproduct of ongoing processes in the brain or body and therefore of limited relevance to psychological theory. Others argue that 1/f scaling reflects a fundamental aspect of all physiological and cognitive functions, namely, that they emerge in the balance of independent versus interdependent component activities. In 4 experiments, series of key-press responses were used to test between these 2 alternative explanations. The critical design feature was to take 2 measures of each key-press response: reaction time and key-contact duration. These measures resulted in 2 parallel series of intrinsic fluctuations for each series of key-press responses. Intrinsic fluctuations exhibited 1/f scaling in both reaction times and key-contact durations, yet the 2 measures were uncorrelated with each other and separately perturbable. These and other findings indicate that 1/f scaling is too pervasive to be idiosyncratic and of limited relevance. It is instead argued that 1/f scaling reflects the coordinative, metastable basis of cognitive function.

Serotonin exerts a wide range of physiological actions on many different lobster tissues. To begin the examination of the role of serotonin in lobsters at a cellular level, we have used immunohistochemical methods to search for presumptive serotonergic neurons, their central and peripheral projections, and their terminal fields of arborization. Whole mount preparations of the ventral nerve cord and various peripheral nerve structures have been used for these studies. With these tissues, more than 100 cell bodies have been found that show serotonin-like immunoreactivity. Although a few of the cell bodies are located peripherally (near the pericardial organs, a well known crustacean neurohemal organ), the vast majority are located in central ganglia. Every ganglion in the ventral nerve cord contains at least one immunoreactive cell body. The projections of many of the neurons have been traced, and we have constructed a map of the system of serotonin-immunoreactive cell bodies, fibers, and nerve endings. In addition, a dense plexus of nerve endings showing serotonin-like immunoreactivity surrounds each of the thoracic second roots in the vicinity of groups of peripheral neurosecretory neurons. These peripheral nerve plexuses originate from central neurons of the ventral nerve cord. In some cases we have been able to trace processes from particular central cell bodies directly to the peripheral nerve root plexuses; in other cases we have traced ganglionic neuropil regions to these peripheral endings.

BackgroundAdult neurogenesis, the production and integration of new neurons into circuits in the brains of adult animals, is a common feature of a variety of organisms, ranging from insects and crustaceans to birds and mammals. In the mammalian brain the 1st-generation neuronal precursors, the astrocytic stem cells, reside in neurogenic niches and are reported to undergo self-renewing divisions, thereby providing a source of new neurons throughout an animal's life. In contrast, our work shows that the 1st-generation neuronal precursors in the crayfish (Procambarus clarkii) brain, which also have glial properties and lie in a neurogenic niche resembling that of vertebrates, undergo geometrically symmetrical divisions and both daughters appear to migrate away from the niche. However, in spite of this continuous efflux of cells, the number of neuronal precursors in the crayfish niche continues to expand as the animals grow and age. Based on these observations we have hypothesized that (1) the neuronal stem cells in the crayfish brain are not self-renewing, and (2) a source external to the neurogenic niche must provide cells that replenish the stem cell pool.ResultsIn the present study, we tested the first hypothesis using sequential double nucleoside labeling to track the fate of 1st- and 2nd-generation neuronal precursors, as well as testing the size of the labeled stem cell pool following increasing incubation times in 5-bromo-2'-deoxyuridine (BrdU). Our results indicate that the 1st-generation precursor cells in the crayfish brain, which are functionally analogous to neural stem cells in vertebrates, are not a self-renewing population. In addition, these studies establish the cycle time of these cells. In vitro studies examining the second hypothesis show that Cell Tracker™ Green-labeled cells extracted from the hemolymph, but not other tissues, are attracted to and incorporated into the neurogenic niche, a phenomenon that appears to involve serotonergic mechanisms.ConclusionsThese results challenge our current understanding of self-renewal capacity as a defining characteristic of all adult neuronal stem cells. In addition, we suggest that in crayfish, the hematopoietic system may be a source of cells that replenish the niche stem cell pool.

Neuronal plasticity and synaptic remodeling play important roles during the development of the invertebrate nervous system. In addition, structural neuroplasticity as a result of long-term environmental changes, behavioral modifications, age, and experience have been demonstrated in the brains of sexually mature insects. In adult vertebrates, persistent neurogenesis is found in the granule cell layer of the mammalian hippocampus and the subventricular zone, as well as in the telencephalon of songbirds, indicating that persistent neurogenesis, which is presumably related to plasticity and learning, may be an integral part of the normal biology of the mature brain. In decapod crustaceans, persistent neurogenesis among olfactory projection neurons is a common principle that shapes the adult brain, indicating a remarkable degree of life-long structural plasticity. The present study closes a gap in our knowledge of this phenomenon by describing the continuous cell proliferation and gradual displacement of proliferation domains in the central olfactory pathway of the American lobster Homarus americanus from early embryonic through larval and juvenile stages into adult life. Neurogenesis in the deutocerebrum was examined by the in vivo incorporation of bromodeoxyuridine, and development and structural maturation of the deutocerebral neuropils were studied using immunohistochemistry against Drosophila synapsin. The role of apoptotic cell death in shaping the developing deutocerebrum was studied using the terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling method, combined with immunolabeling using an antiphospho histone H3 mitosis marker. Our results indicate that, in juvenile and adult lobsters, birth and death of olfactory interneurons occur in parallel, suggesting a turnover of these cells. When the persistent neurogenesis and concurrent death of interneurons in the central olfactory pathway of the crustacean brain are taken into account with the life-long turnover of olfactory receptor cells in crustacean antennules, a new, highly dynamic picture of olfaction in crustaceans emerges.