The mitral cell is the primary output neuron and central relay in the olfactory bulb of vertebrates. The morphology of these cells has been studied extensively in mammalian systems and to a lesser degree in teleosts. This study uses retrograde tract tracing and other techniques to characterize the morphology and distribution of mitral cells in the olfactory bulb of adult zebrafish, Danio rerio. These output neurons, located primarily in the glomerular layer and superficial internal cell layer, had variable-shaped somata that ranged in size from 4-18 microm in diameter and 31-96 microm2 in cross-sectional area. The mitral cells exhibited two main types of morphologies with regard to their dendrites: the unidendritic morphology was a single primary dendrite with one or more tufts, but multidendritic cells with several dendritic projections also were seen. The axons of these cells projected to either the medial or the lateral olfactory tract and, in general, the location of the cell on the medial or lateral side of the bulb was indicative of the tract to which it would project. Further, this study shows that the majority of zebrafish mitral cells likely innervate a single glomerulus rather than multiple glomeruli. This information is contrary to the multiple innervation pattern suggested for all teleost mitral cells. Our findings suggest that mitral cells in zebrafish may be more similar to mammalian mitral cells than previously believed, despite variation in size and structure. This information provides a revised anatomical framework for olfactory processing studies in this key model system.

It is now well established that neurogenesis in the rodent subgranular zone of the hippocampal dentate gyrus continues throughout adulthood. Neuroblasts born in the dentate subgranular zone migrate into the granule cell layer, where they differentiate into neurons known as dentate granule cells. Suppression of neurogenesis by irradiation or genetic ablation has been shown to disrupt synaptic plasticity in the dentate gyrus and impair some forms of hippocampus-dependent learning and memory. Using a recently developed transgenic mouse model for suppressing neurogenesis, we sought to determine the long-term impact of ablating neurogenesis on synaptic plasticity in young-adult mice. Consistent with previous reports, we found that ablation of neurogenesis resulted in significant deficits in dentate gyrus long-term potentiation (LTP) when examined at a time proximal to the ablation. However, the observed deficits in LTP were not permanent. LTP in the dentate gyrus was restored within 6 wk and this recovery occurred in the complete absence of neurogenesis. The recovery in LTP was accompanied by prominent changes within the dentate gyrus, including an increase in the survival rate of newborn cells that were proliferating just before the ablation and a reduction in inhibitory input to the granule cells of the dentate gyrus. These findings suggest that prolonged suppression of neurogenesis in young-adult mice results in wide-ranging compensatory changes in the structure and dynamics of the dentate gyrus that function to restore plasticity.adult neurogenesis | thymidine kinase | metaplasticity | miniature inhibitory postsynaptic currents F orebrain neurogenesis persists into adulthood in the subgranular zone (SGZ) of the hippocampal dentate gyrus in rodents (1-4). Under normal conditions (i.e., in the absence of overt pathology) neuroblasts that arise in the SGZ migrate a short distance into the dentate granule cell layer (GCL) and differentiate into dentate granule cells (DGCs), where they subsequently reach functional maturity (5, 6). The birth, integration, and survival of DGCs are modulated by environmental enrichment (7), exercise (8), stress (9, 10), hippocampus-dependent learning (11), and direct manipulation of neuronal activity (12, 13). In addition, adult-born DGCs respond preferentially in hippocampus-dependent memory tasks (14) and display increased synaptic plasticity relative to mature DGCs (15,16).The correlation of increased DGC neurogenesis with cognitively demanding tasks has led to the hypothesis that adult-born neurons are integral participants in hippocampus-dependent memory processing and behavior. The role of adult-born DGCs in hippocampal function has been studied at the behavioral level in rodents after suppressing neurogenesis with antimitotic agents (17, 18), radiation (19), or genetic targeting (19)(20)(21)(22). These studies indicate that adult-born DGCs are necessary for some hippocampus-dependent tasks, although results have been inconsistent and vary by rodent species and strain, behavioral ...

Forebrain neurogenesis persists throughout life in the rodent subventricular zone (SVZ) and hippocampal dentate gyrus (DG). Several strategies have been employed to eliminate adult neurogenesis and thereby determine whether depleting adult-born neurons disrupts specific brain functions, but some approaches do not specifically target neural progenitors. We have developed a transgenic mouse line to reversibly ablate adult neural stem cells and suppress neurogenesis. The nestin-tk mouse expresses herpes simplex virus thymidine kinase (tk) under the control of the nestin 2nd intronic enhancer, which drives expression in neural progenitors. Administration of ganciclovir (GCV) kills actively dividing cells expressing this transgene. We found that peripheral GCV administration suppressed SVZ-olfactory bulb and DG neurogenesis within two weeks but caused systemic toxicity. Intracerebroventricular GCV infusion for 28 days nearly completely depleted proliferating cells and immature neurons in both the SVZ and DG without systemic toxicity. Reversibility of the effects after prolonged GCV infusion was slow and partial. Neurogenesis did not recover 2 weeks after cessation of GCV administration, but showed limited recovery 6 weeks after GCV that differed between the SVZ and DG. Suppression of neurogenesis did not inhibit antidepressant responsiveness of mice in the tail suspension test. These findings indicate that SVZ and DG neural stem cells differ in their capacity for repopulation, and that adult-born neurons are not required for antidepressant responses in a common behavioral test of antidepressant efficacy. The nestin-tk mouse should be useful for studying how reversible depletion of adult neurogenesis influences neurophysiology, other behaviors, and neural progenitor dynamics.

Abstract. The male reproductive system of the caterpillar‐hunting wasp Ancistrocerus antilope was composed of the testes, seminal vesicles, accessory glands, the penis, and the connecting ducts. The testes, which were paired structures lined with a thin layer of squamous epithelium, were composed of several lobes that contain developing spermatozoa. These spermatozoa began as spermatocytes in the distal portion of the testicular lobe and, as they matured, they moved proximally toward the seminal vesicles. Upon maturation, the spermatozoa were long, thin structures with a nucleus of ∼10–15 μm in length and 1–2 μm in width. Their tails, which were not morphologically distinct from their heads at the gross level, averaged 388±25 μm in length. Once the spermatozoa had fully developed, they traveled from the testes to the seminal vesicles, where they were stored. The walls of the seminal vesicles were composed of a ciliated, pseudostratified, columnar epithelium. A layer of fibrous connective tissue termed the testicular capsule encapsulated both the testes and the seminal vesicles. Upon ejaculation, the spermatozoa moved through the deferent ducts, where they combined with granular fluid from the accessory glands. From here, they traveled via the ejaculatory duct to the penis, where they were expelled. In this study, these anatomical descriptions are discussed in reference to other species of the Hymenoptera.

The removal of afferent input to the olfactory bulb by both cautery and chemical olfactory organ ablation in adult zebrafish results in a significant decrease in volume of the ipsilateral olfactory bulb. To examine the effects of deafferentation at a cellular level, primary output neurons of the olfactory bulb, the mitral cells, were investigated using retrograde tract tracing with fluorescent dextran using ex vivo brain cultures. Morphological characteristics including the number of major dendritic branches, total length of dendritic branches, area of the dendritic arbor, overall dendritic complexity, and optical density of the arbor were used to determine the effects of deafferentation on mitral cell dendrites. Following 8 weeks of permanent deafferentation there were significant reductions in the total length of dendritic branches, the area of the dendritic arbor, and the density of fine processes in the dendritic tuft. With 8 weeks of chronic, partial deafferentation there were significant reductions in all parameters examined, including a modified Sholl analysis that showed significant decreases in overall dendritic complexity. These results show the plasticity of mitral cell dendritic structures in the adult brain and provide information about the response of these output neurons following the loss of sensory input in this key model system.