Neurogenesis and gliogenesis in the spinal cord of turtles

CRMP4 is required for the positioning and maturation of newly generated neurons in adult mouse hippocampus

Adult neurogenesis is a phenomenon in which neural stem cells differentiate and mature to generate new neurons in the adult brain. In mammals, the sites where adult neurogenesis occurs are limited to the subgranular zone (SGZ) of the hippocampal dentate gyrus and the subventricular zone. In the hippocampus, newly generated neurons migrate into the granule cell layer (GCL) and are integrated into neural circuits. Previous studies have revealed that CRMP4, a member of the CRMP family, is expressed in immature neurons in the hippocampal SGZ of the adult brain. However, the role of CRMP4 in adult neurogenesis is unknown. To study the role of CRMP4 in hippocampal adult neurogenesis, we compared adult neurogenesis between wild type and CRMP4-/- mice. In CRMP4-/- mice, the number of doublecortin (DCX)-positive cells was comparable to that in wild-type mice, and some DCX-positive cells were ectopically located in the granule cell layer, suggesting that CRMP4 is involved in the migration of adult neurogenesis. In addition, the number of calretinin-positive new neurons in the SGZ was significantly increased, whereas the number of EdU/NeuN-double positive neurons was decreased in CRMP4-/- mice, suggesting that CRMP4 plays an important role in neuronal maturation. Because CRMP4 is expressed in immature neurons, its expression may regulate the migration from the SGZ to the GCL during neuronal maturation in hippocampal adult neurogenesis.

Different functional bio-scaffolds share similar neurological mechanism to promote locomotor recovery of canines with complete spinal cord injury

Many studies have shown that rodents exhibit a certain degree of spontaneous motor function recovery even if they suffer from spinal cord complete transection injury. However, the characteristics of spontaneous locomotor recovery and its associated neurobiological mechanisms are unclear. In this study, we observed that spontaneous locomotor function recovery of hind limbs could also be detected in a canine thoracic (T8) spinal cord complete transection model. In addition, the spontaneous locomotor recovery of canines could be further promoted by chronic implantation of Taxol- or human bone marrow mesenchymal stem cell-modified bio-scaffolds. Moreover, functional bio-scaffolds implantation promoted locomotor outcome could be significantly weakened (drop to the spontaneous recovery level) but not totally abolished by resection in the lesion site. The neurological mechanism for functional bio-scaffolds improved locomotor outcome was primarily dependent on the formation of neuronal bridging but not the long-distance regeneration of descending motor axons throughout the lesion gap. Besides that, we found that spontaneously achieved locomotor recovery of hind limbs was unable to be weaken by repetitive resection of the lesion area, indicating the mechanism for spontaneous locomotor recovery was independent on functional neurological bridging throughout the lesion gap.

Spinal Cord Stem Cells In Their Microenvironment: The Ependyma as a Stem Cell Niche

The ependyma of the spinal cord is currently proposed as a latent neural stem cell niche. This chapter discusses recent knowledge on the developmental origin and nature of the heterogeneous population of cells that compose this stem cell microenviroment, their diverse physiological properties and regulation. The chapter also reviews relevant data on the ependymal cells as a source of plasticity for spinal cord repair.

Emergence of Serotonergic Neurons After Spinal Cord Injury in Turtles

Plasticity of neural circuits takes many forms and plays a fundamental role in regulating behavior to changing demands while maintaining stability. For example, during spinal cord development neurotransmitter identity in neurons is dynamically adjusted in response to changes in the activity of spinal networks. It is reasonable to speculate that this type of plasticity might occur also in mature spinal circuits in response to injury. Because serotonergic signaling has a central role in spinal cord functions, we hypothesized that spinal cord injury (SCI) in the fresh water turtle Trachemys scripta elegans may trigger homeostatic changes in serotonergic innervation. To test this possibility we performed immunohistochemistry for serotonin (5-HT) and key molecules involved in the determination of the serotonergic phenotype before and after SCI. We found that as expected, in the acute phase after injury the dense serotonergic innervation was strongly reduced. However, 30 days after SCI the population of serotonergic cells (5-HT+) increased in segments caudal to the lesion site. These cells expressed the neuronal marker HuC/D and the transcription factor Nkx6.1. The new serotonergic neurons did not incorporate the thymidine analog 5-bromo-2'-deoxyuridine (BrdU) and did not express the proliferating cell nuclear antigen (PCNA) indicating that novel serotonergic neurons were not newborn but post-mitotic cells that have changed their neurochemical identity. Switching towards a serotonergic neurotransmitter phenotype may be a spinal cord homeostatic mechanism to compensate for the loss of descending serotonergic neuromodulation, thereby helping the outstanding functional recovery displayed by turtles. The 5-HT_1A receptor agonist (±)-8-Hydroxy-2-dipropylaminotetralin hydrobromide (8-OH-DPAT) blocked the increase in 5-HT+ cells suggesting 5-HT_1A receptors may trigger the respecification process.

Bridging the gap with functional collagen scaffolds: tuning endogenous neural stem cells for severe spinal cord injury repair

Severe spinal cord injury (SCI) induces massive proliferation of spinal cord neural stem cells (NSCs), which are considered a promising cell source for therapeutic neural repair.

Telencephalic-olfactory bulb ventricle wall organization in Austrolebias charrua: Cytoarchitecture, proliferation dynamics, neurogenesis and migration

Adult neurogenesis participates in fish olfaction sensitivity in response to environmental challenges. Therefore, we investigated if several populations of stem/progenitor cells that are retained in the olfactory bulbs (OB) may constitute different neurogenic niches that support growth and functional demands. By electron microscopy and combination cell proliferation and lineage markers, we found that the telencephalic ventricle wall (VW) at OB level of Austrolebias charrua fish presents three neurogenic niches (transitional 1, medial 2 and ventral 3). The main cellular types described in other vertebrate neurogenic niches were identified (transient amplifying cells, stem cells and migrating neuroblasts). However, elongated vimentin/BLBP+ radial glia were the predominant cells in transitional and ventral zones. Use of halogenated thymidine analogs chloro- and iodo-deoxyuridine administered at different experimental times showed that both regions have the highest cell proliferation and migration rates. Zone 1 migration was toward the OB and telencephalon, whereas in zone 3, migration was directed toward the OB rostral portion constituting the equivalent of the mammal rostral migratory band. Medial zone (MZ) has fewer proliferating non-migrant cells that are the putative stem cells as indicated by short and long proliferation assays as well as cell lineage markers. Sparse migration observed suggests MZ may collaborate with VW growth. Scanning electron microscopy evidenced that the whole VW has only monociliated cells with remarkable differences in cilium length among regions. In OB there are monociliated cells with dwarf cilium whereas ventral telencephalon shows long cilium. Summarizing, we identified three neurogenic niches that might serve different functional purposes.

GABAergic signalling in a neurogenic niche of the turtle spinal cord

The region that surrounds the central canal (CC) in the turtle spinal cord is a neurogenic niche immersed within already functional circuits, where radial glia expressing brain lipid binding protein (BLBP) behave as progenitors. The behaviour of both progenitors and neuroblasts within adult neurogenic niches must be regulated to maintain the functional stability of the host circuit. In the brain, GABA plays a major role in this kind of regulation but little is known about GABAergic signalling in neurogenic niches of the postnatal spinal cord. Here we explored the action of GABA around the CC of the turtle spinal cord by combining patch-clamp recordings of CC-contacting cells, immunohistochemistry for key components of GABAergic signalling and Ca(2+) imaging. Two potential sources of GABA appeared around the CC: GABAergic terminals and CC-contacting neurones. GABA depolarized BLBP(+) progenitors via GABA transporter-3 (GAT3) and/or GABA(A) receptors. In CC-contacting neurones, GABA(A) receptor activation generated responses ranging from excitation to inhibition. This functional heterogeneity appeared to originate from different ratios of activity of the Na(+)-K(+)-2Cl(-) co-transporter (NKCC1) and the K(+)-Cl(-) co-transporter (KCC2). In both progenitors and immature neurones, GABA induced an increase in intracellular Ca(2+) that required extracellular Ca(2+) and was blocked by the selective GABA(A) receptor antagonist gabazine. Our study shows that GABAergic signalling around the CC shares fundamental properties with those in the embryo and adult neurogenic niches, suggesting that GABA may be part of the mechanisms regulating the production and integration of neurones within operational spinal circuits in the turtle.

Immunohistological markers for proliferative events, gliogenesis, and neurogenesis within the adult hippocampus

Biologists long believed that, once development is completed, no new neurons are produced in the forebrain. However, as is now firmly established, new neurons can be produced at least in two specific forebrain areas: the subventricular zone (SVZ) and the dentate gyrus (DG) of the hippocampal formation. Neurogenesis within the adult DG occurs constitutively throughout postnatal life, and the rate of neurogenesis within the DG can be altered under various physiological and pathophysiological conditions. The process of adult neurogenesis within the DG is a multi-step process (proliferation, differentiation, migration, targeting, and synaptic integration) that ends with the formation of a post-mitotic functionally integrated new neuron. Various markers are expressed during specific stages of adult neurogenesis. The availability of such markers allows the time-course and fate of newly born cells to be followed within the DG in a detailed and precise fashion. Several of the available markers (e.g., PCNA, Ki-67, PH3, MCM2) are markers for proliferative events, whereas others are more specific for early phases of neurogenesis and gliogenesis within the adult DG (e.g., nestin, GFAP, Sox2, Pax6). In addition, markers are available allowing events to be distinguished that are related to later steps of gliogenesis (e.g., vimentin, BLBP, S100beta) or neurogenesis (e.g., NeuroD, PSA-NCAM, DCX).

Cell proliferation and cytoarchitectural remodeling during spinal cord reconnection in the fresh-water turtle Trachemys dorbignyi

In fresh-water turtles, the bridge connecting the proximal and caudal stumps of transected spinal cords consists of regenerating axons running through a glial cellular matrix. To understand the process leading to the generation of the scaffold bridging the lesion, we analyzed the mitotic activity triggered by spinal injury in animals maintained alive for 20-30 days after spinal cord transection. Flow cytometry and bromodeoxyuridine (BrdU)-labeling experiments revealed a significant increment of cycling cells around the lesion epicenter. BrdU-tagged cells maintained a close association with regenerating axons. Most dividing cells expressed the brain lipid-binding protein (BLBP). Cells with BrdU-positive nuclei expressed glial fibrillary acidic protein. As spinal cord regeneration involves dynamic cell rearrangements, we explored the ultra-structure of the bridge and found cells with the aspect of immature oligodendrocytes forming an embryonic-like microenvironment. These cells supported and ensheathed regenerating axons that were recognized by immunocytological and electron-microscopical procedures. Since functional recovery depends on proper impulse transmission, we examined the anatomical axon-glia relationships near the lesion epicenter. Computer-assisted three-dimensional models revealed helical axon-glial junctions in which the intercellular space appeared to be reduced (5-7 nm). Serial-sectioning analysis revealed that fibril-containing processes provided myelinating axon sheaths. Thus, disruption of the ependymal layer elicits mitotic activity predominantly in radial glia expressing BLBP on the lateral aspects of the ependyma. These cycling cells seem to migrate and contribute to the bridge providing the main support and sheaths for regenerating axons.

Enigmatic central canal contacting cells: immature neurons in "standby mode"?

The region that surrounds the central canal of the spinal cord derives from the neural tube and retains a substantial degree of plasticity. In turtles, this region is a neurogenic niche where newborn neurons coexist with precursors, a fact that may be related with the endogenous repair capabilities of low vertebrates. Immunohistochemical evidence suggests that the ependyma of the mammalian spinal cord may contain cells with similar properties, but their actual nature remains unsolved. Here, we combined immunohistochemistry for cell-specific markers with patch-clamp recordings to test the hypothesis that the ependyma of neonatal rats contains immature neurons similar to those in low vertebrates. We found that a subclass of cells expressed HuC/D neuronal proteins, doublecortin, and PSA-NCAM (polysialylated neural cell adhesion molecule) but did not express NeuN (anti-neuronal nuclei). These immature neurons displayed electrophysiological properties ranging from slow Ca(2+)-mediated responses to fast repetitive Na(+) spikes, suggesting different stages of maturation. These cells originated in the embryo, because we found colocalization of neuronal markers with 5-bromo-2'-deoxyuridine when injected during embryonic day 7-17 but not in postnatal day 0-5. Our findings represent the first evidence that the ependyma of the rat spinal cord contains cells with molecular and functional features similar to immature neurons in adult neurogenic niches. The fact that these cells retain the expression of molecules that participate in migration and neuronal differentiation raises the possibility that the ependyma of the rat spinal cord is a reservoir of immature neurons in "standby mode," which under some circumstances (e.g., injury) may complete their maturation to integrate spinal circuits.

Neural reconnection in the transected spinal cord of the freshwater turtle Trachemys dorbignyi

This paper provides the first evidence that freshwater turtles are able to reconnect their completely transected spinal cords, leading to some degree of recovery of the motor functions lost after injury. Videographic analysis showed that some turtles (5 of 11) surviving more than 20 days after injury were able to initiate stepping locomotion. However, the stepping movements were slower than those of normal animals, and swimming patterns were not restored. Even though just 45% of the injured turtles recovered their stepping patterns, all showed axonal sprouting beyond the lesion site. Immunocytochemical and electron microscope images revealed the occurrence of regrowing axons crossing the severed region. A major contingent of the axons reconnecting the cord originated from sensory neurons lying in dorsal ganglia adjacent to the lesion site. The axons bridging the damaged region traveled on a cellular scaffold consisting of brain lipid-binding protein (BLBP)- and glial fibrillary acidic protein (GFAP)-positive cells and processes. Serotonergic varicose nerve fibers and endings were found at early stages of the healing process at the epicenter of the lesion. Interestingly, the glial scar commonly found in the damaged central nervous system of mammals was absent. In contrast, GFAP- and BLBP-positive processes were found running parallel to the main axis of the cord accompanying the crossing axons.

Preliminary evidence of neuronal regeneration in the anoxia tolerant vertebrate brain

Postnatal neurogenesis in response to stroke or ischemia is currently of great medical interest. In this study, we investigated the potential for neurogenesis in an anoxia tolerant vertebrate in response to global ischemia. The results suggest sustained neurogenesis in the turtle that increases after ischemic damage, thus revealing a potential physiological adaptation to repeated anoxia-reoxygenation events. This finding further emphasizes the common vertebrate phenomenon of postnatal neurogenesis, with the capacity for extensive regeneration of neurons apparent in some reptilian species.

Immunohistological markers for staging neurogenesis in adult hippocampus

Neurogenesis in the adult dentate gyrus (DG) of the hippocampus occurs constitutively throughout postnatal life, and the rate of neurogenesis within the DG can be altered under various physiological and pathophysiological conditions. Adult neurogenesis includes the process in which the division of a precursor cell takes place and the multi-step process (proliferation, differentiation, migration, targeting, and synaptic integration) that ends with the formation of a postmitotic functionally integrated new neuron. During specific time-frames of adult neurogenesis, various markers are expressed that correlate with the differentiation steps along the pathway from early progenitor cells to newly generated postmitotic neurons within the DG. Markers that are currently widely used for the investigation of adult hippocampal neurogenesis are: glial fibrillary acidic protein, nestin, Pax6, NeuroD, PSA-NCAM, doublecortin, TUC-4, Tuj-1, and calretinin. The discovery and development of specific markers that allow the time-course and fate of neurons to be followed during adult neurogenesis in a detailed and precise fashion are not only helpful for gaining further insights into the genesis of new neurons in the hippocampus, but also might be applicable to the development of strategies for therapeutic interventions.

Beyond anoxia: the physiology of metabolic downregulation and recovery in the anoxia-tolerant turtle

The freshwater turtle Trachemys scripta is among the most anoxia-tolerant of vertebrates, a true facultative anaerobe able to survive without oxygen for days at room temperature to weeks or months during winter hibernation. Our good friend and colleague Peter Lutz devoted nearly 25 years to the study of the physiology of anoxia tolerance in these and other model organisms, promoting not just the basic science but also the idea that understanding the physiology and molecular mechanisms behind anoxia tolerance provides insights into critical survival pathways that may be applicable to the hypoxic/ischemic mammalian brain. Work by Peter and his colleagues focused on the factors which enable the turtle to enter a deep hypometabolic state, including decreases in ion flux ("channel arrest"), increases in inhibitory neuromodulators like adenosine and GABA, and the maintenance of low extracellular levels of excitatory compounds such as dopamine and glutamate. Our attention has recently turned to molecular mechanisms of anoxia tolerance, including the upregulation of such protective factors as heat shock proteins (Hsp72, Hsc73), the reversible downregulation of voltage gated potassium channels, and the modulation of MAP kinase pathways. In this review we discuss three phases of anoxia tolerance, including the initial metabolic downregulation over the first several hours, the long-term maintenance of neuronal function over days to weeks of anoxia, and finally recovery upon reoxygenation, with necessary defenses against reactive oxygen stress.

Ontogeny of 5-HT neurons in the brainstem of the lamprey, Petromyzon marinus

This study examined the spatial and temporal distribution of serotonin-immunoreactive (5-HT-ir) neurons in the brainstem of Petromyzon marinus at three developmental stages, larval, postmetamorphic, and reproductive. Computer-assisted 3-D reconstructions were made of the three main 5-HT-ir neuron groups. The rostralmost brainstem group was located near the posterior commissure, the second group at the isthmus, and the third group in the bulbar area. For each of those groups, the distribution of the 5-HT-ir neurons was very similar in the three developmental stages examined, suggesting that the 5-HT system is relatively mature early in larval animals. The soma of 5-HT-ir neurons increased in size and their dendritic fields increased in complexity with development. Furthermore, the number of 5-HT-ir neurons in each group increased significantly from the larval to the reproductive stage. To determine whether this was due to the genesis of 5-HT neurons, bromodeoxyuridine (BrdU) was injected into larval, metamorphosing, and postmetamorphic lampreys. These experiments revealed a few neurons colocalizing BrdU and 5-HT in metamorphosing animals. Taken together, the present results suggest that 5-HT neurons increase in number during maturation and that neurogenesis could, at least partially, contribute to the appearance of new 5-HT cells at different developmental stages.

Cell proliferation in the Rana catesbeiana auditory medulla over metamorphic development

During metamorphic development, bullfrogs (Rana catesbeiana) undergo substantial morphological, anatomical, and physiological changes as the animals prepare for the transition from a fully-aquatic to a semi-terrestrial existence. Using BrdU incorporation and immunohistochemistry, we quantify changes in cell proliferation in two key auditory brainstem nuclei, the dorsolateral nucleus and the superior olivary nucleus, over the course of larval and early postmetamorphic development. From hatchling through early larval stages, numbers of proliferating cells increase in both nuclei, paralleling the overall increase in total numbers of cells available for labeling. Numbers of proliferating cells in the superior olivary nucleus decrease during the late larval and deaf periods, and significantly increase during metamorphic climax. Proliferating cells in the dorsolateral nucleus increase in number from hatchling to late larval stages, decrease during the deaf period, and increase during climax. In both nuclei, numbers of proliferating cells decrease during the postmetamorphic froglet stage, despite increases in the number of cells available for label. Newly generated cells express either glial- or neural-specific phenotypes beginning between 1 week and 1 month post-BrdU injection, respectively, while some new cells express gamma-aminobutyric acid within 2 days of mitosis. Our data show that these auditory nuclei dramatically up-regulate mitosis immediately prior to establishment of a transduction system based on atmospheric hearing.

Developmental changes of the GABA-immunoreactive fibers in the lamprey spinal cord

The changes in distribution and number of GABA immunoreactive (GABA-ir) fibers from postembryonic stages to adulthood in the lamprey spinal cord white matter were studied by using immunocytochemical techniques. From prolarvae to adult spawning animals there was an increase of the number of GABA-ir fibers. Three phases can be distinguished: (a) from prolarvae to middle size larvae (around 50 mm in body length) an increase in the number of GABA-ir fibers per section is observed. Furthermore, an adult-like pattern of GABA-ir fibers distribution is established during this phase. (b) Then, the number of GABA-ir fibers remains stable until metamorphosis, the end of the larval period. (c) Finally, in young postmetamorphic and adult animals the number of GABA-ir fibers is higher than in larvae. These observations, joined to the changes previously reported in the GABA-ir neurons, indicate that at least parts of the GABA inhibitory component of the spinal locomotor network is reorganized during the lamprey life cycle and it may indicate different inhibitory requirements in the locomotor network.

Glutamate receptor antagonists and growth factors modulate dentate granule cell neurogenesis in organotypic, rat hippocampal slice cultures

Generation of dentate granule cells and its modulation by glutamate receptor antagonists, growth factors and pilocarpine-induced seizure-like activity was investigated in rat hippocampal slice cultures derived from 1-week-old rats and grown for 2 weeks. Focussing on the dentate granule cell layer facing CA1 and the immediate subgranular zone, exposure for 3 days to the NMDA receptor blocking agents MK-801 (10 microM) or APV (25 microM) in the culture medium, increased the number of TOAD-64/Ulip/CRMP-4 (TUC-4)-positive cells as counted in the slice cultures at the end of the 3-day treatment period. Exposure to IGF-I (200 ng/ml) and EGF (20 ng/ml) also increased the number of TUC-4-positive cells. Combining APV with IGF-I/EGF had an additive effect. Similar results were obtained by 3 days treatment with the AMPA receptor antagonist CNQX (25 microM). Surprisingly, addition of 5 mM pilocarpine reduced the number of TUC-4-positive cells, just as combining pilocarpine with the neurogenesis-stimulating compounds, prevented or reduced the increase of TUC-4-positive cells. None of the treatments were found to induce dentate granule cell death within the observed period. Labeling of dividing cells by adding 5-bromo-2-deoxyuridine (BrdU) to the culture medium did not result in cells double-labeled with BrdU and TUC-4. The induced increase in TUC-4-positive cells therefore represent neuronal differentiation of existing neural precursor cells when investigated at the 3-day time point. We conclude that 3 days treatment of 2-week-old hippocampal slice cultures with IGF-I and EGF and NMDA and AMPA glutamate receptor antagonists increase granule cell neurogenesis from preexisting neural precursors.

Functional and molecular clues reveal precursor-like cells and immature neurones in the turtle spinal cord

In lower vertebrates, some cells contacting the central canal (CC) retain the ability to proliferate, leading the reconstruction of the spinal cord after injury. A better understanding about the nature of these cells could contribute to the development of novel strategies for spinal cord repair. Here, by combining light and electron microscopy, immunocytochemistry and patch-clamp recordings, we provide evidence supporting the presence of precursor-like cells and immature neurones contacting the CC of juvenile turtles. A class of cells expressed the ependymal and glial cell marker S100 and displayed morphological and electrophysiological features of radial glia: relatively low input resistance, high resting potential, lack of active membrane properties and extensive dye-coupling. A second class of S100 reactive cells were characterized by a higher input resistance and outward rectification. Finally, some CC-contacting cells expressed HuC/D - a marker of immature neurones - and fired action potentials. The coexistence of cells with functional properties of precursor-like cells and immature neurones suggests that the region surrounding the CC is a site of active neurogenesis. It remains to be demonstrated by lineage analysis whether, as in the embryonic cerebral cortex, radial glia are the progenitor cells in the turtle spinal cord.

Development of GABA-immunoreactive cells in the spinal cord of the sea lamprey,P. marinus

The lamprey spinal cord increases in length and size during all its life cycle; thus, it is expected that new cells will be generated. This expectation suggests that the locomotor circuits must be continuously remodeled. Key elements in the cellular network controlling locomotor behavior are inhibitory cells. Here, we studied the gamma-aminobutyric acid-immunoreactive (GABA-ir) cells in the lamprey spinal cord during postembryonic development. Three major populations of GABA-ir cells were identified according to their distribution: those located in the gray matter, those contacting the cerebrospinal liquid (LC cells), and those located in the white matter. The results show (1). the number of GABA-ir cells per segment increase from prolarvae (<10 mm) to adulthood; (2). the lower number of GABA-ir cells in 100 microm of spinal cord is 66 +/- 7, found in premetamorphic larvae, and the highest is 107 +/- 6, found in postmetamorphic animals; (3). the gray matter and LC GABA-ir cells show different variations in number depending on the developmental period. Thus, in the 10-mm larvae, the gray matter GABA-ir cells are more abundant than LC cells, whereas in the young postmetamorphic specimens, the contrary occurs. Most of the GABA-ir cells located in the white matter were classified as edge cells. They increase in number from the beginning of the prolarval period, where there are not white matter-positive cells, to the middle larval period, where there are 9 +/- 4 GABA-ir edge cells per segment. This value was unaltered in later periods, where GABA-ir edge cells represent 20-30% of the total number of edge cells per segment. The increase in number of GABA-ir cells in these populations during a specific point of the lamprey life cycle may indicate different inhibitory requirements of the locomotor circuit at different developmental periods.

Environment temperature affects cell proliferation in the spinal cord and brain of juvenile turtles

The spinal cords and brains--comprising dorsal cortex (DC), medial cortex (MC) and diencephalon (Dien)--of juvenile turtles acclimated to warm temperature [27-30 degrees C; warm-acclimated turtles (WATs)] revealed higher density values of bromodeoxyuridine-labeled cells (BrdU-LCs) than those acclimated to a cooler environment [5-14 degrees C; cold-acclimated turtles (CATs)]. Both populations were under the influence of the seasonal daily light-dark rhythms. Pronounced differences between WATs and CATs (independent t-test; confidence level, P<0.01) were found in the central area of the spinal gray matter and in the ependymal epithelium lining the brain ventricles. Forebrain regions (DC, MC and Dien) also revealed significant differences between WATs and CATs (independent t-test; confidence level, P<0.01-0.05). Unexplored biological clocks that may be affecting cell proliferation were equalized by performing paired experiments involving one WAT and one CAT. Both animals were injected on the same day at the same time and both were sacrificed 24 h later. These experiments confirmed that a warm environment increased cell proliferation in the CNS of turtles. Double- and triple-labeling experiments involving anti-BrdU antibody together with anti-glial protein antibodies revealed that temperature modulates not only cell populations expressing glial markers but also other cells that do not express them. As expected, in the case of short post-injection (BrdU) surviving time points, no cells were found colabeling for BrdU and NeuN (neuronal marker). The probable direct effect of temperature on the cell division rate should be analyzed together with potential indirect effects involving increased motor activity and increased food intake. The fate of the increased BrdU-LCs (death, permanence as progenitor cells or differentiation following neuronal or glial lines) remains a matter for further investigation. Results are discussed in the light of current opinions concerned with post-natal neurogenesis in vertebrates.