Transient effects of nerve injury on estimates of sensory neuron number in juvenile bullfrog

Neurogenesis in the lamprey central nervous system following spinal cord transection

After spinal cord transection, lampreys recover functionally and axons regenerate. It is not known whether this is accompanied by neurogenesis. Previous studies suggested a baseline level of nonneuronal cell proliferation in the spinal cord and rhombencephalon (where most supraspinal projecting neurons are located). To determine whether cell proliferation increases after injury and whether this includes neurogenesis, larval lampreys were spinally transected and injected with 5-bromo-2&prime-deoxyuridine (BrdU) at 0-3 weeks posttransection. Labeled cells were counted in the lesion site, within 0.5 mm rostral and caudal to the lesion, and in the rhombencephalon. One group of animals was processed in the winter and a second group was processed in the summer. The number of labeled cells was greater in winter than in summer. The lesion site had the most BrdU labeling at all times, correlating with an increase in the number of cells. In the adjacent spinal cord, the percentage of BrdU labeling was higher in the ependymal than in nonependymal regions. This was also true in the rhombencephalon but only in summer. In winter, BrdU labeling was seen primarily in the subventricular and peripheral zones. Some BrdU-labeled cells were also double labeled by antibodies to glial-specific (antikeratin) as well as neuron-specific (anti-Hu) antigens, indicating that both gliogenesis and neurogenesis occurred after spinal cord transection. However, the new neurons were restricted to the ependymal zone, were never labeled by antineurofilament antibodies, and never migrated away from the ependyma even at 5 weeks after BrdU injection. They would appear to be cerebrospinal fluid-contacting neurons.

Stereological and allometric studies on neurons and axo-dendritic synapses in superior cervical ganglia

The superior cervical ganglion (SCG) plays an important role in neuropathies including Horner's syndrome, stroke, and epilepsy. While mammalian SCGs seem to share certain organizational features, they display natural differences related to the animal size and side and the complexity and synaptic coverage of their dendritic arborizations. However, apart from the rat SCG, there is little information concerning the number of SCG neurons and synapses, and the nature of relationships between body weight and the numbers and sizes of neurons and synapses remain uncertain. In the recognition of this gap in the literature, in this chapter, we reviewed the current knowledge on the SCG structure and its remodeling during postnatal development across a plethora of large mammalian species, focusing on exotic rodents and domestic animals. Instrumentally, we present stereology as a state-of-the-art 3D technology to assess the SCG 3D structure unbiasedly and suggest future research directions on this topic.

SCG postnatal remodelling--hypertrophy and neuron number stability--in Spix's yellow-toothed cavies (Galea spixii)

Whilst a fall in neuron numbers seems a common pattern during postnatal development, several authors have nonetheless reported an increase in neuron number, which may be associated with any one of a number of possible processes encapsulating either neurogenesis or late maturation and incomplete differentiation. Recent publications have thus added further fuel to the notion that a postnatal neurogenesis may indeed exist in sympathetic ganglia. In the light of these uncertainties surrounding the effects exerted by postnatal development on the number of superior cervical ganglion (SCG) neurons, we have used state-of-the-art design-based stereology to investigate the quantitative structure of SCG at four distinct timepoints after birth, viz., 1-3 days, 1 month, 12 months and 36 months. The main effects exerted by ageing on the SCG structure were: (i) a 77% increase in ganglion volume; (ii) stability in the total number of the whole population of SCG nerve cells (no change--either increase or decrease) during post-natal development; (iii) a higher proportion of uninucleate neurons to binucleate neurons only in newborn animals; (iv) a 130% increase in the volume of uninucleate cell bodies; and (v) the presence of BrdU positive neurons in animals at all ages. At the time of writing our results support the idea that neurogenesis takes place in the SCG of preás, albeit it warrants confirmation by further markers. We also hypothesise that a portfolio of other mechanisms: cell repair, maturation, differentiation and death may be equally intertwined and implicated in the numerical stability of SCG neurons during postnatal development.

Cell proliferation in the lamprey central nervous system

After spinal cord transection, axons regenerate both in larval and adult lampreys. It is not known to what degree cells proliferate, even in the uninjured animal. Therefore, we have determined the prevalence of mitosis in the lamprey central nervous system (CNS). Bromodeoxyuridine (BrdU) was injected and incorporated for 4 hours into 2- to 5-year-old larvae, animals undergoing metamorphosis, and young adults. Labeled cells were counted in the rhombencephalon (where most supraspinal projecting neurons are located) and spinal cord. A mitotic index (MI) was calculated as the percentage of nuclei that were labeled. There was a seasonal variation in mitotic activity, with higher MIs occurring in summer. Within the summer, there was an additional transient spike in mitosis, especially in the rhombencephalon. There was no correlation between age and MI within the range of developmental stages examined. Baseline MIs in the rhombencephalon and spinal cord were approximately 0.15% and 0.20%, respectively. In most animals, the highest mitotic rates in both the rhombencephalon and spinal cord were seen in the ependyma, but many labeled cells were found in nonependymal regions as well. During the summer spike, almost all of the additional mitosis in the rhombencephalon was in the ependyma, but this finding was not true in the spinal cord. Many BrdU-labeled cells in the spinal cord and rhombencephalon were also stained by monoclonal antibodies specific for lamprey glial keratin but were never labeled by anti-neurofilament antibodies. These results suggest that (1) neurogenesis is uncommon in the lamprey CNS; (2) during most of the year, baseline gliogenesis occurs mainly in the ependyma with substantial contribution by nonependymal areas. During the summer, a spike of mitotic activity occurs in the ependyma of the rhombencephalon and throughout the spinal cord.

Neuroprotective Effect of Deprenyl in Sensory Neurons of Axotomized Dorsal Root Ganglion

Spinal motoneuron neuroprotection by deprenyl was previously reported; the present study was carried out to evaluate neuroprotectivity in the dorsal root ganglion sensory neuron. The total neuron counts were calculated, and the axotomized sensory neurons of the dorsal root ganglion were significantly lower than those of the unaxotomized sides. Three secondary and three tertiary parameters were used. The secondary parameters were: the percentages of sensory neuron increase at the axotomized side (PNIA) and at the unaxotomized side (PNIU), and the percentage of neuronal response (PNR). The tertiary parameters were: the percentages of maximal response at the axotomized side (PMRA) and at the unaxotomized side (PMRU), and the percentage of maximal relative response (PMRR). Nonlinear statistical analysis using Gaussian, quadratic and logistic models of the tertiary parameters suggested that the data were bell-shape, which indicated that the data were biphasic. The data were divided into ascending and descending sets, and linear regression. They were analyzed according to Bent-hyperbola model and the ascending set was considered as a neurotrophic phase, while the descending one as a neurotoxic phase. The slops of PMRA were higher than that of PMRU, which indicates that the axotomized neurons were more sensitive than the unaxotomized neurons to the protective and neurotoxic effect of deprenyl. Moreover, the results showed that deprenyl had a proliferative effect on the dorsal root ganglion sensory neuron.

Sensory neuron addition in juvenile rat: time course and specificity

The time course and specificity of neuron addition to lumbar dorsal root ganglia (DRGs) L(4)-L(6) of rats was investigated. By using methods validated by three-dimensional reconstructions, profile counts in paraffin sections of nucleoli within a nucleus were 36% greater in 100-day-old (P100) rats than in 1-day-old (P1) rats. Adult values were reached by P50. Added neurons fell disproportionately into the population of neurons whose size was below that of the mean size within the ganglion. The biochemical characteristics of small neurons were used to determine whether added neurons fall into particular subpopulations. In DRGs, L(3) and L(4), the number of neurons immunoreactive to substance P (SP) or calcitonin-gene-related peptide (CGRP) or that bound the lectin isolectin B4 (IB4) was determined. Between P5 and P100, the number of SP-stained neurons increased by 2,280 (40% increase), CGRP-stained neurons increased by 6,080 (70% increase), and IB4-stained neurons increased by 6,900 (90% increase). The increase in the number of neurons stained for CGRP or IB4 was more than twice the number of neurons found to be added to these ganglia, indicating that coexpression of these markers as well as neuron number may be developmentally regulated during postnatal life.

Increase in descending brain-spinal cord projections with age in larval lamprey: implications for spinal cord injury

The purpose of this study was to determine whether new descending brain-spinal cord projections are added with age in larval lamprey and might contribute substantially to restoration of these projections following spinal cord injury. Retrograde horseradish peroxidase (HRP) labeling of descending brain neurons was performed in "young" and "old" larval lamprey that differed in age by at least one year. In old larval lamprey, significantly more descending brain neurons projected to specific rostral levels of the spinal cord than in young animals. Furthermore, in young and old lamprey, the main morphological change in Müller and Mauthner cells was an increase in soma size. The major conclusion from the present study is that in larval lamprey, some new brain-spinal cord projections are added with age that could be due to axonal elongation by preexisting brain neurons and/or descending projections from new neurons (i.e., neurogenesis or maturation of incompletely differentiated neurons). Following spinal cord transections, the numbers of descending projections were not significantly different than those in normal, unlesioned animals. Thus, some new descending projections are added with age, but at a relatively slow rate, and the rate does not appear to be affected significantly by spinal cord transections. Together, the present results and those from our recent double-labeling study suggest that following spinal cord transection in larval lamprey, axonal regeneration by descending brain neurons, rather than the relatively slow addition of new brain-spinal cord projections with age, probably accounts for the majority of restored projections and recovery of locomotor function

Neurogenesis in postnatal mouse dorsal root ganglia

Neurogenesis continues in various regions of the central nervous system (CNS) throughout life. As the mitogen basic fibroblast growth factor (bFGF) can proliferate neuronal precursors of CNS neurons in culture, and is also upregulated within adult dorsal root ganglia following axotomy, it is possible that the postnatal dorsal root ganglia contain bFGF-responsive neuronal precursors. We undertook cell culture of postnatal mouse dorsal root ganglia to demonstrate neurogenesis. Basic FGF induced a cellular proliferative response in dorsal root ganglia cell culture. After 2 weeks in serum-free medium containing bFGF, neurons were rarely observed. However, following removal of bFGF and addition of trophic factors, many cells were observed that morphologically resembled dorsal root ganglia neurons, stained for neuronal markers, and generated action potentials. Furthermore, bromodeoxyuridine, used as a marker of cytogenesis, was detected in neurofilament-160(+) and/or microtubule-associated protein-2(+) cells that morphologically resembled neurons. In addition to bFGF, epidermal growth factor, nerve growth factor, and sonic hedgehog were also capable of generating spherical cell clusters that contained cells that stained for neuronal markers following the addition of trophic factors. These results suggest that early postnatal dorsal root ganglia contain neural precursors that appear to proliferate in response to various factors and can then be induced to differentiate into neurons. In conclusion, the existence of neural precursors and the possibility of neurogenesis in postnatal dorsal root ganglia may provide a greater range of plasticity available to somatosensory systems during growth or following injury, perhaps to replace ineffectual or dying neurons.

Neuron addition and enlargement in juvenile and adult animals

Locomotion requires bilateral symmetry of neural circuitry in the spinal cord. Although not well understood, the mechanisms responsible for establishing and maintaining this symmetry must balance the numbers, sizes, and connectivity of the neurons on both sides of the spinal cord. Those mechanisms do not cease to function after embryogenesis, since there is substantial evidence that these properties continue to change as juvenile animals grow to adult size. We review the evidence that spinal neuron number and size increase in growing juvenile frogs and mammals. We postulate that these increases are regulated by both local and systemic factors. In addition, we discuss evidence that axotomy of spinal sensory and motor neurons also enlists local and systemic regulatory factors, some of which may also be operative in normal growth and development.