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Neuron addition and neurogenesis are not interchangeable terms.

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THE ANATOMICAL RECORD (NEW ANAT.) 265:159, 2001
LETTERS TO THE EDITOR
Neuron Addition and Neurogenesis Are Not
Interchangeable Terms
G
euna et al. (2000) call attention to the possibility that recent findings of neurogenesis
in the central nervous system of adult
animals could be applicable also in
the periphery, particularly to the dorsal root ganglion (DRG). Citing the
century-long history of reports of increases in DRG neurons with age,
these investigators call for more research into the possibility of continuous neurogenesis of sensory neurons.
However, in doing so, Geuna et al.
(2000) assume that neurogenesis is
the only explanation for increased
numbers of neurons. Evidence is accumulating that late differentiation of
committed but immature neurons accounts for neuron addition.
Results obtained in both amphibians and mammals fail to support the
possibility of neurogenesis in the
DRG. In the bullfrog (Rana catesbeiana), neuron number increases twoto threefold in lumbar spinal DRGs
(St. Wecker and Farel, 1994) and in
sympathetic ganglia (St. Wecker et al.,
1995) and approximately 40% among
motoneurons (Farel, 1987; Farel et al.,
1993) as juvenile frogs grow to adult
size. Yet, extensive searches for evidence that [3H]thymidine is incorporated into neurons during the period
of neuron addition provided no evidence for neurogenesis. Putative incompletely differentiated neurons
have been identified among motor
(Farel, 1987; Farel et al., 1993) and
sensory (Meeker and Farel, 1997) neuron pools. Among motoneurons,
quantitative studies show an inverse
© 2001 Wiley-Liss, Inc.
relationship between the numbers of
incompletely differentiated motoneurons and the number of patent motoneurons. In the DRG, the increase in
the number of sensory neuron somata
occurs in the absence of increases in
the number of peripherally or centrally projecting axons (Meeker and
Farel, 1997).
Studies in mammals have shown
that increases in DRG neuron number
occur in rat (Cecchini et al., 1993;
Popken and Farel, 1997) but not in
mouse (Berg and Farel, 2000). In rat,
as in frog, no evidence has been found
for neurogenesis in the DRGs of juvenile or adult animals (Cecchini et al.,
1999; Ciarioni et al., 2000). Negative
evidence cannot be used to rule out a
hypothesis, but in the absence of confirmatory data, alternative explanations must be considered.
Geuna et al. (2000) are well justified
in their enthusiasm for recent reports
of neuron addition in the DRGs of
postnatal rat, but it is important that
their call for more research into the
mechanisms of neuron addition in
DRG not be limited to the possibility
of neurogenesis.
LITERATURE CITED
Berg JS, Farel PB. 2000. Developmental
regulation of sensory neuron number
and limb innervation in the mouse. Dev
Brain Res 29:21-30.
Cecchini T, Cuppini R, Ciaroni S, Del
Grande P. 1993. Increased number of
dorsal root ganglion neurons in vitaminE-deficient rats. Somatosens Mot Res 10:
433-443.
Cecchini T, Ferri P, Ciaroni S, Cuppini R,
Ambrogini P, Papa S, Del Grande P.
1999. Postnatal proliferation of DRG
non-neuronal cells in vitamin E-deficient
rats. Anat Rec 256:109-115.
Ciaroni S, Cecchini T, Cuppini R, Ferri P,
Ambrogini P, Bruno C, Del Grande P.
2000. Are there proliferating neuronal
precursors in adult rat dorsal root ganglia? Neurosci Lett 281:69-71.
Farel PB. 1987. Motoneuron number in the
lumbar lateral motor column of larval
and adult bullfrogs. J Comp Neurol 261:
266-276.
Farel PB, Wray SE, Meeker ML. 1993.
Size-related increase in motoneuron
number: evidence for late differentiation. Dev Brain Res 71:169-179.
Geuna S, Borrione P, Fornaro M, Giacobini-Robecchi MG. 2000. Neurogenesis
and stem cells in adult mammalian dorsal root ganglia. Anat Rec (New Anat)
261:139-140.
Meeker ML, Farel PB. 1997. Neuron addition during growth of the postmetamorphic bullfrog: sensory neuron and axon
number. J Comp Neurol 389:569-576.
Popken GJ, Farel PB. 1997. Sensory neuron number in neonatal and adult rats
estimated by means of stereologic and
profile-based methods. J Comp Neurol
386:8-15.
St. Wecker PG, Farel PB. 1994. Hindlimb
sensory neuron number increases with
body size. J Comp Neurol 342:430-438.
St. Wecker PG, Baek JK, Farel PB. 1995.
Principal neurons of the lumbar sympathetic ganglia increase in number with
body size. J Comp Neurol 357:117-123.
Paul B. Farel
Department of Cell and Molecular
Physiology, CB7545
School of Medicine
University of North Carolina
Chapel Hill, NC 27599-7545
Fax: 919-966-6927
E-mail: farel@med.unc.edu
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