Regulation of neuron numbers in Xenopus laevis: effects of hormonal manipulation altering size at metamorphosis

Activational vs. organizational effects of sex steroids and their role in the evolution of reproductive behavior: Looking to foot-flagging frogs and beyond

Sex steroids play an important role in regulation of the vertebrate reproductive phenotype. This is because sex steroids not only activate sexual behaviors that mediate copulation, courtship, and aggression, but they also help guide the development of neural and muscular systems that underlie these traits. Many biologists have therefore described the effects of sex steroid action on reproductive behavior as both "activational" and "organizational," respectively. Here, we focus on these phenomena from an evolutionary standpoint, highlighting that we know relatively little about the way that organizational effects evolve in the natural world to support the adaptation and diversification of reproductive behavior. We first review the evidence that such effects do in fact evolve to mediate the evolution of sexual behavior. We then introduce an emerging animal model - the foot-flagging frog, Staurois parvus - that will be useful to study how sex hormones shape neuromotor development necessary for sexual displays. The foot flag is nothing more than a waving display that males use to compete for access to female mates, and thus the neural circuits that control its production are likely laid down when limb control systems arise during the developmental transition from tadpole to frog. We provide data that highlights how sex steroids might organize foot-flagging behavior through its putative underlying mechanisms. Overall, we anticipate that future studies of foot-flagging frogs will open a powerful window from which to see how sex steroids influence the neuromotor systems to help germinate circuits that drive signaling behavior. In this way, our aim is to bring attention to the important frontier of endocrinological regulation of evolutionary developmental biology (endo-evo-devo) and its relationship to behavior.

Nitric oxide synthase expression and cell changes in dorsal root ganglia and spinal dorsal horn of developing and adultRana esculenta indicate a role of nitric oxide in limb metamorphosis

Metamorphosis of amphibians requires reconfiguration of sensory and locomotor neural networks. In view of such plastic changes and implications of nitric oxide (NO) in neural developmental shaping, we examined via histochemistry and immunohistochemistry its synthetic enzyme nitric oxide synthase (NOS) in dorsal root ganglia (DRGs) and dorsal horn of the developing and adult frog Rana esculenta. In limb DRGs, NOS positivity was first and selectively detected just before limb bud appearance, increased during metamorphosis, and was then down-regulated. In adulthood, NOS was expressed in some DRG neurons at all segmental levels. Similar features were detected in the dorsal horn neuropil. In limb DRGs, cell counts in Nissl-stained sections revealed a twofold increase of differentiated neurons during metamorphosis and an additional twofold increase in adulthood. Perikaryal sizes in limb DRGs did not vary during metamorphosis but increased and were more heterogeneous in the adult frog, probably reflecting adaptation to body size. NOS and cell changes during metamorphosis were much less marked in DRGs at other levels. Carbocyanine tracing documented selective labeling of NOS-expressing hindlimb DRG neurons from the spinal nerve at the time of initiation of hindlimb movements. The findings show that, in limb DRG neurons, NOS parallels cell differentiation and limb development during metamorphosis. The data also provide evidence of NOS expression in DRG cells innervating the hindlimbs when sensorimotor circuits become functionally mature. This study indicates a key role of NO production in the maturation of sensory functions that subserves in amphibians the transition from swimming to tetrapod locomotion.

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.

Breed differences in deafferentation-induced neuronal cell death and shrinkage in chick cochlear nucleus

Removal of functional presynaptic input can result in a variety of changes in postsynaptic neurons in the central nervous system, including altered metabolism, changes in neuronal cell size, and even death of the postsynaptic cell. Age-dependent neuronal cell death and shrinkage has been documented in second order auditory neurons in the chick brainstem (nucleus magnocellularis, NM) following cochlea removal (Born and Rubel, 1985. J. Comp. Neurol. 231, 435-445). Here we examined whether the extent of neuronal cell death and shrinkage is also breed-dependent. We performed unilateral cochlea removal on both hatchling and adult birds of either a broiler breed (Arbor Acres Cross) or egg layer breed (Hy-Line, H and N) and killed birds one week later. Changes in neuronal cell number and cross sectional area were determined from Nissl-stained sections. We observed 25% neuronal cell loss and a 15-20% decrease in neuronal cross sectional area after cochlea removal in either broiler or egg layer hatchling birds. In adult birds, however, neuronal cell loss is breed-dependent. Adult egg layer birds lose an average of 37% of NM neurons after cochlea removal, while adult broiler birds show no cell loss. In both breeds of adult birds, cochlea removal results in a 20% decrease in neuronal cross sectional area. These results suggest that analysis of differences between breeds as well as ages of birds will prove fruitful in determining how afferent input controls neuronal survival and metabolism.

Variation and symmetry in the lumbar and thoracic dorsal root ganglion cell populations of newly metamorphosed Xenopus laevis

The sizes of the lumbar and thoracic dorsal root ganglion cell populations in normally developing newly metamorphosed Xenopus laevis were measured in order to determine whether these neuron populations have the same characteristics as the hindlimb motoneuron population (i.e., large individual as well as sibling group differences, striking bilateral symmetry, and a rough correspondence between neuron number and body size that suggests some peripheral control of cell number during normal development (Sperry, J. Comp. Neurol. 264:250-267). Among animals from three sibling groups, the total numbers of thoracic and lumbar ganglion cells are highly variable and symmetrical, although symmetry is not uniformly present at the level of individual ganglion pairs. Significant sibling group differences in neuron number are also present. Metamorphic body size and cell number in the thoracic but not in the lumbar ganglia are significantly correlated. The motoneurons innervating the hindlimbs were also counted and measured in the same animals. While variable as well as symmetrical, motoneuron number and metamorphic body size are correlated in only two of the three sibling groups. Interestingly, the numbers of motoneurons and lumbar ganglion cells, two populations of neurons whose sizes one might predict would be significantly correlated in normally developing animals, are not correlated. The relationship between these observations and currently held views concerning how neuron numbers might be controlled during normal development is discussed.

Effects of an ectopic hindlimb on the brachial motoneurons in Xenopus

A forelimb bud of Xenopus tadpoles was replaced with the much larger hindlimb but at developmental stage 50, prior to the onset of the normal period of motoneuron death. At the conclusion of the motoneuron death period, there were generally no significant differences between the total numbers and nuclear area distributions of the brachial motoneurons supplying the ectopic hindlimb, and the remaining forelimb. It was concluded that factors in addition to the amount of muscle, or premuscle in the limb may be important in determining the totals and sizes of surviving motoneurons.

Neuronal overload in the developing anuran lateral motor column in response to limb removal and thyroid hormone

The lateral motor column (LMC) in the anuran spinal cord normally undergoes a dramatic reduction in motor neuron number during development. At least two factors influence this process: the limb target which is required for the progression of cell loss, and thyroid hormone, a requisite for metamorphosis. This study has examined the relative and combined effects of limb amputation and exogenous thyroxine, initiated at the onset of normal rapid cell loss in Rana pipiens tadpoles, in regulating neuron number in the lumbosacral LMC. Thyroxine treatment or unilateral limb amputation temporarily resulted in significantly more LMC neurons than in untreated controls. Extraordinary numbers of motor neurons persisted through metamorphic climax when both treatments were combined. Population sizes frequently exceeded the maximum number of neurons observed prior to the onset of natural cell loss. Moreover, thyroxine-treated tadpoles contained increased numbers of mitotic figures in the ventricular zone of the spinal cord and significantly more newly generated cells in the LMC, as revealed by 3H-thymidine autoradiography. These findings suggest that thyroxine-potentiated mitogenesis promotes greater numbers of new motor neurons to the LMC while, simultaneously, target removal delays the loss of extant cells. It is proposed that this interaction effectively maintains an immature state in the LMC so that neuronal "decisions" for survival and the consequent loss of target-deprived neurons are postponed far longer than previously reported.

Lumbar lateral motor column development in triploid Xenopus laevis

The effects of increasing ploidy on the development of the lumbar lateral motor column (L-LMC) in Xenopus laevis were investigated in order to determine how early events contribute to producing the significant difference in the average number of motoneurons present in diploid and triploid animals after cell death (Sperry: J. Comp. Neurol. 277:499-508, '88). From naturally occurring diploid and experimentally produced triploid siblings at two stages prior to significant amounts of neuronal cell death, at one stage during the peak period of cell death, and at one stage after cell death, the L-LMC motoneurons were counted and nuclear cross-sectional areas were measured. At stages before and after cell death, the average nuclear cross-sectional areas of motoneurons and of other cells that were also measured were greater in the triploids, while the average number of motoneurons and motoneuron density (the mean number of cells per section) were less. Average body size and average motor column length in diploid and triploid animals were equal at each of the stages. The general characteristics of L-LMC development that have been widely noted in diploids, an increase in cell size accompanied by a decrease in cell number, were also observed in the triploid animals. However, not only were these general features present in the triploids, but the increase in average motoneuron size and the decrease in average motoneuron number in diploids and triploids were roughly equal when scaled to the general differences in nuclear size or to the difference in the average number of motoneurons present prior to cell death.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of increasing ploidy on the lumbar lateral motor column and hindlimb of newly metamorphosed Xenopus laevis: a comparison of diploid and triploid siblings

This study was undertaken to determine how increasing ploidy in Xenopus laevis affected the size of the lumbar lateral motor column (L-LMC) motoneuron population, the size of representative hindlimb muscles, and the relationship between these features in animals at the completion of metamorphosis. Triploids were produced by exposing fertilized diploid eggs to increased hydrostatic pressure. In the triploids, L-LMC motoneuron number was significantly reduced and motoneuron nuclear cross-sectional area was significantly increased. Both L-LMC length and the total L-LMC size (neuron number x mean nuclear size) were roughly equal in diploids and triploids. No ploidy-related differences in fiber number were observed in two representative thigh muscles. In diploid animals, motoneuron number is significantly correlated with both muscle fiber number and with body size. The latter two variables are also significantly correlated with one another, making it possible that a feature related to muscle fiber number or one related to body size or both are significant in determining motoneuron number. In triploid animals, motoneuron number was significantly correlated with body size but not with muscle fiber number. This suggests that the feature significant in determining motoneuron number may be one related to body size rather than to muscle fiber number. If a feature related to muscle fiber number were the primary determinant of motoneuron number, one would have expected in addition similar average changes in the two variables in comparing diploids and triploids. That this was not observed provides further reason to suspect muscle fiber numbers may not be a primary determinant of motoneuron number. In both diploids and triploids, total L-LMC size (a value combining neuron number and neuron size) was highly correlated with body size, but again, not with muscle fiber number. The average total L-LMC size and the average body size were equal in diploids and triploids while average motoneuron number was significantly different. What this suggests is that in discussing possible mechanisms to account for correspondences between central and peripheral sizes, the relevant variable for the former may be total L-LMC size rather than motoneuron number.

The development of cholinergic neurons

Motoneuron precursors acquire some principles of their spatial organization early in their cell lineage, probably at the blastula stage. A predisposition to the cholinergic phenotype in motoneurons and some neural crest cells is detectable at the gastrula to neurula stages. Cholinergic expression is evident upon cessation of cell division. Cholinergic neurons can synthesize ACh during their migration and release ACh from their growth cones prior to target contact or synapse formation. Neurons of different cell lineages can express the cholinergic phenotype, suggesting the importance of secondary induction. Early cholinergic commitment can be modified or reversed until later in development when it is amplified during interaction with target. Motoneurons extend their axons and actively sort out in response to local environmental cues to make highly specific connections with appropriate muscles. The essential elements of the matching mechanism are not species-specific. A certain degree of topographic matching is present throughout the nervous system. In dissociated cell culture, most topographic specificity is lost due to disruption of local environmental cues. Functional cholinergic transmission occurs within minutes of contact between the growth cone and a receptive target. These early contacts contain a few clear vesicles but lack typical ultrastructural specializations and are physiologically immature. An initial stabilization of the nerve terminal with a postsynaptic AChR cluster is not prevented by blocking ACh synthesis, electrical activity, or ACh receptors, but AChR clusters are not induced by non-cholinergic neurons. After initial synaptic contact, there is increasing deposition of presynaptic active zones and synaptic vesicles, extracellular basal lamina and AChE, and postjunctional ridges over a period of days to weeks. There is a concomitant increase in m.e.p.p. frequency, mean quantal content, metabolic stabilization of AChRs, and maturation of single channel properties. At the onset of synaptic transmission, cell death begins to reduce the innervating population of neurons by about half over a period of several days. If target tissue is removed, almost all neurons die. If competing neurons are removed or additional target is provided, cell death is reduced in the remaining population. Pre- or postsynaptic blockade of neuromuscular transmission postpones cell death until function returns.(ABSTRACT TRUNCATED AT 400 WORDS)

Excessive numbers of axons after early enucleation and blockade of metamorphosis in the oculomotor nerve of Xenopus laevis

The number of axons in the oculomotor (OM) nerve of Xenopus laevis tadpoles was counted in unoperated and in unilaterally enucleated animals, raised in 0.4 g/l thiourea (TU), a thyroid-blocking agent, which arrested their development at premetamorphosis. In unoperated animals the number of axons starts to decrease with metamorphosis. When raised in TU, the tadpoles do not metamorphose and show no axon loss; rather, there is a moderate increase in axon number (13%) after 6 months of thiourea-treatment. Thus metamorphosis is necessary for the initiation of axon loss. In unilaterally enucleated tadpoles, increased axon loss occurs at metamorphosis in the OM nerve of the operated side, but the contralateral OM nerve shows no loss at all. When these animals are treated with TU, there is, as compared with the effects of the TU-treatment in unoperated animals, an increase of 7% in the ipsilateral side and of 28% in the contralateral one. Thus thyroxine blockade prevents the effects of unilateral enucleation and induces an excessive number of axons during the period observed. We postulate that through blockade of metamorphosis, axon elimination is arrested, while their production continues, and that these effects are accentuated by manipulation of the axonal target.

Relationship between natural variations in motoneuron number and body size in Xenopus laevis: a test for size matching

During normal development, tadpoles of Xenopus laevis demonstrate large variations in body size that are carried through metamorphosis. This variation in size exists at the stages when lumbar lateral motor column (L-LMC) motoneurons are produced and when neuronal cell death in this neuron population occurs. Body size, hindlimb size, motoneuron number, and motoneuron size (i.e., neuron nuclear cross-sectional area) were measured in animals from three developmental stages: one prior to significant amounts of cell death, one at the peak rate of cell death, and one after cell death. The hypothesis that neuron population size is matched to peripheral size was tested by using the natural size variation found at each of these stages. The ranges of values for the measurements at the three stages were large. Significant correlations between body size and motoneuron number, as well as between motoneuron number and muscle fiber number, were present after cell death. Since these correlations emerged as cell death reduced neuron numbers, size matching may have occurred and cell death may have adjusted the L-LMC motoneuron population's size to variation in body size. In addition to the correlations between body size and motoneuron number at the end of cell death, neuron numbers before and after cell death were significantly correlated among groups of siblings. The possibility that the number of neurons after cell death was also influenced by differences in the number of L-LMC progenitors is discussed.

Motoneuron number in the lumbar lateral motor column of larval and adult bullfrogs

Motoneuron number in the lumbar lateral motor column of the bullfrog, Rana catesbeiana, was investigated through the course of premetamorphic development and in postmetamorphic frogs. Motoneurons were distinguished on the basis of histological characteristics into two classes, type L (less differentiated) and type M (more differentiated). The number of type L motoneurons on each side showed a precipitous decline between stages V and VI (6,300 to 2,500) and a slower rate of loss until stage XI (to 550). Type M motoneurons increased in number between stages V and VII (560 to 2,775) and declined precipitously between stages VII and VIII to a value similar to that of juvenile frogs (1,100). These changes in motoneuron number do not correspond to the formation of myotubes or to the appearance of contractile properties in hindlimb muscles. The development of myotubes in the hindlimb occurs only after total motoneuron number has declined by 35%. Similarly, hindlimb muscle contraction develops after the early decline in type L motoneuron number and is restricted to proximal thigh at the peak of type M motoneuron number. In postmetamorphic frogs, a weak (r = 0.44) but statistically significant correlation was found between type M motoneuron number and body length. In the largest frogs (greater than 15 cm body length), 1262 +/- 157 (mean +/- s.d.) motoneurons were present, whereas the smallest frogs (less than 5 cm body length) had 1099 +/- 98 motoneurons. These results are not consistent with previous findings that the variance of motoneuron number among small frogs is greater than that among larger frogs. The present results are thus inconsistent with explanations of size-related differences in motoneuron number that are based on selection of small frogs with greater number of motoneurons for survival. The increase in motoneuron number may be due to a slow addition of newly born motoneurons to the LMC or to the differentiation of existing motoneurons. The latter possibility is supported by the finding that the number of presumptive type L profiles is less in larger frogs.