Influence of reduced neuron pool on the magnitude of naturally occurring motor neuron death

Cell loss in the inferior olive of the staggerer mutant mouse is an indirect effect of the gene

Staggerer (sg) is an autosomal recessive mutation in mouse that causes severe cerebellar atrophy. In this mutant, the Purkinje cell (PC) number is reduced by about 75% and the remaining Purkinje cells have a reduced dendritic arbor and an ectopic location. Previous analysis of staggerer chimeras has demonstrated that the Purkinje cell phenotypes are all direct consequences of the cell-autonomous action of the staggerer gene. The two major afferents to the Purkinje cell are also affected. Virtually all of the granule cells die by the end of the first postnatal month. This death, however has been shown to be an indirect consequence of mutant gene action. The second major afferent system is from the cells of the inferior olive that projects to the main trunks of the Purkinje cell dendrite via the climbing fiber system. Quantitative studies of cell number in the inferior olive have shown that the number of cells is reduced by about 62% in adult sg/sg mutants. We report here the results of our quantitative analysis of three staggerer chimeras. beta-glucuronidase activity was used as an independent cell marker. Our findings demonstrate that inferior olive cell death in staggerer mutant mice is an indirect effect of staggerer gene action. Thus as for the granule cells, the loss of olivary neurons most likely results from a target related cell death.

Neurotrophic interactions in the development of spinal cord motoneurons

The final number of spinal cord motoneurons is attained by a two-step process involving the proliferation of precursor cells and the loss by cell death of a proportion (approximately 50%) of the post-mitotic neurons. Although the mechanisms responsible for the proliferation of stereotyped numbers of motoneurons are not understood, considerable evidence from in vitro as well as in vivo studies indicates that the second step in attaining population size (cell death) is controlled by the interaction of motoneurons with both their efferent targets and their afferent inputs. Target influences on motoneuron survival are thought to be regulated by muscular activity and by competition for limited amounts of neurotrophic factors derived from striated skeletal muscles. However, evidence that such putative neurotrophic factors actually modulate motoneuron survival in vivo has been lacking. Using crude and partially purified extracts from embryonic hindlimbs (Days 8-9) we have found that the treatment of chick embryos in ovo with these agents during the normal cell death period (Days 5-10) rescues a significant number of motoneurons from degeneration. Kidney or lung extracts and heat-inactivated hindlimb extracts were ineffective. The survival-inducing activity of partially purified extract was dose dependent and developmentally regulated. The survival of sensory, sympathetic and a population of cholinergic sympathetic preganglionic neurons was unaffected by treatment with hindlimb extract. The massive motoneuron death that occurs after early target (hindlimb) removal was partially ameliorated by daily treatment with the hindlimb extract. Survival-inducing activity of the extract is lost after trypsin treatment. Taken collectively these results indicate that a target-derived protein or polypeptide neurotrophic factor is involved in the regulation of motoneuron survival in vivo.

Influence of the postsynaptic target on the functional properties of neurons in the adult mammalian central nervous system

In this review we have attempted to summarize present knowledge concerning the regulatory role of target cells on the expression and maintenance of the neuronal phenotype during adulthood. It is well known that in early developmental stages the survival of neurons is maintained by specific neurotrophic factors derived from their target tissues. Neuronal survival is not the only phenotype that is regulated by target-derived neurotrophic factors since the expression of electrophysiological and cytochemical properties of neurons is also affected. However, a good deal of evidence indicates that the survival of neurons becomes less dependent on their targets in the adult stage. The question is to what extent are target cells still required for the maintenance of the pre-existing or programmed state of the neuron; i.e., what is the functional significance of target-derived factors during maturity? Studies addressing this question comprise a variety of neuronal systems and technical approaches and they indicate that trophic interactions, although less apparent, persist in maturity and are most easily revealed by experimental manipulation. In this respect, research has been directed to analyzing the consequences of disconnecting a group of neurons from their target-by either axotomy or selective target removal using different neurotoxins-and followed (or not) by the implant of a novel target, usually a piece of embryonic tissue. Numerous alterations have been described as taking place in neurons following axotomy, affecting their morphology, physiology and metabolism. All these neuronal properties return to normal values when regeneration is successful and reinnervation of the target is achieved. Nevertheless, most of the changes persist if reinnervation is prevented by any procedure. Although axotomy may represent, besides target disconnection, a cellular lesion, alternative approaches (e.g., blockade of either the axoplasmic transport or the conduction of action potentials) have been used yielding similar results. Moreover, in the adult mammalian central nervous system, neurotoxins have been used to eliminate a particular target selectively and to study the consequences on the intact but target-deprived presynaptic neurons. Target depletion performed by excitotoxic lesions is not followed by retrograde cell death, but targetless neurons exhibit several modifications such as reduction in soma size and in the staining intensity for neurotransmitter-synthesizing enzymes. Recently, the oculomotor system has been used as an experimental model for evaluating the functional effects of target removal on the premotor abducens internuclear neurons whose motoneuronal target is destroyed following the injection of toxic ricin into the extraocular medial rectus muscle. The functional characteristics of these abducens neurons recorded under alert conditions simultaneously with eye movements show noticeable changes after target loss, such as a general reduction in firing frequency and a loss of the discharge signals related to eye position and velocity. Nevertheless, the firing pattern of these targetless abducens internuclear neurons recovers in parallel with the establishment of synaptic contacts on a presumptive new target: the small oculomotor internuclear neurons located in proximity to the disappeared target motoneurons. The possibility that a new target may restore neuronal properties towards a normal state has been observed in other systems after axotomy and is also evident from experiments of transplantation of immature neurons into the lesioned central nervous system of adult mammals. It can be concluded that although target-derived factors may not control neuronal survival in the adult nervous system, they are required for the maintenance of the functional state of neurons, regulating numerous aspects of neuronal structure, chemistry and electro-physiology.(ABSTRUCT TRUNCATED)

Apoptosis of the midline glia during Drosophila embryogenesis: a correlation with axon contact

We have examined cell death within lineages in the midline of Drosophila embryos. Approximately 50% of cells within the anterior, middle and posterior midline glial (MGA, MGM and MGP) lineages died by apoptosis after separation of the commissural axon tracts. Glial apoptosis is blocked in embryos deficient for reaper, where greater than wild-type numbers of midline glia (MG) are present after stage 12. Quantitative studies revealed that MG death followed a consistent temporal pattern during embryogenesis. Apoptotic MG were expelled from the central nervous system and were subsequently engulfed by phagocytic haemocytes. MGA and MGM survival was apparently dependent upon proper axonal contact. In embryos mutant for the commissureless gene, a decrease in axon-glia contact correlated with a decrease in MGA and MGM survival and accelerated the time course of MG death. In embryos mutant for the slit gene, MGA and MGM maintained contact with longitudinally and contralaterally projecting axons and MG survival was comparable to that in wild-type embryos. The initial number of MG within individual ventral nerve cord segments was increased by ectopic expression of the rhomboid gene, without changing axon number. Extra MGA and MGM were eliminated from the ventral nerve cord by apoptosis to restore wild-type numbers of midline glia. Ectopic rhomboid expression also shifted MGA and MGM cell death to an earlier stage of embryogenesis. One possible explanation is that axon-glia contact or communication promotes survival of the MG and that MG death may result from a competition for available axon contact.

Sixth Annual Stuart Reiner Memorial Lecture: embryonic development of nerve and muscle

This article provides a basic scheme of sequential anatomic and some physiologic events occurring during the course of embryonic development of motor neurons and muscles, leading to the establishment of mature nerve-muscle relationships. Motor neurons and muscles begin their development independently and during embryogenesis they become dependent on each other for further development and survival. Aspects of development which occur independently and those requiring mutual interactions are identified. The development of motor neurons is discussed with respect to their production, projection, neuromuscular transmission, myelination, sprouting, survival, and death. The development of muscles is discussed with respect to the origin, differentiation, and muscle fiber types. Discussion on the development of neuromuscular junction includes differentiation of presynaptic nerve terminal, postsynaptic components, and elimination of multiple axons.

The survival of developing neurons: a review of afferent control

Developmental cell death is a major event of neurogenesis, and emphasis has systematically been placed on the roles of either the peripheral targets or central postsynaptic neurons in the control of neuronal survival. In this article, the main types of experimental design used to test the control of neuronal death by the afferent supply are compared with analogous data indicating neurotrophic support by the targets. It is argued that targets and afferents may have equivalent roles and interact in the control of neuron numbers during development of the vertebrate nervous system. Possible mechanisms of anterograde trophic control include contact-mediated cell interactions, activity-dependent processes mediated by neurotransmitters or neuromodulators, modulation of the levels of cytoplasmic free calcium and the involvement of neurotrophic factors.

The role of target size in neuronal survival

A loss of about half of the trochlear motor neurons occurs during the course of normal development in duck and quail embryos. The role of the size of the target muscle in controlling the number of surviving motor neurons was examined by making motor neurons innervate targets either larger or smaller in size than their normal target. In one experiment the smaller trochlear motor neuron pool of the quail embryo was forced to innervate the larger superior oblique muscle of the duck embryo. This was accomplished by grafting the midbrain of a quail embryo in the place of the midbrain of a duck embryo. Results indicated that no additional quail trochlear motor neurons were rescued in spite of a considerable increase in target size. In another experiment the larger trochlear motor neuron pool of the duck embryo was made to innervate the smaller superior oblique muscle of the quail embryo. This resulted in loss of some additional neurons; however, the number of surviving motor neurons was not proportionate to the reduction in target size. These experiments failed to provide support for the hypothesis that the size of the target muscle controls the number of surviving motor neurons. Although contact with target is necessary for survival of neurons, factors other than the number or size of target cells are involved in the control of motor neuron numbers during development.

The relationship of intramuscular nerve branching and synaptogenesis to motoneuron survival

The target has been considered for some time to play a major role in allowing neurons to survive the period of naturally occurring cell death. For the motoneurons that innervate the chick limb, evidence is presented that suggests access to target-derived trophic factor via intramuscular nerve branches and synapses may be important in regulating neuronal survival. Alterations in branching and synapse formation produced by activity blockade as well as by alteration of adhesion molecule function are shown to result in changes in motoneuron survival consistent with the proposed hypothesis. The relevance of these observations to the numerical-matching hypothesis of vertebrate neuronal cell death is also considered.

Influence of altered afferent input on the number of trochlear motor neurons during development

A loss of about half of the trochlear motor neurons occurs during the course of normal development. The present investigation was undertaken to examine the role of afferent input in regulating the number of surviving or dying trochlear motor neurons. A majority of the afferent input to the trochlear nucleus comes from the vestibular nuclei of the hindbrain via the medial longitudinal fasciculus. Portions of the hindbrain were lesioned in duck embryos on embryonic day 3, considerably prior to the time motor neurons send their axons out and cell death begins. The effectiveness of hindbrain lesion was verified by electron microscopical examination of synapses. There was a significant decrease in the number of synapses on trochlear motor neurons following hindbrain lesion. Cell counts made after the period of cell death indicated a significant decrease in the final number of surviving trochlear motor neurons. Cell counts made prior to the onset of cell death indicated that there was a drastic reduction in the initial number of trochlear motor neurons produced in hindbrain lesion embryos. In spite of a significant reduction in the initial number of neurons, the percentage loss of neurons was about the same as during normal development. Since trochlear motor neurons are generated prior to the formation of afferent synapses on them, it is unlikely that the reduction in the number of motor neurons initially produced is due to reduced afferent synaptic input. Since the percentage of cell loss in hindbrain lesion and normal embryos is about the same, it seems that the magnitude of cell death is genetically programmed.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of grafting a smaller target muscle on the magnitude of naturally occurring trochlear motor neuron death during development

About half of the motor neurons produced by some neural centers die during the course of normal development. It is thought that the size of the target muscle determines the number of surviving motor neurons. Previously, we tested the role of target size in limiting the number of survivors by forcing neurons to innervate a larger target (Sohal et al., '86). Results did not support the size-matching hypothesis because quail trochlear motor neurons innervating duck superior oblique muscle were not rescued. We have now performed the opposite experiment, i.e., forcing neurons to innervate a smaller target. By substituting the embryonic forebrain region of the duck with the same region of the quail before cell death begins, chimera embryos were produced that had a smaller quail superior oblique muscle successfully innervated by the trochlear motor neurons of the duck. The number of surviving trochlear motor neurons in chimeras was significantly higher than in the normal quail but less than in the normal duck. The smaller target resulted in some additional loss of neurons, suggesting that the target size may regulate neuron survival to a limited extent. Failure to achieve neuron loss corresponding to the reduction in target size suggests that there must be other factors that regulate neuron numbers during development.

Synapse formation on trochlear motor neurons in relation to naturally occurring cell death during development

About half of the trochlear motor neurons die during the course of normal development. The present study was undertaken to determine whether the afferent synapses form before the onset of motor neuron death and also to determine whether the number of synapses differs between the healthy and degenerating trochlear motor neurons. Brains of duck embryos from days 10 to 20 were prepared for quantitative electron microscopical observations on synaptogenesis. Results indicate that synapses form on the trochlear motor neuron soma before cell death begins suggesting that afferent input is in a position to exert an influence on survival or death of motor neurons. There were no significant differences in the number of synapses between the healthy and dying neurons during the period of cell death. This observation suggests that the mechanism by which afferent synapses could be involved in neuron survival or death is not related to the number of synapses on the cell soma. The number of synapses on the cell process, synaptic transmission and/or molecules released at the synapses are likely candidates for the mechanism of action of afferent input.

Cell loss in the inferior olive of the staggerer mutant mouse is an indirect effect of the gene

Staggerer (sg) is an autosomal recessive mutation in mouse that causes severe cerebellar atrophy. In this mutant, the Purkinje cell (PC) number is reduced by about 75% and the remaining Purkinje cells have a reduced dendritic arbor and an ectopic location. Previous analysis of staggerer chimeras has demonstrated that the Purkinje cell phenotypes are all direct consequences of the cell-autonomous action of the staggerer gene. The two major afferents to the Purkinje cell are also affected. Virtually all of the granule cells die by the end of the first postnatal month. This death, however, has been shown to be an indirect consequence of mutant gene action. The second major afferent system is from the cells of the inferior olive that project to the main trunks of the Purkinje cell dendrite via the climbing fiber system. Quantitative studies of cell number in the inferior olive have shown that the number of cells is reduced by about 62% in adult sg/sg mutants. We report here the results of our quantitative analysis of three staggerer chimeras. beta-glucuronidase activity was used as an independent cell marker. Our findings demonstrate that inferior olive cell death in staggerer mutant mice is an indirect effect of staggerer gene action. Thus, as for the granule cells, the loss of olivary neurons most likely results from a target related cell death.

Synapse formation on quail trochlear neurons transplanted in duck embryos before naturally occurring motor neuron death

About half of the trochlear motor neurons in duck and quail embryos die during normal development. In a previous study the role of target muscle in controlling the number of surviving motor neurons was investigated by reducing the number of neurons innervating the muscle. This was accomplished by removing the midbrain of the duck embryo and grafting in its place the midbrain of the quail embryo before motor neuron death begins. It was observed that the number of surviving trochlear motor neurons in the quail-duck chimera embryos was not significantly different from that of the normal quail. The present investigation was undertaken to determine whether trochlear motor neurons in the chimera embryos received afferent synapses. Brains of duck, quail and chimera embryos on days 16 and 20 were processed for electron microscopical observations. Synapses formed on motor neurons of the chimera embryos. Surprisingly, synapses on motor neurons of quail differed from those of duck, both qualitatively and quantitatively. Synapses on the motor neurons of the chimera embryos developed in a fashion similar to that for the duck motor neurons. Our failure to rescue trochlear motor neurons in the chimera embryos suggests that the developing motor neurons may respond to a larger target muscle only if they received a normal complement of afferent synaptic input.

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.

Numerical matching in the mammalian CNS: lack of a competitive advantage of early over late-generated cerebellar granule cells

In this report we use postnatal 3H-thymidine injections to test whether granule cells that are generated early in postnatal cerebellar development and whose axons have access to their Purkinje cell target beginning in the first postnatal week have an advantage over granule cells generated 9 days later in the competition for target-related stabilization. In the wild-type mouse, 3-5% of the adult granule cell population is labeled by injection of 3H-thymidine at either postnatal day 4 (P4) or P13. In the lurcher mutant, however, over 40% of the surviving granule cells are labeled by P4 injection while less than 1% are labeled after a P13 injection. Together, these results suggest that time of target contact is a critical factor in the competition for neuronal survival. The results from the lurcher chimeras, however, reveal that the situation is likely to be more complicated. In all chimeras examined, with target sizes ranging from 3 to 108% of wild type, equivalent numbers of granule cells were labeled at P4 and P13. These data lead to the contradictory conclusion that, in this experimental situation, early generated granule cells do not have a competitive advantage over later-generated granule cells. The results are discussed in terms of various models of target stabilization. We propose that, of the various hypotheses, our results are best explained by postulating two distinct mechanisms for developmental cell death. Supporting evidence for this hypothesis from other neuronal systems is also briefly reviewed.

The regulation of intramuscular nerve branching during normal development and following activity blockade

In vertebrates, approximately 50% of the lumbosacral motoneurons die during a short period of development that coincides with synaptogenesis in the limb. Although it has been postulated that these motoneurons die because they fail to obtain adequate trophic support from the muscles, it is not clear how this factor is supplied. The mechanism by which activity blockade prevents motoneurons cell death is also unknown. In order to begin to understand the nature of these proposed trophic interactions, we have examined the temporal sequence of axonal invasion and ramification within two muscles of the chick hindlimb, the predominantly slow iliofibularis and the fast posterior iliotibialis, during the cell death period. We found striking differences in intramuscular nerve ingrowth and branching between fast and slow muscle. We also observed differences in the molecular composition of fast and slow myotubes that may contribute to the nerve pattern differences. In addition, we observed a progressive increase in the degree of intramuscular nerve fasciculation as well as a precise temporal sequence of nerve branching. The earliest detectable response to chronic curarization was a dramatic decrease in the degree of intramuscular nerve fasciculation. Activity blockade also greatly enhanced nerve branching within the muscles from the time that nerve branches normally formed, and, additionally, interfered with the normal cessation of axon growth. Our results support the idea that nerve endings are the sites of trophic uptake. Furthermore, although our results do not allow us to exclude other activity-dependent influences on motoneuron survival, they suggest the following testable hypotheses: (1) the normal regulation of motoneuron survival may result from the precise control of intramuscular nerve branching, (2) activity blockade may increase motoneuron survival by enhancing intramuscular nerve branching, and (3) anything which affects this complex process of nerve branching may also alter motoneuron survival.

Afferent control of neuron numbers in the developing brain

In this study we tested whether the quantitative matching of developing neuronal populations may depend on the size of the afferent supply. Partial deafferentation of the middle division of the parabigeminal nucleus (PBm) was produced before the period of naturally occurring cell death, by reducing the neuronal population of the superior colliculus following partial lesions or eye removal. The number of neurons surviving cell death in the PBm was linearly related to the number of its afferent neurons. This result supports the hypothesis that neurotrophic control by the afferent supply during the period of natural neuronal death is a major determinant of the number of neurons in the developing brain.

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)

Quantitative relationships between motoneuron and muscle development in Xenopus laevis: implications for motoneuron cell death and motor unit formation

A common approach to the study of neural regression has been to correlate the timing of cell loss with other events such as target development. Most of these studies have areas of uncertainty. First, the analysis is normally carried out on groups of neurons that innervate a variety of targets. Second, there are some doubts about the reliability of light microscopic quantitation of muscle development. In this study, the period of cell death in the semimembranosus motor pool of Xenopus laevis has been estimated and correlated with an electron microscopic study of the development of the semimembranosus. The period of cell death of semimembranosus motoneurons was estimated on the basis of their position in the spinal cord and from the number of myelinated axons in the semimembranosus motor nerve. The semimembranosus motor pool contained approximately 70 motoneurons and was located 17-37% along the rostrocaudal axis of the lumbar cord. Cell loss from this motor pool occurred between stages 53-54 and 56, whereas cell death in the entire lumbar cord extended beyond stage 58. Primary myogenesis occurred between stages 53 and 54 in the semimembranosus. There was then a hiatus in myotube production until secondary myogenesis began around stage 56. It is concluded that secondary myotubes are not involved in regulating motoneuron cell death and that the number of primary myotube clusters is similar in magnitude to the number of motoneurons that will ultimately survive the period of cell death. The implications of these observations for theories of cell death and motor unit formation are discussed.

Mammalian motoneuron development: effect of peripheral deprivation on motoneuron numbers in a marsupial

In nonmammalian vertebrates, the survival of developing motoneurons is dependent on their contacting appropriate target cells. It is generally accepted that developing mammalian motoneurons have a similar dependency on their target, but as yet there is little experimental evidence to support this contention. This is mainly because of the difficulty of experimenting on eutherian embryos. We have, therefore, been studying neuronal development in the tammar (an Australian marsupial) as its nervous system is immature at birth. Radical or partial removal of hindlimb buds from newborn tammars resulted in an increased motoneuron cell death. The motoneurons which survived in the operated tammars did so by innervating muscle remnants. In the instances where a group of muscles was totally removed, the corresponding motonuclei appeared to be totally lost. This study supports the hypothesis that mammalian motoneurons must contact their appropriate muscle in order to survive through the period of natural neuronal cell death.

Development of postsynaptic-like specializations of the neuromuscular synapse in the absence of motor nerve

It was previously reported that the acetylcholine receptor clusters and acetylcholinesterase appear on embryonic superior oblique muscle cells developing in vivo without motor nerve contacts. The objective of this study was to examine whether some other components of neuromuscular junction also form on muscle cells developing in vivo in the absence of motor neurons. In the present study, postsynaptic specializations such as junctional folds, postsynaptic density and basal lamina were studied in normal and aneural muscles. The superior oblique muscle of duck embryos was made aneural by permanent destruction of trochlear motor neurons by cauterizing midbrain on embryonic day 7; 3 days before the motor neurons normally project their axons into the muscle. Normal and aneural muscles from embryonic days 10 to 25 were processed for electron microscopy. The results indicate that morphological specializations such as junction-like folds, postsynaptic-like density, and basal lamina also develop in the absence of motor neuron contacts. Whether the differentiation of specialized synaptic basal lamina is dependent on the presence of motor neurons was examined by utilizing a monoclonal antibody against heparan sulfate proteoglycan. Immunohistochemical studies indicate that specialized synaptic basal lamina differentiates in the absence of motor neurons. Thus, the mechanism of development of postsynaptic components of neuromuscular junction in this muscle is not dependent on motor neuron contacts. These results also suggest that the postsynaptic cell plays a more active role in synapse formation than previously realized. The results are discussed in relation to the control of synapse numbers by the postsynaptic cell.

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.