Analysis of moving supernumerary limbs of Xenopus laevis

What determines motor neuron number? Slow scaling of facial motor neuron numbers with body mass in marsupials and primates

How does the number of motor neurons in the brain correlate with the muscle mass to be controlled in the body? Numbers of motor neurons are known to be adjusted during development by cell death, but the change in the percentage of surviving motor neurons in response to experimental changes in target muscle mass is relatively small. Here we address the quantitative matching between final numbers of motor neurons in the facial nucleus and body mass (which we use as a proxy for the muscle mass). In 22 marsupial species, we found that the number of facial motor neurons is strongly correlated with body mass, and scales across species as a power function of body mass with a very small exponent of 0.184, which is close to the exponent found in primates from previously published data. With such an exponent, doubling the body mass is accompanied by a modest increase of only 14% in numbers of facial motor neurons, while halving body mass results in a decrease of only 12%. These numbers are remarkably similar to the 15-20% increase or 8% decrease in the number of spinal cord motor neurons that results from experimental or natural doubling or reducing by half the target muscle field of birds and amphibians. The scaling rule presented here might thus account for the quantitative matching of motor neurons to their target muscle mass in evolution. With this small scaling exponent, our data also raise the possibility that larger animals will have larger motor units.

Motoneuron and muscle fibre counts in normal and bilaterally innervated Xenopus hindlimbs

Motoneurons of the Lumbar Lateral Motor Column (LMC) and muscle fibres of gastrocnemius and tibialis anterior were counted in juvenile Xenopus frogs, including normal animals and those reared with a single bilaterally innervated hindlimb (monopodal frogs). In many monopodal frogs, the single hind limb becomes hyperinnervated by a large number of motoneurons on the contralateral side in addition to the normal ipsilateral number, even after the completion of cell death. In frogs with hyper-innervated limbs, muscle fibre number was evaluated beyond that expected for a normal population, but this increase was not commensurate with the quantity of extra innervation. When taken together with previous findings which showed that the supporting capacity of individual fibres was not elevated, the conclusion is that the ability of the single limb to support extra motoneurons cannot be completely explained by a commensurate increased proliferation of muscle fibres resulting from the operation or the bilateral innervation. The results give further evidence against the hypothesis that motoneuron numbers are controlled solely by peripheral competition. The study also provides evidence that muscle fibre numbers are regulated in part by the quantity of motor innervation received by the limb.

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.

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.

Quantitative compensation by lateral motor column neurons in response to four functional hindlimbs in a frog tadpole

An analysis of the lateral motor columns of a frog tadpole with 4 functional unilateral hindlimbs revealed a 58% increase in the number of neurons on the overloaded side as compared to the control side of the spinal cord. An accompanying, apparently selective, increase in the number of very large neurons suggests that spinal motor centers respond to peripheral overload through a variable combination of neuronal number and size in providing for adequate innervation.

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

Xenopus laevis tadpoles reared in a 0.01% solution of 6-n-propyl-2-thiouracil (PTU) are blocked in their development at larval stage 54 but continue to increase in size. When released from the effects of PTU they metamorphose into frogs of sizes significantly larger than those of their untreated siblings. Using this size difference to examine the hypothesis that neuron numbers are matched to the size of their postsynaptic targets during neuronal cell death, we measured the following on stage 66 frogs metamorphosing from PTU-treated and untreated tadpoles: lumbar lateral motor column (L-LMC) motoneuron number and mean nuclear cross-sectional area; thoracic and lumbar dorsal root ganglion (DRG) cell number and mean nuclear cross-sectional area; and muscle fiber number in two representative thigh muscles. A few measurements of neuron number and cell size were also made on untreated and PTU-treated stage 54 tadpoles. The most striking correlations observed were not between peripheral size and neuron numbers but between peripheral size and neuron size. Motoneuron numbers were not increased in the PTU-treated animals, perhaps because the increase in peripheral size involved an increase in muscle fiber diameter rather than an increase in muscle fiber number. Thoracic DRG cell number, but not the sum of thoracic and lumbar DRG cell numbers, was increased. In general, our findings do not support the hypothesis that neuron numbers are matched to peripheral size by a process regulating the amount of cell death that occurs during metamorphic stages in Xenopus laevis.

Hyperplasia in the spinal sensory system of the frog. I. Plasticity in the most caudal dorsal root ganglion

Increases in the amount of periphery available for innervation have been achieved by the unilateral removal of hindlimb dorsal root ganglion (DRGs) in Rana pipiens, a procedure which generally results in a compensatory cell number increase (hyperplasia) in the DRGs which remain. We have found that the hyperplastic response is extremely variable, and we have investigated various factors which might control its production. Our findings indicate, however, that the pattern of DRGs removed, the animal's age at the time of removal, and the survival period are not strictly related to the production of hyperplasia in hindlimb DRGs. Special emphasis has been placed on DRG 10, the caudalmost DRG which normally innervates the cloaca and sends a small projection to the hindlimb. This DRG displayed dramatic cell number increases of up to 564%. In addition, several unique features of the hyperplastic response have been observed in DRG 10. This DRG showed increases in cell number on both the operated and the unoperated sides. It showed hyperplasias in animals subjected to ganglionectomy past metamorphosis as well as during larval development. Finally the production of DRG 10 hyperplasias exclusively occurred in male pre- and postmetamorphic animals. To account for these distinctive features of DRG 10 hyperplasia, baseline studies of the normal course of proliferation and cell death in DRG 10 were undertaken. They reveal no fundamental developmental differences between DRG 10 and other hindlimb DRGs. Other mechanisms responsible for these unusual features of developmental plasticity in DRG 10 are discussed.

Postmetamorphic changes in the lumbar lateral motor column in relation to muscle growth in the toad, Bufo americanus

Motoneuron number and size (nuclear cross-sectional area) were measured from serially sectioned spinal cords of Bufo americanus to investigate the relation between changes in the lumbar lateral motor column (L-LMC) and postmetamorphic increases in hindlimb muscle fiber number. Previous studies of neuron number in a variety of anuran species reported a correlation between number and body size, suggesting the possible addition of neurons during growth. Our results show a poor correlation between motoneuron number and body size with at most a 25% increase in neuron number occurring over the body size range where previous work had shown a hindlimb muscle fiber increase of ten to 20-fold. Thus, most new muscle fibers must be incorporated into motor units that exist at metamorphosis. Motoneurons, but not ependymal cells, showed a significant size increase with increasing body size; this is perhaps related to an increased motor unit size that results from axonal sprouting. The range of variation of L-LMC cell numbers in newly metamorphosed toads was nearly equal to that of all other toads examined. This suggests that the weak correlations between motoneuron number and size observed in this and previous studies may be due to differential survival of individuals with larger neuron populations rather than to postmetamorphic addition of motoneurons. Our findings also show a strong bilateral correlation of motoneuron numbers, a finding suggesting that factors other than peripheral size may be important in regulating motoneuron number.

Topographic and morphometric effects of bilateral embryonic eye removal on the optic tectum and nucleus isthmus of the leopard frog

Rana pipiens were raised through metamorphosis after extirpation of both eye primordia at Shumway embryonic stage 17 (Shumway '40). The visual connections between the isthmic nuclei and the optic tectum were examined in these animals using horseradish peroxidase (HRP) histochemistry. Isthmo-tectal projections are normally aligned with the primary retinotectal map. We asked whether these connections would develop normal topographic organization in the absence of normal retinal input. HRP was formed into a solid pellet (congruent to 200-500 micrometer diameter) and inserted into one tectal lobe on the tip of a fine metal probe. The procedure produced relatively restricted retrograde label in somas and dendrites in both isthmi nuclei. In the nucleus isthmus ipsilateral to the tectal lobe receiving the HRP pellet, processes of tecto-isthmi neurons were labeled by anterograde transport. The topography of the isthmo-tectal and tecto-isthmic projections were identical in the developmentally enucleated animals and in normal frogs, even though eye removal severely reduced the volume of the optic tecta and the isthmi nuclei. Thus our analyses indicate that retinal contacts do not play an active role in the development of the positional or polarity cues that are involved in "mapping" projections between central visual nuclei. These results are discussed in the context of peripheral specification of central connections and in terms of models that have recently been proposed to explain the development of the retinotectal system.

Location of motoneurons supplying individual muscles in normal and grafted supernumerary limbs of Xenopus laevis

The purpose of this study was to investigate innervation of transplanted supernumerary hindlimbs in the frog (Xenopus laevis). Motoneurons innervating identified muscles in normal and supernumerary limbs were located by the method of retrograde transport of HRP after intramuscular injection. In the lumbar spinal cord of normal Xenopus, motoneurons supplying medial hindlimb muscles, which are derived from the ventral muscle mass during development, are located at the medial end of the motor column; those innervating lateral, dorsally-derived muscles, lie at the lateral end of the motor column. In animals with supernumerary limbs, motoneurons supplying the transplant usually occupied the same mediolateral position as those supplying the same muscle in the normal limb. However, the rostrocaudal location of these motor pools exhibited greater flexibility. When the transplant was innervated by a rostral nerve of the lumbar plexus, motoneurons supplying gastrocnemius could be located in a region of the spinal cord whose motoneurons do not normally innervate this muscle. There is thus no rigid requirement that gastrocnemius motoneurons be located at specific segmental levels. Motoneurons supplying gastrocnemius in the normal limb on the experimental side showed normal rostrocaudal distributions, indicating little rearrangement of these motor pools. Dorsal root ganglion cells labeled after HRP injection could be concentrated in a ganglion which normally supplies little or no innervation to the injected muscle. The location of these cells confirmed the segmental source of sensory innervation of the extra limb; i.e., there was no stray innervation. Animals with supernumerary limbs exhibited little or no increase in the number of motoneurons on the extra limb side. In contrast, dorsal root ganglion cell populations exhibited a large increase on the experimental side.

Movements of supernumerary hindlimbs after innervation by single lumbar spinal nerves of Xenopus laevis

Lumbar spinal nerves S8, S9, and S10, together innervating normal hindlimbs in Xenopus laevis, were tested to cause coordinated movements in grafted hindlimbs. It could be shown that this ability is mainly restricted to lumbar nerve S9.

[Examination of Weiss's modulation hypothesis in regenerating hind limb nerves ofXenopus laevis tadpoles]

1. In tadpoles ofXenopus laevis (stage 56), either the sciatic or the crural nerve was transsected in the right hind limb ("Ortsbein"). The central nerve stump was deviated and connected with the peripheral sciatic nerve stump of theautoplastically transplanted left hind limb. 2. Up to final observations between stage 62 and stage 66 the grafts developed "herkunftsgemäß" into normal, differentiated hind limbs. In all cases the deviated nerves grew into the grafts by regeneration and established the characteristic branching pattern of the sciatic nerve. 3. In grafted hind limbsone type of positional defect arises alongside normally placed hind limbs: permanent extension in the hip and knee joints. This positional defect may have been caused by the blocking syndrome, due to the operation. There is no explanation for functional defects in the hip and knee joints of normally placed hind limbs. The result of innervation of the nerves grown into the graft is judged on the basis of the motor function originating in the crurotarsal joint of the grafts, because of positional and functional defects in proximal joints. 4. Motor function in the crurotarsal joint is more complete after innervation by the sciatic nerve than after innervation by the crural nerve. The sciatic nerve can moreover produce coordinated swimming movements in the crurotarsal joints of all grafts. After innervation by the crural nerve onlyun-coordinated movements are to be seen in the crurotarsal joints. These results demonstrate that the corresponding motor neurons, originally innervating hip flexors, are not capable of myotypic response. The modulation hypothesis of Weiss can therefore be rejected in this case.

A comparison of the reflex organization of thoracic and lumbar segments in the frog spinal cord

Both lumbar dorsal root (DR) primary afferents and descending fibers in the lateral column (LC) make monosynaptic connections with ipsilateral lumbar motoneurons in the frog spinal cord. We have determined that LC fibers also make monosynaptic connections with thoracic motoneurons, while thoracic primary afferents do not. The central reflex time (CRT) for the lumbar DR-VR pathway was 2.5 +/- 0.3 msec, but the CRT for the thoracic DR-VR pathway was 8.5 +/- 2.6 msec. By using a conditioning-test paradigm we have been able to determine that the earliest sign of segmental synaptic transmission in thoracic motoneurons occurs only after a delay of 6.1 +/- 0.9 msec. These results correlate very well with the different morphological characteristics of lumbar and thoracic segments. We have also investigated the central organization of lumbar and thoracic segments and found that the segmental polysynaptic input to motoneurons is more diffuse in thoracic than in lumbar segments. The intersegmental reflexes between thoracic and lumbar segments provide additional evidence for a more diffuse organization in thoracic segments.

Reduction of the naturally occurring motor neuron loss by enlargement of the periphery

Motor hyperplasia following the enlargement of the periphery by implantation of a supernumerary leg is not due to "remote control" of proliferation, as shown by motor neuron counts in 6-day chick embryos. We have tested the alternative hypothesis that we are dealing with reduction of the naturally occurring cell death. In normal development, the lumbar lateral motor column (l.m.c.) undergoes motor neuron degeneration resulting in a cell loss of at least 40%, which occurs between six and one-half and nine and one-half days. Following transplantation of supernumerary legs, cases selected for vigorous motility showed a numerical difference between experimental and contralateral (control) sides amounting to +11.0% to +27.5%. The transplants were innervated by varying combinations of thoracic and rostral lumbar nerves. We interpret our data in terms of survival of motor neurons which normally would have failed in a competition at the periphery but which were sustained by the enlarged peripheral fields. Our data do not permit a decision between the two alternatives: competition for synaptic sites or for a trophic agent. The surviving motor neurons are not limited to the rostral segments of the motor column but in most instances distributed along its entire rostro-caudal extent, implying a redistribution of all l.m.c. axons. The term "hyperplasia" is no longer appropriate for the phenomenon under consideration and should be replaced by the term "hypothanasia.""