Birth dates of trigeminal motoneurons and metamorphic reorganization of the jaw myoneural system in frogs

Development of central respiratory control in anurans: The role of neurochemicals in the emergence of air-breathing and the hypoxic response

Physiological and environmental factors impacting respiratory homeostasis vary throughout the course of an animal's lifespan from embryo to adult and can shape respiratory development. The developmental emergence of complex neural networks for aerial breathing dates back to ancestral vertebrates, and represents the most important process for respiratory development in extant taxa ranging from fish to mammals. While substantial progress has been made towards elucidating the anatomical and physiological underpinnings of functional respiratory control networks for air-breathing, much less is known about the mechanisms establishing these networks during early neurodevelopment. This is especially true of the complex neurochemical ensembles key to the development of air-breathing. One approach to this issue has been to utilize comparative models such as anuran amphibians, which offer a unique perspective into early neurodevelopment. Here, we review the developmental emergence of respiratory behaviours in anuran amphibians with emphasis on contributions of neurochemicals to this process and highlight opportunities for future research.

Respiratory motoneuron properties during the transition from gill to lung breathing in the American bullfrog

Amphibian respiratory development involves a dramatic metamorphic transition from gill to lung breathing and coordination of distinct motor outputs. To determine whether the emergence of adult respiratory motor patterns was associated with similarly dramatic changes in motoneuron (MN) properties, we characterized the intrinsic electrical properties of American bullfrog trigeminal MNs innervating respiratory muscles comprising the buccal pump. In premetamorphic tadpoles (TK stages IX-XVIII) and adult frogs, morphometric analyses and whole cell recordings were performed in trigeminal MNs identified by fluorescent retrograde labeling. Based on the amplitude of the depolarizing sag induced by hyperpolarizing voltage steps, two MN subtypes (I and II) were identified in tadpoles and adults. Compared with type II MNs, type I MNs had larger sag amplitudes (suggesting a larger hyperpolarization-activated inward current), greater input resistance, lower rheobase, hyperpolarized action potential threshold, steeper frequency-current relationships, and fast firing rates and received fewer excitatory postsynaptic currents. Postmetamorphosis, type I MNs exhibited similar sag, enhanced postinhibitory rebound, and increased action potential amplitude with a smaller-magnitude fast afterhyperpolarization. Compared with tadpoles, type II MNs from frogs received higher-frequency, larger-amplitude excitatory postsynaptic currents. Input resistance decreased and rheobase increased postmetamorphosis in all MNs, concurrent with increased soma area and hyperpolarized action potential threshold. We suggest that type I MNs are likely recruited in response to smaller, buccal-related synaptic inputs as well as larger lung-related inputs, whereas type II MNs are likely recruited in response to stronger synaptic inputs associated with larger buccal breaths, lung breaths, or nonrespiratory behaviors involving powerful muscle contractions.

Xenopus pancreas development

Understanding how the pancreas develops is vital to finding new treatments for a range of pancreatic diseases, including diabetes and pancreatic cancer. Xenopus is a relatively new model organism for the elucidation of pancreas development, and has already made contributions to the field. Recent studies have shown benefits of using Xenopus for understanding both early patterning and lineage specification aspects of pancreas organogenesis. This review focuses specifically on Xenopus pancreas development, and covers events from the end of gastrulation, when regional specification of the endoderm is occurring, right through metamorphosis, when the mature pancreas is fully formed. We have attempted to cover pancreas development in Xenopus comprehensively enough to assist newcomers to the field and also to enable those studying pancreas development in other model organisms to better place the results from Xenopus research into the context of the field in general and their studies specifically. Developmental Dynamics 238:1271-1286, 2009. (c) 2009 Wiley-Liss, Inc.

Behavioral transformations during metamorphosis: remodeling of neural and motor systems

During insect metamorphosis, neural and motor systems are remodeled to accommodate behavioral transformations. Nerve and muscle cells that are required for larval behavior, such as crawling, feeding and ecdysis, must either be replaced or respecified to allow adult emergence, walking, flight, mating and egg-laying. This review describes the types of cellular changes that occur during metamorphosis, as well as recent attempts to understand how they are related to behavioral changes and how they are regulated. Within the periphery, many larval muscles degenerate at the onset of metamorphosis and are replaced by adult muscles, which are derived from myoblasts and, in some cases, remnants of the larval muscle fibers. The terminal processes of many larval motoneurons persist within the periphery and are essential for the formation of adult muscle fibers. Although most adult sensory neurons are born postembryonically, a subset of larval proprioceptive neurons persist to participate in adult behavior. Within the central nervous system, larval neurons that will no longer be necessary die and some adult interneurons are born postembryonically. By contrast, all of the adult motoneurons, as well as some interneurons and modulatory neurons, are persistent larval cells. In accordance with their new behavioral roles, these neurons undergo striking changes in dendritic morphology, intrinsic biophysical properties, and synaptic interactions.

Metamorphosis in drosophila and other insects: the fate of neurons throughout the stages

The nervous system of insects is profoundly reorganised during metamorphosis, affecting the fate of different types of neuron in different ways. Almost all adult motor neurons derive from larval motor neurons that are respecified for adult functions. A subset of larval motor neurons, those which mediate larval- or ecdysis-specific behaviours, die before and immediately after eclosion, respectively. Many adult interneurons develop from larval interneurons, whereas those related to complex adult sense organs originate during larval life from persisting embryonic neuroblasts. Sensory neurons of larvae and adults derive from essentially two distinct sources. Larval sensory neurons are formed in the embryonic integument and - with few exceptions - die during metamorphosis. Their adult counterparts, on the other hand, arise from imaginal discs. Special emphasis is given in this review to the metamorphic remodelling of persisting neurons, both at the input and output levels, and to the associated behavioural changes. Other sections deal with the programmed death of motor neurons and its causes, as well as with the metamorphic interactions between motor neurons and their target muscles. Remodelling and apoptosis of these two elements appear to be under independent ecdysteroid control. This review focusses on the two most thoroughly studied holometabolous species, the fruitfly Drosophila melanogaster and the tobacco hornworm moth Manduca sexta. While Manduca has a long tradition in neurodevelopmental studies due to the identification of many of its neurons, Drosophila has been increasingly used to investigate neural reorganisation thanks to neurogenetic tools and molecular approaches. The wealth of information available emphasises the strength of the insect model system used in developmental studies, rendering it clearly the most important system for studies at the cellular level.

Reutilization of trigeminal motoneurons during amphibian metamorphosis

Indirect evidence suggests that trigeminal motoneurons (Vmns) in Rana pipiens innervate distinct myofiber populations in tadpoles and adult frogs. Redeployment occurs when the larval myofibers die and are replaced during metamorphosis. To directly test this hypothesis, DiI was injected into the larval muscle and Fast Blue into the replacement myofiber population. Over 95% of the Vmns contained both tracers, providing support for the innervation of sequential targets by the same motoneurons.

Steroid control of muscle remodeling during metamorphosis in Manduca sexta

During metamorphosis in the tobacco hornworm, Manduca sexta, the abdominal body-wall muscle DEO1 is remodeled to form the adult muscle DE5. The degeneration of muscle DEO1 involves the dismantling of its contractile apparatus followed by the degeneration of muscle nuclei. As some nuclei are degenerating, others begin to incorporate 5-bromodeoxyuridine (BrdU), indicating the onset of nuclear proliferation. This proliferation is initially most evident at the site where the motoneuron contacts the muscle remnant. The developmental events involved in muscle remodeling are under the control of the steroid hormones, the ecdysteroids. The loss of the contractile elements of the larval muscle requires the rise and fall of the prepupal peak of ecdysteroids, whereas the subsequent loss of muscle nuclei is influenced by the slight rise in ecdysteroids seen after pupal ecdysis. Incorporation of BrdU by muscle nuclei depends on both the adult peak of the ecdysteroids and contact with the motoneuron. Unilateral axotomy blocks proliferation within the rudiment, but it does not block its subsequent differentiation into a very thin muscle in the adult.

Redeployment of trigeminal motor axons during metamorphosis

As a consequence of the degeneration and replacement of the jaw muscle fibers in the leopard frog, Rana pipiens, trigeminal motoneurons innervate different targets before and after metamorphosis. This investigation examined the morphological correlates of the reassignment of trigeminal motoneurons during the initial phases of myofiber turnover. Specifically, silver-cholinesterase histochemistry and electron microscopy were used to 1) identify the fate of motor axons within the neuromuscular junctions (NMJs) applied to degenerating larval myofibers and 2) to determine the origin(s) of the motor axons that innervate the postmetamorphic muscle fibers of the jaw. The results demonstrate that the NMJs are retained on larval myofibers throughout their degeneration and are readily identifiable on the residual larval basal laminae that remain after involution of the sarcoplasm. Light and electron microscopic observations provide evidence that both pre- and post-synaptic elements are present on the degenerating fibers. Furthermore, morphometric analyses indicate that the preponderance (86%) of motor axons supplying adult muscle fibers originates from the larval NMJs. This condition suggests that metamorphic redeployment of trigeminal motoneurons occurs through the resumption of growth at the axon terminal supplying larval muscle rather than through the proximal collateralization of these axons and resorption of larval terminals.

Patterns of serotonin and SCP immunoreactivity during metamorphosis of the nervous system of the red abalone, Haliotis rufescens

Larvae of the red abalone, Haliotis rufescens, rely on external chemical cues to trigger metamorphosis; thus, the timing of metamorphosis is dependent upon the larva's chance encounter with the appropriate substrate. We examined the effect of the timing of metamorphosis on the development of the central nervous system (CNS), concentrating on the pattern of serotonin and small cardioactive peptide- (SCP) immunopositive neurons in the cerebral ganglia. By 4 days postfertilization the cerebral ganglion has five pairs of serotonin-immunoreactive (IR) neurons, one pair of which (the V cells) innervate the velum. This complement of cells remains stable for as long as the larval stage persists but metamorphosis causes the rapid loss of the V cells. In the case of SCP-IR neurons, one pair is present prior to metamorphic competency, but as larvae continue to age in the absence of inducing cues, additional pairs are gradually added. Metamorphosis causes an acceleration in SCP-IR neuron addition. This separation of developmental patterns is well adapted for the inherent uncertainty of the timing of metamorphosis in abalone larvae.

Continuous growth of the motor system in the axolotl

During growth of the axolotl, motor neurons, and muscle fibres are added to the motor system. By double labelling neurons with tritiated thymidine and retrogradely transported HRP, we show that some motor neurons are born at postembryonic stages. Further analysis of motor neurons with the aid of HRP reveals this population of newly born cells relatively frequently in small (5-7 cm long) axolotls, but only rarely in large (7-13 cm long) axolotls. Evidence is presented that suggests that these immature cells are in the process of migrating from close to the ependyma out to the ventral horn. HRP transport also reveals growth cones of advancing axons within spinal nerves in animals up to 6 cm in length. Cell counts by light and electron microscopic methods show that muscle fibres are generated throughout larval life in the iliotibialis, a typical limb muscle. This analysis provides data consistent with the notion that new muscle fibres are added from a localised growth zone situated at the superficial edge of the muscle. These results are discussed in terms of the correlation between continuous growth of the motor system and the ability of the axolotl to functionally repair lesions to the peripheral nervous system.

Retrofitting larval neuromuscular circuits in the metamorphosing frog

Maturation of vertebrate neuromuscular systems typically occurs in a continuous, orderly progression. After an initial period of developmental adjustment by means of cell death and axonal pruning, relatively stable relationships, with only subtle modifications, are maintained between motoneurons and their appropriate targets throughout life. However, among a restricted group of vertebrates (amphibians and especially the anuran amphibians) the sequential maturation of neuromuscular systems is altered by an abrupt reordering of the basic body plan that encompasses cellular changes in all tissues from skeleton to nervous system. Many anuran amphibians possess neuromuscular circuits that are remarkable by virtue of their complete reorganization during the brief span of metamorphosis. During this period motor systems initially designed for the behavioral patterns of aquatic tadpoles are adjusted to meet the drastically different motor activities of postmetamorphic terrestrial life. This adjustment involves the deletion of neural elements mediating larval specific activities, the accelerated maturation of neural circuits eliciting adult-specific activities and the retrofitting of larval neuromuscular components to serve postmetamorphic behaviors. This review focuses on the cellular events associated with the neuromuscular adaptation in the jaw complex during metamorphosis of the leopard frog, Rana pipiens. As part of the metamorphic reorganization of the jaw apparatus there is a complete turnover of the myofiber complement of the adductor mandibulae musculature. Trigeminal motoneurons initially deployed to the larval myofibers are redirected to new muscle fibers. Simultaneously the cellular geometry and synaptic input to these motoneurons is revamped. These changes suggest that trigeminal neuromuscular circuitry established during embryogenesis is updated during metamorphosis and reused to provide the basis for adult jaw motor activity that is far different than its larval counterpart.

The gastropod nervous system in metamorphosis

Many gastropods, including the sea hare Aplysia californica, undergo metamorphosis in passing from the larval to the juvenile phases of their life cycle. During metamorphosis, the gastropod nervous system is affected by both progressive and regressive neuronal events. In addition to this metamorphic reorganization, the nervous system appears to be centrally involved in initiating metamorphosis. We propose that gastropods not only possess temporally distinct neuronal adaptations for the specific needs of the larval and juvenile phases, but also another transient neuronal adaptation specialized to subserve the metamorphic episode.

Mesencephalic fifth nucleus cell responses to thyroid hormone: one population or two?

Hypophysectomized Rana pipiens tadpoles 3-6 months old were placed in dl-thyroxine (T4) solutions of 4 to 200 micrograms/l for 1-18 days and fixed 1 day after removal from the hormone solution. Exposure times varied inversely with T4 concentration. Mesencephalic fifth nucleus (M-V) cells were counted on both sides, and cell and nuclear sizes were drawn and measured for each tadpole. Changes in M-V cell characteristics correlated well with T4 exposure times and concentrations, as did changes in external tadpole morphology. All T4 concentrations were effective. Cells and nuclei were distinctly larger in all T4-treated groups. The changes were greatest for cell sizes, less for nuclear dimensions, and still less for nucleo-cytoplasmic ratios. Significant changes were seen for minimum and maximum sizes as well as for the mean values. The greatest mean changes were seen at dosages of 50 micrograms/l for 7-9 days. Mean M-V cell numbers are significantly smaller in hypophysectomized tadpoles than in controls (about 420 vs. 650). Thyroid hormone treatment of the hypophysectomized animals abolishes much of the deficit, though M-V cell deaths at the higher concentrations and longer treatment times reduce the apparent increase in numbers. Are the additional cells obtained through cell division, or do they represent a preexisting subpopulation of prospective M-V cells that require stimulation by thyroid hormone for their full differentiation?

Mauthner cells maintain their lumbar projection in adult frog

We have used retrograde labeling with horseradish peroxidase (HRP) in the bullfrog. Rana catesbeiana, to determine whether Mauthner (M) cells maintain a projection to the lumbar spinal cord in adult bullfrogs. We found that M cells persist in the adult bullfrog and maintain a projection to the lumbar spinal cord, despite the degeneration of much of their afferent input and of their motoneuronal targets in the spinal cord.

Development of the abducens nuclei in the Xenopus laevis

The development of the main (nVI) and the accessory abducens (nVIa) nuclei was studied with the horseradish peroxidase and cobaltic-lysine labeling techniques in Xenopus laevis tadpoles. In earliest labeling was obtained at stage 39, and neuroblasts of both nuclei formed two separate groups according to their definitive positions in relation to other rhombencephalic structures in this young age of development. Conspicuous morphological differences were observed between the two nuclei: the accessory abducens neuroblasts were twice as big as the abducens neuroblasts and the characteristic nVIa 'knee' was present from this time of the first successful labeling. The two different dendritic arborization patterns, which clearly distinguished the abducens neurons from the accessory abducens neurons, gradually developed in tadpoles. It is suggested that the form and position of abducens and accessory abducens neurons are determined at a prefunctional stage, probably before the beginning of axonal outgrowth, and neurobiotaxis may not play the role attributed previously in the differentiation of these two nuclei.

Sex-specific neuronal respecification during the metamorphosis of the genital segments of the tobacco hornworm moth Manduca sexta

At metamorphosis, the terminal abdominal segments of larvae of the moth Manduca sexta transform into either male or female genitalia. At the start of this transformation, the larval muscles degenerate but their remains may persist to form the scaffolding on which the new adult muscles differentiate. The survival and subsequent orientation of larval muscle remnants is determined by the sex of the individual and is independent of motor innervation at the start of metamorphosis. Many of the larval motoneurons persist through metamorphosis and innervate the skeletal muscle of the adult. The survival of particular motoneurons is also sex-dependent and correlated with the survival of its respective muscle remnant. No new skeletal motoneurons arise postembryonically, so all of the adult skeletal muscle motoneurons are derived from preexisting larval skeletal muscle motoneurons. The fates during metamorphosis are more complex for the visceral muscle motoneurons. Those innervating the adult hindgut of both sexes are identical and are derived from the larval hindgut motoneurons. Other hindgut motoneurons in the larva switch targets during metamorphosis and come to innervate the oviduct in adult females or perish in adult males. Other regions of the reproductive tract become innervated by adult-specific cells that differentiate during metamorphosis. These cells come from distinct lineages in males and females.

Myofiber turnover is used to retrofit frog jaw muscles during metamorphosis

Metamorphic reorganization of the head in anuran amphibians entails abrupt restructuring of the jaw complex as larval feeding structures are transformed into their adult configurations. In this morphometric study, light microscopy wa used to analyze the larval maturation and metamorphic transfiguration of the adductor jaw muscles in the leopard frog (Rana pipiens). Larval jaw muscles, first established during embryogenesis, continue to grow by fiber addition until prometamorphosis, stage XII. Thereafter, fiber number remains stable but additional muscle growth continues by hypertrophy of the individual fibers until metamorphic climax. During metamorphic stages XIX-XXIII, a complete involution of all larval myofibers occurs. Simultaneously, within the same muscle beds, a second wave of myogenesis produces myoblasts which are the precursors of adult jaw myofibers. New muscle fibers continue to be added to these muscles well after the completion of metamorphosis; however, the total duration of the postmetamorphic myogenic period has not been defined. These observations provide clear evidence that the entir population of primary myofibers used in larval oral activity disappears from the adductor muscle beds and is replaced by a second wave of myogenesis commencing during climax. These findings indicate that the adductor jaw muscles are prepared for adult feeding by a complicated cellular process that retrofits existing muscle beds with a completely new complement of myofibers.

Trigeminal motoneurons in frogs develop a new dendritic field during metamorphosis

During metamorphosis neural systems that regulate larval behavior are altered to adult patterns. Identified strategies used to produce this alteration include neuronal deletions, neuronal additions, and the reconfiguration of existing neural circuits. This investigation focused on the structural alterations that occur in frog trigeminal motoneurons during metamorphic climax. These neurons are of special interest because they mediate the vastly different muscular activities associated with larval and adult food capture. We examined the development of the trigeminal motoneurons in anuran larvae, devoting special attention to the elaboration of different dendritic fields during metamorphic climax. When horseradish peroxidase (HRP) back-filled motoneurons in adults were scrutinized by Sholl analysis, they were found to have two spatially discrete dendritic domains, one extending ventrally and laterally, the other projecting dorsomedially into the periventricular cells. The ventrolateral dendritic field alone is represented in the motoneurons of premetamorphic larvae. The dorsomedial dendritic field first appears at the beginning of metamorphic climax and is rapidly elaborated during the terminal stages of larval development.

An ultrastructural comparison of neuromuscular junctions in normal and developmentally arrested Rana pipiens larvae: limited maturation in the absence of metamorphosis

Neuromuscular junctions in the rectus abdominis muscles of normal and developmentally arrested Rana pipiens larvae were studied with freeze fracture and conventional electron microscopy to determine whether structural aspects of junctional maturation depend on metamorphosis. Comparison was made between junctions in premetamorphic larvae 1-3 months old and junctions in larvae that had remained in premetamorphosis for more than a year (more than four times as long as normal). In most respects, junctions from the two groups of larvae were similar. Unlike adult junctions, nerve-muscle contacts in both larval groups were pleomorphic and often involved more than one neuronal process; Schwann cell processes very rarely extended between nerve and muscle. Active zone structure ranged from total disorganization to an adult pattern of highly ordered double rows of particles aligned over junctional folds. Only quantitative analysis revealed differences between junctions in old and young larvae. The older larvae had fewer nerve-muscle contact sites involving multiple neuronal elements and a higher ratio of active zone length to presynaptic membrane area, although the mean active zone length was the same in the two groups. The results indicate that the maturation of junctional shape, the branching pattern of the axons, and the relationship of presynaptic axons to Schwann cells must be directly or indirectly dependent on the hormonal or behavioral changes associated with metamorphosis.

Neurogenesis in reptilian cortical structures: 3H-thymidine autoradiographic analysis

Histogenesis was studied in forebrain cortical areas of two reptiles, Emys orbicularis and Lacerta trilineata, by using tritiated thymidine autoradiography. Four areas were considered: the dorsomedial, the general (dorsal), and the lateral cortices, and the dorsal ventricular ridge (DVR). The bulk of neurogenesis in these four pallial fields proceeds within a short period of 8-9 days, between developmental stages 15 and 18 in Emys and stages 32-34 in Lacerta. Lateral-to-medial as well as anterior-to-posterior tangential gradients of histogenesis are present in both species. Radial neurogenetic gradients are directed from outside to inside, except in the medial cortex of lizards, where no radial gradient is seen. This pattern of histogenesis in the cortex of turtles and lizards is comparable to that in mammals in terms of timing and tangential, areal variations. It might represent a "common denominator" of cortical histogenesis. However, in contrast to the mammalian cortex, which develops according to an inside to outside, "inverted" pattern, radial neurogenesis in the cortex of turtles and lizards follows an outside-to-inside gradient. These observations suggest that the inside-out gradient of cortical neurogenesis has been acquired during evolution of the synapsid radiation from stem reptiles to mammals, and that it may be related to the development of radial cortical architectonics.

Growth and death of cells of the mesencephalic fifth nucleus in Xenopus laevis larvae

Positions, numbers, and cell and nuclear sizes of mesencephalic fifth nucleus (M-V) cells were determined in Xenopus laevis larvae in stages 47 through the end of metamorphosis at stage 66. M-V cells may be found at every tectal level, from the rostralmost section almost to the caudal pole, and in the anterior medullary velum. A large majority of the cells lie between 15 and 65% caudad of the rostral tip of the tectum. At anterior and middle tectal levels the cells lie lateral to but mainly above the optic ventricle. At posterior levels, to which the ventricle does not extend, a few cells may be seen at middle and lower tectal levels, as if in transition to the anterior medullary velum. At stage 47 fewer than ten cells are seen in each animal. The numbers rise to 20-40 by stage 50, and are uniformly above 100 after stage 51. Initially many M-V cells were small, i.e., 6-7 microns in diameter, but grew to a mean diameter of about 19 microns at stage 59, with a maximum value of 29 microns. A single individual at stage 57 had 581 cells. The peak of mean cell numbers, 387, occurred at stage 59, which was also the stage with the highest mean values for nuclear and cell sizes. Pyknotic M-V cells at low frequency were seen at stages 55 and 57, and at all stages thereafter. Cell death frequency peaked at stage 62, but continued through stage 66. By stage 66 mean cell numbers had been reduced to about 240, indicating survival of about 60% of cells present at stage 59.

Growth and metamorphosis of the rectus abdominis muscle in Rana pipiens

Cross sections through the middle segment of the anuran rectus abdominis muscle were analyzed morphometrically at nine stages of development, from early larval life through full maturity. The numbers, sizes, and relative distributions of twitch and slow muscle fibers, newly differentiated fibers, degenerating fibers, and satellite cells were determined at each stage. The data indicate that the muscle increases slowly in size and fiber content during early larval life. New fibers appear to form primarily along the medial margin of the muscle. During mid-larval stages, when thyroid hormone levels are rising, new fibers form throughout the medial portion of the muscle. At a slightly later stage, fibers in the lateral region of the muscle begin to degenerate. Structurally normal presynaptic elements are present on both degenerating fibers and the empty basal laminae of fibers that had been removed by phagocytes. Both fiber formation and fiber loss slow during midmetamorphic climax, at the time when thyroid hormone levels reach a peak in anurans and begin to decline. Degenerating fibers appear within the body of the muscle at the end of metamorphosis. By the end of the second postmetamorphic month, neither degenerating nor newly differentiated fibers are present. The muscle continues to grow through adult life primarily by fiber hypertrophy.

Maturation and recycling of trigeminal motoneurons in anuran larvae

Development of the trigeminal motor system was analyzed in Rana pipiens larvae and adults. The aim of this investigation was to determine the postmetamorphic fate of the primary motoneurons that innervate the larval jaw muscles. Specifically, we wanted to ascertain whether these neurons were deleted in conjunction with their muscular targets during metamorphosis or reused to innervate the adult jaw muscles. Cell counts and horseradish peroxidase tracer were used to distinguish between these two possibilities. The number of trigeminal motoneurons was relatively constant in premetamorphic and prometamorphic larvae. A small reduction in the cellular complement of the motor nucleus occurred during metamorphic climax, but the majority (approximately equal to 90%) of the primary motoneurons were retained from the larval to the adult nervous system. The cell loss may represent motoneurons that innervated specific larval muscles that have no adult successors and thus the entire myoneural unit degenerates. Retrograde tracers indicated that all trigeminal motoneurons extended axons into the jaw muscles of both premetamorphic larvae and adult frogs. These observations provide further support for the recycling of the trigeminal motoneurons.