Retrofitting larval neuromuscular circuits in the metamorphosing frog

Structure and metamorphic remodeling of the larval nervous system and musculature of Phoronis pallida (Phoronida)

The structure of the larval nervous system and the musculature of Phoronis pallida were studied, as well as the remodeling of these systems at metamorphosis. The serotonergic portion of the apical ganglion is a U-shaped field of cell bodies that send projections into a central neuropil. The majority of the serotonergic cells are (at least) bipolar sensory cells, and a few are nonsensory cells. Catecholaminergic cell bodies border the apical ganglion. The second (hood) sense organ develops at competence and is composed of bipolar sensory cells that send projections into a secondary neuropil. Musculature of the competent larva includes circular and longitudinal muscle fibers of the body wall, as well as elevators and depressors of the tentacles and hood. The juvenile nervous system and musculature are developed prior to metamorphosis and are integrated with those of the larva. Components of the juvenile nervous system include a diffuse neural net of serotonergic cell bodies and fibers and longitudinal catecholaminergic fibers. The juvenile body wall musculature consists of longitudinal fibers that overlie circular muscle fibers, except in the cincture regions, where this pattern is reversed. Metamorphosis is initiated by the larval neuromuscular system but is completed by the juvenile neuromuscular system. During metamorphosis, the larval nervous system and the musculature undergo cell death, and the larval tentacles and gut are remodeled into the juvenile arrangement. Although the phoronid nervous system has often been described as deuterostome-like, these data show that several cytological aspects of the larval and juvenile neuromuscular systems also have protostome (lophotrochozoan) characteristics.

Mandibular arch musculature of anuran tadpoles, with comments on homologies of amphibian jaw muscles

This study analyzes the structure of the mandibular arch musculature in larval, metamorphic, and postmetamorphic anurans of 26 species and makes comparisons with larvae of three caudate and one gymnophione species. Major transformations in early evolution of anuran larvae comprise, for example, the powering of the larval upper jaw cartilages by relocating insertion sites of mandibular arch levators; splitting of some larval muscles into two muscles or muscle heads (m. intermandibularis, m. lev. mand. externus, m. lev. mand. longus); evolution of a muscle invading the lower lip of the oral disk (m. mandibulolabialis), and shift of origin of the internus and longus muscles from dorsal on the cranium to sites on the ventral otic capsule and palatoquadrate, respectively. In all these characters, Ascaphus truei shares the plesiomorphic conditions with caudates. The larva of Xenopus laevis is remarkable because the insertion pattern of three larval mandibular muscles anticipates the postmetamorphic condition of frogs in general and also resembles the caudate condition. Discoglossids, bombinatorids, pelobatids, and neobatrachians are largely similar in their muscle arrangements. The filter-feeding microhylids, however, have most clearly modified the general neobatrachian pattern. Past conflicts in the interpretation and naming of muscles can be attributed to the implicit or explicit homology assumptions used. In particular, the muscles' relations to the branches of the trigeminal nerve have been the dominant criteria for inferring homology and has led to inconsistencies. This concept is questioned herein. It is observed that the relative position of the ramus mandibularis (V(3)) is more variable interspecifically in anuran larvae than previously thought. The relations of the nerve branches and muscles in larvae are maintained during metamorphosis. Considering the muscle pattern to be more conserved in interspecific comparisons than the position of the nerve branches results in a new interpretation of muscle homologies and a hypothesis of jaw muscle evolution in amphibians that is more parsimonious than earlier views. A new, simplified terminology for the jaw musculature is proposed that is applicable for larvae and adults. It maximizes information content and reflects the hypothesized homologies of amphibian jaw muscles.

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.

lin-14 regulates the timing of synaptic remodelling in Caenorhabditis elegans

In the nematode Caenorhabditis elegans six GABAergic motor neurons, known as DDs, remodel their patterns of synaptic connectivity during larval development. DD remodelling involves a complete reversal of the direction of information flow within nerve processes without marked changes in process morphology. We used a marker localized in vivo to DD presynaptic zones to analyse how the timing of DD remodelling is controlled. In wild-type animals, DDs remodel their synaptic outputs within a 3-5-hour period at the end of the first larval stage. We show that the heterochronic gene lin-14, which controls the timing of stage-specific cell lineages, regulates the timing of DD synaptic output remodelling. In lin-14 loss-of-function mutants, DDs remodel precociously. The degree of precocious remodelling is correlated with the level of lin-14 activity. Expression of lin-14(+) in the DDs of lin-14-null mutants rescues the precocious remodelling, indicating that lin-14 can act cell-autonomously. Consistent with this hypothesis, LIN-14 protein levels decrease in the DDs before remodelling. Our observations reveal a role of heterochronic genes in non-dividing cells, and provide an example of cell-autonomous respecification of neuronal connectivity.

Development of anuran locomotion: ethological and neurophysiological considerations

There are dramatic quantitative and qualitative differences in the locomotor behavior of larval and juvenile frogs. Larvae (tadpoles) are primarily herbivourous and rely heavily on locomotion via undulations to acquire food and avoid predation. After metamorphosis, juvenile frogs adopt a carnivorous lifestyle and capture prey and avoid predators by remaining motionless in a place of concealment. When they must move, frogs locomote by means of ballistic hops or by more conventional walking. However, locomotion of both tadpoles and frogs can be considered of two fundamental functional types: (a) startle and escape; and (b) sustained locomotion. Neural mechanisms underlying startle responses and sustained locomotion in larvae and juveniles are described and possible ontogenetic relationships those behaviors are proposed. The role of different parts of the nervous system in the ontogeny of locomotion, as well as nonneuronal factors, are described. Results show that the transition from tadpole-like behavior to frog-like behavior is not a simple function of maturation of central locomotor controls. Rather, it results from a complex interaction of central nervous system maturation, morphological change, and a change in habitat preference. Examples of similar multidimensional control of behavioral ontogeny in other species are described, and it is argued that to understand the ontogeny of behavior, one must investigate contributions made at all levels, from the neuronal to the environmental.

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.

Cranial ontogeny in the direct-developing frog,Eleutherodactylus coqui (anura: Leptodactylidae), analyzed using whole-mount immunohistochemistry

Direct development in amphibians is an evolutionarily derived life-history mode that involves the loss of the free-living, aquatic larval stage. We examined embryos of the direct-developing anuran Eleutherodactylus coqui (Leptodactylidae) to evaluate how the biphasic pattern of cranial ontogeny of metamorphosing species has been modified in the evolution of direct development in this lineage. We employed whole-mount immunohistochemistry using a monoclonal antibody against the extracellular matrix component Type II collagen, which allows visualization of the morphology of cartilages earlier and more effectively than traditional histological procedures; these latter procedures were also used where appropriate. This represents the first time that initial chondrogenic stages of cranial development of any vertebrate have been depicted in whole-mounts. Many cranial cartilages typical of larval anurans, e.g., suprarostrals, cornua trabeculae, never form in Eleutherodactylus coqui. Consequently, many regions of the skull assume an adult, or postmetamorphic, morphology from the inception of their development. Other components, e.g., the lower jaw, jaw suspensorium, and the hyobranchial skeleton, initially assume a mid-metamorphic configuration, which is subsequently remodeled before hatching. Thirteen of the adult complement of 17 bones form in the embryo, beginning with two bones of the jaw and jaw suspensorium, the angulosplenial and squamosal. Precocious ossification of these and other jaw elements is an evolutionarily derived feature not found in metamorphosing anurans, but shared with some direct-developing caecilians. Thus, in Eleutherodactylus cranial development involves both recapitulation and repatterning of the ancestral metamorphic ontogeny.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurometamorphosis

This article introduces this special issue of the Journal of Neurobiology by reviewing several basic issues in metamorphosis as they specifically relate to the nervous system. It promotes the idea that metamorphic changes in the nervous system (neurometamorphosis) represent adaptive restructurings rather than recapitulations of evolutionary transitions. It introduces, but leaves unresolved, the question of whether neurometamorphosis is achieved primarily as a delayed phase of embryonic neurogenesis or as a special neurogenic period. It points out that respecification of old neurons and the addition of new neurons are the main contributory pathway of neural restructuring at metamorphosis, that respecification can be dramatic and seems to be preferred over the elimination and replacement of particular neurons. It also highlights the question of how much the central rewiring during metamorphosis is driven by trophic interactions with the changing body of the metamorphic animal and to what extent neurometamorphosis is driven by the direct action of metamorphic hormones on the neural elements themselves. Finally, this article introduces the question of the cellular and molecular pathways of neurometamorphosis, from the role of the nervous system in triggering the event to the receptor mediated changes in gene expression. Further details on all of these issues are to be found in the articles that make up the rest of this special issue.