Sesamoids in Anurans: New Data,Old Issues

Sesamoids are skeletal elements rarely considered in studies of the vertebrate skeleton. In this work, we integrate ontogenetic data of anuran sesamoids in two species (Leptodactylus latinasus and Pleurodema cf. guayapae), the related structures (tendons, muscles, and joints) in L. latinasus, and a survey of sesamoid distribution in 185 anuran taxa. Our main goals are: (1) to contribute to the knowledge of the comparative anatomy of sesamoids in tetrapods; (2) to provide additional developmental evidence to interpret the ontogenetic pattern of sesamoids in anurans, as a key to elucidate that of tetrapods in general; (3) to provide data about tendon development in relation to sesamoid development in anurans for the first time; and (4) to propose a pattern of anuran sesamoid distribution. The homologies of sesamoids across tetrapods are discussed here. Observations were made in cleared and stained skeletal whole‐mounts. Fifty‐four sesamoids were found in anurans, thirty‐seven of which occur in L. latinasus. The traditional point of view of embedded sesamoids always resulting from biomechanical stimuli of a previously existing tendon is not sustained by our data. Many sesamoids arise before the differentiation of a tendinous tissue. Our survey results in a data set where the two big anuran clades, Hyloides (12 families) and Ranoides (14 families), were represented. The matrix has 38% missing entries. Most of the surveyed sesamoids have multiple origins, with only three of them (about 19%) having one origin. Anat Rec 293:1646–1668, 2010. © 2010 Wiley‐Liss, Inc.     

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

Drastic alterations in oral behavior characterize metamorphosis of anuran amphibians. Changes cascade through all components of the jaw apparatus from bone to muscle to nerve. In this investigation, tritiated thymidine autoradiography was used to determine the production schedule of the trigeminal motoneurons in the leopard frog, Rana pipiens. The time of origin of these neurons and their subsequent fate are of special interest given the breakdown of the larval jaw muscles and the de novo generation of adult muscle fibers during metamorphosis. Specifically, we wanted to learn whether trigeminal motoneurons are added, deleted, or reused during metamorphic climax. The entire complement of trigeminal motoneurons was produced over a 4-day span commencing at embryonic stage 13 and terminating at stage 20. Newly formed neurons are added to the primordial trigeminal nucleus in an orderly pattern. Firstborn neurons settle in the ventrorostral region of the nucleus; cells with progressively later birth dates were added in a posterodorsal direction. No additional trigeminal motoneurons are generated during larval maturation or at metamorphosis, thus indicating that the same population of neurons is present throughout the lifespan of the animal. From these observations we suggest that, during metamorphosis, the trigeminal motoneurons that supply the larval muscles switch their allegiance to the newly formed adult jaw muscles. This change of peripheral targets can be viewed as a respecification of the trigeminal motoneurons.     

Development of the amphibian oculomotor complex: Evidences for migration of oculomotor motoneurons across the midline

Summary The development of the oculomotor nucleus in five species of salamanders and one anuran species was investigated with tracing techniques. The data presented support the hypothesis that oculomotor motoneurons innervating the superior rectus muscle migrate across the midline. In the salamander Pleurodeles waltl, only ipsilateral oculomotor motoneurons are labeled in early development. Later, these neurons extend dendrites toward the contralateral side into the ventral tegmental neuropil, after which there is displacement of their nuclei (neuronal somata) across the midline. Cell bodies can be observed directly at the midline. In adult Salamandra salamandra, motoneurons innervating the superior rectus muscle are seen occasionally at the midline and on the ipsilateral side, with dendrites toward the contralateral side. Motoneurons on the ipsilateral side do not display these features. In Pleurodeles, developmental brain processes are slowed down, and the sequence of development of the contralateral subnucleus, which can be clearly observed, supports the migration hypothesis. In Xenopus laevis and most other species of salamanders this process is accelerated.     

Masculinization of toad pretrigeminal nucleus by androgens

The magnocellular portion of the pretrigeminal nucleus (a structure involved in vocalization) of male toads contains larger cells than does the female nucleus. Testosterone propionate injections caused essentially complete, and dihydrotestosterone injections incomplete, masculinization of the female nucleus. Estradiol benzoate injections had little or no effect. The androgen effects are therefore largely direct rather than involving extensive aromatization of androgen to estrogen.     

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.     

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.     

Tuning of the Tectorial Membrane in the Basilar Papilla of the Northern Leopard Frog

The basilar papilla (BP) in the frog inner ear is a relatively simple auditory receptor. Its hair cells are embedded in a stiff support structure, with the stereovilli connecting to a flexible tectorial membrane (TM). Acoustic energy passing the papilla presumably causes displacement of the TM, which in turn deflects the stereovilli and stimulates the hair cells. Auditory neurons that contact the BP’s hair cells are known to have nearly identical characteristic frequencies and frequency selectivity. In this paper, we present optical measurements of the mechanical response of the TM. Results were obtained from five specimens. The TM displacement was essentially in phase across the membrane, with the largest amplitudes occurring near the hair cells. The response was tuned to a frequency near 2 kHz. The phase accumulated over at least 270° across the measured frequencies. The tuning quality Q_10dB values were calculated; the average Q_10dB was 2.0 ± 0.8 (standard deviation). Our results are comparable to those of neural-tuning curves in the same and a similar species. Also, they are in agreement with the response of an associated structure—the contact membrane—in a closely related species. Our data provides evidence for a mechanical basis for the frequency selectivity of the frog’s BP.     

Biomechanical properties of anuran long bones: correlations with locomotor modes and habitat use

Long bones are subjected to mechanical loads during locomotion that will influence their biomechanical properties through a feedback mechanism (the bone mechanostat). This mechanism adapts the spatial distribution of the mineralized tissue to resist compression, bending and torsion. Among vertebrates, anurans represent an excellent group to study long bone properties because they vary widely in locomotor modes and habitat use, which enforce different skeletal loadings. In this study, we hypothesized that (a) the cortical bone mass, density and design of anuran femur and tibiofibula would reflect the mechanical influences of the different locomotor modes and habitat use, and (b) the relationships between the architectural efficiency of cortical design (cross-sectional moments of inertia) and the intrinsic stiffness of cortical tissue [cortical mineral density; the 'distribution/quality' (d/q) relationship] would describe some inter-specific differences in the efficiency of the bone mechanostat to improve bone design under different mechanical loads. To test this hypothesis, we determined tomographic (peripheral quantitative computed tomography) indicators of bone mass, mineralization, and design along the femur and tibiofibula of four anuran species with different modes of locomotion and use of habitat. We found inter-specific differences in all measures between the distal and proximal ends and mid-diaphysis of the bones. In general, terrestrial-hopper species had the highest values. Arboreal-walker species had the lowest values for all variables except for cortical bone mineral density, which was lowest in aquatic-swimmer species. The d/q relationships showed similar responses of bone modeling as a function of cortical stiffness for aquatic and arboreal species, whereas terrestrial-hoppers had higher values for moments of inertia regardless of the tissue compliance to be deformed. These results provide new evidence regarding the significant role of movement and habitat use in addition to the biomechanical properties of long bones within a morpho-functional and comparative context in anuran species.     

Relation of binaural interaction and spectro-temporal characteristics in the auditory midbrain of the grassfrog

The relation between binaural interaction type and spectro-temporal characteristics was studied for single units in the auditory midbrain of the grassfrog. Tonal and continuous wideband noise ensembles have been used as stimuli. Spectro-temporal sensitivities were determined for ipsi-, contra- and bilateral stimulus presentation by a closed sound system. Binaural interaction was classified in monaural EO (one ear excitatory), binaural EE (both ears excitatory) and EI (one ear excitatory, the other inhibitory) and purely inhibitory categories. Binaural interaction appeared to be rather invariant to alterations in stimulus intensity and type. A very clear correlation was observed between best frequency and binaural interaction type: EE units are predominantly of high best frequency, whereas EI units are predominantly of low best frequency. The correlation with latency was less significant: EE units tended to have somewhat shorter latencies that EI units. EO units take an intermediate position. Comparisons of ipsi-, contra- and bilateral spectro-temporal sensitivities, revealed differences in best frequency, latency and temporal discharge pattern. In some units a complex interplay of excitatory and inhibitory monaural influences was demonstrated. A number of units was recorded, which were characterized by multiple activation or suppression areas. The majority of these units exhibited frequency-dependent binaural interaction types. In some units it was noticed that binaural interaction type can be dependent on state of adaptation. A comparison of binaural interaction types of neighbouring units provided only weak evidence for a binaural organization in the anuran auditory midbrain, since simultaneously recorded pairs shared the same binaural interaction type only slightly more than expected by mere chance (chi 2-test, P less than 0.10).