Search

Metamorphosis of cranial cartilages in anuran amphibians constitutes one of the most dramatic and extensive ontogenetic transformations in vertebrates. We quantitatively examined the role of thyroid hormone (3,3',5-triiodo-L-thyronine; T3) in mediating gross aspects of this morphological repatterning in the skull of the Oriental fire-bellied toad, Bombina orientalis. T3 was administered via plastic (Elvax) micropellets in three treatment dosages (2.5, 0.25, and 0.025 microgram) and one control dosage (0 microgram) to tadpoles of three Gosner developmental stages--28/29, 30/31, and 32/33; tadpoles were recoved up to 8 d (treatment and control dosages) or 14 d (control dosage) later. Response of larval cartilages to exogenous T3 was dosage dependent but not implant-stage dependent; chondrogenic tissues that participate in metamorphic transformation are competent to respond to T3 well before they normally do. Metamorphic effects of T3 were visible within 2 d; in most treatment groups, the normal metamorphic sequence was two-thirds complete after 8 d. While T3 also induced precocious ossification, the normal temporal relation between bone formation and cartilage transformation was dissociated in experimental groups. Morphological integration between cartilage and bone during cranial metamorphosis is at least partly the result of each tissue responding independently to endocrine factors.

Species of the salamander genus Plethodon have a characteristically uniform morphology. Morphological conservation at the level of interspecific comparisons, however, is not always reflected within species. Perhaps the most extreme example of intraspecific variation is the recent description of extensive variability in limb-skeletal patterning both within and between populations of the widespread species P. cinereus. We utilized limb regeneration following experimental amputation as a tool to examine whether naturally occurring variant skeletal patterns result from limb loss and regeneration in nature, and to assay the intrinsic (i.e., genetic) component of between-individual variation in mesopodial patterning. We observed the following. First, regenerate patterns are strikingly different from native patterns: interelement fusions in regenerates are typically between proximodistally adjacent cartilages, whereas interelement fusions in native variant limbs occur exclusively between laterally adjacent cartilages. Fusions also are over ten times more frequent in regenerates than in native limbs. Second, there is no strong correlation between native limb pattern (typical vs. variant) and the regenerate pattern. We conclude that variability in field-collected P. cinereus reflects extensive intrapopulation variation in limb-skeletal patterning during original limb development, rather than regeneration in nature, and that limb regeneration analysis provides no evidence of a strong genetic component to between-individual variation. Finally, unusual mesopodial patterns produced during limb regeneration may be related to the mechanical factors impinging on the regenerating limb in this terrestrial species.

Patterns of growth and variation of the appendicular skeleton were examined in Thorius, a speciose genus of minute terrestrial plethodontid salamanders from southern Mexico. Observations were based primarily on ontogenetic series of each of five species that collectively span the range of adult body size in the genus; samples of adults of each of seven additional species provided supplemental estimates of the full range of variation of limb skeletal morphology. Limbs are generally reduced, i.e., pedomorphic, in both overall size and development, and they are characterized by a pattern of extreme variation in the composition of the limb skeleton, especially mesopodial elements, both within and between species. Fifteen different combinations of fused carpal or tarsal elements are variably present in the genus, producing at least 18 different overall carpal or tarsal arrangements, many of which occur in no other plethodontid genus. As many as four carpal or tarsal arrangements were observed in single population samples of each of several; five tarsal arrangements were observed in one population of T. minutissimus. Left-right asymmetry of mesopodial arrangement in a given specimen is also common. In contrast, several unique, nonpedomorphic features of the limb skeleton, including ossification of the typically cartilaginous adult mesopodial elements and ontogenetic increase in the degree of ossification of long bones, are characteristic of all species and distinguish Thorius from most related genera. They form part of a mechanism of determinate skeletal growth that restricts skeletal growth after sexual maturity. Interspecific differences in the timing of the processes of appendicular skeletal maturation relative to body size are well correlated with interspecific differences in mean adult size and size at sexual maturity, suggesting that shifts in the timing of skeletal maturation provide a mechanism of achieving adult size differentiation among species. Processes of skeletal maturation that confer determinate skeletal growth in Thorius are analogous to those typical of most amniotes - both groups exhibit ontogenetic reduction and eventual disappearance of the complex of stratified layers of proliferating and maturing cartilage in long bone epiphyses - but, unlike most amniotes, Thorius lacks secondary ossification centers. Thus, the presence of secondary ossification centers cannot be used as a criterion for establishing determinate skeletal growth in all vertebrates., (Copyright © 1982 Wiley-Liss, Inc.)

The sequence of appearance of the 17 different skull bones in the oriental fire-bellied toad, Bombina orientalis, is described. Data are based primarily on samples of ten or 11 laboratory-reared specimens of each of 11 Gosner developmental stages (36-46) representing middle through late metamorphosis. Ossification commences as early as stage 37 (hind limb with all five toes distinct), but the full complement of adult bones is not attained until stage 46 (metamorphosis complete). Number of bones present at intermediate stages is poorly correlated with external morphology. As many as four Gosner developmental stages elapse before a given bone is present in all specimens following the stage at which it may first appear. The modal ossification sequence is frontoparietal, exoccipital, parasphenoid, septomaxilla, premaxilla, vomer, nasal, maxilla, angulosplenial, dentary, squamosal, quadratojugal, pterygoid, prootic, interfrontal, sphenethmoid, and mentomeckelian. Most specimens are consistent with this sequence, despite the poor correlation between cranial ossification and external development as assayed by Gosner stage. The timing of cranial ossification in Bombina orientalis differs in many respects from that described for two other, distantly related anurans, the leopard frog (Rana pipiens) and the western toad (Bufo boreas). These include the total number and sequence of appearance of bones, and the timing of ossification relative to the development of external morphology. Interspecific variation may reflect differences in the timing of the tissue interactions known to underlie skeletal differentiation and evolution., (Copyright © 1984 Wiley-Liss, Inc.)

Procedures for revealing the three-dimensional arrangement of calcareous sclerites, spicules, or ossicles embedded within connective tissue in formalin fixed invertebrates are described. Spicules are stained with alizarin red S following maceration of preserved animals or colonies with either trypsin or KOH solutions. Connective tissue is stained with alcian blue in different samples prior to maceration. Stained animals or colonies are cleared in glycerin. This method for revealing spicular structure and arrangement and the gross morphology of connective tissues offers several advantages over either scanning electron microscopy or reconstruction from serial sections.

In anuran amphibians, cranial bones typically first form at metamorphosis when they rapidly invest or replace the cartilaginous larval skull. We describe early development of the first three bones to form in the Oriental fire-bellied toad, Bombina orientalis--the parasphenoid, the frontoparietal, and the exoccipital--based on examination of serial sections. Each of these bones is fully differentiated by Gosner stage 31 (hindlimb in paddle stage) during premetamorphosis. This is at least six Gosner developmental stages before they are first visible in whole-mount preparations at the beginning of prometamorphosis. Thus, developmental events that precede and mediate the initial differentiation of these cranial osteogenic sites occur very early in metamorphosis--a period generally considered to lack significant morphological change. Subsequent development of these centers at later stages primarily reflects cell proliferation and calcified matrix deposition, possibly in response to increased circulating levels of thyroid hormone which are characteristic of later metamorphic stages. Interspecific differences in the timing of cranial ossification may reflect one or both of these phases of bone development. These results may qualify the use of whole-mount preparations for inferring the sequence and absolute timing of cranial ossification in amphibians.