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Notes: [Auszug] Atavisms — the reappearance of ancestral characteristics in individual members of a species — serve to remind us that the genetic and developmental information originally used in the production of such characteristics has not been lost during evolution but lies quiescent within the ...

Notes: Summary We examined the role of thyroid hormone (TH) in mediating cranial ossification during metamorphosis in the Oriental fire-bellied toad, Bombina orientalis. Exogenous T3 (3,3′,5-triiodo-L-thyronine) was administered in three treatment dosages (0.025, 0.25, and 2.5 μg) plus a control dosage via plastic micropellets implanted within the dermis of tadpoles of three Gosner developmental stages: 28/29, 30/31, 32/33. Tadpoles were recovered after 2, 4, 6, and 8 d, and scored for the presence of three bones —median parasphenoid and paired frontoparietals and exoccipitals—as seen in cleared-and-stained, whole-mount preparations. T3 induced precocious ossification in both a stage-dependent and a dosage-dependent manner; stage dependence corresponded precisely with the degree of osteogenic differentiation at the time of hormone administration. Precocious ossification thus was due to the T3-promoted growth and calcified matrix deposition of these centers. Differential TH sensitivity among osteogenic sites may underlie both the temporal cranial ossification sequences characteristic of metamorphosing amphibians as well as sequence differences commonly observed among taxa.

Notes: Abstract  Most craniofacial membrane bones are derived from neural crest (NC) cells. Interaction between NC cells and epithelium, and cellular condensation, are two major events that lead NC cells to become osteoblasts that deposit membrane bone. Unlike endochondral bone, membrane bone formation is not preceded by cartilage formation in normal development. However, chondrogenic potential in membrane bone is evidenced by several cartilage-associated phenomena in vivo. Furthermore, in vitro, periosteal cells of some membrane bones express cartilage phenotype gene products and even differentiate into chondrocytes. Hence, membrane bone periosteal cells can undergo chondrogenic differentiation. The precursor of chondrogenic cells in membrane bone is not clear: chondrocytes were proposed to arise from unipotential chondroprogenitor cells, bi- or multipotential progenitor cells, or differentiated osteogenic cells. There is experimental support for each, but studies on clonal and cell cultures provided more support for a common precursor of both chondro- and osteogenic cells. Moreover, in periostea, chondrogenesis probably arises from a differentiated cell type. Membrane bone formation in periostea may include a transient cell stage that is able to undergo both osteo- and chondrogenesis. Osteogenesis would be the normal pathway, but chondrogenesis can be evoked in certain microenvironments. It is not known whether microenvironmental factors trigger chondrogenesis through a universal molecular mechanism, nor is the molecule that triggers chondrogenesis known. Expression of neural cell adhesion molecule (NCAM) is down-regulated during commitment of periostal cells for secondary chondrogenesis, suggesting a possible regulatory role for NCAM in the alternative differentiation pathways of periosteal cells.

Notes: Summary Neural crestectomies were performed on neurula stage medaka embryos to remove neural crest with tungsten needles from one of five anteriorly located zones. The embryos were allowed to develop to stage 35 (immediately posthatching) larvae, then cleared and stained for cartilage. An analysis of changes to the head skeletons indicated that most of the anterior neurocranium and the entire viscerocranium received neural crest contributions during development. The elements involved included; the lamina orbitonasalis of the nasal capsule, the trabeculae, Meckels' cartilage and the quadrate of the lower jaw, the pterygoid process, the orbital cartilages and the epiphyseals of the neurocranial roof, as well as all the elements of the hyoid and branchial arches. By further analysis of only those neural crest ablations which produced alterations to the head skeleton, the neural crest cells which contributed to the development of each element were mapped. They originated principally, from one of three regions; the mesencephalon (second most anterior zone removed, number II), the preotic rhombencephalon (zone III), or the postotic rhombencephalon (zone IV). Neural crest from the level of the prosencephalon (zone I) was not chondrogenic nor was neural crest from the fifth region (zone V) which extended beyond the 5th to about the 8th or 10th somite and marked the anterior end of trunk neural crest. The data are discussed and are found to be consistent with the results from other vertebrates and support the central role of the neural crest in the development and evolution of the vertebrate bead skeleton.

Notes: 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.

Notes: Among vertebrates, some teleosts are unique in having bone which lacks osteocytes embedded in the matrix. The fate of cells that secrete the matrix of these acellular bones has not been investigated thoroughly. Histological and fluorescent microscopic analysis of the vertebral bone of Oryzias latipes demonstrated that acellularity is not a secondary appearance of an early cellular bone during ontogeny. Vertebral bone is devoid of cells embedded in the matrix throughout development. Cells that secrete bone matrix do not become trapped in their own secretion. Instead, they always remain as a surface layer over the outer surface of the bone. Fluorescent microscopic visualization of tetracycline injected into growing fish demonstrated that bone was only deposited by osteoblasts lining the outer surface of the bone; no deposition of bone took place on the inner surface.

Notes: A series of studies by Edgeworth demonstrated that cranial muscles of gnathostome fishes are embryologically of somitic origin, originating from the mandibular, hyoid, branchial, epibranchial, and hypobranchial muscle plates. Recent experimental studies using quail-chick chimeras support Edgeworth's view on the developmental origin of cranial muscles. One of his findings, the existence of the premyogenic condensation constrictor dorsalis in teleost fishes, has also been confirmed by molecular developmental studies. Therefore, developmental mechanisms for patterning of cranial muscles, as described and implicated by Edgeworth, may serve as structural entities or regulatory phenomena responsible for developmental and evolutionary changes. With Edgeworth's and other studies as background, muscles in the ventral gill arch region of batoid fishes are analyzed and compared with those of other gnathostome fishes. The spiracularis is regarded as homologous at least within batoid fishes, but its status within elasmobranchs remains unclear; developmental modifications of the spiracularis proper are evident in some batoid fishes and in several shark groups. The peculiar ventral extension of the spiracularis in electric rays and some stingrays may represent convergence, probably facilitating ventilation and/or feeding in both groups. The evolutionary origin of the “internus” and “externus” remains uncertain, despite the fact that a variety of forms of the constrictor superficiales ventrales in batoid fishes indicates an actual medio-ventral extension of the “externus.” The intermandibularis is probably present only in electric rays. The “X” muscle occurs only in electric rays and is considered to be Edgeworth's intermandibularis profundus. Its association with the adductor mandibular complex in narkinidid and narcinidid electric rays may relate to its functional role in lower jaw movement. Contrary to common belief, in most batoid fishes as well as some sharks, muscles that originate from the branchial muscle plate and extend medially in the ventral gill arches do exist: the medial extension of the interbranchiales in most batoid fishes and some sharks and the “Y” muscle in the pelagic stingrays Myliobatos and Rhinoptera. The latter is another example of the medial extension of the “internus.” Whether the interbranchiales and “Y” muscle are homologous within elasmobranchs and whether homologous with the obliques ventrales and/or transversi ventrales of osteichthyan fishes await further research. Four hypobranchial muscles are recognized in batoid fishes: the coracomandibularis, coracohyoideus, coracoarcualis, and coracohyomandibularis. The coracohyoideus is discrete from the coracoarcualis; its complete structural separation from the latter occurs in several groups of batoid fishes. The sternohyoideus of osteichthyan fishes is regarded as a partially developed, continuous bundle of muscle whose counterpart in chondrichthyan fishes appears to be the fully developed rectus cervicus in holocephalans and the squaloid shark Isistius. The coracoarculais is, therefore, present structurally and possibly functionally as a discrete muscle only in elasmobranchs. Although the coracohyomandibularis has been regarded as unique in batoid fishes, the first coracobranchialis in the sawshark Pristiophorus may represent the coracohyomandibularis. The conceptual frameworks and results of the development and evolution of cranial muscles presented here emphasize the importance of molecular and experimental embryological studies and integration of these areas with comparative anatomical and functional studies. Edgeworth's contributions remain as a remarkable achievement in muscle biology. © 1992 Wiley-Liss, Inc.

Notes: 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.

Notes: Haematoxylin, Alcian Blue-Chlorantine Fast Red (ABCR) and the Ralis osteoid-specific stain were employed to closely follow the histogenesis of the tibia of the embryonic chick so as to provide an accurate description of the onset of ossification.An overview of the major cytological events preceding osteogenesis in the tibia was obtained from hindlimbs of embryos of H. H. (Hamburger and Hamilton, '51) stages 16-26 (2.5-5 days of incubation) stained with ABCR. A description of the cytological changes in the periosteum as it develops from the perichondrium and an analysis of the timing of the onset of osteoid deposition was obtained from the tibiae of accurately aged and staged embryos of H. H. stages 28-32 (5.5-8 days). These tibiae were stained specifically for the detection of osteoid:the freshly-secreted, unmineralized product of fully-differentiated osteoblasts. The perichondrium transformed into a bi-layered periosteum at H. H. late stage 29 (6.5 days) while osteoid was first detected adjacent to the hypertrophic cartilage of H. H. stage 30 (6.5-7 days) tibial diaphyses.These results, correlated with the immunoflourescent studies of Von der Mark et al. ('76a,b), which revealed the presence of Type I (bone-type) collagen-synthesizing cells in the perichondria of tibiae from embryos of H. H. stage 28 (5.5-6 days), demonstrated that the onset of determination of cells for osteogenesis and the cytodifferentiation of the periosteum are not temporally coupled.