The role of embryo movement in the development of the furcula

The pectoral girdle is a complex structure which varies in its morphology between species. A major component in birds is the furcula, which can be considered equivalent to a fusion of the paired clavicles found in many mammals, and the single interclavicle found in many reptiles. These elements are a remnant of the dermal skeleton and the only intramembranous bones in the trunk. Postnatally, the furcula plays important mechanical roles by stabilising the shoulder joint and acting as a mechanical spring during flight. In line with its mechanical role, previous studies indicate that, unlike many other intramembranous bones, furcula growth during development can be influenced by mechanical stimuli. This study investigated the response of individual aspects of furcula growth to both embryo immobilisation and hypermotility in the embryonic chicken. The impact of altered incubation temperature, which influences embryo motility, on crocodilian interclavicle development was also explored. We employed whole-mount bone and cartilage staining and 3D imaging by microCT to quantify the impact of rigid paralysis, flaccid paralysis and hypermobility on furcula growth in the chicken, and 3D microCT imaging to quantify the impact of reduced temperature (32-28 °C) and motility on interclavicle growth in the crocodile. This revealed that the growth rates of the clavicular and interclavicular components of the furcula differ during normal development. Total furcula area was reduced by total unloading produced by flaccid paralysis, but not by rigid paralysis which maintains static loading of embryonic bones. This suggests that dynamic loading, which is required for postnatal bone adaptation, is not a requirement for prenatal furcula growth. Embryo hypermotility also had no impact on furcula area or arm length. Furcula 3D shape did, however, differ between groups; this was marked in the interclavicular component of the furcula, the hypocleideum. Hypocleideum length was reduced by both methods of immobilisation, and interclavicle area was reduced in crocodile embryos incubated at 28 °C, which are less motile than embryos incubated at 32 °C. These data suggest that the clavicular and interclavicle components of the avian furcula respond differently to alterations in embryo movement, with the interclavicle requiring both the static and dynamic components of movement-related loading for normal growth, while static loading preserved most aspects of clavicle growth. Our data suggest that embryo movement, and the mechanical loading this produces, is important in shaping these structures during development to suit their postnatal mechanical roles.

Effect of species-specific sound stimulation on the development and hatching of broiler chicks

1. Previous research has reported that chicken embryos develop a functionary auditory system during incubation and that prenatal sound may play an important role in embryo development and alter the hatch time. In this study the effects of prenatal auditory stimulation on hatch process, hatch performance, the development of embryo and blood parameters were investigated. 2. Four batches of Ross 308 broiler breeder eggs were incubated either in control or in sound-stimulated groups. The sound-stimulated embryos were exposed to a discontinuous sound of species-specific calls by means of a speaker at 72 dB for 16 h a day: maternal calls from d 10 to d 19 of incubation time and embryo/chick calls from d 19 until hatching. The species-specific sound was excluded from the control group. 3. The onset of hatch was delayed in the sound-stimulated group compared to the controls. This was also supported by comparison of the exact hatching time of individual focal chicks within the two groups. However, the sound-stimulated embryos had a lower hatchability than the control group, mainly due to significantly increased numbers of late deaths. 4. The embryos exhibited a similar growth pattern between the sound-stimulated group and the control group. Although sound exposure decreased body weight at d 16, no consistent effect of sound on body weight at incubation stage was observed. Species-specific sound stimulation also had no impact on chick quality, blood values and plasma corticosterone concentrations during hatch.

Mechanoadaptation of developing limbs: shaking a leg

The proportion of total limb length taken up by the individual skeletal elements (limb proportionality), varies widely between species. These diverse skeletal forms have evolved to allow for a range of limb uses and they first emerge as the embryo develops, to achieve the characteristic skeletal architecture of each species. During this time, the developing skeleton experiences mechanical loading as a result of embryonic muscle contraction. The possibility that adaptation to such mechanical input may allow embryos to coordinate the appearance of skeletal design with their expanding range of movements has so far received little attention. This is surprising, given the critical role exerted by embryo movement in normal skeletal development; stage-specific in ovo immobilisation of embryonic chicks results in joint contractures and a reduction in longitudinal bone growth in the limbs. Epigenetic mechanisms allow for selective activation of genes in response to environmental signals, resulting in the production of phenotypic complexity in morphogenesis; mechanical loading of bone during movement appears to be one such signal. It may be that 'mechanosensitive' genes under regulation of mechanical input adjust proportionality along the bone's proximo-distal axis, introducing a level of phenotypic plasticity. If this hypothesis is upheld, species with more elongated distal limb elements will have a greater dependence on mechanical input for the differences in their growth, and mechanosensitive bone growth in the embryo may have evolved as an additional source of phenotypic diversity during skeletal development.

Effect of transportation duration of 1-day-old chicks on postplacement production performances and pododermatitis of broilers up to slaughter age

This experiment studied the effect of transportation duration of 1-d-old chicks on dehydration, mortality, production performance, and pododermatitis during the growout period. Eggs from the same breeder flock (Ross PM3) were collected at 35, 45, and 56 wk of age, for 3 successive identical experiments. In each experiment, newly hatched chicks received 1 of 3 transportation duration treatments from the hatchery before placement in the on-site rearing facility: no transportation corresponding to direct placement in less than 5 min (T00), or 4 (T04) or 10 h (T10) of transportation. The chicks were housed in 35-m(2) pens (650 birds each) and reared until 35 d old. Hematocrit and chick BW were measured on sample chicks before and after transportation. During the growout period, bird weight, feed uptake, and feed conversion ratio were measured weekly until slaughter. Transportation duration affected BW; T00 groups had a significantly higher BW than T04 and T10 transported birds but this effect lasted only until d 21. No clear effect on hematocrit, feed uptake, feed conversion ratio, or mortality was observed for birds transported up to 10 h. The decrease in weight in T10 birds was associated with less severe pododermatitis. Increasing age of the breeder flock was correlated with reduced egg fertility and hatchability, and also with higher quality and BW of hatched chicks. Chicks from older breeders also exhibited reduced mortality during the growout period.

Cartilage to bone transitions in health and disease

Aberrant redeployment of the 'transient' events responsible for bone development and postnatal longitudinal growth has been reported in some diseases in what is otherwise inherently 'stable' cartilage. Lessons may be learnt from the molecular mechanisms underpinning transient chondrocyte differentiation and function, and their application may better identify disease aetiology. Here, we review the current evidence supporting this possibility. We firstly outline endochondral ossification and the cellular and physiological mechanisms by which it is controlled in the postnatal growth plate. We then compare the biology of these transient cartilaginous structures to the inherently stable articular cartilage. Finally, we highlight specific scenarios in which the redeployment of these embryonic processes may contribute to disease development, with the foresight that deciphering those mechanisms regulating pathological changes and loss of cartilage stability will aid future research into effective disease-modifying therapies.

The retinal pigment epithelium of the eye regulates the development of scleral cartilage

The majority of vertebrate species have a layer of hyaline cartilage within the fibrous sclera giving an extra degree of support to the eyeball. In chicks, this is seen as a cuplike structure throughout the scleral layer. However, the mechanisms that control the development of scleral cartilage are largely unknown. Here we have studied the phases of scleral cartilage development and characterised expression profiles of genes activated during the cartilage differentiation programme. CART1 and SOX9, the earliest markers of pre-committed cartilage, are expressed in the mesenchyme surrounding the optic cup. Later AGGRECAN, a matrix protein expressed during chondrocyte differentiation, is also expressed. The expression of these genes is lost following early removal of the optic cup, suggesting a role for this tissue in inducing scleral cartilage. By grafting young retinal pigment epithelium (RPE) and retina into cranial mesenchyme in vivo, it was found that RPE alone has the ability to induce cartilage formation. There are some exceptions within the vertebrates where scleral cartilage is not present; one such example is the placental mammals. However, we found that the cartilage differentiation pathway is initiated in mice as seen by the expression of Cart1 and Sox9, but expression of the later cartilage marker Aggrecan is weak. Furthermore, cartilage forms in mouse peri-ocular mesenchyme micromass culture. This suggests that the process halts in vivo before full differentiation into cartilage, but that murine scleral mesenchyme has retained the potential to make cartilage in vitro. RA, Wnts and Bmps have been linked to the cartilage development process and are expressed within the developing RPE. We find that RA may have a role in early scleral cartilage development but is not likely to be the main factor involved. These data reveal the course of scleral cartilage formation and highlight the key role that the optic cup plays in this process. The driving element within the optic cup is almost certainly the retinal pigmented epithelium.

Evidence that the canonical Wnt signalling pathway regulates deer antler regeneration

Wnt signalling regulates many developmental processes, including the fate specification, polarity, migration, and proliferation of cranial neural crest. The canonical Wnt pathway has also been shown to play an important role in bone physiology and there is evidence for its recapitulation during organ regeneration in lower vertebrates. This study explores the role of the Wnt signalling pathway in deer antlers, frontal bone appendages that are the only mammalian organs capable of regeneration. Immunocytochemistry was used to map the distribution of the activated form of beta-catenin ((a)betaCAT). A low level of (a)betaCAT staining was detected in chondrocytes and in osteoblasts at sites of endochondral bone formation. However, (a)betaCAT was localised in cellular periosteum and in osteoblasts in intramembranous bone, where it co-localised with osteocalcin. The most intense (a)betaCAT staining was in dividing undifferentiated cells in the mesenchymal growth zone. Antler progenitor cells (APCs) were cultured from this region and when the canonical Wnt pathway was inhibited at the level of Lef/TCF by epigallocatechin gallate (EGCG), the cell number decreased. TUNEL staining revealed that this was as a result of increased apoptosis. Activation of the pathway by lithium chloride (LiCl) had no effect on cell number but inhibited alkaline phosphate activity (ALP), a marker of APC differentiation, whereas EGCG increased ALP activity. This study demonstrates that beta-catenin plays an important role in the regulation of antler progenitor cell survival and cell fate. It also provides evidence that beta-catenin's function in regulating bone formation by osteoblasts may be site-specific.

Fishing for gene function--endocrine modelling in the zebrafish

The use of zebrafish (Danio rerio) in scientific research is growing rapidly. It initially became popular as a model of vertebrate development because zebrafish embryos develop rapidly and are transparent. In the past 5 years, the sequencing of the zebrafish genome has increased the profile of zebrafish research even further, expanding into other areas such as pharmacology, cancer research and drug discovery. The use of zebrafish in endocrine research has mainly been confined to the study of the development of endocrine organs. However, it is likely to be a useful model in other areas of endocrinology, as there are a wide variety of both forward and reverse genetic techniques that can be employed in the zebrafish to decipher gene function in disease states. In this review, we compare the endocrine system of the zebrafish to mouse and human, demonstrating that the systems are sufficiently similar for zebrafish to be employed as a model for endocrine research. We subsequently review the repertoire of genetic techniques commonly employed in the zebrafish model to understand gene function in vertebrate development and disease. We anticipate that the use of these techniques will make the zebrafish a prominent model in endocrine research in the coming years.

Afuni, a novel transforming growth factor-β gene is involved in arm regeneration by the brittle star Amphiura filiformis

The bone morphogenetic proteins (BMPs) are a family of the transforming growth factor-beta (TGF-beta) superfamily that perform multiple roles during vertebrate and invertebrate development. Here, we report the molecular cloning of a novel BMP from regenerating arms of the ophiuroid Amphiura filiformis. The theoretically translated amino acid sequence of this novel BMP has high similarity to that of the sea urchin BMP univin. This novel BMP has been named afuni. Whole-mount in situ hybridisation implicates afuni in arm regeneration. Expression occurs in distinct proximal and distal regions of late regenerates (3- and 5-week postablation). These sites are at different stages of regeneration, suggesting multiple roles for this gene in adult arm development. Cellular expression of this gene occurs in migratory cells within the radial water canal (RWC) of regenerating and nonregenerating arms. These migrating coelomocytes suggest a key role for the coelomic RWC as a source of the cellular material for use in arm regeneration by A. filiformis.

Connexin43 gap junction protein plays an essential role in morphogenesis of the embryonic chick face

Normal outgrowth and fusion of facial primordia during vertebrate development require interaction of diverse tissues and co-ordination of many different signalling pathways. Gap junction channels, made up of subunits consisting of connexin proteins, facilitate communication between cells and are implicated in embryonic development. Here we describe the distribution of connexin43 and connexin32 gap junction proteins in the developing chick face. To test the function of connexin43 protein, we applied antisense oligodeoxynucleotides that specifically reduced levels of connexin43 protein in cells of early chick facial primordia. This resulted in stunting of primordia outgrowth and led to facial defects. Furthermore, cell proliferation in regions of facial primordia that normally express high levels of connexin43 protein was reduced and this was associated with lower levels of Msx-1 expression. Facial defects arise when retinoic acid is applied to the face of chick embryos at later stages. This treatment also resulted in significant reduction in connexin43 protein, while connexin32 protein expression was unaffected. Taken together, these results indicate that connexin43 plays an essential role during early morphogenesis and subsequent outgrowth of the developing chick face.

A population of caudally migrating cranial neural crest cells: functional and evolutionary implications

The deployment of the cranial neural crest is central to the patterning of the skeletomuscular elements of the vertebrate head, with cranial muscles invariably attaching to skeletal elements formed by crest from the same axial level. Here we demonstrate, through gene expression analysis, ablation studies and fate-mapping, the existence of a population of caudally migrating cranial crest that arise from the postotic neural tube. As with the rest of the postotic crest, these cells express the transcription factor Mafb, and this marker can be used to highlight their posterior migration. They pass out between the anterior somite and the otic vesicle, before turning caudally and running along the base of the somites. With long-term fate mapping, we show that these cells migrate to the clavicle and settle at the site of formation of the attachment point for the cleidohyoid muscle. As such, the influence of the cranial neural crest in organising skeletomuscular connectivity seems to extend beyond the head into the trunk. These results are of further importance as they help explain how, even though the pectoral girdle and the skull became physically dissociated during tetrapod evolution, skeletomuscular connectivity has been maintained.

The evolution of the pectoral girdle

The pectoral girdle articulates the forelimb with the axial skeleton in all vertebrates with paired anterior appendages. The structure of the pectoral girdle and its position along the axial skeleton has changed significantly during vertebrate evolution. These morphological changes have been well described, but there is little comparative embryology to indicate how these changes may have occurred. It is equally obscure how the muscles that connect the head with the pectoral girdle have maintained appropriate attachments even though these 2 structures have become separated. Here I review the changes in the pectoral girdle across different vertebrate taxa, indicating, where known, the developmental mechanisms underlying these changes. I also suggest how the muscular connections between the head and pectoral girdle have been maintained between these once adjacent bones, displaced during vertebrate evolution.

Fate map of the developing chick face: analysis of expansion of facial primordia and establishment of the primary palate

Developing facial primordia change shape substantially in stages leading up to primary palate formation. We investigated expansion of cell populations within each of the four facial primordia of chick embryos between HH-stages 20 and 28, by using DiI labelling. Populations of cells centred around the nasal pits in the upper face, the midline of the paired mandibular primordia in the lower face, and at sites of fusion contribute most to overall expansion. Abundant Msx-1 transcripts are found in regions of high expansion, and Fgf-8 transcripts are seen in ectoderm associated with some of these regions. Many cell populations display preferential expansion along one axis. Maxillary and mandibular primordia cell populations expand along the proximodistal axis, whereas at the distal tip of the frontonasal mass, cell populations expand mediolaterally. Thus outgrowth occurs at the tips of mandibular and maxillary primordia, but at the base of the frontonasal mass. At regions where adjacent primordia abut each other, we found bidirectional movement of cells between primordia, unidirectional movement or could detect no movement at all. Regions of highest expansion in each primordium have the highest percentage of S phase labelled cells. Cell death occurs in some regions of low expansion but it seems likely that cell rearrangements and intercalations also contribute to shaping. These rearrangements could be associated with stretching of the primordia by neighbouring tissues. Treatment of chick embryos with retinoic acid causes clefts of the primary palate (Tamarin et al. [1984] J. Embryol. Exp. Morphol. 84:105-123). We found a decrease in expansion of cell populations that normally contribute to primary palate formation but surprisingly little ectopic cell death. Expansion of other cell populations in the treated upper face was more even rather than directed. This further supports the idea that tension exerted by neighbouring tissues plays a major role in global shaping of the upper face.