Somite formation and patterning

Insight into unique somitogenesis of yak (Bos grunniens) with one additional thoracic vertebra

Background

The yak is a species of livestock which is crucial for local communities of the Qinghai-Tibet Plateau and adjacent regions and naturally owns one more thoracic vertebra than cattle. Recently, a sub-population of yak termed as the Jinchuan yak has been identified with over half its members own a thoracolumbar vertebral formula of T15L5 instead of the natural T14L5 arrangement. The novel T15L5 positioning is a preferred genetic trait leading to enhanced meat and milk production. Selective breeding of this trait would have great agricultural value and exploration of the molecular mechanisms underlying this trait would both accelerate this process and provide us insight into the development and regulation of somitogenesis.

Results

Here we investigated the genetic background of the Jinchuan yak through resequencing fifteen individuals, comprising five T15L5 individuals and ten T14L5 individuals with an average sequencing depth of > 10X, whose thoracolumbar vertebral formulae were confirmed by anatomical observation. Principal component analysis, linkage disequilibrium analysis, phylogenetic analysis, and selective sweep analysis were carried out to explore Jinchuan yak’s genetic background. Three hundred and thirty candidate markers were identified as associated with the additional thoracic vertebrae and target sequencing was used to validate seven carefully selected markers in an additional 51 Jinchuan yaks. The accuracies of predicting 15 thoracic vertebrae and 20 thoracolumbar vertebrae with these 7 markers were 100.00 and 33.33% despite they both could only represent 20% of all possible genetic diversity. Two genes, PPP2R2B and TBLR1, were found to harbour the most candidate markers associated with the trait and likely contribute to the unique somitic number and identity according to their reported roles in the mechanism of somitogenesis.

Conclusions

Our findings provide a clear depiction of the Jinchuan yak’s genetic background and a solid foundation for marker-assistant selection. Further exploitation of this unique population and trait could be promoted with the aid of our genomic resource.

Making muscle: skeletal myogenesis in vivo and in vitro

Skeletal muscle is the largest tissue in the body and loss of its function or its regenerative properties results in debilitating musculoskeletal disorders. Understanding the mechanisms that drive skeletal muscle formation will not only help to unravel the molecular basis of skeletal muscle diseases, but also provide a roadmap for recapitulating skeletal myogenesis in vitro from pluripotent stem cells (PSCs). PSCs have become an important tool for probing developmental questions, while differentiated cell types allow the development of novel therapeutic strategies. In this Review, we provide a comprehensive overview of skeletal myogenesis from the earliest premyogenic progenitor stage to terminally differentiated myofibers, and discuss how this knowledge has been applied to differentiate PSCs into muscle fibers and their progenitors in vitro.

Elevated CD26 Expression by Skin Fibroblasts Distinguishes a Profibrotic Phenotype Involved in Scar Formation Compared to Gingival Fibroblasts

Compared to skin, wound healing in oral mucosa is faster and produces less scarring, but the mechanisms involved are incompletely understood. Studies in mice have linked high expression of CD26 to a profibrotic fibroblast phenotype, but this has not been tested in models more relevant for humans. We hypothesized that CD26 is highly expressed by human skin fibroblasts (SFBLs), and this associates with a profibrotic phenotype distinct from gingival fibroblasts (GFBLs). We compared CD26 expression in human gingiva and skin and in gingival and hypertrophic-like scar-forming skin wound healing in a pig model, and used three-dimensional cultures of human GFBLs and SFBLs. In both humans and pigs, nonwounded skin contained abundantly CD26-positive fibroblasts, whereas in gingiva they were rare. During skin wound healing, CD26-positive cells accumulated over time and persisted in forming hypertrophic-like scars, whereas few CD26-positive cells were present in the regenerated gingival wounds. Cultured human SFBLs displayed significantly higher levels of CD26 than GFBLs. This was associated with an increased expression of profibrotic genes and transforming growth factor-β signaling in SFBLs. The profibrotic phenotype of SFBLs partially depended on expression of CD26, but was independent of its catalytic activity. Thus, a CD26-positive fibroblast population that is abundant in human skin but not in gingiva may drive the profibrotic response leading to excessive scarring.

Targeted Deletion of Btg1 and Btg2 Results in Homeotic Transformation of the Axial Skeleton

Btg1 and Btg2 encode highly homologous proteins that are broadly expressed in different cell lineages, and have been implicated in different types of cancer. Btg1 and Btg2 have been shown to modulate the function of different transcriptional regulators, including Hox and Smad transcription factors. In this study, we examined the in vivo role of the mouse Btg1 and Btg2 genes in specifying the regional identity of the axial skeleton. Therefore, we examined the phenotype of Btg1 and Btg2 single knockout mice, as well as novel generated Btg1^-/-;Btg2^-/- double knockout mice, which were viable, but displayed a non-mendelian inheritance and smaller litter size. We observed both unique and overlapping phenotypes reminiscent of homeotic transformation along the anterior-posterior axis in the single and combined Btg1 and Btg2 knockout animals. Both Btg1^-/- and Btg2^-/- mice displayed partial posterior transformation of the seventh cervical vertebra, which was more pronounced in Btg1^-/-;Btg2^-/- mice, demonstrating that Btg1 and Btg2 act in synergy. Loss of Btg2, but not Btg1, was sufficient for complete posterior transformation of the thirteenth thoracic vertebra to the first lumbar vertebra. Moreover, Btg2^-/- animals displayed complete posterior transformation of the sixth lumbar vertebra to the first sacral vertebra, which was only partially present at a low frequency in Btg1^-/- mice. The Btg1^-/-;Btg2^-/- animals showed an even stronger phenotype, with L5 to S1 transformation. Together, these data show that both Btg1 and Btg2 are required for normal vertebral patterning of the axial skeleton, but each gene contributes differently in specifying the identity along the anterior-posterior axis of the skeleton.

Specification of chondrocytes and cartilage tissues from embryonic stem cells

Osteoarthritis primarily affects the articular cartilage of synovial joints. Cell and/or cartilage replacement is a promising therapy, provided there is access to appropriate tissue and sufficient numbers of articular chondrocytes. Embryonic stem cells (ESCs) represent a potentially unlimited source of chondrocytes and tissues as they can generate a broad spectrum of cell types under appropriate conditions in vitro. Here, we demonstrate that mouse ESC-derived chondrogenic mesoderm arises from a Flk-1(-)/Pdgfrα(+) (F(-)P(+)) population that emerges in a defined temporal pattern following the development of an early cardiogenic F(-)P(+) population. Specification of the late-arising F(-)P(+) population with BMP4 generated a highly enriched population of chondrocytes expressing genes associated with growth plate hypertrophic chondrocytes. By contrast, specification with Gdf5, together with inhibition of hedgehog and BMP signaling pathways, generated a population of non-hypertrophic chondrocytes that displayed properties of articular chondrocytes. The two chondrocyte populations retained their hypertrophic and non-hypertrophic properties when induced to generate spatially organized proteoglycan-rich cartilage-like tissue in vitro. Transplantation of either type of chondrocyte, or tissue generated from them, into immunodeficient recipients resulted in the development of cartilage tissue and bone within an 8-week period. Significant ossification was not observed when the tissue was transplanted into osteoblast-depleted mice or into diffusion chambers that prevent vascularization. Thus, through stage-specific manipulation of appropriate signaling pathways it is possible to efficiently and reproducibly derive hypertrophic and non-hypertrophic chondrocyte populations from mouse ESCs that are able to generate distinct cartilage-like tissue in vitro and maintain a cartilage tissue phenotype within an avascular and/or osteoblast-free niche in vivo.

Mir-125a-5p-mediated regulation of Lfng is essential for the avian segmentation clock

Somites are embryonic precursors of the axial skeleton and skeletal muscles and establish the segmental vertebrate body plan. Somitogenesis is controlled in part by a segmentation clock that requires oscillatory expression of genes including Lunatic fringe (Lfng). Oscillatory genes must be tightly regulated at both the transcriptional and posttranscriptional levels for proper clock function. Here, we demonstrate that microRNA-mediated regulation of Lfng is essential for proper segmentation during chick somitogenesis. We find that mir-125a-5p targets evolutionarily conserved sequences in the Lfng 3' UTR and that preventing interactions between mir-125a-5p and Lfng transcripts in vivo causes abnormal segmentation and perturbs clock activity. This provides strong evidence that microRNAs function in the posttranscriptional regulation of oscillatory genes in the segmentation clock. Further, this demonstrates that the relatively subtle effects of microRNAs on target genes can have broad effects in developmental situations that have critical requirements for tight posttranscriptional regulation.

Molecular diagnosis of vertebral segmentation disorders in humans

BACKGROUND

Vertebral malformations contribute substantially to the pathophysiology of kyphosis and scoliosis, common health problems associated with back and neck pain, disability, cosmetic disfigurement and functional distress.

OBJECTIVE

To provide an overview of the current understanding of vertebral malformations, at both the clinical level and the molecular level, and factors that contribute to their occurrence.

METHODS

The literature related to the following was reviewed: recent advances in the understanding of the molecular embryology underlying vertebral development and relevance to elucidation of etiologies of several known human vertebral malformation syndromes; outcomes of molecular studies elucidating genetic contributions to congenital and sporadic vertebral malformations; and complex interrelationships between genetic and environmental factors that contribute to the pathogenesis of isolated syndromic and non-syndromic congenital vertebral malformations.

RESULTS/CONCLUSION

Expert opinions extend to discussion of the importance of establishing improved classification systems for vertebral malformation, future directions in molecular and genetic research approaches to vertebral malformation and translational value of research efforts to clinical management and genetic counseling of affected individuals and their families.

Ndrg2 regulates vertebral specification in differentiating somites

It is generally thought that vertebral patterning and identity are globally determined prior to somite formation. Relatively little is known about the regulators of vertebral specification after somite segmentation. Here, we demonstrated that Ndrg2, a tumor suppressor gene, was dynamically expressed in the presomitic mesoderm (PSM) and at early stage of differentiating somites. Loss of Ndrg2 in mice resulted in vertebral homeotic transformations in thoracic/lumbar and lumbar/sacral transitional regions in a dose-dependent manner. Interestingly, the inactivation of Ndrg2 in osteoblasts or chondrocytes caused defects resembling those observed in Ndrg2(-/-) mice, with a lower penetrance. In addition, forced overexpression of Ndrg2 in osteoblasts or chondrocytes also conferred vertebral defects, which were distinct from those in Ndrg2(-/-) mice. These genetic analyses revealed that Ndrg2 modulates vertebral identity in segmented somites rather than in the PSM. At the molecular level, combinatory alterations of the amount of Hoxc8-11 gene transcripts were detected in the differentiating somites of Ndrg2(-/-) embryos, which may partially account for the vertebral defects in Ndrg2 mutants. Nevertheless, Bmp/Smad signaling activity was elevated in the differentiating somites of Ndrg2(-/-) embryos. Collectively, our findings unveiled Ndrg2 as a novel regulator of vertebral specification in differentiating somites.

Laminins, via heparan sulfate proteoglycans, participate in zebrafish myotome morphogenesis by modulating the pattern of Bmp responsiveness

In zebrafish, Hedgehog-induced Engrailed expression defines a muscle fibre population that includes both slow and fast fibre types and exhibits an organisational role on myotome and surrounding tissues, such as motoneurons and lateral line. This Engrailed-positive population is restricted in the myotome to a central domain. To understand how this population is established, we have analysed the phenotype of the sly/lamc1 mutation in the Laminin γ1 chain that was shown to specifically affect Engrailed expression in pioneers. We find that the sly mutation affects Engrailed expression in the entire central domain and that Hedgehog signalling does not mediate this effect. We show that Bmp-responding cells are excluded from the central domain and that this pattern is modulated by laminins, but not by Hedgehog signalling. Knockdown of Bmp signalling rescues Engrailed expression in the sly mutant and ectopically activates Engrailed expression in slow and fast lineages in wild-type embryos. Last, extracellular matrix-associated heparan sulfate proteoglycans are absent in sly and their enzymatic removal mimics the sly phenotype. Our results therefore show that laminins, via heparan sulfate proteoglycans, are instrumental in patterning Bmp responsiveness and that Bmp signalling restricts Engrailed expression to the central domain. This study underlines the importance of extracellular cues for the precise spatial modulation of cell response to morphogens.

The segmentation clock mechanism moves up a notch

The vertebrate segmentation clock is a molecular oscillator that regulates the periodicity of somite formation. Three signalling pathways have been proposed to underlie the molecular mechanism of the oscillator, namely the Notch, Wnt and Fgf pathways. Characterizing the roles and hierarchy of these three pathways in the oscillator mechanism is currently the focus of intense research. Recent publications report the first identification of a molecular mechanism involved in the regulation of the pace of this oscillator. We review these and other recent findings regarding the interaction between the three pathways in the oscillator mechanism that have significantly expanded our understanding of the segmentation clock.

Hox genes and regional patterning of the vertebrate body plan

Several decades have passed since the discovery of Hox genes in the fruit fly Drosophila melanogaster. Their unique ability to regulate morphologies along the anteroposterior (AP) axis (Lewis, 1978) earned them well-deserved attention as important regulators of embryonic development. Phenotypes due to loss- and gain-of-function mutations in mouse Hox genes have revealed that the spatio-temporally controlled expression of these genes is critical for the correct morphogenesis of embryonic axial structures. Here, we review recent novel insight into the modalities of Hox protein function in imparting specific identity to anatomical regions of the vertebral column, and in controlling the emergence of these tissues concomitantly with providing them with axial identity. The control of these functions must have been intimately linked to the shaping of the body plan during evolution.

Homozygous inactivating mutations in the NKX3-2 gene result in spondylo-megaepiphyseal-metaphyseal dysplasia

Spondylo-megaepiphyseal-metaphyseal dysplasia (SMMD) is a rare skeletal dysplasia with only a few cases reported in the literature. Affected individuals have a disproportionate short stature with a short and stiff neck and trunk. The limbs appear relatively long and may show flexion contractures of the distal joints. The most remarkable radiographic features are the delayed and impaired ossification of the vertebral bodies as well as the presence of large epiphyseal ossification centers and wide growth plates in the long tubular bones. Numerous pseudoepiphyses of the short tubular bones in hands and feet are another remarkable feature of the disorder. Genome wide homozygosity mapping followed by a candidate gene approach resulted in the elucidation of the genetic cause in three new consanguineous families with SMMD. Each proband was homozygous for a different inactivating mutation in NKX3-2, a homeobox-containing gene located on chromosome 4p15.33. Striking similarities were found when comparing the vertebral ossification defects in SMMD patients with those observed in the Nkx3-2 null mice. Distinguishing features were the asplenia found in the mutant mice and the radiographic abnormalities in the limbs only observed in SMMD patients. The absence of the latter anomalies in the murine model may be due to the perinatal death of the affected animals. This study illustrates that NKX3-2 plays an important role in endochondral ossification of both the axial and appendicular skeleton in humans. In addition, it defines SMMD as yet another skeletal dysplasia with autosomal-recessive inheritance and a distinct phenotype.

Illuminating cell-cycle progression in the developing zebrafish embryo

By exploiting the cell-cycle-dependent proteolysis of two ubiquitination oscillators, human Cdt1 and geminin, which are the direct substrates of SCF(Skp2) and APC(Cdh1) complexes, respectively, Fucci technique labels mammalian cell nuclei in G(1) and S/G(2)/M phases with different colors. Transgenic mice expressing these G(1) and S/G(2)/M markers offer a powerful means to investigate the coordination of the cell cycle with morphogenetic processes. We attempted to introduce these markers into zebrafish embryos to take advantage of their favorable optical properties. However, although the fundamental mechanisms for cell-cycle control appear to be well conserved among species, the G(1) marker based on the SCF(Skp2)-mediated degradation of human Cdt1 did not work in fish cells, probably because the marker was not ubiquitinated properly by a fish E3 ligase complex. We describe here the generation of a Fucci derivative using zebrafish homologs of Cdt1 and geminin, which provides sweeping views of cell proliferation in whole fish embryos. Remarkably, we discovered two anterior-to-posterior waves of cell-cycle transitions, G(1)/S and M/G(1), in the differentiating notochord. Our study demonstrates the effectiveness of using the Cul4(Ddb1)-mediated Cdt1 degradation pathway common to all metazoans for the development of a G(1) marker that works in the nonmammalian animal model.

Developmental control of segment numbers in vertebrates

Segmentation or metamery in vertebrates is best illustrated by the repetition of the vertebrae and ribs, their associated skeletal muscles and blood vessels, and the spinal nerves and ganglia. The segment number varies tremendously among the different vertebrate species, ranging from as few as six vertebrae in some frogs to as many as several hundred in some snakes and fish. In vertebrates, metameric segments or somites form sequentially during body axis formation. This results in the embryonic axis becoming entirely segmented into metameric units from the level of the otic vesicle almost to the very tip of the tail. The total segment number mostly depends on two parameters: (1) the control of the posterior growth of the body axis during somitogenesis-more same-size segments can be formed in a longer axis and (2) segment size--more smaller--size segments can be formed in a same-size body axis. During evolution, independent variations of these parameters could explain the huge diversity in segment numbers observed among vertebrate species. These variations in segment numbers are accompanied by diversity in the regionalization of the vertebral column. For example, amniotes can exhibit up to five different types of vertebrae: cervical, thoracic, lumbar, sacral and caudal, the number of which varies according to the species. This regionalization of the vertebral column is controlled by the Hox family of transcription factors. We propose that during development, dissociation of the Hox- and segmentation-clock-dependent vertebral patterning systems explains the enormous diversity of vertebral formulae observed in vertebrates.

Anatomical transformation in mammals: developmental origin of aberrant cervical anatomy in tree sloths

Mammalian cervical count has been fixed at seven for more than 200 million years. The rare exceptions to this evolutionary constraint have intrigued anatomists since the time of Cuvier, but the developmental processes that generate them are unknown. Here we evaluate competing hypotheses for the evolutionary origin of cervical variants in Bradypus and Choloepus, tree sloths that have broken the seven cervical vertebrae barrier independently and in opposite directions. Transitional and mediolaterally disjunct anatomy characterizes the cervicothoracic vertebral boundary in each genus, although polarities are reversed. The thoracolumbar, lumbosacral, and sacrocaudal boundaries are also disrupted, and are more extreme in individuals with more extreme cervical counts. Hypotheses of homologous, homeotic, meristic, or associational transformations of traditional vertebral column anatomy are not supported by these data. We identify global homeotic repatterning of abaxial relative to primaxial mesodermal derivatives as the origin of the anomalous cervical counts of tree sloths. This interpretation emphasizes the strong resistance of the "rule of seven" to evolutionary change, as morphological stasis has been maintained primaxially coincident with the generation of a functionally longer (Bradypus) or shorter (Choloepus) neck.

Progress in the understanding of the genetic etiology of vertebral segmentation disorders in humans

Vertebral malformations contribute substantially to the pathophysiology of kyphosis and scoliosis, common health problems associated with back and neck pain, disability, cosmetic disfigurement, and functional distress. This review explores (1) recent advances in the understanding of the molecular embryology underlying vertebral development and relevance to elucidation of etiologies of several known human vertebral malformation syndromes; (2) outcomes of molecular studies elucidating genetic contributions to congenital and sporadic vertebral malformation; and (3) complex interrelationships between genetic and environmental factors that contribute to the pathogenesis of isolated syndromic and nonsyndromic congenital vertebral malformation. Discussion includes exploration of the importance of establishing improved classification systems for vertebral malformation, future directions in molecular and genetic research approaches to vertebral malformation, and translational value of research efforts to clinical management and genetic counseling of affected individuals and their families.

Establishment of Hox vertebral identities in the embryonic spine precursors

The vertebrate spine exhibits two striking characteristics. The first one is the periodic arrangement of its elements-the vertebrae-along the anteroposterior axis. This segmented organization is the result of somitogenesis, which takes place during organogenesis. The segmentation machinery involves a molecular oscillator-the segmentation clock-which delivers a periodic signal controlling somite production. During embryonic axis elongation, this signal is displaced posteriorly by a system of traveling signaling gradients-the wavefront-which depends on the Wnt, FGF, and retinoic acid pathways. The other characteristic feature of the spine is the subdivision of groups of vertebrae into anatomical domains, such as the cervical, thoracic, lumbar, sacral, and caudal regions. This axial regionalization is controlled by a set of transcription factors called Hox genes. Hox genes exhibit nested expression domains in the somites which reflect their linear arrangement along the chromosomes-a property termed colinearity. The colinear disposition of Hox genes expression domains provides a blueprint for the regionalization of the future vertebral territories of the spine. In amniotes, Hox genes are activated in the somite precursors of the epiblast in a temporal colinear sequence and they were proposed to control their progressive ingression into the nascent paraxial mesoderm. Consequently, the positioning of the expression domains of Hox genes along the anteroposterior axis is largely controlled by the timing of Hox activation during gastrulation. Positioning of the somitic Hox domains is subsequently refined through a crosstalk with the segmentation machinery in the presomitic mesoderm. In this review, we focus on our current understanding of the embryonic mechanisms that establish vertebral identities during vertebrate development.

A gradient of Shh establishes mutually repressing somitic cell fates induced by Nkx3.2 and Pax3

Wnt and Sonic Hedgehog (Shh) signals are known to pattern the somite into dermomyotomal, myotomal and sclerotomal cell fates. By employing explants of presomitic mesoderm cultured with constant levels of Wnt3a conditioned medium and increasing levels of Shh, we found that differing levels of Shh signaling elicit differing responses from somitic cells: the lowest level of Shh signaling allows dermomyotomal gene expression, intermediate levels induce loss of dermomyotomal markers and activation of myogenic differentiation, and higher levels induce loss of myotomal markers and activation of sclerotomal gene expression. In addition, we have found that in the presence of high levels of Wnt signaling, instead of inducing sclerotomal markers, Shh signals act to maintain the expression of dermomyotomal and myotomal markers. One of the sclerotomal genes induced by high levels of Shh signaling is Nkx3.2. Forced expression of Nkx3.2 blocks somitic expression of the dermomyotomal marker Pax3 both in vitro and in vivo. Conversely, forced expression of Pax3 in somites can block Shh-mediated induction of sclerotomal gene expression and chondrocyte differentiation in vitro. Thus we propose that varying levels of Shh signaling act in a morphogen-like manner to elicit differing responses from somitic cells, and that Pax3 and Nkx3.2 set up mutually repressing cell fates that promote either dermomyotome/myotome or sclerotome differentiation, respectively.

Mutations in the MESP2 gene cause spondylothoracic dysostosis/Jarcho-Levin syndrome

Spondylothoracic dysostosis (STD), also known as Jarcho-Levin syndrome (JLS), is an autosomal-recessive disorder characterized by abnormal vertebral segmentation and defects affecting spine formation, with complete bilateral fusion of the ribs at the costovertebral junction producing a "crab-like" configuration of the thorax. The shortened spine and trunk can severely affect respiratory function during early childhood. The condition is prevalent in the Puerto Rican population, although it is a panethnic disorder. By sequencing a set of candidate genes involved in mouse segmentation, we identified a recessive E103X nonsense mutation in the mesoderm posterior 2 homolog (MESP2) gene in a patient, of Puerto Rican origin and from the Boston area, who had been diagnosed with STD/JLS. We then analyzed 12 Puerto Rican families with STD probands for the MESP2 E103X mutation. Ten patients were homozygous for the E103X mutation, three patients were compound heterozygous for a second nonsense mutation, E230X, or a missense mutation, L125V, which affects a conserved leucine residue within the bHLH region. Thus, all affected probands harbored the E103X mutation. Our findings suggest a founder-effect mutation in the MESP2 gene as a major cause of the classical Puerto Rican form of STD/JLS.

Segmental patterning of the vertebrate embryonic axis

The body axis of vertebrates is composed of a serial repetition of similar anatomical modules that are called segments or metameres. This particular mode of organization is especially conspicuous at the level of the periodic arrangement of vertebrae in the spine. The segmental pattern is established during embryogenesis when the somites--the embryonic segments of vertebrates--are rhythmically produced from the paraxial mesoderm. This process involves the segmentation clock, which is a travelling oscillator that interacts with a maturation wave called the wavefront to produce the periodic series of somites. Here, we review our current understanding of the segmentation process in vertebrates.

Identification of oscillatory genes in somitogenesis from functional genomic analysis of a human mesenchymal stem cell model

During somitogenesis, oscillatory expression of genes in the notch and wnt signaling pathways plays a key role in regulating segmentation. These oscillations in expression levels are elements of a species-specific developmental mechanism. To date, the periodicity and components of the human clock remain unstudied. Here we show that a human mesenchymal stem/stromal cell (MSC) model can be induced to display oscillatory gene expression. We observed that the known cycling gene HES1 oscillated with a 5 h period consistent with available data on the rate of somitogenesis in humans. We also observed cycling of Hes1 expression in mouse C2C12 myoblasts with a period of 2 h, consistent with previous in vitro and embryonic studies. Furthermore, we used microarray and quantitative PCR (Q-PCR) analysis to identify additional genes that display oscillatory expression both in vitro and in mouse embryos. We confirmed oscillatory expression of the notch pathway gene Maml3 and the wnt pathway gene Nkd2 by whole mount in situ hybridization analysis and Q-PCR. Expression patterns of these genes were disrupted in Wnt3a(tm1Amc) mutants but not in Dll3(pu) mutants. Our results demonstrate that human and mouse in vitro models can recapitulate oscillatory expression observed in embryo and that a number of genes in multiple developmental pathways display dynamic expression in vitro.

Beta-catenin activation is necessary and sufficient to specify the dorsal dermal fate in the mouse

Dorsal dermis and epaxial muscle have been shown to arise from the central dermomyotome in the chick. En1 is a homeobox transcription factor gene expressed in the central dermomyotome. We show by genetic fate mapping in the mouse that En1-expressing cells of the central dermomyotome give rise to dorsal dermis and epaxial muscle and, unexpectedly, to interscapular brown fat. Thus, the En1-expressing central dermomyotome normally gives rise to three distinct fates in mice. Wnt signals are important in early stages of dermomyotome development, but the signal that acts to specify the dermal fate has not been identified. Using a reporter transgene for Wnt signal transduction, we show that the En1-expressing cells directly underneath the surface ectoderm transduce Wnt signals. When the essential Wnt transducer beta-catenin is mutated in En1 cells, it results in the loss of Dermo1-expressing dorsal dermal progenitors and dermis. Conversely, when beta-catenin was activated in En1 cells, it induces Dermo1 expression in all cells of the En1 domain and disrupts muscle gene expression. Our results indicate that the mouse central dermomyotome gives rise to dermis, muscle, and brown fat, and that Wnt signalling normally instructs cells to select the dorsal dermal fate.

Molecular characterization of the rostral-most somites in early somitic stages of the chick embryo

Segmentation consists on the progressive formation of repetitive embryonic structures, named somites, which are formed from the most rostral part of the presomitic mesoderm. Somites are subdivided into anterior and posterior compartments and several genes are differentially expressed in either compartment. This has provided evidence for the importance of establishing the anterior-posterior polarity within each somite, which is critical for the correct segmented pattern of the adult vertebrate body. Although all somites appear morphologically similar, fate map studies have shown that the first 4 somites do not give rise to segmented structures, in contrast to more posterior ones. Moreover, in several somitogenesis-related mutants the anterior somites are not affected while posterior somites present clear defects or do not form at all. Altogether these data suggest relevant differences between rostral and caudal somites. In order to check for molecular differences between anterior and posterior somites, we have performed a detailed expression pattern analysis of several Notch signalling related genes. For the first time, we show that the somitic expression pattern profile is not the same along the anterior-posterior axis and that the differences are not observed always at the same somite level.

Did the first chordates organize without the organizer?

Models of vertebrate development frequently portray the organizer as acting on a largely unpatterned embryo to induce major components of the body plan, such as the neural plate and somites. Recent experiments examining the molecular and genetic basis of major inductive events of vertebrate embryogenesis force a re-examination of this view. These newer observations, along with a proposed revised fate map for the frog Xenopus laevis, suggest a possible reconciliation between the seemingly disparate mechanisms present in the ontogeny of the common chordate body plan of vertebrate and invertebrate chordates. Here, we review data from vertebrates and from an ascidian urochordate and propose that the organizer was not present at the base of the chordate lineage, but could have been a later innovation in the lineage leading to vertebrates, where its role was more permissive than instructive.

Hox genes specify vertebral types in the presomitic mesoderm

We show here that expression of Hoxa10 in the presomitic mesoderm is sufficient to confer a Hox group 10 patterning program to the somite, producing vertebrae without ribs, an effect not achieved when Hoxa10 is expressed in the somites. In addition, Hox group 11-dependent vertebral sacralization requires Hoxa11 expression in the presomitic mesoderm, while their caudal differentiation requires that Hoxa11 is expressed in the somites. Therefore, Hox gene patterning activity is different in the somites and presomitic mesoderm, the latter being very prominent for Hox gene-mediated patterning of the axial skeleton. This is further supported by our finding that inactivation of Gbx2, a homeobox-containing gene expressed in the presomitic mesoderm but not in the somites, produced Hox-like phenotypes in the axial skeleton without affecting Hox gene expression.

Asymmetric cell divisions are concentrated in the dermomyotome dorsomedial lip during epaxial primary myotome morphogenesis

To determine if somitic stem cell pools could be identified by an intrinsic difference in mitotic behaviour, the orientation of mitoses in the dermomyotome epithelium was analysed. We describe a concentration of apico-basal mitoses within the dermomyotome dorsomedial lip (DML). The occurrence of apico-basal divisions is closely associated with asymmetric localisation of the notch pathway factor numb, allowing description of such divisions as asymmetric. In contrast, planar divisions, occurring in the plane of the epithelium, are symmetric. Further, we show that the DML environmental niche is sufficient to promote numb expression in epaxial dermomyotome tissue that does not normally express this factor. These data provide, for the first time, a non-retrospective tracing analysis of the mechanism by which the DML fulfils the stem-cell pool role it plays during epaxial primary myotome morphogenesis.

Hedgehog signaling is required for commitment but not initial induction of slow muscle precursors

In zebrafish, skeletal muscle precursors can adopt at least three distinct fates: fast, non-pioneer slow, or pioneer slow muscle fibers. Slow muscle fibers develop from adaxial cells and depend on Hedgehog signaling. We analyzed when precursors become committed to their fates and the step(s) along their differentiation pathway affected by Hedgehog. Unexpectedly, we find that embryos deficient in Hedgehog signaling still contain postmitotic adaxial cells that differentiate into fast muscle fibers instead of slow. We show that by the onset of gastrulation, slow and fast muscle precursors are already spatially segregated but uncommitted to their fates until much later, in the segmental plate when slow precursors become independent of Hedgehog. In contrast, pioneer and non-pioneer slow muscle precursors share a common lineage from the onset of gastrulation. Our results demonstrate that slow muscle precursors form independently of Hedgehog signaling and further provide direct evidence for a multipotent muscle precursor population whose commitment to the slow fate depends on Hedgehog at a late stage of development when postmitotic adaxial cells differentiate into slow muscle fibers.

The chick embryo: a leading model in somitogenesis studies

The vertebrate body is built on a metameric organization which consists of a repetition of functionally equivalent units, each comprising a vertebra, its associated muscles, peripheral nerves and blood vessels. This periodic pattern is established during embryogenesis by the somitogenesis process. Somites are generated in a rhythmic fashion from the presomitic mesoderm and they subsequently differentiate to give rise to the vertebrae and skeletal muscles of the body. Somitogenesis has been very actively studied in the chick embryo since the 19th century and many of the landmark experiments that led to our current understanding of the vertebrate segmentation process have been performed in this organism. Somite formation involves an oscillator, the segmentation clock whose periodic signal is converted into the periodic array of somite boundaries by a spacing mechanism relying on a traveling threshold of FGF signaling regressing in concert with body axis extension.

Integrins in the mouse myotome: Developmental changes and differences between the epaxial and hypaxial lineage

Integrins are cellular adhesion receptors that mediate signaling and play key roles in the development of multicellular organisms. However, their role in the cellular events leading to myotome formation is completely unknown. Here, we describe the expression patterns of the alpha1, alpha4, alpha5, alpha6, and alpha7 integrin subunits in the mouse myotome and correlate them with the expression of several differentiation markers. Our results indicate that these integrin subunits may be differentially involved in the various phases of myogenic determination and differentiation. A detailed characterization of the myogenic cell types expressing the alpha4 and alpha6 subunits showed a regionalization of the myotome and dermomyotome based on cell-adhesion properties. We conclude that alpha6beta1 may be an early marker of epaxial myogenic progenitor cells. In contrast, alpha4beta1 is up-regulated in the intercalated myotome after myocyte differentiation. Furthermore, alpha4beta1 is expressed in the hypaxial dermomyotome and is maintained by early hypaxial myogenic progenitor cells colonizing the myotome.

Smad-dependent recruitment of a histone deacetylase/Sin3A complex modulates the bone morphogenetic protein-dependent transcriptional repressor activity of Nkx3.2

We have previously shown that Nkx3.2, a transcriptional repressor that is expressed in the sclerotome and developing cartilage, can activate the chondrocyte differentiation program in somitic mesoderm in a bone morphogenetic protein (BMP)-dependent manner. In this work, we elucidate how BMP signaling modulates the transcriptional repressor activity of Nkx3.2. We have found that Nkx3.2 forms a complex, in vivo, with histone deacetylase 1 (HDAC1) and Smad1 and -4 in a BMP-dependent manner. The homeodomain and NK domain of Nkx3.2 support the interaction of this transcription factor with HDAC1 and Smad1, respectively, and both of these domains are required for the transcriptional repressor activity of Nkx3.2. Furthermore, the recruitment of an HDAC/Sin3A complex to Nkx3.2 requires that Nkx3.2 interact with Smad1 and -4. Indeed, Nkx3.2 both fails to associate with the HDAC/Sin3A complex and represses target gene transcription in a cell line lacking Smad4, but it performs these functions if exogenous Smad4 is added to these cells. While prior work has indicated that BMP-dependent Smads can support transcriptional activation, our findings indicate that BMP-dependent Smads can also potentiate transcriptional repression, depending upon the identity of the Smad-interacting transcription factor.

Intrinsic signals regulate the initial steps of myogenesis in vertebrates

In vertebrates, despite the evidence that extrinsic factors induce myogenesis in naive mesoderm, other experiments argue that the initiation of the myogenic program may take place independent of these factors. To resolve this discrepancy, we have re-addressed this issue, using short-term in vivo microsurgery and culture experiments in chick. Our results show that the initial expression of the muscle-specific markers Myf5 and MyoD is regulated in a mesoderm-autonomous fashion. The reception of a Wnt signal is required for MyoD, but not Myf5 expression; however, we show that the source of the Wnt signal is intrinsic to the mesoderm. Gain- and loss-of-function experiments indicate that Wnt5b, which is expressed in the presomitic mesoderm, represents the MyoD-activating cue. Despite Wnt5b expression in the presomitic mesoderm, MyoD is not expressed in this tissue: our experiments demonstrate that this is due to a Bmp inhibitory signal that prevents the premature expression of MyoD before somites form. Our results indicate that myogenesis is a multistep process which is initiated prior to somite formation in a mesoderm-autonomous fashion; as somites form, influences from adjacent tissues are likely to be required for maintenance and patterning of early muscles.

Characterization of Nkx3.2 DNA binding specificity and its requirement for somitic chondrogenesis

We have previously shown that Nkx3.2, a member of the NK class of homeoproteins, functions as a transcriptional repressor to promote somitic chondrogenesis. However, it has not been addressed whether Nkx3.2 can bind to DNA in a sequence-specific manner and whether DNA binding by Nkx3.2 is required for its biological activity. In this work, we employed a DNA binding site selection assay, which identified TAAGTG as a high affinity Nkx3.2 binding sequence. Sequence-specific binding of Nkx3.2 to the TAAGTG motif in vitro was confirmed by electrophoretic mobility shift assays, and mutagenesis of this sequence revealed that HRAGTG (where H represents A, C, or T, and R represents A or G) comprises the consensus DNA binding site for Nkx3.2. Consistent with these findings, the expression of a reporter gene containing reiterated Nkx3.2 binding sites was repressed in vivo by Nkx3.2 co-expression. In addition, we have generated a DNA nonbinding point mutant of Nkx3.2 (Nkx3.2-N200Q), which contains an asparagine to glutamine missense mutation in the homeodomain. Interestingly, despite being defective in DNA binding, Nkx3.2-N200Q still retains its intrinsic transcriptional repressor function. Finally, we demonstrate that unlike wild-type Nkx3.2, Nkx3.2-N200Q is unable to activate the chondrocyte differentiation program in somitic mesoderm, indicating that DNA binding by Nkx3.2 is critical for this factor to induce somitic chondrogenesis.

A new view of patterning domains in the vertebrate mesoderm

The musculoskeletal system of vertebrates is derived from the embryonic mesoderm. Its structures are categorized as epaxial or hypaxial based on their adult position and innervation. The epaxial/hypaxial terminology is also used to describe regions of the embryonic somites based on fate mapping of somitic derivatives. However, the adult, functional distinctions are not fully consistent with the changing embryonic environments of mesodermal populations during morphogenesis, and the traditional terminology loses accuracy when used to describe certain mutant phenotypes. Here we describe a new terminology naming two mesodermal environments defined by the lineage of the included cells. We discuss how mutant phenotypes may be better explained by consideration of the embryonic context in which genes take their effect and argue that the recognition of these embryonic territories clarifies description and discussion of the morphogenesis and patterning of the musculoskeletal system.

Defects in cervical vertebrae in boric acid-exposed rat embryos are associated with anterior shifts of hox gene expression domains

BACKGROUND

Previously, we showed that prenatal exposure to boric acid (BA), an industrial agent with large production, causes alterations of the axial skeleton in rat embryos, reminiscent of homeotic transformations. Indeed, Sprague-Dawley rats exposed in utero to BA on gestation day 9 (GD 9) had only six, rather than the normal seven, cervical vertebrae. This finding, observed in 91% of GD 21 fetuses, suggests posterior transformations of vertebrae. The present study attempts to determine if these skeletal alterations could be explained by modifications of the hox code, involved in the establishment of positional information along the craniocaudal axis of the embryo.

METHODS

Pregnant rats were treated by gavage with BA (500 mg/kg, twice) on GD 9. Embryos were collected on GD 11 or GD 13.5 and processed for in situ hybridization. Several hox genes were selected according to the position of their cranial limit of expression in the cervical and thoracic region.

RESULTS

At GD 13.5, we detected a cranial shift of the anterior limit of expression of hoxc6 and hoxa6. We observed no difference between control and treated embryos in the location of the cranial limit of expression of the other genes: hoxd4, hoxa4, hoxc5, and hoxa5.

CONCLUSIONS

Our results demonstrate that following in utero exposure to BA on GD 9, a disturbance of the expression of hox genes involved inthe specification of most anterior vertebrae is observed at GD 13.5. Based on their expression domain and on their implication in the definition of the cervicothoracic vertebral boundary, it is likely that the anteriorization of hoxc6 and hoxa6 reported here is correlated to the morphological phenotype observed in BA-exposed fetuses at GD 21.

Differential regulation of the chick dorsal thoracic dermal progenitors from the medial dermomyotome

The chick dorsal feather-forming dermis originates from the dorsomedial somite and its formation depends primarily on Wnt1 from the dorsal neural tube. We investigate further the origin and specification of dermal progenitors from the medial dermomyotome. This comprises two distinct domains: the dorsomedial lip and a more central region (or intervening zone) that derives from it. We confirm that Wnt1 induces Wnt11 expression in the dorsomedial lip as previously shown, and show using DiI injections that some of these cells, which continue to express Wnt11 migrate under the ectoderm, towards the midline, to form most of the dorsal dermis. Transplantation of left somites to the right side to reverse the mediolateral axis confirms this finding and moreover suggests the presence of an attractive or permissive environment produced by the midline tissues or/and a repellent or inadequate environment by the lateral tissues. By contrast, the dorsolateral dermal cells just delaminate from the surface of the intervening space, which expresses En1. Excision of the axial organs or the ectoderm, and grafting of Wnt1-secreting cells, shows that, although the two populations of dermal progenitors both requires Wnt1 for their survival, the signalling required for their specification differs. Indeed Wnt11 expression relies on dorsal neural tube-derived Wnt1, while En1 expression depends on the presence of the ectoderm. The dorsal feather-forming dermal progenitors thus appear to be differentially regulated by dorsal signals from the neural tube and the ectoderm, and derive directly and indirectly from the dorsomedial lip. As these two dermomyotomal populations are well known to also give rise to epaxial muscles, an isolated domain of the dermomyotome that contains only dermal precursors does not exist and none of the dermomyotomal domains can be considered uniquely as a dermatome.

Persistent myogenic capacity of the dermomyotome dorsomedial lip and restriction of myogenic competence

The dorsomedial lip (DML) of the somite dermomyotome is the source of cells for the early growth and morphogenesis of the epaxial primary myotome and the overlying dermomyotome epithelium. We have used quail-chick transplantation to investigate the mechanistic basis for DML activity. The ablated DML of chick wing-level somites was replaced with tissue fragments from various mesoderm regions of quail embryos and their capacity to form myotomal tissue assessed by confocal microscopy. Transplanted fragments from the epithelial sheet region of the dermomyotome exhibited full DML growth and morphogenetic capacity. Ventral somite fragments (sclerotome), head paraxial mesoderm or non-paraxial (lateral plate) mesoderm tested in this assay were each able to expand mitotically in concert with the surrounding paraxial mesoderm, although no myogenic potential was evident. When ablated DMLs were replaced with fragments of the dermomyotome ventrolateral lip of wing-level somites or pre-somitic mesoderm (segmental plate), myotome development was evident but was delayed or otherwise limited in some cases. Timed DML ablation-replacement experiments demonstrate that DML activity is progressive throughout the embryonic period (to at least E7) and its continued presence is necessary for the complete patterning of each myotome segment. The results of serial transplantation and BrdU pulse-chase experiments are most consistent with the conclusion that the DML consists of a self-renewing population of progenitor cells that are the primary source of cells driving the growth and morphogenesis of the myotome and dermomyotome in the epaxial domain of the body.

Cell coherence during production of the presomitic mesoderm and somitogenesis in the mouse embryo

In this study, we investigated (in the early mouse embryo) the clonal properties of precursor cells which contribute to the segmented myotome, a structure derived from the somites. We used the laacZ method of single cell-labelling to visualise clones born before segmentation and bilateralisation. We found that clones which contribute to several segments both unilateral and bilateral were regionalised along the mediolateral axis and that their mediolateral position was maintained in successive adjacent segments. Furthermore, clones contributed to all segments, from their most anterior to their most posterior borders. Therefore, it appears that mediolateral regionalisation of myotomal precursor cells is a property established before bilateralisation of the presomitic mesoderm and that coherent clonal growth accompanies cell dispersion along both the mediolateral and anteroposterior axes. These findings in the mouse correlate well with what is known in the chick, suggesting conservation of the mode of production and distribution of the cells of the presomitic mesoderm. However, in addition, we also found that the mediolateral contribution of a clone is already determined in the pool of self-renewing cells that produces the myotomal precursor cells and thus that this pool is itself regionalised. Finally, we found that bilateral clones exhibit symmetry in right and left sides in the embryo at all levels of the mediolateral axis of the myotome. All these properties indicate synchrony and symmetry of formation of the presomitic mesoderm on both sides of the embryo leading to formation of a static embryonic structure with few cell movements. We suggest that sequential production of groups of cells with an identical clonal origin for both sides of the embryo from a single pool of self-renewing cells, coupled with aquisition of static cell behaviour, could play a role in colinearity of expression of Hox genes and in the segmentation system of higher vertebrates.

Shh establishes an Nkx3.2/Sox9 autoregulatory loop that is maintained by BMP signals to induce somitic chondrogenesis

Prior work has established that transient Shh signals from the notochord and floor plate confer a competence in somitic tissue for subsequent BMP signals to induce chondrogenesis. We have therefore proposed that Shh induces a factor(s) that renders somitic cells competent to chondrify in response to subsequent BMP signals. Recently, we have shown that forced expression of Nkx3.2, a transcriptional repressor induced by Shh, is able to confer chondrogenic competence in somites. In this work, we show that administration of Shh or forced Nkx3.2 expression induces the expression of the transcription factor Sox9 in the somitic tissue. Forced expression of Sox9 can, in turn, induce robust chondrogenesis in somitic mesoderm, provided that BMP signals are present. We have found that in the presence of BMP signals, Sox9 and Nkx3.2 induce each other's expression. Thus, Nkx3.2 may promote axial chondrogenesis by derepressing the expression of Sox9 in somitic mesoderm. Furthermore, forced expression of either Sox9 or Nkx3.2 not only activates expression of cartilage-specific genes in somitic mesoderm, but also promotes the proliferation and survival of the induced chondrocytes in the presence of BMP signals. However, unlike Nkx3.2, Sox9 is able to induce de novo cartilage formation in non-cartilage-forming tissues. Our findings suggest that Shh and BMP signals work in sequence to establish a positive regulatory loop between Sox9 and Nkx3.2, and that Sox9 can subsequently initiate the chondrocyte differentiation program in a variety of cellular environments.

Two linkedhairy/Enhancer of split-related zebrafish genes,her1andher7, function together to refine alternating somite boundaries

The formation of somites, reiterated structures that will give rise to vertebrae and muscles, is thought to be dependent upon a molecular oscillator that may involve the Notch pathway. hairy/Enhancer of split related [E(spl)]-related (her or hes) genes, potential targets of Notch signaling, have been implicated as an output of the molecular oscillator. We have isolated a zebrafish deficiency, b567, that deletes two linked her genes, her1 and her7. Homozygous b567 mutants have defective somites along the entire embryonic axis. Injection of a combination of her1 and her7 (her1+7) morpholino modified antisense oligonucleotides (MOs) phenocopies the b567 mutant somitic phenotype, indicating that her1 and her7 are necessary for normal somite formation and that defective somitogenesis in b567 mutant embryos is due to deletion of her1 and her7. Analysis at the cellular level indicates that somites in her1+7-deficient embryos are enlarged in the anterior-posterior dimension. Weak somite boundaries are often found within these enlarged somites which are delineated by stronger, but imperfect, boundaries. In addition, the anterior-posterior polarity of these enlarged somites is disorganized. Analysis of her1 MO-injected embryos and her7 MO-injected embryos indicates that although these genes have partially redundant functions in most of the trunk region, her1 is necessary for proper formation of the anteriormost somites and her7 is necessary for proper formation of somites posterior to somite 11. By following somite development over time, we demonstrate that her genes are necessary for the formation of alternating strong somite boundaries. Thus, even though two potential downstream components of Notch signaling are lacking in her1+7-deficient embryos, somite boundaries form, but do so with a one and a half to two segment periodicity.

Segmentation defects of Notch pathway mutants and absence of a synergistic phenotype in lunatic fringe/radical fringe double mutant mice

The Notch signaling pathway is important in regulating formation and anterior-posterior patterning of somites in vertebrate embryos. Here we show that distinct segmentation defects are displayed in embryos mutant for the Notch pathway genes Notch1, Lunatic fringe (Lfng), Delta-like 1 (Dll1), and Delta-like 3 (Dll3). Lfng-deficient mice and Dll3-deficient mice exhibit very similar defects, and marker analysis suggests that progression of the segmentation clock is disrupted in Dll3 mutants. We also show that Radical fringe (Rfng)-deficient mice exhibit no obvious phenotypic defects. To assess whether the absence of a phenotype in Rfng-deficient mice was the result of functional redundancy with the Lfng gene, we generated Lfng/Rfng double homozygous mutant mice. These mice exhibit the skeletal defects normally observed in Lfng-deficient mice, but we detected no obvious synergistic or additive effects in the double mutant animals.

Clonal separation and regionalisation during formation of the medial and lateral myotomes in the mouse embryo

In vertebrates, muscles of the back (epaxial) and of the body wall and limbs (hypaxial) derive from precursor cells located in the dermomyotome of the somites. In this paper, we investigate the mediolateral regionalisation of epaxial and hypaxial muscle precursor cells during segmentation of the paraxial mesoderm and myotome formation, using mouse LaacZ/LacZ chimeras. We demonstrate that precursors of medial and lateral myotomes are clonally separated in the mouse somite, consistent with earlier studies in birds. This clonal separation occurs after segmentation of the paraxial mesoderm. We then show that myotome precursors are mediolaterally regionalised and that this regionalisation precedes clonal separation between medial and lateral precursors. Strikingly, the properties of myotome precursors are remarkably similar in the medial and lateral domains. Finally, detailed analysis of our clones demonstrates a direct spatial relationship between the myocytes in the myotome and their precursors in the dermomyotome, and earlier in the somite and presomitic mesoderm, refuting several models of myotome formation, based on permanent stem cell systems or extensive cell mingling. This progressive mediolateral regionalisation of the myotome at the cellular level correlates with progressive changes in gene expression in the dermomyotome and myotome.

FGF signaling controls somite boundary position and regulates segmentation clock control of spatiotemporal Hox gene activation

Vertebrate segmentation requires a molecular oscillator, the segmentation clock, acting in presomitic mesoderm (PSM) cells to set the pace at which segmental boundaries are laid down. However, the signals that position each boundary remain unclear. Here, we report that FGF8 which is expressed in the posterior PSM, generates a moving wavefront at which level both segment boundary position and axial identity become determined. Furthermore, by manipulating boundary position in the chick embryo, we show that Hox gene expression is maintained in the appropriately numbered somite rather than at an absolute axial position. These results implicate FGF8 in ensuring tight coordination of the segmentation process and spatiotemporal Hox gene activation.

The early neural plate rules over the mesoderm

In this issue of Developmental Cell, Richard Harland and colleagues describe evidence that an inductive interaction between the neural plate and the paraxial mesoderm regulates somite development and somite size.

Biological timing and the clock metaphor: oscillatory and hourglass mechanisms

Living organisms have developed a multitude of timing mechanisms--"biological clocks." Their mechanisms are based on either oscillations (oscillatory clocks) or unidirectional processes (hourglass clocks). Oscillatory clocks comprise circatidal, circalunidian, circadian, circalunar, and circannual oscillations--which keep time with environmental periodicities--as well as ultradian oscillations, ovarian cycles, and oscillations in development and in the brain, which keep time with biological timescales. These clocks mainly determine time points at specific phases of their oscillations. Hourglass clocks are predominantly found in development and aging and also in the brain. They determine time intervals (duration). More complex timing systems combine oscillatory and hourglass mechanisms, such as the case for cell cycle, sleep initiation, or brain clocks, whereas others combine external and internal periodicities (photoperiodism, seasonal reproduction). A definition of a biological clock may be derived from its control of functions external to its own processes and its use in determining temporal order (sequences of events) or durations. Biological and chemical oscillators are characterized by positive and negative feedback (or feedforward) mechanisms. During evolution, living organisms made use of the many existing oscillations for signal transmission, movement, and pump mechanisms, as well as for clocks. Some clocks, such as the circadian clock, that time with environmental periodicities are usually compensated (stabilized) against temperature, whereas other clocks, such as the cell cycle, that keep time with an organismic timescale are not compensated. This difference may be related to the predominance of negative feedback in the first class of clocks and a predominance of positive feedback (autocatalytic amplification) in the second class. The present knowledge of a compensated clock (the circadian oscillator) and an uncompensated clock (the cell cycle), as well as relevant models, are briefly re viewed. Hourglass clocks are based on linear or exponential unidirectional processes that trigger events mainly in the course of development and aging. An important hourglass mechanism within the aging process is the limitation of cell division capacity by the length of telomeres. The mechanism of this clock is briefly reviewed. In all clock mechanisms, thresholds at which "dependent variables" are triggered play an important role.