System to identify individual somites and their derivatives in the developing mouse embryo

Fgf4 maintains Hes7 levels critical for normal somite segmentation clock function

During vertebrate development, the presomitic mesoderm (PSM) periodically segments into somites, which will form the segmented vertebral column and associated muscle, connective tissue, and dermis. The periodicity of somitogenesis is regulated by a segmentation clock of oscillating Notch activity. Here, we examined mouse mutants lacking only Fgf4 or Fgf8, which we previously demonstrated act redundantly to prevent PSM differentiation. Fgf8 is not required for somitogenesis, but Fgf4 mutants display a range of vertebral defects. We analyzed Fgf4 mutants by quantifying mRNAs fluorescently labeled by hybridization chain reaction within Imaris-based volumetric tissue subsets. These data indicate that FGF4 maintains Hes7 levels and normal oscillatory patterns. To support our hypothesis that FGF4 regulates somitogenesis through Hes7, we demonstrate genetic synergy between Hes7 and Fgf4, but not with Fgf8. Our data indicate that Fgf4 is potentially important in a spectrum of human Segmentation Defects of the Vertebrae caused by defective Notch oscillations.

Developmental origin and morphogenesis of the diaphragm, an essential mammalian muscle

The diaphragm is a mammalian skeletal muscle essential for respiration and for separating the thoracic and abdominal cavities. Development of the diaphragm requires the coordinated development of muscle, muscle connective tissue, tendon, nerves, and vasculature that derive from different embryonic sources. However, defects in diaphragm development are common and the cause of an often deadly birth defect, Congenital Diaphragmatic Hernia (CDH). Here we comprehensively describe the normal developmental origin and complex spatial-temporal relationship between the different developing tissues to form a functional diaphragm using a developmental series of mouse embryos genetically and immunofluorescently labeled and analyzed in whole mount. We find that the earliest developmental events are the emigration of muscle progenitors from cervical somites followed by the projection of phrenic nerve axons from the cervical neural tube. Muscle progenitors and phrenic nerve target the pleuroperitoneal folds (PPFs), transient pyramidal-shaped structures that form between the thoracic and abdominal cavities. Subsequently, the PPFs expand across the surface of the liver to give rise to the muscle connective tissue and central tendon, and the leading edge of their expansion precedes muscle morphogenesis, formation of the vascular network, and outgrowth and branching of the phrenic nerve. Thus development and morphogenesis of the PPFs is critical for diaphragm formation. In addition, our data indicate that the earliest events in diaphragm development are critical for the etiology of CDH and instrumental to the evolution of the diaphragm. CDH initiates prior to E12.5 in mouse and suggests that defects in the early PPF formation or their ability to recruit muscle are an important source of CDH. Also, the recruitment of muscle progenitors from cervical somites to the nascent PPFs is uniquely mammalian and a key developmental innovation essential for the evolution of the muscularized diaphragm.

Closure of the vertebral canal in human embryos and fetuses

The vertebral column is the paradigm of the metameric architecture of the vertebrate body. Because the number of somites is a convenient parameter to stage early human embryos, we explored whether the closure of the vertebral canal could be used similarly for staging embryos between 7 and 10 weeks of development. Human embryos (5-10 weeks of development) were visualized using Amira 3D^® reconstruction and Cinema 4D^® remodelling software. Vertebral bodies were identifiable as loose mesenchymal structures between the dense mesenchymal intervertebral discs up to 6 weeks and then differentiated into cartilaginous structures in the 7th week. In this week, the dense mesenchymal neural processes also differentiated into cartilaginous structures. Transverse processes became identifiable at 6 weeks. The growth rate of all vertebral bodies was exponential and similar between 6 and 10 weeks, whereas the intervertebral discs hardly increased in size between 6 and 8 weeks and then followed vertebral growth between 8 and 10 weeks. The neural processes extended dorsolaterally (6th week), dorsally (7th week) and finally dorsomedially (8th and 9th weeks) to fuse at the midthoracic level at 9 weeks. From there, fusion extended cranially and caudally in the 10th week. Closure of the foramen magnum required the development of the supraoccipital bone as a craniomedial extension of the exoccipitals (neural processes of occipital vertebra 4), whereas a growth burst of sacral vertebra 1 delayed closure until 15 weeks. Both the cranial- and caudal-most vertebral bodies fused to form the basioccipital (occipital vertebrae 1-4) and sacrum (sacral vertebrae 1-5). In the sacrum, fusion of its so-called alar processes preceded that of the bodies by at least 6 weeks. In conclusion, the highly ordered and substantial changes in shape of the vertebral bodies leading to the formation of the vertebral canal make the development of the spine an excellent, continuous staging system for the (human) embryo between 6 and 10 weeks of development.

Developmental origins of precocial forelimbs in marsupial neonates

Marsupial mammals are born in an embryonic state, as compared with their eutherian counterparts, yet certain features are accelerated. The most conspicuous of these features are the precocial forelimbs, which the newborns use to climb unaided from the opening of the birth canal to the teat. The developmental mechanisms that produce this acceleration are unknown. Here we show that heterochronic and heterotopic changes early in limb development contribute to forelimb acceleration. Using Tbx5 and Tbx4 as fore- and hindlimb field markers, respectively, we have found that, compared with mouse, both limb fields arise notably early during opossum development. Patterning of the forelimb buds is also accelerated, as Shh expression appears early relative to the outgrowth of the bud itself. In addition, the forelimb fields and forelimb myocyte allocation are increased in size and number, respectively, and migration of the spinal nerves into the forelimb bud has been modified. This shift in the extent of the forelimb field is accompanied by shifts in Hox gene expression along the anterior-posterior axis. Furthermore, we found that both fore- and hindlimb fields arise gradually during gastrulation and extension of the embryonic axis, in contrast to the appearance of the limb fields in their entirety in all other known cases. Our results show a surprising evolutionary flexibility in the early limb development program of amniotes and rule out the induction of the limb fields by mature structures such as the somites or mesonephros.

Lack of the mesodermal homeodomain protein MEOX1 disrupts sclerotome polarity and leads to a remodeling of the cranio-cervical joints of the axial skeleton

Meox1 and Meox2 are two related homeodomain transcription factor genes that together are essential for the development of all somite compartments. Here we show that mice homozygous for Meox1 mutations alone have abnormalities that are restricted to the sclerotome and its derivatives. A prominent and consistent phenotype of these mutations is a remodeling of the cranio-cervical joints whose major feature is the assimilation of the atlas into the basioccipital bone so that the skull rests on the axis. These abnormalities can be traced back to changes in the relative rates of cell proliferation in the rostral and caudal sclerotome compartments, and they are associated with alterations in the expression of at least three transcription factor genes, Tbx18, Uncx, and Bapx1. As previously observed for Bapx1, MEOX1 protein occupies evolutionarily conserved promoter regions of Tbx18 and Uncx, suggesting that Meox1 regulates these genes at least in part directly. Hence, Meox1 is part of a regulatory circuit that serves an essential, non-redundant function in the maintenance of rostro-caudal sclerotome polarity and axial skeleton formation.

The NLRR gene family and mouse development: Modified differential display PCR identifies NLRR-1 as a gene expressed in early somitic myoblasts

During vertebrate embryogenesis, the somites form by segmentation of the trunk mesoderm, lateral to the neural tube, in an anterior to posterior direction. Analysis of differential gene expression during somitogenesis has been problematic due to the limited amount of tissue available from early mouse embryos. To circumvent these problems, we developed a modified differential display PCR technique that is highly sensitive and yields products that can be used directly as in situ hybridisation probes. Using this technique, we isolated NLRR-1 as a gene expressed in the myotome of developing somites but not in the presomitic mesoderm. Detailed expression analysis showed that this gene was expressed in the skeletal muscle precursors of the myotome, branchial arches and limbs as well as in the developing nervous system. Somitic expression occurs in the earliest myoblasts that originate from the dorsal lip in a pattern reminiscent of the muscle determination gene Myf5, but not at the ventral lip, indicating that NLRR-1 is expressed in a subset of myotome cells. The NLRR genes comprise a three-gene family encoding glycosylated transmembrane proteins with external leucine-rich repeats, a fibronectin domain, an immunoglobulin domain and short intracellular tails capable of mediating protein-protein interaction. Analysis of NLRR-3 expression revealed regulated expression in the neural system in developing ganglia and motor neurons. NLRR-2 expression appears to be predominately confined to the adult. The regulated embryonic expression and cellular location of these proteins suggest important roles during mouse development in the control of cell adhesion, movement or signalling.

Hox code in embryos of Chinese soft-shelled turtlePelodiscus sinensis correlates with the evolutionary innovation in the turtle

Turtles have the most unusual body plan of the amniotes, with a dorsal shell consisting of modified ribs. Because this morphological change in the ribs can be described as an axial-level specific alteration, the evolution of the turtle carapace should depend on changes in the Hox code. To identify turtle-specific changes in developmental patterns, we cloned several Hox genes from the Chinese soft-shelled turtle, Pelodiscus sinensis, examined their expression patterns during embryogenesis, and compared them with those of chicken and mouse embryos. We detected possibly turtle-specific derived traits in Hoxc-6 expression, which is restricted to the paraxial part of the embryo; in the expression of Hoxa-5 and Hoxb-5, the transcripts of which were detected only at the cervical level; and in Hoxc-8 and Hoxa-7 expression, which is shifted anteriorly relative to that of the other two amniote groups. From the known functions of the Hox orthologs in model animals, these P. sinensis-specific changes apparently correlate with specializations in the turtle-specific body plan.

Expression of retinaldehyde dehydrogenase II and sequential activation of 5' Hoxb genes in the mouse caudal hindbrain

The precise anterior boundaries of Hox expression domains are critical for correct antero-posterior (A-P) patterning of the vertebrate longitudinal axis. Retinoic acid (RA) signalling has been shown to play an important role in the specification of pre-otic rhombomere boundaries, and in the regulation of 3' Hox expression within this territory. In addition, we recently showed that RA signalling controls 5'Hoxb gene expression in the caudal hindbrain, which had not been discovered before. We show here that the expression domain of these 5'Hoxb genes undergoes a sequential, colinear rostral expansion between E9.5 and E11.5 in the caudal hindbrain, and that this differential expansion occurs just rostrally to the localisation of the transcripts for the RA biosynthetic enzyme Raldh2 in the cervical mesenchyme.

The initial somitic phase of Myf5 expression requires neither Shh signaling nor Gli regulation

Myf5, the skeletal muscle determination gene, is first expressed in the dorso-medial aspect of the somite under the control of an element we have called the early epaxial enhancer. It has subsequently been reported that this enhancer is a direct target of Shh signaling mediated by Gli transcription factors (Gustafsson et al. 2002). We here demonstrate that activation of Myf5 expression depends on neither Shh function nor an intact Gli binding site, although the Gli site is necessary for continuation of expression. We suggest that the discrepancy is due to the existence of specific interactions between the enhancer and the Myf5 promoter.

Embryonic expression of cholesterogenic genes is restricted to distinct domains and colocalizes with apoptotic regions in mice

Cholesterol biosynthesis has been assumed to be an ubiquitous process in vertebrate organisms. Here we present data demonstrating that expression of key enzymes of cholesterol biosynthesis is restricted to specific tissues during embryonic development. Distinct expression starts in the dorsal neural tube at embryonic day 8 and is later detected in dorsal root and cephalic ganglia, in the pharyngeal pouches and limb buds. In the limb, expression becomes progressively restricted to interdigital regions during differentiation. Caspase3 whole mount immunostaining revealed that cholesterol biosynthesis colocalizes with apoptotic regions that are targets of the morphogenic signal Sonic hedgehog. This expression pattern correlates closely with the shared phenotypic features of cholesterol biosynthesis and hedgehog mutants.

Multiple levels of transcriptional and post-transcriptional regulation are required to define the domain of Hoxb4 expression

Hox genes are key determinants of anteroposterior patterning of animal embryos, and spatially restricted expression of these genes is crucial to this function. In this study, we demonstrate that expression of Hoxb4 in the paraxial mesoderm of the mouse embryo is transcriptionally regulated in several distinct phases, and that multiple regulatory elements interact to maintain the complete expression domain throughout embryonic development. An enhancer located within the intron of the gene (region C) is sufficient for appropriate temporal activation of expression and the establishment of the correct anterior boundary in the paraxial mesoderm (somite 6/7). However, the Hoxb4 promoter is required to maintain this expression beyond 8.5 dpc. In addition, sequences within the 3' untranslated region (region B) are necessary specifically to maintain expression in somite 7 from 9.0 dpc onwards. Neither the promoter nor region B can direct somitic expression independently, indicating that the interaction of regulatory elements is crucial for the maintenance of the paraxial mesoderm domain of Hoxb4 expression. We further report that the domain of Hoxb4 expression is restricted by regulating transcript stability in the paraxial mesoderm and by selective translation and/or degradation of protein in the neural tube. Moreover, the absence of Hoxb4 3'-untranslated sequences from transgene transcripts leads to inappropriate expression of some Hoxb4 transgenes in posterior somites, indicating that there are sequences within region B that are important for both transcriptional and post-transcriptional regulation.

The early epaxial enhancer is essential for the initial expression of the skeletal muscle determination geneMyf5but not for subsequent, multiple phases of somitic myogenesis

Vertebrate myogenesis is controlled by four transcription factors known as the myogenic regulatory factors (MRFs): Myf5, Mrf4, myogenin and MyoD. During mouse development Myf5 is the first MRF to be expressed and it acts by integrating multiple developmental signals to initiate myogenesis. Numerous discrete regulatory elements are involved in the activation and maintenance of Myf5 gene expression in the various muscle precursor populations, reflecting the diversity of the signals that control myogenesis. Here we focus on the enhancer that recapitulates the first phase of Myf5 expression in the epaxial domain of the somite, in order to identify the subset of cells that first transcribes the gene and therefore gain insight into molecular, cellular and anatomical facets of early myogenesis. Deletion of this enhancer from a YAC reporter construct that recapitulates the Myf5 expression pattern demonstrates that this regulatory element is necessary for expression in the early epaxial somite but in no other site of myogenesis. Importantly, Myf5 is subsequently expressed in the epaxial myotome under the control of other elements located far upstream of the gene. Our data suggest that the inductive signals that control Myf5 expression switch rapidly from those that impinge on the early epaxial enhancer to those that impinge on the other enhancers that act later in the epaxial somite, indicating that there are significant changes in either the signalling environment or the responsiveness of the cells along the rostrocaudal axis. We propose that the first phase of Myf5 epaxial expression, driven by the early epaxial enhancer in the dermomyotome, is necessary for early myotome formation, while the subsequent phases are associated with cytodifferentiation within the myotome.

Expression of the myogenic regulatory factor Mrf4 precedes or is contemporaneous with that of Myf5 in the somitic bud

The development of skeletal muscle in vertebrate embryos is controlled by a transcriptional cascade involving the four myogenic regulatory factors. In the somites of the mouse embryo the order of expression is thought to be Myf5, Myogenin, Mrf4 and MyoD. We have re-examined the expression pattern of Mrf4 and show that in the hypaxial domain of thoracic somites (the somitic bud) Mrf4 expression precedes or is contemporaneous with that of Myf5, suggesting that this transcription factor plays a hitherto unsuspected role in myogenesis.

Pax1/Pax9-Related genes in an agnathan vertebrate, Lampetra japonica: expression pattern of LjPax9 implies sequential evolutionary events toward the gnathostome body plan

Among the transcription factor gene families, Pax genes play important and unique roles in morphological patterning of animal body plans. Of these, Group I Pax genes (Pax1 and Pax9) are expressed in the endodermal pharyngeal pouches in many groups of deuterostomes, and vertebrates seem to have acquired more extensive expression domains in embryos. To understand the evolution of Pax1/Pax9-related genes in basal groups of vertebrates, their cognates were isolated from the Japanese marine lamprey, Lampetra japonica. RT-PCR of larval lamprey cDNA yielded two different fragments containing vertebrate Pax1- and Pax9-like paired domains. The Pax9 orthologue was isolated and named LjPax9. Whole-mount in situ hybridization revealed that this gene was expressed in endodermal pharyngeal pouches, mesenchyme of the velum (the oral pumping apparatus) and the hyoid arch, and the nasohypophysial plate, but not in the somitic mesoderm of the lamprey embryo. These expression patterns could be regarded as a link between the basal chordates and the gnathostomes and are consistent with the phylogenetic position of the lamprey. Especially, the appearance of neural crest seemed to be the basis of velar expression. Homology of the velum and the jaw is also discussed based on the LjPax9 expression in the first pharyngeal pouch and in the velar mesenchyme. We conclude that Pax9 genes have sequentially expanded into new expression domains through evolution as more complicated body plans emerged.

Paradox segmentation along inter- and intrasomitic borderlines is followed by dysmorphology of the axial skeleton in the open brain (opb) mouse mutant

In open brain (opb) mutant embryos, developmental defects of the trunk spinal cord were spatially correlated with severe defects of the epaxial somite derivatives including sclerotomes, whereas hypaxial somite derivatives are much less affected. Later in development, the neural arches (epaxial sclerotome derivatives) formed but were severely disorganized, and also the distal ribs (hypaxial sclerotome derivatives) were malformed. Adjacent neural arches and vertebral bodies were often fused where joints should have formed suggesting defects of the intrasomitic borderlines. Moreover, neural arches frequently and ribs sometimes were split into halves at distinct levels along the dorso-ventral body axis. This suggests that 'resegmentation' of sclerotomes across the somite borders did not completely occur. These prominent skeletal defects were preceded by reduced expression of Pax1 along the intrasomitic borderlines, and incomplete maintenance of somite borders between central sclerotome moieties. The defects of the axial skeleton were accompanied by segmentation defects of the myotomes which were split distally, and also partly fused from adjacent segments across somite borders. The segmentation defects observed suggest that in opb mutants both segmental borderlines, the somite borders and the intrasomitic borderlines (fissures), were affected and behaved paradoxically.

Computer-based three-dimensional visualization of developmental gene expression

A broad understanding of the relationship between gene activation, pattern formation and morphogenesis will require adequate tools for three-dimensional and, perhaps four-dimensional, representation and analysis of molecular developmental processes. We present a novel, computer-based method for the 3D visualization of embryonic gene expression and morphological structures from serial sections. The information from these automatically aligned 3D reconstructions exceeds that from single-section and whole-mount visualizations of in situ hybridizations. In addition, these 3D models of gene-expression patterns can become a central component of a future developmental database designed for the collection and presentation of digitized, morphological and gene-expression data. This work is accompanied by a web site (http://www.univie.ac.at/GeneEMAC).

Targeted inactivation of Hoxb8 affects survival of a spinal ganglion and causes aberrant limb reflexes

Hoxb8 mutant mice were generated by inserting the lacZ coding sequence in frame with the first exon of Hoxb8. These mice express a fusion protein with a functional beta-galactosidase activity instead of Hoxb8. Mutant embryos were analyzed for anatomical changes. The results indicate that Hoxb8 is not an indispensable regulator of A-P patterning in the forelimb, unlike suggested by our Hoxb8 gain of function experiments (Charité J, DeGraaff W, Shen S, Deschamps J. Cell 1994;78:589-601). The null mutant phenotypic traits include degeneration of the second spinal ganglion (C2), an abnormality opposite to the alteration in the gain of function transgenic mice. Subtle changes in the thoracic part of the vertebral column were observed as well. Adult homozygous mutants exhibit an abnormal clasping reflex of the limbs.

The homeobox gene Msx1 is expressed in a subset of somites, and in muscle progenitor cells migrating into the forelimb

In myoblast cell cultures, the Msx1 protein is able to repress myogenesis and maintain cells in an undifferentiated and proliferative state. However, there has been no evidence that Msx1 is expressed in muscle or its precursors in vivo. Using mice with the nlacZ gene integrated into the Msx1 locus, we show that the reporter gene is expressed in the lateral dermomyotome of brachial and thoracic somites. Cells from this region will subsequently contribute to forelimb and intercostal muscles. Using Pax3 gene transcripts as a marker of limb muscle progenitor cells as they migrate from the somites, we have defined precisely the somitic origin and timing of cell migration from somites to limb buds in the mouse. Differences in the timing of migration between chick and mouse are discussed. Somites that label for Msx1(nlacZ)transgene expression in the forelimb region partially overlap with those that contribute Pax3-expressing cells to the forelimb. In order to see whether Msx1 is expressed in this migrating population, we have grafted somites from the forelimb level of Msx1(nlacZ)mouse embryos into a chick host embryo. We show that most cells migrating into the wing field express the Msx1(nlacZ)transgene, together with Pax3. In these experiments, Msx1 expression in the somite depends on the axial position of the graft. Wing mesenchyme is capable of inducing Msx1 transcription in somites that normally would not express the gene; chick hindlimb mesenchyme, while permissive for this expression, does not induce it. In the mouse limb bud, the Msx1(nlacZ)transgene is downregulated prior to the activation of the Myf5 gene, an early marker of myogenic differentiation. These observations are consistent with the proposal that Msx1 is involved in the repression of muscle differentiation in the lateral half of the somite and in limb muscle progenitor cells during their migration.

Developmental morphology of the head mesoderm and reevaluation of segmental theories of the vertebrate head: evidence from embryos of an agnathan vertebrate, Lampetra japonica

Due to the peculiar morphology of its preotic head, lampreys have long been treated as an intermediate animal which links amphioxus and gnathostomes. To reevaluate the segmental theory of classical comparative embryology, mesodermal development was observed in embryos of a lamprey, Lampetra japonica, by scanning electron microscopy and immunohistochemistry. Signs of segmentation are visible in future postotic somites at an early neurula stage, whereas the rostral mesoderm is unsegmented and rostromedially confluent with the prechordal plate. The premandibular and mandibular mesoderm develop from the prechordal plate in a caudal to rostral direction and can be called the preaxial mesoderm as opposed to the caudally developing gastral mesoderm. With the exception of the premandibular mesoderm, the head mesodermal sheet is secondarily regionalized by the otocyst and pharyngeal pouches into the mandibular mesoderm, hyoid mesoderm, and somite 0. The head mesodermal components never develop into cephalic myotomes, but the latter develop only from postotic somites. These results show that the lamprey embryo shows a typical vertebrate phylotype and that the basic mesodermal configuration of vertebrates already existed prior to the split of agnatha-gnathostomata; lamprey does not represent an intermediate state between amphioxus and gnathostomes. Unlike interpretations of theories of head segmentation that the mesodermal segments are primarily equivalent along the axis, there is no evidence in vertebrate embryos for the presence of preotic myotomes. We conclude that mesomere-based theories of head metamerism are inappropriate and that the formulated vertebrate head should possess the distinction between primarily unsegmented head mesoderm which includes preaxial components at least in part and somites in the trunk which are shared in all the known vertebrate embryos as the vertebrate phylotype.

Isolation and embryonic expression of the novel mouse gene Hic1, the homologue of HIC1, a candidate gene for the Miller-Dieker syndrome

The human gene HIC1 (hypermethylated in cancer) maps to chromosome 17p13.3 and is deleted in the contiguous gene disorder Miller-Dieker syndrome (MDS) [Makos-Wales et al. (1995) Nature Med., 1, 570-577; Chong et al. (1996) Genome Res., 6, 735-741]. We isolated the murine homologue Hic1, encoding a zinc-finger protein with a poxvirus and zinc-finger (POZ) domain and mapped it to mouse chromosome 11 in a region exhibiting conserved synteny to human chromosome 17. Comparison of genomic and cDNA sequences predicts two exons for the murine Hic1. The second exon exhibits 88% identity to the human HIC1 on DNA level. During embryonic development, Hic1 is expressed in mesenchymes of the sclerotomes, lateral body wall, limb and cranio-facial regions embedding the outgrowing peripheral nerves during their differentiation. During fetal development, Hic1 additionally is expressed in mesenchymes apposed to precartilaginous condensations, at many interfaces to budding epithelia of inner organs, and weakly in muscles. We observed activation of Hic1 expression in the embryonic anlagen of many tissues displaying anomalies in MDS patients. Besides lissencephaly, MDS patients exhibit facial dysmorphism and frequently additional birth defects, e.g. anomalies of the heart, kidney, gastrointestinal tract and the limbs (OMIM 247200). Thus, HIC1 activity may correlate with the defective development of the nose, jaws, extremities, gastrointestinal tract and kidney in MDS patients.