The divergent homeobox gene Hex exhibits three notable expression patterns during early mouse development. Initially Hex is expressed in the primitive endoderm of the implanting blastocyst but by 5.5 ...dpc its transcripts are present only in a small patch of visceral endoderm at the distal tip of the egg cylinder. Lineage analysis shows that these cells move unilaterally to assume an anterior position while continuing to express Hex. The primitive streak forms on the opposite side of the egg cylinder from this anterior Hex expression domain approximately 24 hours after the initial anterior movement of the distal visceral endoderm. Thus, Hex expression marks the earliest unequivocal molecular anteroposterior asymmetry in the mouse embryo and indicates that the anteroposterior axis of the embryo develops from conversion of a proximodistal asymmetry established in the primitive endoderm lineage. Subsequently, Hex is expressed in the earliest definitive endoderm to emerge from the streak and its expression within the gut strongly suggests that the ventral foregut is derived from the most anterior definitive endoderm and that the liver is probably the most anterior gut derivative. Hex is also an early marker of the thyroid primordium. Within the mesoderm, Hex is transiently expressed in the nascent blood islands of the visceral yolk sac and later in embryonic angioblasts and endocardium. Comparison with flk-1 (T. P. Yamaguchi et al., Development 118, 489-498, 1993) expression indicates that Hex is also an early marker of endothelial precursors but its expression in this progenitor population is much more transient than that of flk-1, being downregulated once endothelial cell differentiation commences.
The anterior aspect of the mouse primitive streak resembles the organizer of Xenopus and chick in terms of its developmental fate, ability to alter pattern in the chick limb bud and with respect to ...the repertoire of genes that its constituent cells express. However, until now there has been no direct evidence that the mouse node organizes pattern during gastrulation, nor that the exceptionally small mouse embryonic egg cylinder can be induced to form a second axis. Grafts of transgenically marked midgastrulation mouse node, or node labelled with DiI, to a posterolateral location in a host embryo of the same developmental stage results in the induction of a second neural axis and the formation of ectopic somites. The graft gives rise predominantly to notochord and endoderm tissue whereas the neurectoderm and somites are mainly of host origin. The ectopic notochord formed is derived solely from the donor node which suggests that the node can serve as a 'stem cell' source of axial mesoderm. This is corroborated by the observation that labelling in situ the population of cells on the outer surface of the mid-gastrulation node with DiI results in continuous labelling of the notochord. DiI-labelled cells are present throughout the notochord from a rostral boundary in the cranial region to its most caudal extreme and the node itself always remains labelled.
Anterior patterning in mouse Beddington, Rosa S.P; Robertson, Elizabeth J
Trends in genetics,
07/1998, Letnik:
14, Številka:
7
Journal Article
Recenzirano
The anteroposterior axis of the vertebrate embryo becomes explicit during gastrulation, the process that converts a relatively featureless embryonic precursor population into new tissues assembled ...into a recognisable body pattern. Vertebrate embryos arrive at gastrulation in very different states in terms of their size, cell number and reliance on factors inherited from the unfertilized egg. However, they emerge from gastrulation looking very similar, and there is now extensive molecular genetic evidence to indicate that the bare essentials of the gastrulation process have been well conserved during evolution. Here, we review recent findings in the mouse that suggest that anterior identity is, in fact, established before gastrulation starts. They suggest that it is first manifest in extraembryonic tissue and that this tissue is essential for the embryo to develop normal anterior structures, such as the forebrain. We also argue that this precocious anterior pattern could have a counterpart in other non-mammalian vertebrates.
Studying the formation of the embryonic axes in mammals poses a special problem because patterning of the embryo itself is a relatively late event in development. In the mouse, explicit pattern in ...the tissue that will generate the future organism only becomes apparent one-third of the way through a gestational period of 3 weeks. This tardiness of eutherian mammals is due to viviparity. The earliest stages of their development are necessarily preoccupied with the generation of extraembryonic tissues essential for survival in utero. This means that the conceptus, comprising both the extraembryonic and embryonic components, is already a complex structure and implanted in the uterus by the time the embryo itself begins to acquire obvious pattern. For this reason, the architecture of the conceptus as a whole cannot be ignored when considering the origin of the embryonic axes of mammals. This contrasts with the development of most invertebrates and anamniote vertebrates, where, from the moment of fertilization, embryogenesis is wholly concerned with defining the pattern of the future organism. Indeed, in many cases, this patterning begins during oogenesis when molecular asymmetries are laid down in the egg. In this review, we will explore some of the characteristics of mouse development relevant to axis development. We will consider whether the pattern that develops in the mouse conceptus influences pattern that appears later in the embryo, and we will discuss what is known about how the definitive axes of the embryo (anterior-posterior A-P; dorsal-ventral D-V; and left-right L-R) are laid down during mammalian development.
The homeobox gene Hex is expressed in the anterior visceral endoderm (AVE) and rostral definitive endoderm of early mouse embryos. Later, Hex transcripts are detected in liver, thyroid and ...endothelial precursor cells. A null mutation was introduced into the Hex locus by homologous recombination in embryonic stem cells. Hex mutant embryos exhibit varying degrees of anterior truncation as well as liver and thyroid dysplasia. The liver diverticulum is formed but migration of hepatocytes into the septum transversum fails to occur. Development of the thyroid is arrested at the thyroid bud stage at 9.5 dpc. Brain defects are restricted to the rostral forebrain and have a caudal limit at the zona limitans intrathalamica, the boundary between dorsal and ventral thalamus. Analysis of Hex(-/-) mutants at early stages shows that the prospective forebrain ectoderm is correctly induced and patterned at 7.5 days post coitum (dpc), but subsequently fails to develop. AVE markers are expressed and correctly positioned but development of rostral definitive endoderm is greatly disturbed in Hex(-/-) embryos. Chimeric embryos composed of Hex(-/-) cells developing within a wild-type visceral endoderm show forebrain defects indicating that Hex is required in the definitive endoderm. All together, these results demonstrate that Hex function is essential in definitive endoderm for normal development of the forebrain, liver and thyroid gland.
A strategy based on the gene trap was developed to prescreen mouse embryonic stem cells for insertional mutations in genes encoding secreted and membrane-spanning proteins. The "secretory trap" ...relies on capturing the N-terminal signal sequence of an endogenous gene to generate an active β-galactosidase fusion protein. Insertions were found in a cadherin gene, an unc6-related laminin (netrin) gene, the sek receptor tyrosine kinase gene, and genes encoding two receptor-linked protein-tyrosine phosphatases, LAR and PTPκ. Analysis of homozygous mice carrying insertions in LAR and PTPκ showed that both genes were effectively disrupted, but neither was essential for normal embryonic development.
Shortly after implantation the mouse embryo comprises three tissue layers. The founder tissue of the embryo proper, the epiblast, forms a radially symmetric cup of epithelial cells that grows in ...close apposition to the extra-embryonic ectoderm and the visceral endoderm. This simple cylindrical structure exhibits a distinct molecular pattern along its proximal-distal axis. The anterior-posterior axis of the embryo is positioned later by coordinated cell movements that rotate the pre-existing proximal-distal axis. The transforming growth factor-β family member Nodal is known to be required for formation of the anterior-posterior axis. Here we show that signals from the epiblast are responsible for the initiation of proximal-distal polarity. Nodal acts to promote posterior cell fates in the epiblast and to maintain molecular pattern in the adjacent extra-embryonic ectoderm. Both of these functions are independent of Smad2. Moreover, Nodal signals from the epiblast also pattern the visceral endoderm by activating the Smad2-dependent pathway required for specification of anterior identity in overlying epiblast cells. Our experiments show that proximal-distal and subsequent anterior-posterior polarity of the pregastrulation embryo result from reciprocal cell-cell interactions between the epiblast and the two extra-embryonic tissues.
The expression pattern of bone morphogenetic protein-7 (BMP-7) in the hindbrain region of the headfold and early somite stage developing mouse embryo suggests a role for BMP-7 in the patterning of ...this part of the cranial CNS. In chick embryos it is thought that BMP-7 is one of the secreted molecules which mediates the dorsalizing influence of surface ectoderm on the neural tube, and mouse surface ectoderm has been shown to have a similar dorsalizing effect. While we confirm that BMP-7 is expressed in the surface ectoderm of mouse embryos at the appropriate time to dorsalize the neural tube, we also show that at early stages of hindbrain development BMP-7 transcripts are present in paraxial and ventral tissues, within and surrounding the hindbrain neurectoderm, and only later does expression become restricted to a dorsal domain. To determine more directly the effect that BMP-7 may have on the developing hindbrain we have grafted COS cells expressing BMP-7 into the ventrolateral mesoderm abutting the neurectoderm in order to prolong BMP-7 expression in the vicinity of ventral hindbrain. Three distinct actvities of BMP-7 are apparent. Firstly, as expected from previous work in chick, BMP-7 can promote dorsal characteristics in the neural tube. Secondly, we show that it can also attenuate the expression of sonic hedgehog (Shh) in the floorplate without affecting Shh expression in the notochord. Finally, we find that ectopic BMP-7 appears to promote growth of the neurectoderm. These findings are discussed with respect to possible timing mechanisms necessary for the coordination of hindbrain dorsoventral patterning.
We report a new role for Wnt signaling in the vertebrate embryo: the induction of neural tissue from ectoderm. Early expression of mouse wnt8, Xwnt8, beta-catenin, or dominant-negative GSK3 induces ...the expression of neural-specific markers and inhibits the expression of Bmp4 in Xenopus ectoderm. We show that Wnt8, but not the BMP antagonist Noggin, can inhibit Bmp4 expression at early gastrula stages. Furthermore, inhibition of beta-catenin activity in the neural ectoderm of whole embryos by a truncated TCF results in a decrease in neural development. Therefore, we suggest that a cleavage-stage Wnt signal normally contributes to an early repression of Bmp4 on the dorsal side of the embryo and sensitizes the ectoderm to respond to neural inducing signals from the organizer. The Wnt targets Xnr3 and siamois have been shown previously to have neuralizing activity when overexpressed. However, antagonists of Wnt signaling, dnXwnt8 and Nxfrz8, inhibit Wnt-mediated Xnr3 and siamois induction, but not neural induction, suggesting an alternative mechanism for Bmp repression and neuralization. Conversely, dnTCF blocks both Wnt-mediated Xnr3 and neural induction, suggesting that both pathways require this transcription factor.
We have previously shown that familial septo-optic dysplasia (SOD), a syndromic form of congenital hypopituitarism involving optic nerve hypoplasia and agenesis of midline brain structures, is ...associated with homozygosity for an inactivating mutation in the homeobox gene HESX1/Hesx1 in man and mouse. However, as most SOD/congenital hypopituitarism occurs sporadically, the possible contribution of HESX1 mutations to the aetiology of these cases is presently unclear. Interestingly, a small proportion of mice heterozygous for the Hesx1 null allele show a milder SOD phenocopy, implying that heterozygous mutations in human HESX1 could underlie some cases of congenital pituitary hypoplasia with or without midline defects. Accordingly, we have now scanned for HESX1 mutations in 228 patients with a broad spectrum of congenital pituitary defects, ranging in severity from isolated growth hormone deficiency to SOD with panhypopituitarism. Three different heterozygous missense mutations were detected in individuals with relatively mild pituitary hypoplasia or SOD, which display incomplete penetrance and variable phenotype amongst heterozygous family members. Gel shift analysis of the HESX1-S170L mutant protein, which is encoded by the C509T mutated allele, indicated that a significant reduction in relative DNA binding activity results from this mutation. Segregation analysis of a haplotype spanning 6.1 cM, which contains the HESX1 locus, indicated that only one HESX1 mutation was present in the families containing the C509T and A541G mutations. These results demonstrate that some sporadic cases of the more common mild forms of pituitary hypoplasia have a genetic basis, resulting from heterozygous mutation of the HESX1 gene.
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