Vertebrate segmentation initiates with the subdivision of the paraxial mesoderm into a regular array of somites. Recent evidence suggests that the segmentation clock — a biochemical oscillator acting ...in the unsegmented paraxial mesoderm cells in most vertebrates — controls cyclic Notch signalling, resulting in periodic formation of somite boundaries.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Notch signalling is best known for its role in lateral inhibition, where it acts to prevent differentiation of cells neighbouring one that has ‘won out’ in a competition to differentiate. Recent ...results suggest that Notch signalling can work in the opposite way, and promote differentiation of the receiving cells.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Developmental clocks are hypothetical embryonic time-measuring devices — some are run by oscillators, whereas otherdepend on rate-limiting processes. Their existence has been deduced from recent ...studies of the timing of the midblastula transition, the opening of the Hox cluster during organogenesis, and oligodendrocyte progenitor differentiation; however, the mechanisms underlying their function remain largely unknown.
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In vertebrates, the primary segmented tissue of the body axis is the paraxial mesoderm, which lies bilaterally to the axial organs, neural tube and notochord. The segmental pattern of the paraxial ...mesoderm is established during embryogenesis through the production of the somites which are transient embryonic segments giving rise to the vertebrae, the skeletal muscles and the dorsal dermis. Somitogenesis can be subdivided into three major phases (see Fig. 1). First a growth phase during which new paraxial mesoderm cells are produced by a growth zone (epiblast and blastopore margin or primitive streak and later on tail bud) and become organized as two rods of mesenchymal tissue,forming the presomitic mesoderm. Second a patterning phase occuring in the PSM, during which the segmental pattern is established at the molecular level. Third, the somitic boundaries are formed during the morphological segmentation phase. In all vertebrates, all cells of the paraxial mesoderm, during their maturation in the PSM, go successively through these three phases, which are tightly regulated at the spatio-temporal level. The first phase of paraxial mesoderm production falls out of the scope of this review, as it essentially pertains to the gastrulation process. Here, I essentially discuss the segmental patterning phase in vertebrates. Recent data suggest that establishment of the segmental pattern relies on a clock and wavefront mechanism which has been conserved in vertebrates. Furthermore, conservation of this system could extend to invertebrates, suggesting that the clock and wavefront is an ancestral mechanism.
Somitogenesis: segmenting a vertebrate McGrew, Michael J; Pourquié, Olivier
Current opinion in genetics & development,
08/1998, Volume:
8, Issue:
4
Journal Article
Peer reviewed
The partitioning of the vertebrate body into a repetitive series of segments, or somites, requires the spatially and temporally co-ordinated behaviour of mesodermal cells. To date, it remains unknown ...how applicable our knowledge of the genetic mechanisms governing
Drosophila segmentation will be to that of vertebrates, though recent results indicate some degree of conservation. Genetic studies in the mouse point to a major role for the Notch-Delta signalling pathway in somite formation. Furthermore, a molecular clock may be ‘ticking’ in the presomitic mesoderm.
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IJS, IMTLJ, KILJ, KISLJ, NUK, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
It has been shown previously that in the chick embryo the cell adhesion molecule BEN/SC1/DM-GRASP is expressed by neurons in the inferior olive (IO) and by their terminal axonal arbors in the ...cerebellar cortex, the climbing fibers (Porquié et al., 1992b). Here, new information on the expression of BEN during the formation of the olivocerebellar projection adds the important notion that BEN is also expressed by the cerebellar targets of inferior olivary axons, Purkinje cells (PCs) and deep nuclear neurons. This expression is transient, starting at E7-E8 and vanishing shortly after hatching. More importantly, BEN expression is restricted to precise subsets of IO neurons and PCs. In the cerebellar cortex, BEN-immunoreactive (BEN-IR) structures are not found randomly but are distributed according to a reproducible pattern of parasagittal stripes. A maximum of four distinct sagittal stripes is found in each lobule, along the whole rostrocaudal extent of the cerebellum. Moreover, BEN-expressing stripes belong to two classes; one contains BEN-IR climbing fibers terminating on BEN-IR PCs and the other, more frequent class is solely composed of BEN-IR climbing fibers. Organotypic cultures of isolated cerebella have shown that the expression of BEN in the IO and in the cerebellum arise independently, probably because of an intrinsic developmental program. Thus, the cell adhesion molecule BEN meets all criteria for a recognition molecule involved in the formation of the olivocerebellar projection.
The regular spacing of somites during vertebrate embryogenesis involves a dynamic gradient of FGF signaling that controls the timing of maturation of cells in the presomitic mesoderm (PSM). How the ...FGF signal is transduced by PSM cells is unclear. Here, we first show that the FGF gradient is translated into graded activation of the extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) pathway along the PSM in the chicken embryo. Using in ovo electroporation of PSM cells, we demonstrate that constitutive activation of ERK signaling in the PSM blocks segmentation by preventing maturation of PSM cells, thus phenocopying the overexpression of FGF8. Conversely, inhibition of ERK phosphorylation mimics a loss of function of FGF signaling in the PSM. Interestingly, video microscopy analysis of cell movements shows that ERK regulates the motility of PSM cells, suggesting that the decrease of cell movements along the PSM enables mesenchymal PSM cells to undergo proper segmentation. Together, our data demonstrate that ERK is the effector of the gradient of FGF in the PSM that controls the segmentation process.
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The segmented body plan of vertebrate embryos arises through segmentation of the paraxial mesoderm to form somites. The tight temporal and spatial control underlying this process of somitogenesis is ...regulated by the segmentation clock and the FGF signaling wavefront. Here, we report the cyclic mRNA expression of
Snail1 and
Snail2 in the mouse and chick presomitic mesoderm (PSM), respectively. Whereas
Snail genes' oscillations are independent of NOTCH signaling, we show that they require WNT and FGF signaling. Overexpressing
Snail2 in the chick embryo prevents cyclic
Lfng and
Meso1 expression in the PSM and disrupts somite formation. Moreover, cells misexpressing
Snail2 fail to express
Paraxis, remain mesenchymal, and are thereby inhibited from undergoing the epithelialization event that culminates in the formation of the epithelial somite. Thus,
Snail genes define a class of cyclic genes that coordinate segmentation and PSM morphogenesis.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Vertebrate somitogenesis is associated with a molecular oscillator, the segmentation clock, which is defined by the periodic expression of genes related to the Notch pathway such as hairy1 and hairy2 ...or lunatic fringe (referred to as the cyclic genes) in the presomitic mesoderm (PSM). Whereas earlier studies describing the periodic expression of these genes have essentially focussed on later stages of somitogenesis, we have analysed the onset of the dynamic expression of these genes during chick gastrulation until formation of the first somite. We observed that the onset of the dynamic expression of the cyclic genes in chick correlated with ingression of the paraxial mesoderm territory from the epiblast into the primitive streak. Production of the paraxial mesoderm from the primitive streak is a continuous process starting with head mesoderm formation, while the streak is still extending rostrally, followed by somitic mesoderm production when the streak begins its regression. We show that head mesoderm formation is associated with only two pulses of cyclic gene expression. Because such pulses are associated with segment production at the body level, it suggests the existence of, at most, two segments in the head mesoderm. This is in marked contrast to classical models of head segmentation that propose the existence of more than five segments. Furthermore, oscillations of the cyclic genes are seen in the rostral primitive streak, which contains stem cells from which the entire paraxial mesoderm originates. This indicates that the number of oscillations experienced by somitic cells is correlated with their position along the AP axis.
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