Signals from Trunk Paraxial Mesoderm Induce Pronephros Formation in Chick Intermediate Mesoderm

A common form of evolutionary variation between vertebrate taxa is the different numbers of segments that contribute to various regions of the anterior-posterior axis; cervical vertebrae, thoracic vertebrae, etc. The term ‘transposition’ is used to describe this phenomenon. Genetic experiments with homeotic genes in mice have demonstrated that Hox genes are in part responsible for the specification of segmental identity along the anterior-posterior axis, and it has been proposed that an axial Hox code determines the morphology of individual vertebrae (Kessel, M. and Gruss, P. (1990) Science 249, 347–379). This paper presents a comparative study of the developmental patterns of homeobox gene expression and developmental morphology between animals that have homologous regulatory genes but different morphologies. The axial expression boundaries of 23 Hox genes were examined in the paraxial mesoderm of chick, and 16 in mouse embryos by in situ hybridization and immunolocalization techniques. Hox gene anterior expression boundaries were found to be transposed in concert with morphological boundaries. This data contributes a mechanistic level to the assumed homology of these regions in vertebrates. The recognition of mechanistic homology supports the historical homology of basic patterning mechanisms between all organisms that share these genes.

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her1, a zebrafish pair-rule like gene, acts downstream of notch signalling to control somite development

Development ◽

10.1242/dev.126.13.3005 ◽

1999 ◽

Vol 126 (13) ◽

pp. 3005-3014 ◽

Cited By ~ 8

Author(s):

C. Takke ◽

J.A. Campos-Ortega

Keyword(s):

Active Form ◽

Notch Signalling ◽

Paraxial Mesoderm ◽

Presomitic Mesoderm ◽

Essentially Normal ◽

Early Embryos ◽

Somite Formation ◽

Genes Encoding ◽

Ectopic Activation ◽

Pair Rule

During vertebrate embryonic development, the paraxial mesoderm becomes subdivided into metameric units known as somites. In the zebrafish embryo, genes encoding homologues of the proteins of the Drosophila Notch signalling pathway are expressed in the presomitic mesoderm and expression is maintained in a segmental pattern during somitogenesis. This expression pattern suggests a role for these genes during somite development. We misexpressed various zebrafish genes of this group by injecting mRNA into early embryos. RNA encoding a constitutively active form of notch1a (notch1a-intra) and a truncated variant of deltaD [deltaD(Pst)], as well as transcripts of deltaC and deltaD, the hairy-E(spl) homologues her1 and her4, and groucho2 were tested for their effects on somite formation, myogenesis and on the pattern of transcription of putative downstream genes. In embryos injected with any of these RNAs, with the exception of groucho2 RNA, the paraxial mesoderm differentiated normally into somitic tissue, but failed to segment correctly. Activation of notch results in ectopic activation of her1 and her4. This misregulation of the expression of her genes might be causally related to the observed mesodermal defects, as her1 and her4 mRNA injections led to effects similar to those seen with notch1a-intra. deltaC and deltaD seem to function after subdivision of the presomitic mesoderm, since the her gene transcription pattern in the presomitic mesoderm remains essentially normal after misexpression of delta genes. Whereas notch signalling alone apparently does not affect myogenesis, zebrafish groucho2 is involved in differentiation of mesodermal derivatives.

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Onset of the segmentation clock in the chick embryo: evidence for oscillations in the somite precursors in the primitive streak

Development ◽

10.1242/dev.129.5.1107 ◽

2002 ◽

Vol 129 (5) ◽

pp. 1107-1117 ◽

Cited By ~ 3

Author(s):

Caroline Jouve ◽

Tadahiro Iimura ◽

Olivier Pourquie

Keyword(s):

Continuous Process ◽

Notch Pathway ◽

Primitive Streak ◽

The Body ◽

Paraxial Mesoderm ◽

Segmentation Clock ◽

Head Mesoderm ◽

Dynamic Expression ◽

Mesoderm Formation ◽

Cyclic Gene

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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Myogenesis in paraxial mesoderm: preferential induction by dorsal neural tube and by cells expressing Wnt-1

Development ◽

10.1242/dev.121.11.3675 ◽

1995 ◽

Vol 121 (11) ◽

pp. 3675-3686 ◽

Cited By ~ 40

Author(s):

H.M. Stern ◽

A.M. Brown ◽

S.D. Hauschka

Keyword(s):

Neural Tube ◽

Muscle Development ◽

Paraxial Mesoderm ◽

Myogenic Response ◽

Dorsal Neural Tube ◽

Ventral Neural Tube ◽

Myogenic Induction ◽

Inductive Activity

Previous studies have demonstrated that the neural tube/notochord complex is required for skeletal muscle development within somites. In order to explore the localization of myogenic inducing signals within the neural tube, dorsal or ventral neural tube halves were cultured in contact with single somites or pieces of segmental plate mesoderm. Somites and segmental plates cultured with the dorsal half of the neural tube exhibited 70% and 85% myogenic response rates, as determined by immunostaining for myosin heavy chain. This response was slightly lower than the 100% response to whole neural tube/notochord, but was much greater than the 30% and 10% myogenic response to ventral neural tube with and without notochord. These results demonstrate that the dorsal neural tube emits a potent myogenic inducing signal which accounts for most of the inductive activity of whole neural tube/notochord. However, a role for ventral neural tube/notochord in somite myogenic induction was clearly evident from the larger number of myogenic cells induced when both dorsal neural tube and ventral neural tube/notochord were present. To address the role of a specific dorsal neural tube factor in somite myogenic induction, we tested the ability of Wnt-1-expressing fibroblasts to promote paraxial mesoderm myogenesis in vitro. We found that cells expressing Wnt-1 induced a small number of somite and segmental plate cells to undergo myogenesis. This finding is consistent with the localized dorsal neural tube inductive activity described above, but since the ventral neural tube/notochord also possesses myogenic inductive capacity yet does not express Wnt-1, additional inductive factors are likely involved.

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