QUICK SEARCH:   [advanced] Author: Keyword(s):
Home     Help     Feedback     Subscriptions     Archive     Search     Table of Contents

First published online 5 January 2005
doi: 10.1242/dev.01603

This Article Summary Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Wilson, R. Articles by Leptin, M. Search for Related Content PubMed PubMed Citation Articles by Wilson, R. Articles by Leptin, M.

FGF signalling and the mechanism of mesoderm spreading in Drosophila embryos

Institut für Genetik, Universität zu Köln, Weyertal 121, 50931 Köln, Germany

View larger version (137K): [in a new window]   Fig. 1. Events during mesoderm spreading. (A,B) The events that occur during the dispersal of the mesoderm. (A) Following invagination, the mesoderm flattens down onto the surface of the ectoderm (arrows). The first cells make contact with the ectoderm symmetrically on either side of the midline (arrowheads) and accumulate high levels of the activated form of MAPK (cells marked in blue). (B) The mesodermal cells subsequently undergo an epithelial to mesenchymal transition, divide and spread out away from the site of invagination to cover the surface of the ectoderm (arrows). A gradient of activated MAPK is observed in the mesoderm with the highest levels at the edge of the mesodermal sheet (cells shaded from light to dark blue). (C) A drawing of a cross section through an embryo at an early stage of mesodermal spreading to illustrate the regions shown in D and E (red boxes). (D,E) Electron micrographs of mesodermal cells (m) showing cytoplasmic extensions (arrowheads) protuding towards the ectoderm (e).

View larger version (128K): [in a new window]   Fig. 2. Mesoderm spreading in mutant embryos. (A-F) Ectodermal cell fate and mesoderm morphogenesis. Cross sections of embryos showing the development of the mesoderm in embryos with altered ectodermal cell fates. The embryos have been stained to reveal the di-phosphorylated form of the MAP kinase ERK. Each set of panels show an early and late timepoint during the flattening of the mesoderm. (A,B) Sections of brk, sog mutant embryos. (C,D) Sections of Egfr (top18A) mutant embryos. (E,F) Sections of brk, sog; Egfr (top18A) triple mutant embryos. Arrowheads indicate mesodermal cells in which high levels of the di-phosphorylated form of ERK are present. (G,H) Sections of embryos stained with antibodies directed against Twist to reveal mesodermal cells. (G) A section of a late dof, stg double mutant embryo. (H) Mesoderm development in a late htl mutant embryo expressing an activated form of Htl throughout the mesoderm.

View larger version (134K): [in a new window]   Fig. 3. Activation of MAPK and rescue of Eve-positive cells in the mesoderm of htl mutants. Embryos were stained with antibodies directed against dp-ERK (left), or Even-skipped (right). (A,H) Whole-mounts of wild-type embryos; (B-G,I-N) enlarged views of the trunk regions of htl mutant embryos. (B,I) The homozygous htl mutant phenotype. (C-G,J-N) The effect of expressing transgenes in the mesoderm of htl mutant embryos. (C,J) Htl; (D,K) an activated form of Htl; (E,L) an activated form of Btl; (F,M) an activated form of EGFR; (G,N) an activated form of PVR.

View larger version (140K): [in a new window]   Fig. 4. Rescue of MAPK activation and spreading at early stages of mesoderm morphogenesis. Sections of mutants expressing transgenes in the mesoderm stained to reveal activated MAPK. (A) Expression of Htl in the mesoderm of an htl mutant embryo completely rescues mesoderm spreading. (B) An activated form of the EGF receptor expressed in an htl mutant fails to promote interaction of the mesoderm with the ectoderm and does not stimulate activation of MAPK within the mesoderm at this developmental stage. (C) An activated form of Raf does not induce mesoderm spreading or the activation of MAPK in a dof mutant embryo. Note that we chose a slightly older embryo, showing that even at a stage when the mesoderm collapses onto the ectoderm no MAPK activation is detectable. (D-I) Sections from a young embryo are shown on the left of each row, with older embryos towards the right. (D-F) Expression of an activated form of Htl rescues both early contact of the mesoderm with the ectoderm and the activation of MAPK, notably at early stages this is only evident in the mesodermal cells that make contact with the ectoderm. (G-I) Expression of an activated form of Btl also rescues mesoderm spreading, but the onset of spreading occurs slightly later than in wild-type embryos. Activated MAPK is only observed in mesodermal cells that are in contact with the ectoderm.

View larger version (135K): [in a new window]   Fig. 6. The role of Rho-family GTPases in mesoderm spreading. (A) A section through an embryo that lacks the function of Rac2. The mesoderm flattens down onto the ectoderm as in wild type, but does not adhere to the ectoderm properly. (B) The effect of reducing the maternal dose of RhoA. The interaction of the mesoderm with the ectoderm is not affected, but the shape of the cells in the mesoderm is abnormal. (C,D) Embryos derived from mothers with reduced dose of (C) Rac1 and Rac2, or (D) Rac1, Rac2 and Mtl. In these mutants, contact between the mesoderm and the ectoderm fails to be established properly. (E) Embryos that lack the materal and zygotic function of the Rac effector Pak show no effect upon the initation of mesodermal spreading. (F) A section of a homozygous pebble mutant embryo. Contact of the mesoderm with the ectoderm does not occur, and MAPK activation is not observed within the mesoderm at this stage of development. Embryos shown in A-E were stained with Twist and the embryo in Fwas stained with anti-dp-ERK.

View larger version (126K): [in a new window]   Fig. 5. Mesoderm spreading is initiated in the absence of MAPK activation. Cross sections of embryos stained for Twist to reveal the development of the mesoderm (brown). (A) A wild-type embryo. (B) A homozygous dof1 mutant embryo. (C-F) Mesoderm development in embryos derived from germline mutant clones lacking both the maternal and zygotic function of (C) Dsor1LH110, (D) Ras85Dc40B, (E) cswE(sev)1A-eOP and (F) dShc13G.

View larger version (54K): [in a new window]   Fig. 7. Models for the signalling events and cell movements during early mesoderm spreading. (A) Diagram of part of the invaginated mesodermal tube (light grey) and the underlying ectoderm (dark grey) showing a model of events during the establishment of contact between mesoderm and ectoderm. Left panel: FGF receptors on the mesodermal cell close to the ectoderm bind FGF secreted by the ectoderm (red dots), dimerise, undergo autophosphorylation and phosphorylate Dof (shown here as part of the receptor complex). However, FGF receptor activation alone is not sufficient to activate MAPK (dashed grey arrow) or lead to cell shape changes, either because it does not deliver a sufficiently strong signal or because of the presence of an inhibitor (Inhib.). It is postulated that an event at the cell surface (yellow star) is also needed that can only occur in the vicinity of the ectoderm and depends on FGF signalling, the Rho exchange factor Pebble and Rac GTPases. As a result of this event (right panel), the activation of MAPK becomes possible, either by suppression of the postulated inhibitor, or by strengthening the MAPK signalling pathway (large black arrow). Cell-shape changes ensue that bring the next cell into the vicinity of the ectoderm and allow it to undergo the same process. The requirement for the membrane event can be overridden by overexpression of FGF. (B) Diagram of the invaginated mesoderm before (left) and after (right) it has begun to spread. Depending on the mechanism by which cells move away from the site of invagination, the cells marked in red and yellow end up in different positions (right): (i) if the first mesodermal cells to make contact with the ectoderm (yellow) become the leading edge of a migrating cell sheet, these cells would end up in a more dorsal position than the cells starting at a position more distant from the ectoderm (red); and (ii) if cells that make contact with the ectoderm become stationary and other cells crawl over them, then the first cells to make contact with the ectoderm will remain near the site of invagination, while other mesodermal cells will end up in more dorsal positions.