Gastrulation

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Gastrulation is a stage in early animal development marked by extensive cell migration[1]. When cells are reorganised in the gastrula they develop into distinct germ layers: the Mesoderm, Ectoderm and Endoderm. Together they give the body the basis of its organs and create the external form of developing animals[2]. This positioning is controlled by Morphogens, secreted proteins which determine differentiation and result in a change in the cell shape. 

Gastrulation can be divided into three main processes, which are morphogenetic and have been conserved evolutionarily: Ingression, Epiboly, and Convergence and Extension. 

Contents

Ingression

Mesodermal and endodermal cells move via the primitive streak, a hollow sphere of cells which forms early in development[3]. Cells move downwards and outwards as individual cells to form the middle layer. Prior to this, mesodermal and endodermals cells are stored in the epithelium[4]. This process involves the cytoskeleton due to the formation of the intermediate filament network and rearrangement of microtubules to radiate from a centrosome [5]

Epiboly

This is a thinning process whereby cell layers spread in the early embryo. Cells change the way they are arranged; this is achieved by Radial intercalation where cell layers merge and cells intermix as they move away from the blastula[6]Actin is important in this process, with formation of rings of filamentation having been observed[7]

Convergence and Extension

Once cell layers have thinned the germ layers are lengthened anteroposteriorly and narrowed along the dorso-ventral axis. Cells intercalate by polarised radial intercalation. Convergenve and extension occurs in the Ectoderm and Mesoderm

Signalling in gastrulation

Cell adhesion and microtubule skeletal systems drive the main processes of gastrulation[8]. Forming cell-cell and cell-matrix complexes is crucial in development and mediated by CadherinsProtocadherins and the tight junction. Although gastrulation movements are evoluntionarily conserved there are distinctions in mechanisms which have been observed in model organisms such as C. elegansDrosophila melanogasterXenopusZebrafishGallus gallus (chickens) and Mus musculus (mice). Signalling molecules in gastrulation are numerous. Processes of gastrulation do not occur in isolation, so many genes work in parallel. Wnt and Wnt 3 are particularly crucial. controlling body patterning and cell differentiation, proliferation and migration. Their role in this was discovered through mutations in Drosophila melanogaster and later found to be a cause of breast cancer in mice. 

References

  1. Hartl, D. and Ruvolo, M. (2012) Genetics: Analysis of Genes and Genomes. pp 482-483 8th Edition. United States: Jones and Bartlett Publishers, Inc.
  2. Solnica-Krezel, L. and Sepich, D. (2012) ‘Gastrulation: Making and Shaping Germ Layers’, Annual Review of Cell and Developmental Biology, 28(1)
  3. Hartl and Ruvolo, 2012, pp. 482–483
  4. Solnica-Krezel and Sepich, 2012, pp. 687–71
  5. Thiery, J. P., Acloque, H., Huang, R. Y. J. and Nieto, A. (2009) ‘Epithelial-Mesenchymal Transitions in Development and Disease’, Cell, 139(5).
  6. Solnica-Krezel and Sepich, 2012, pp. 687–717
  7. Cheng, J., Miller, A. and Webb, S. (2004) ‘Organization and function of microfilaments during late epiboly in zebrafish embryos’, Developmental Dynamics, 231(2).
  8. Nelson, W. (2009) ‘Remodeling Epithelial Cell Organization: Transitions Between Front-Rear and Apical-Basal Polarity’, Cold Spring Harbor Perspectives in Biology, 1(1).

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