Tissue segregation in the early vertebrate embryo
Introduction
Early animal development follows a stereotypical sequence of events: The fertilized egg first undergoes a series of repeated cleavages to produce a multicellular structure called the blastula. A combination of patterning signals subdivides the blastula into different regions, determining the germ layers and creating “organizing” centres that further pattern the germ layers along the dorsal-ventral and anterior-posterior axes. Gastrulation then extensively remodels this prepatterned blastula, repositioning the various regions in order to build the general body plan. This remodelling requires a tight coordination between acquisition of cell fate and regulation of cellular properties responsible for morphogenesis, such as cell division, polarity, adhesion and motility.
While the general principles underlying morphogenetic processes are valid for all metazoans, the vertebrates have acquired some major distinctive features that directly impact early development. One of them is a multi-layered organization (Fig. 1). This organization has two major consequences on morphogenesis of the vertebrate early embryo: Firstly, it creates two categories of cells, superficial cells exposed to the “outside”, and deep cells confined to the inside (Fig. 1B). We will see how this has been exploited by evolution to produce distinct cell types. Secondly, gastrulation now involves internalization of massive groups of cells “flowing” inside the embryo. Directly related to this multi-layered organization, the vertebrate embryo had also come up with new ways to segregate tissues and maintain their integrity.
Another characteristic of vertebrates, shared with our remote prochordate relatives, is the notochord, a rod-like structure that arises from the dorsal mesodermal midline and sets the primitive body axis. The notochord eventually degenerates in adult vertebrates, but it plays a central role during embryonic development in patterning the dorsal structures, in particular the nervous system and the paraxial mesoderm, also called presomitic mesoderm (PSM). The development of these two tissues is covered in two other chapters of this issue (Pujades, Naganathan & Oates).
In this chapter, I will summarize our current understanding of the early segregation processes. I will first focus on the amphibian Xenopus embryo, where these processes are best understood, and which may be considered as the archetypical model of early vertebrate development. I will start with the segregation between inner and outer layers of the blastula. We will see that this special type of segregation results is a consequence of asymmetric division of epithelial cells. I will then review the process of separation of the germ layers during gastrulation, followed by individualization of the notochord. In both cases, tissue boundaries are formed by a local increase in cortical tension and decrease of cell adhesion, controlled by the same cell-cell contact cues. I will then discuss the similarities and divergences observed in the fish and mouse embryos, and highlight some key questions that remain unsolved.
Section snippets
Pioneering experiments
The ability of cell populations to segregate from each other is an amazing property that has fascinated early embryologists (e.g. [1,2]). The process has been most extensively studied by Johannes Holtfreter using amphibian embryos [3,4]. By systematically mixing cells dissociated from various tissues, in all possible combinations, he discovered fundamental features of cell sorting: Firstly, combining cells from any two tissues would create compact aggregates, demonstrating the existence of a
The polarity switch of the egg membrane
The oocyte grows in the comfortable ambient of the uterus, where it is provided with plentiful of nutrients. In oviparous animals, the laid egg is abruptly exposed to the external hostile environment, and must resist desiccation when exposed to air, or the extreme osmotic pressures of fresh or sea water. Embryonic development then relies on internal supplies and exchanges with the outer medium are restricted to few components, the most essential being oxygen. During oocyte maturation and
Segregation of the EVL and its equivalence to Xenopus superficial layer
Compared to prochordates or Xenopus, the geometry of the Zebrafish egg is strongly distorted due to the large volume of the yolk, now confined to a huge single syncytial cell. As a consequence of the meroblastic mode of cleavage, the entire organism derives from the animally located blastoderm. Note however that a purely “superficial” view of fish development would be inaccurate: Indeed, as result of incomplete cleavage, a number of nuclei end up within the superficial region of the yolk, the
Early segregation of the extraembryonic and embryonic tissues
From fish and amphibians to mammals, the vertebrate embryo went through two major innovations, firstly development of extraembryonic tissues, common to all amniotes, then, unique to mammals, implantation in the uterine wall through one of the extraembryonic layers, the trophectoderm (TE). With this switch to a developmental mode that relies on continuous maternal supplies, the formation of the extraembryonic layers became a priority, and indeed the two first segregation events deal with the
Conclusion
We have seen that the earliest process of segregation in amphibian, fish and mouse occurs during cleavage and is based on asymmetric inheritance of the apical egg membrane. The appearance of a separate protective superficial layer had a huge impact on vertebrate development. Firstly, the deep layer, now freed of the rigid constrains of the epithelial organization, could explore complex three-dimensional morphogenetic movements. Furthermore, the deep cells, when coated by the superficial layer,
Declaration of Competing Interest
The author declares that no competing interests exist.
Acknowledgements
I would like to acknowledge David Rozema and Guillaume Desgarceaux for critical reading of the manuscript. F.F. research is supported by an EpiGenMed labex chair of excellence, ANR @traction grant, and ARC Fundation grant 206766.
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