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Gastrulation: Making and Shaping Germ Layers CB28CH26-SolnicaKrezel ARI 5 September 2012 17:28 Gastrulation: Making and Shaping Germ Layers Lila Solnica-Krezel and Diane S. Sepich Department of Developmental Biology, Washington University School of Medicine in St. Louis, St. Louis, Missouri 63110; email: [email protected], [email protected] Annu. Rev. Cell Dev. Biol. 2012. 28:687–717 Keywords First published online as a Review in Advance on cell migration, cell intercalation, adhesion, chemotaxis, planar polarity July 9, 2012 The Annual Review of Cell and Developmental Abstract Biology is online at cellbio.annualreviews.org Gastrulation is a fundamental phase of animal embryogenesis during This article’s doi: which germ layers are specified, rearranged, and shaped into a body 10.1146/annurev-cellbio-092910-154043 plan with organ rudiments. Gastrulation involves four evolutionar- by Reed College on 07/26/13. For personal use only. Copyright c 2012 by Annual Reviews. ily conserved morphogenetic movements, each of which results in a All rights reserved specific morphologic transformation. During emboly, mesodermal and 1081-0706/12/1110-0687$20.00 endodermal cells become internalized beneath the ectoderm. Epibolic movements spread and thin germ layers. Convergence movements nar- row germ layers dorsoventrally, while concurrent extension movements Annu. Rev. Cell Dev. Biol. 2012.28:687-717. Downloaded from www.annualreviews.org elongate them anteroposteriorly. Each gastrulation movement can be achieved by single or multiple motile cell behaviors, including cell shape changes, directed migration, planar and radial intercalations, and cell divisions. Recent studies delineate cyclical and ratchet-like behaviors of the actomyosin cytoskeleton as a common mechanism underlying vari- ous gastrulation cell behaviors. Gastrulation movements are guided by differential cell adhesion, chemotaxis, chemokinesis, and planar polar- ity. Coordination of gastrulation movements with embryonic polarity involves regulation by anteroposterior and dorsoventral patterning sys- tems of planar polarity signaling, expression of chemokines, and cell adhesion molecules. 687 CB28CH26-SolnicaKrezel ARI 5 September 2012 17:28 INTRODUCTION Contents Animals have bodies of diverse shapes with INTRODUCTION.................. 688 internal collections of organs of unique mor- COMPONENT GASTRULATION phology and function. Such sophisticated body MOVEMENTS: architecture is elaborated during embryonic de- MORPHOGENETIC velopment, whereby a fertilized egg undergoes OUTCOMES AND a program of cell divisions, fate specification, UNDERLYING CELL and movements. One key process of embryo- BEHAVIORS..................... 689 genesis is determination of the anteroposterior Emboly . 689 (AP), dorsoventral (DV), and left-right (LR) Epiboly. 691 embryonic axes. Other aspects of embryo- Convergence and Extension . 691 genesis are specification of the germ layers, GASTRULATION MOVEMENTS endoderm, mesoderm, and ectoderm, as well as INMODELORGANISMS........ 693 their subsequent patterning and diversification Caenorhabditis elegans ............... 693 of cell fates along the embryonic axes. These Drosophila melanogaster ............. 693 processes occur very early during development SeaUrchin........................ 695 when most embryos consist of a relatively Zebrafish.......................... 695 small number of morphologically similar cells Frog.............................. 696 arranged in simple structures, such as cell balls Chick............................. 697 or sheets, which can be flat or cup shaped. Mouse............................. 698 The term gastrulation, derived from the Greek MECHANICS OF POLARIZATION word gaster, denoting stomach or gut, is a fun- OF CELL ARCHITECTURE damental process of animal embryogenesis that AND ACTIVITY DURING employs cellular rearrangements and move- GASTRULATION................ 699 ments to reposition and shape the germ layers, Cell Shape and Motility Depend on thus creating the internal organization as well Adhesion and Cytoskeleton. 699 as the external form of developing animals. Apical Constriction and Pulsed Here we discuss both the differences in Actomyosin Contraction . 700 the cellular and molecular mechanisms of gas- CellIntercalation.................. 702 trulation as well as the many similarities that emerge as we learn more about this fascinating by Reed College on 07/26/13. For personal use only. DirectedMigration................ 703 MOLECULAR CUES GUIDING process in model organisms. First, we discuss POLARIZED GASTRULATION the four evolutionarily conserved gastrulation CELLBEHAVIORS.............. 704 movements, epiboly, internalization, conver- Cell-Cell Adhesion. 704 gence, and extension, each of which drives de- fined morphological tissue transformation. Sec- Annu. Rev. Cell Dev. Biol. 2012.28:687-717. Downloaded from www.annualreviews.org Cell-MatrixAdhesion.............. 706 PlanarPolarity..................... 706 ond, we survey cellular mechanisms underlying Chemotaxis........................ 707 these gastrulation movements, including cell Chemokinesis...................... 708 migration, intercalation, epithelial mesenchy- COORDINATION OF mal transition, and cell shape changes. Next, we GASTRULATION discuss the process of gastrulation as it occurs MOVEMENTS WITH BODY in several model organisms, highlighting how AXES............................. 708 they employ epiboly, internalization, conver- OUTLOOK.......................... 710 gence, and extension movements as well as the specific cellular mechanisms deployed. Then we provide a short review of the basic cell prop- erties, including cell adhesion, cortical tension, 688 Solnica-Krezel · Sepich CB28CH26-SolnicaKrezel ARI 5 September 2012 17:28 and cytoskeletal systems, that mediate various structure central to the process of gastrulation, gastrulation cell behaviors. The essence of var- also known as blastoderm margin in fish and ious gastrulation cell movements is their po- primitive streak in amniotes (Keller & David- AP: anteroposterior larized and directional nature that affords the son 2004). Internalization is usually followed by transformation of an amorphous cellular mass migration of endodermal and mesodermal pro- DV: dorsoventral or cell sheet into a highly asymmetric and struc- genitors away from the blastopore as individ- LR: left-right tured body rudiment. We review the significant ual cells (Solnica-Krezel 2005). At the onset of EMT: epithelial to progress achieved in recent years in delineat- gastrulation, prospective mesodermal and en- mesenchymal ing various molecular mechanisms that mediate doderm cells reside in epithelium (Drosophila transition and instruct asymmetric cellular behaviors dur- melanogaster, Caenorhabditis elegans, chick, SMO: ing gastrulation and coordinate morphogenetic mouse) or within tightly packed and adherent Spemann-Mangold organizer movements with embryonic polarity. mesenchymal tissue (frog, fish). Thus, emboly and migration of internalized mesodermal and endodermal cells must involve some form of ep- COMPONENT GASTRULATION ithelial to mesenchymal transition (EMT) (Wu MOVEMENTS: et al. 2007). In this process, epithelial junctions MORPHOGENETIC OUTCOMES are disassembled and cell adhesion molecules AND UNDERLYING CELL are downregulated, while intermediate filament BEHAVIORS network is formed and microtubule network is The process of gastrulation entails a set rearranged from acentrosomal to that radiating of evolutionarily conserved morphogenetic from a centrosome (Thiery et al. 2009). movements, emboly/internalization, epiboly, The variations in the cellular mechanisms convergence, and extension, which are defined that drive internalization include the position by their morphogenetic outcome (Keller of the blastopore in the gastrula and the timing et al. 1991). Emboly, or internalization, is of the EMT with respect to the internaliza- the defining gastrulation movement, which tion (preceding or following it) (Figure 1). transports the prospective mesodermal and Invagination is one type of emboly that occurs endodermal cells beneath the future ectoderm during gastrulation in D. melanogaster. Apical (Figure 1a–j). Epibolic movements spread constriction of ventral midline epithelial cells and thin germ layers (Figure 1d,e,k,l,m). creates a furrow where mesoderm folds inward Convergence movements narrow germ layers (Figure 1b,c) (Kam et al. 1991, Leptin & by Reed College on 07/26/13. For personal use only. dorsolaterally/mediolaterally, whereas con- Roth 1994). As the ventral furrow (blastopore) current extension movements elongate them deepens, taking the nascent mesoderm deep anteroposteriorly (Figures 2 and 3). Impor- inside the embryo, cells break away from the tantly, the same morphogenetic transformation epithelium and start migrating on the internal of tissue, or each of these gastrulation move- layer of the future ectoderm. Involution is Annu. Rev. Cell Dev. Biol. 2012.28:687-717. Downloaded from www.annualreviews.org ments, can be achieved by various motile cell another example of internalization that pre- behaviors or a combination of cell behaviors. cedes EMT. In the extensively studied example Consequently, involvement of a specific gastru- of involution during frog gastrulation, the lation movement in a given animal species does prospective mesoderm and part of endoderm not imply the underlying cellular mechanism, form a cohesive tissue above the prospective which must be experimentally determined. blastopore (Keller 1981). Involution is heralded by apical constriction of
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