Modeling Gastrulation in the Chick Embryo: Formation of the Primitive Streak Bakhtier Vasiev 1 * ¤ , Ariel Balter 2 , Mark Chaplain 1 , James A. Glazier 2 , Cornelis J. Weijer 3 1 Division of Mathematics, University of Dundee, Dundee, United Kingdom, 2 Biocomplexity Institute and Department of Physics, Indiana University, Bloomington, Indiana, United States of America, 3 Wellcome Trust Biocentre, School of Life Sciences, University of Dundee, Dundee, United Kingdom Abstract The body plan of all higher organisms develops during gastrulation. Gastrulation results from the integration of cell proliferation, differentiation and migration of thousands of cells. In the chick embryo gastrulation starts with the formation of the primitive streak, the site of invagination of mesoderm and endoderm cells, from cells overlaying Koller’s Sickle. Streak formation is associated with large-scale cell flows that carry the mesoderm cells overlying Koller’s sickle into the central midline region of the embryo. We use multi-cell computer simulations to investigate possible mechanisms underlying the formation of the primitive streak in the chick embryo. Our simulations suggest that the formation of the primitive streak employs chemotactic movement of a subpopulation of streak cells, as well as differential adhesion between the mesoderm cells and the other cells in the epiblast. Both chemo-attraction and chemo-repulsion between various combinations of cell types can create a streak. However, only one combination successfully reproduces experimental observations of the manner in which two streaks in the same embryo interact. This finding supports a mechanism in which streak tip cells produce a diffusible morphogen which repels cells in the surrounding epiblast. On the other hand, chemotactic interaction alone does not reproduce the experimental observation that the large-scale vortical cell flows develop simultaneously with streak initiation. In our model the formation of large scale cell flows requires an additional mechanism that coordinates and aligns the motion of neighboring cells. Citation: Vasiev B, Balter A, Chaplain M, Glazier JA, Weijer CJ (2010) Modeling Gastrulation in the Chick Embryo: Formation of the Primitive Streak. PLoS ONE 5(5): e10571. doi:10.1371/journal.pone.0010571 Editor: Nick Monk, University of Nottingham, United Kingdom Received October 27, 2009; Accepted April 6, 2010; Published May 11, 2010 Copyright: ß 2010 Vasiev et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: Funding was provided by Biotechnology and Biological Sciences Research Council (BBSRC) Grant 94/E18787, ‘‘An individual based mathematical model of chick embryo gastrulation: developing a virtual embryo computational tool’’; the Biocomplexity Institute and the Indiana University Faculty Research Support Program, NIGMS R01 GM76692 and NIGMS R01 GM077138. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: bnvasiev@liverpool.ac.uk ¤ Current address: Department of Mathematical Sciences, Liverpool University, Liverpool, United Kingdom Introduction Gastrulation is a critical stage in the development of all higher organisms, since it is the stage where the three germ layers, the ectoderm, mesoderm and endoderm, assume their definitive positions in the embryo [1]. Cells proliferate, differentiate and migrate extensively during gastrulation. The chick embryo is a convenient model organism for investigation of amniote gastrula- tion, since it is essentially flat, transparent and develops outside the mother. Cell movement during gastrulation is similar in avians and humans. At the time of egg laying, the chick embryo consists of around twenty to thirty thousand cells. A subset of these cells forms a one-cell-layer thick, quasi-epithelial disk, the epiblast. At the periphery of the embryo, the epiblast sits on top of a rigid, several- cell-thick layer of large mesenchymal cells, which directly contact the underlying yolk. This outer segment of the embryo is known as the Area Opaca (AO). In the central part of the embryo, the Area Pellucida (AP), clusters of a few small rounded cells attach to the ventral (bottom) side of the epiblast, forming the primary hypoblast. During the initial course of development, the primary hypoblast flattens to form an epithelial layer of large thin cells, the hypoblast. The AP epiblast cells give rise to the embryo proper, while the hypoblast and AO form extra-embryonic structures. The band of epithelial cells at the posterior lateral boundary between the AO and the AP has an elongated shape, forming the marginal zone. Initially, the embryo appears circularly symmetric. Then, a group of deep mesenchymal cells at the boundary between the AO and AP in the posterior half of the embryo thicken to form Koller’s Sickle, a darker sickle-shaped or lunate region. Inductive signals from the marginal zone, an anterior-posterior gradient of Vg1 and graded Wnt8c expression in the AO induce nodal expression in the epiblast overlying Koller’s Sickle and the nodal-expressing cells then differentiate to form mesendoderm (Fig. 1A) [2], initiating gastrulation. Gastrulation starts with the formation of the primitive streak (PS), as mesendoderm from the sickle-shaped region at the interface between the AO and AP moves into the posterior midline region of the embryo (Fig. 1). The development of the freshly laid egg (stage EG X) to the formation of a fully extended streak (stage HH4) takes roughly 24 hours. Streak formation is concurrent with large vortical flows of cells in the epiblast (Fig. 1). These vortices rotate in opposite directions—away from the midline in the anterior and towards the midline in the posterior [3,4,5,6,7]. In the posterior of the epiblast, where cell flows meet, the cells start to stack on top of each other and the epiblast becomes several cell- diameters thick, forming the structure visible as the streak (HH1- 2). The streak extends progressively in the anterior direction until PLoS ONE | www.plosone.org 1 May 2010 | Volume 5 | Issue 5 | e10571