Cellular mechanisms of gastrulation: A combined experimental and modelling study

The body plan of all higher organisms develops during gastrulation. Gastrulation is associated with intensive proliferation, differentiation and migration of cells forming the embryo. The chick gastrula is represented by an epithelial sheet of cells, known as the epiblast, in which the mesendoderm is induced in a Sickle shaped region (located in the posterior pole) by signals from the extra-embryonic region. Another cellular layer, the hypoblast, forms beneath the epiblast due to ingression of cells from the epiblast. At the early stages of gastrulation there occur the large coordinated cell movements in the epiblast which result in the formation of the streak along the central midline of the embryo. Cells in the middle of the streak form the so called primitive groove and ingress through it to form the mesendoderm. These processes involve the maintenance of active cell-to-cell signalling which influences both migration and differentiation of cells. The aim of the proposed research is to gain detailed description of the processes involved and to uncover the mechanisms coupling cell-cell signalling with the formation of cellular flows during gastrulation in the chick embryo. To achieve this we will combine quantitative experimentation, extensive quantitative data analysis and mathematical modelling.
In our previous studies we have analysed cellular flows formed in the epiblast at early gastrulation and migration patterns of mesenchyme cells observed at late stages. We also have a large collection of experimental observations combining cellular proliferation, change of cell shapes and expression of morphogens with movement of cells. We have previously developed a mathematical model and explored through computer simulations a number of hypotheses based on differential chemotactic cell movement of cells in epiblast as the mechanism involved in streak formation. It is known that there exist interactions between cells forming epiblast and hypoblast mediated by morphogens expressed only in either one of these two areas. In the research proposed here we extend our previous studies by taking into consideration the movements and signals between the epiblast and hypoblast and we will study their effects on cell flows in the epiblast. We will also study other phenomena associated with gastrulation, namely, the formation of primitive groove, ingression of cells through the groove into the space between epiblast and hypoblast, transformation of ingresses cells into mesenchyme and patterns formed by migrating mesenchyme cells. We expect that the combination of experimental and modelling approaches will lead to the identification of forces resulting in the formation of the groove as well as to the generation of cell flows of lateral epiblast cells towards the streak. These studies will also highlight the role of changes in dynamics in the adhesive properties of differentiating cells while they move towards and ingress through the primitive groove.
Our study of the interplay between cell-cell signalling, cell differentiation, proliferation and migration is not only important to the community of researchers whose interest is focused on embryogenesis but will also be of great importance to scientists whose research is centred on processes such as wound healing, tissue repair and regeneration. Furthermore, in order to progress with the proposed research we will develop several new mathematical and computational techniques which are expected to be of great value for further mathematical investigation of other biological and biomedical/engineering problems.

This study focuses on exploration of the cellular mechanisms that drive differentiation and movement of cells during gastrulation in the chick embryo, using a highly integrated experimental and mathematical modelling approach. The experimental approach will make extensive use of a newly developed membrane targeted GFP strain in combination with a newly build Digital Light Sheet Microscope (DSLM), which allows imaging of large parts of the embryo at enough (0.5um) resolution to distinguish important cellular events such as shape changes, protrusion formation for all cells in the embryo. The mathematical modelling will be performed using two kinds of models: continuous models represented by a system of coupled partial differential equations and an individual-cell based model, an extension of previous joint work on gastrulation.
Using these approaches we will study:
1. The chemical and mechanical interactions of cells in the epiblast and hypoblast and the influence of these interactions to the formation of primitive streak. We will focus on the role of ingression of epiblast cells to form hypoblast during streak formation. The analysis of these tissue interactions will require the further development of our experimental and modelling techniques to three-dimensional objects.
2. The formation of the primitive groove and the mechanism of ingression of mesendoderm epiblast cells. We expect that our experiments and simulations will identify the mechano-chemical signals, mechanisms and forces that result in the formation of primitive groove and of the cell flows of the epiblast towards the streak. We will explore the changes in adhesion dynamics of the cells during their ingression through the streak.
3. The mechano-chemical signals controlling the cellular flows of the mesenchyme cells during their collective migration to their targets and the feedback between signalling and movement and its resulting emergent properties important for tissue formation and morphogenesis.

The research proposed is devoted to the investigation of the mechanisms governing gastrulation, a central process in the development of all higher animals. Findings made here will greatly increase our understanding of how cell-cell signalling directs cellular events, like differentiation, proliferation and migration, to build a complex 3 dimensional tissue structure. This is important for understanding development and the origin and cause of many congenital defects. Gastrulation is core material of many Life Sciences and Medical Text books. Provided the research proposed here will proceed as anticipated some of the findings made here are expected to become textbook material and therefore besides being relevant to the immediate academic circle carrying out this research it will affect many students of medical and life sciences.
The key processes of gastrulation such as directed collective migration, ingression and EMT (epithelial to mesenchymal transition) are also central to understanding other biological processes using similar cellular mechanisms processes such as wound healing, tissue repair and regeneration. Failure of proper control of these mechanisms is key to the development of autoimmune diseases and metastasis of cancer cells. Therefore findings made here will be of direct relevance to researchers in these areas. Finally, understanding the mechanisms by which cells organise themselves into tissues and discovering the signals that control their organisation are essential for the rational use of embryonic stem cells in regenerative medicine. It is by no means clear how suitably primed embryonic stem cells injected into organs (brain, heart) in the body, migrate to the right positions and organise themselves in the correct manner in the target tissues to repair defects in-situ. It is evident that the suucesful manipulation of stem cells will require understanding of the processes that we study here, namely directed cell migration, cell-cell interactions and interactions between behaviour and signalling. Therefore the research proposed here will in the longer term (5-10 years) undoubtedly have many practical applications in these increasingly important areas of medicine and healthcare, affecting researchers and practitioners working in the academic as well as in the commercial private sector.
Important interdisciplinary training will be provided to the PDRAs involved as well as associated PhD and master students. Dr Manli Chuai has worked 6 years as fully qualified doctor in China, completed a PhD in life sciences and now has gained expertise and experience in modelling. The modeller on the project will gain considerable experience in Life Sciences.
The research conducted generates many exquisite images both from the experiments and computer simulations. They have been and will be part of exhibitions in the local, national and international science museums.
The Life Sciences sector has an important economic impact in Dundee, contributing around 16% of the city's GDP. A range of activities and organisations in the city connect scientists with the public. In recognition of the economic and social impact of these interactions, the College of Life Sciences won the BBSRC "Excellence with Impact" Award in 2011.
The University of Liverpool Maths Outreach Team runs activities in schools for pupils and students of all ages. These include the Liverpool Mathematical Societies FunMaths Roadshow, the Dragon Quiz, GCSE revision sessions and workshops. It also organises CPD events for both primary and secondary teachers, and a Pop-Maths Quiz and various competitions and Masterclasses for Year 6 to 13 pupils. The conducted research will be used in these activities as examples of the use of math in the analysis of real-life problems.