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4.3.1 Modes of nuclei migration depending on the cell length ... 374.3.2 Impairement of normal IKNM impacts layer integrity ... 38
4.4 IKNM takes place in the mouse epiblast ... 39
5. Gastrulation in invertebrate and mammalian models: what we know so far ... 40
5.1 Gastrulation in anamniotes models ... 40
5.1.1 Drosophila ... 40
5.1.2 Xenopus embryo ... 41
5.1.3 Zebrafish ... 44
5.2 Gastrulation in amniotes models ... 46
5.2.1 Chick embryo ... 46
5.3 Gastrulation in the mouse model ... 48
6. Multicellular rosettes: a transient structure ... 51
6.1 How do rosettes form? ... 52
6.2 Rosettes in gastrulation ... 53
AIMS OF THE THESIS ... 56
2. MATERIAL AND METHODS ... 57
3. RESULTS ... 65
Chapter 1: Cell rearrangement at the onset of gastrulation and mechanisms of cell ingression through the primitive streak. ... 65Chapter 2: Distinct mesoderm migration phenotypes in extraembryonic and embryonic regions of the early mouse embryo ... 96
Chapter 3: Investigation of the molecular actors implicated in mouse embryo gastrulation ... 100
I. RhoA ... 102
1.Experimental Strategy ... 102
2. RhoAΔepi Mutant shows cell accumulation in the embryonic cavity ... 102
3. RhoAΔepi embryos express mesodermal markers ... 104
4. RhoAΔepi mutant undergoes EMT, yet migration is impaired ... 108
5. RhoAΔepi mutant escapes cell death ... 110
II.Ect2 ... 111
1.Experimental Strategy ... 111
2. Ect2Δepi Mutant shows complete atrophy of the embryonic region ... 112
3.Ect2Δepi mutant cells fail to complete cytokinesis and undergo massive cell death in the epiblast. ... 113
4.Ect2Δepi embryos show impaired anterior-posterior axis and mesoderm specification. ... 115
5. Ect2Δepi epiblast cells EMT is not impaired. ... 117
III. Cyclin-dependant kinase 1 (Cdk1) ... 118
1.Experimental Strategy ... 118
2. Cdk1Δepi Mutant shows complete Atrophy of the embryonic region ... 119
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4. Cdk1Δepi epiblast cells fail to perform EMT ... 1234. CONCLUSION ... 125
1. Rosettes can be observed in the gastrulating epiblast, with a higher frequency on the posterior side, where the primitive streak forms. ... 1252. Mosaic labelling of the epiblast highlighted bottle-shaped cells delaminating in the posterior epiblast, often accompagnied by cell mitosis. ... 126
3. Non-apical mitosis happenS only in the posterior epiblast, and giveS rise to one or two mesodermal cells. ... 126
4. Mesoderm migration differs IN the embryonic or extraembryonic regions. ... 128
5. Depending on location, cells have different transcriptional signature and use differents cytoskeletal components. ... 129
6. Rho GTPase RhoA is essential for cell migration during gastrulation. ... 130
7. loss of Ect2 in the epiblast leads to failure of cytokinesis and an embryonically lethal phenotype. ... 133
8. Cdk1 inactivation in the epiblast impairs cell division and induces cell death in the embryo. ... 134
5. PERSPECTIVES ... 135
1. What is the role of rosettes in gastrulation? ... 1352. Are rosettes linked to the primitive streak formation or only to its initiation? ... 135
3. Is non-apical mitosis a feature associated to the primitive streak? ... 136
4. How is IKNM performed in the mouse epiblast, and does non-apical division contribute to cell delamination?136
5. what is the role of cell division in gastrulation? ... 137
6. How do mesoderm cells orient migration to reach their destination? ... 138
7. What are the cytoskeletal components driving cell movement in the embryo? ... 139
8. Is the formation of lumen-like structures in RhoAΔepi due to loss of apico-basal polarity? ... 140
