Small molecules that interfere with microtubule dynamics, such as Taxol and the Vinca alkaloids, are widely used in cell biology research and as clinical anticancer drugs. However, their activity cannot be restricted to specific target cells, which also causes severe side effects in chemotherapy. Here, we introduce the photostatins, inhibitors that can be switched on and off in vivo by visible light, to optically control microtubule dynamics. Photostatins modulate microtubule dynamics with a subsecond response time and control mitosis in living organisms with single-cell spatial precision. In longer-term applications in cell culture, photostatins are up to 250 times more cytotoxic when switched on with blue light than when kept in the dark. Therefore, photostatins are both valuable tools for cell biology, and are promising as a new class of precision chemotherapeutics whose toxicity may be spatiotemporally constrained using light.

Tissue sculpting during development has been attributed mainly to cellular events through processes such as convergent extension or apical constriction 1 , 2 . Recent work, however, has revealed roles for basement membrane remodelling in global tissue morphogenesis 3 – 5 . Upon implantation, the epiblast and extra-embryonic ectoderm of the mouse embryo become enveloped with a basement membrane. Signalling between the basement membrane and these tissues is critical for cell polarization and the ensuing morphogenesis 6 , 7 . However, the mechanical role of the basement membrane for post-implantation embryogenesis remains unknown. Here, we demonstrate the importance of spatiotemporally regulated basement membrane remodelling during early embryonic development. Specifically, we show that Nodal signalling directs the generation and dynamic distribution of perforations in the basement membrane by regulating expression of matrix metalloproteinases. This basement membrane remodelling facilitates embryo growth before gastrulation. The establishment of the anterior-posterior axis 8 , 9 further regulates basement membrane remodelling by localizing Nodal signalling, and therefore activity of matrix metalloproteinases and basement-membrane perforations, to the posterior side of the embryo. Perforations on the posterior side are essential for primitive streak extension during gastrulation by rendering the prospective primitive streak’s basement membrane more prone to breaching. Thus spatio-temporally regulated basement membrane remodelling contributes to the coordination of embryo growth, morphogenesis and gastrulation.

In mouse embryo gastrulation, epiblast cells delaminate at the primitive streak to form mesoderm and definitive endoderm, through an epithelial-mesenchymal transition. Mosaic expression of a membrane reporter in nascent mesoderm enabled recording cell shape and trajectory through live imaging. Upon leaving the streak, cells changed shape and extended protrusions of distinct size and abundance depending on the neighboring germ layer, as well as the region of the embryo. Embryonic trajectories were meandrous but directional, while extra-embryonic mesoderm cells showed little net displacement. Embryonic and extra-embryonic mesoderm transcriptomes highlighted distinct guidance, cytoskeleton, adhesion, and extracellular matrix signatures. Specifically, intermediate filaments were highly expressed in extra-embryonic mesoderm, while live imaging for F-actin showed abundance of actin filaments in embryonic mesoderm only. Accordingly, Rhoa or Rac1 conditional deletion in mesoderm inhibited embryonic, but not extra-embryonic mesoderm migration. Overall, this indicates separate cytoskeleton regulation coordinating the morphology and migration of mesoderm subpopulations.

Laser ablation of centrosomes in one-cell Caenorhabditis elegans embryos shows that chromatids can segregate independently of centrosomes and also independently of the activity of kinetochore microtubules during mitosis. CLASP and RanGTP are required to generate this centrosome-independent force, whereas SPD-1 and BMK-1 act as brakes to oppose it.

In the gastrulating mouse embryo, epiblast cells delaminate at the primitive streak to form mesoderm and definitive endoderm, through an epithelial-mesenchymal transition.Mosaic expression of a membrane reporter in nascent mesoderm enabled recording cell shape and trajectory through live imaging. Upon leaving the streak, cells changed shape and extended protrusions of distinct size and abundance depending on the neighboring germ layer, as well as the region of the embryo. Embryonic trajectories were meandrous but directional, while extra-embryonic mesoderm cells showed little net displacement.Embryonic and extra-embryonic mesoderm transcriptomes highlighted distinct guidance, cytoskeleton, adhesion, and extracellular matrix signatures. Specifically, intermediate filaments were highly expressed in extra-embryonic mesoderm, while live imaging for F-actin showed abundance of actin filaments in embryonic mesoderm only. Accordingly, RhoA or Rac1 conditional deletion in mesoderm inhibited embryonic, but not extra-embryonic mesoderm migration.Overall, this indicates separate cytoskeleton regulation coordinating the morphology and migration of mesoderm subpopulations.

At gastrulation, a subpopulation of epiblast cells constitutes a transient posteriorly located structure called the primitive streak, where cells that undergo epithelial–mesenchymal transition make up the mesoderm and endoderm lineages. Mouse embryo epiblast cells were labelled ubiquitously or in a mosaic fashion. Cell shape, packing, organization and division were recorded through live imaging during primitive streak formation. Posterior epiblast displays a higher frequency of rosettes, some of which associate with a central cell undergoing mitosis. Cells at the primitive streak, in particular delaminating cells, undergo mitosis more frequently than other epiblast cells. In pseudostratified epithelia, mitosis takes place at the apical side of the epithelium. However, mitosis is not restricted to the apical side of the epiblast, particularly on its posterior side. Non‐apical mitosis occurs specifically in the streak even when ectopically located. Posterior non‐apical mitosis results in one or two daughter cells leaving the epiblast layer. Cell rearrangement associated with mitotic cell rounding in posterior epiblast, in particular when non‐apical, might thus facilitate cell ingression and transition to a mesenchymal phenotype.

After fertilisation of the oocyte, the mouse zygote divides three times before undergoing compaction, which allows polarisation of the blastomeres, and the first lineage specification between trophectoderm (destined to become placenta) and inner cell mass (ICM). The second lineage specification occurs at the blastocyst stage, when ICM cells become either epiblast (which will give rise to the embryo proper) or primitive endoderm (an extraembryonic lineage). After implantation, the proximal (site of attachment with the uterus)‐distal axis is established, quickly followed by the determination of the anterior–posterior axis. Formation of the three main germ layers, ectoderm, mesoderm and endoderm, then occurs on the posterior side at the primitive streak, site of gastrulation. However, recent advances in mouse embryo live imaging have shown that lineage specification was more flexible than previously thought. It has also become possible to visualise the early stages of human embryo development. Key Concepts Early mouse embryo development proceeds through a sequence of lineages specification events. Similar signalling pathways are reused at different steps of early development. Positional and mechanical cues play essential roles for cell lineage specification and morphogenesis. The initial steps, up to and including gastrulation, are quite conserved between all amniotes. The main differences at later stages reside in the extraembryonic tissues, as mammals have adapted to internal gestation through the development of a placenta.

After implantation, the mouse embryo undergoes gastrulation and forms mesoderm and endoderm. Mesoderm participates in embryonic and extra-embryonic tissues including the amnion, yolk sac, chorion and allantois, the umbilical cord precursor. Extra-embryonic mesoderm is rich in intermediate filaments. Two-photon live imaging of keratin 8-eYFP knock-in embryos allowed recording nucleation and elongation of keratin filaments, which formed apical cables coordinated across multiple cells in amnion, allantois, and blood islands. Embryos lacking all keratins displayed a deflated exocoelomic cavity, a narrow thick amnion, and a short allantois, indicating a hitherto unknown role for keratin filaments in post-implantation extra-embryonic membranes morphogenesis. Single-cell RNA sequencing of mesoderm cells, microdissected amnion, chorion, and allantois provided an interactive atlas of transcriptomes with germ layer and regional information. Keratin 8high mesenchymal cells in contact with the exocoelom shared a cytoskeleton and adhesion expression profile that might explain the adaptation of extra-embryonic structures to the increasing mechanical pressure.