Epithelial dynamics during early mouse development.

The first epithelia to arise in an organism face the challenge of maintaining the integrity of the newly formed tissue, while exhibiting the behavioral flexibility required for morphogenetic processes to occur effectively. Epithelial cells integrate biochemical and biomechanical cues, both intrinsic and extrinsic, in order to bring about the molecular changes which determine their morphology, behavior and fate. In this review we highlight recent advances in our understanding of the various dynamic processes that the emergent epithelial cells undergo during the first seven days of mouse development and speculate what the future holds in understanding the mechanistic bases for these processes through integrative approaches.

Advances in live imaging early mouse development: exploring the researcher's interdisciplinary toolkit.

Live imaging is an important part of the developmental biologist's armoury of methods. In the case of the mouse embryo, recent advances in several disciplines including embryo culture, microscopy hardware and computational analysis have all contributed to our ability to probe dynamic events during early development. Together, these advances have provided us with a versatile and powerful 'toolkit', enabling us not only to image events during mouse embryogenesis, but also to intervene with them. In this short Spotlight article, we summarise advances and challenges in using live imaging specifically for understanding early mouse embryogenesis.

The Head's Tale: Anterior-Posterior Axis Formation in the Mouse Embryo.

The establishment of the anterior-posterior (A-P) axis is a fundamental event during early development and marks the start of the process by which the basic body plan is laid down. This axial information determines where gastrulation, that generates and positions cells of the three-germ layers, occurs. A-P patterning requires coordinated interactions between multiple tissues, tight spatiotemporal control of signaling pathways, and the coordination of tissue growth with morphogenetic movements. In the mouse, a specialized population of cells, the anterior visceral endoderm (AVE) undergoes a migration event critical for correct A-P pattern. In this review, we summarize our understanding of the generation of anterior pattern, focusing on the role of the AVE. We will also outline some of the many questions that remain regarding the mechanism by which the first axial asymmetry is established, how the AVE is induced, and how it moves within the visceral endoderm epithelium.

Neural induction by the node and placode induction by head mesoderm share an initial state resembling neural plate border and ES cells.

Around the time of gastrulation in higher vertebrate embryos, inductive interactions direct cells to form central nervous system (neural plate) or sensory placodes. Grafts of different tissues into the periphery of a chicken embryo elicit different responses: Hensen's node induces a neural plate whereas the head mesoderm induces placodes. How different are these processes? Transcriptome analysis in time course reveals that both processes start by induction of a common set of genes, which later diverge. These genes are remarkably similar to those induced by an extraembryonic tissue, the hypoblast, and are normally expressed in the pregastrulation stage epiblast. Explants of this epiblast grown in the absence of further signals develop as neural plate border derivatives and eventually express lens markers. We designate this state as "preborder"; its transcriptome resembles embryonic stem cells. Finally, using sequential transplantation experiments, we show that the node, head mesoderm, and hypoblast are interchangeable to begin any of these inductions while the final outcome depends on the tissue emitting the later signals.

The evolution of amniote gastrulation: the blastopore-primitive streak transition.

In the animal kingdom, gastrulation, the process by which the primary germ layers are formed involves a dramatic transformation in the topology of the cells that give rise to all of the tissues of the adult. Initially formed as a mono-layer, this tissue, the epiblast, becomes subdivided through the internalization of cells, thereby forming a two (bi-laminar) or three (tri-laminar) layered embryo. This morphogenetic process coordinates the development of the fundamental body plan and the three-body axes (antero-posterior, dorso-ventral, and left-right) and begins a fundamental segregation of cells toward divergent developmental fates. In humans and other mammals, as well as in avians, gastrulating cells internalize along a structure, called the primitive streak, which builds from the periphery toward the center of the embryo. How these morphogenetic movements are orchestrated and evolved has been a question for developmental biologists for many years. Is the primitive streak a feature shared by the whole amniote clade? Insights from reptiles suggest that the primitive streak arose independently in mammals and avians, while the reptilian internalization site is a structure half-way between an amphibian blastopore and a primitive streak. The molecular machinery driving primitive streak formation has been partially dissected using mainly the avian embryo, revealing a paramount role of the planar cell polarity (PCP) pathway in streak formation. How did the employment of this machinery evolve? The reptilian branch of the amniote clade might provide us with useful tools to investigate the evolution of the amniote internalization site up to the formation of the primitive streak. WIREs Dev Biol 2017, 6:e262. doi: 10.1002/wdev.262 For further resources related to this article, please visit the WIREs website.

Bi-modal strategy of gastrulation in reptiles.

BACKGROUND: Amniote gastrulation is often described with respect to human, mouse and chick development by the presence of the primitive streak, a posterior-to-anterior midline morphological cell ingression feature that has come to define Amniote gastrulation. How this midline, ingression-based strategy of gastrulation evolved from the ancestral blastopore, a circumferential involution event in Anamniotes, is unknown. However, within the Amniote clade there exists a more diverse range of gastrulation strategies than just the primitive streak. Investigating gastrulation in a wider range of Amniotes provides a way to understand evolutionary transition from blastopore to the primitive streak. RESULTS: We analysed early to late gastrulation stages of Chamaeleo calyptratus, showing their unique morphology through confocal imaging of F-actin and laminin-stained embryos to visualise cell morphology and assess basal lamina integrity. We analysed the expression pattern of core mesodermal markers Brachyury and Fgf8 and complimented this analysis with that of the turtle, Trachemys scripta. CONCLUSIONS: Our analysis suggests that reptile gastrulation is bi-modal; primary internalization occurs anteriorly by means of an incomplete blastopore-like opening, while posteriorly the cells undergo ingression in the Brachyury-expressing blastoporal plate. This strategy stands mid-way between Anamniotes and Avians/Mammals, suggesting that blastoporal plate is a precursor of the avian primitive streak. Developmental Dynamics 244:1144-1157, 2015. © 2015 Wiley Periodicals, Inc.

Heading forwards: anterior visceral endoderm migration in patterning the mouse embryo.

The elaboration of anterior-posterior (A-P) pattern is one of the earliest events during development and requires the precisely coordinated action of several players at the level of molecules, cells and tissues. In mammals, it is controlled by a specialized population of migratory extraembryonic epithelial cells, the anterior visceral endoderm (AVE). The AVE is a signalling centre that is responsible for several important patterning events during early development, including specifying the orientation of the A-P axis and the position of the heart with respect to the brain. AVE cells undergo a characteristic stereotypical migration which is crucial to their functions.

Distinct steps of neural induction revealed by Asterix, Obelix and TrkC, genes induced by different signals from the organizer.

The amniote organizer (Hensen's node) can induce a complete nervous system when grafted into a peripheral region of a host embryo. Although BMP inhibition has been implicated in neural induction, non-neural cells cannot respond to BMP antagonists unless previously exposed to a node graft for at least 5 hours before BMP inhibitors. To define signals and responses during the first 5 hours of node signals, a differential screen was conducted. Here we describe three early response genes: two of them, Asterix and Obelix, encode previously undescribed proteins of unknown function but Obelix appears to be a nuclear RNA-binding protein. The third is TrkC, a neurotrophin receptor. All three genes are induced by a node graft within 4-5 hours but they differ in the extent to which they are inducible by FGF: FGF is both necessary and sufficient to induce Asterix, sufficient but not necessary to induce Obelix and neither sufficient nor necessary for induction of TrkC. These genes are also not induced by retinoic acid, Noggin, Chordin, Dkk1, Cerberus, HGF/SF, Somatostatin or ionomycin-mediated Calcium entry. Comparison of the expression and regulation of these genes with other early neural markers reveals three distinct "epochs", or temporal waves, of gene expression accompanying neural induction by a grafted organizer, which are mirrored by specific stages of normal neural plate development. The results are consistent with neural induction being a cascade of responses elicited by different signals, culminating in the formation of a patterned nervous system.