Origin, fate, and function of the components of the avian germ disc region and early blastoderm: Role of ooplasmic determinants

Identification of chicken LOC420478 as Bucky ball equivalent and potential germ plasm organizer in birds

Bucky ball was identified as germ plasm organizer in zebrafish and has proven crucial for Balbiani body condensation. A synteny comparison identified an uncharacterized gene locus in the chicken genome as predicted avian counterpart. Here, we present experimental evidence that this gene locus indeed encodes a ‘Bucky ball’ equivalent in matured oocytes and early embryos of chicken. Heterologous expression of Bucky ball fusion proteins both from zebrafish and chicken with a fluorescent reporter revealed unique patterns indicative for liquid–liquid phase separation of intrinsically disordered proteins. Immuno-labeling detected Bucky ball from oocytes to blastoderms with diffuse distribution in matured oocytes, aggregation in first cleavage furrows, and co-localization to the chicken vasa homolog (CVH). Later, Bucky ball translocated to the cytoplasm of first established cells, and showed nuclear translocation during the major zygotic activation together with CVH. Remarkably, during the phase of area pellucida formation, Bucky ball translocated back into the cytoplasm at stage EGK VI, whereas CVH remained within the nuclei. The condensation of Bucky ball and co-localization with CVH in cleavage furrows and nuclei of the centrally located cells strongly suggests chicken Bucky ball as a germ plasm organizer in birds, and indicate a special importance of the major zygotic activation for germline specification.

Cellular processes driving gastrulation in the avian embryo

Gastrulation consists in the dramatic reorganisation of the epiblast, a one-cell thick epithelial sheet, into a multilayered embryo. In chick, the formation of the internal layers requires the generation of a macroscopic convection-like flow, which involves up to 50,000 epithelial cells in the epiblast. These cell movements locate the mesendoderm precursors into the midline of the epiblast to form the primitive streak. There they acquire a mesenchymal phenotype, ingress into the embryo and migrate outward to populate the inner embryonic layers. This review covers what is currently understood about how cell behaviours ultimately cause these morphogenetic events and how they are regulated. We discuss 1) how the biochemical patterning of the embryo before gastrulation creates compartments of differential cell behaviours, 2) how the global epithelial flows arise from the coordinated actions of individual cells, 3) how the cells delaminate individually from the epiblast during the ingression, and 4) how cells move after the ingression following stereotypical migration routes. We conclude by exploring new technical advances that will facilitate future research in the chick model system.

Vertebrate Axial Patterning: From Egg to Asymmetry

The emergence of the bilateral embryonic body axis from a symmetrical egg has been a long-standing question in developmental biology. Historical and modern experiments point to an initial symmetry-breaking event leading to localized Wnt and Nodal growth factor signaling and subsequent induction and formation of a self-regulating dorsal "organizer." This organizer forms at the site of notochord cell internalization and expresses primarily Bone Morphogenetic Protein (BMP) growth factor antagonists that establish a spatiotemporal gradient of BMP signaling across the embryo, directing initial cell differentiation and morphogenesis. Although the basics of this model have been known for some time, many of the molecular and cellular details have only recently been elucidated and the extent that these events remain conserved throughout vertebrate evolution remains unclear. This chapter summarizes historical perspectives as well as recent molecular and genetic advances regarding: (1) the mechanisms that regulate symmetry-breaking in the vertebrate egg and early embryo, (2) the pathways that are activated by these events, in particular the Wnt pathway, and the role of these pathways in the formation and function of the organizer, and (3) how these pathways also mediate anteroposterior patterning and axial morphogenesis. Emphasis is placed on comparative aspects of the egg-to-embryo transition across vertebrates and their evolution. The future prospects for work regarding self-organization and gene regulatory networks in the context of early axis formation are also discussed.

Germline development in amniotes: A paradigm shift in primordial germ cell specification

In the field of germline development in amniote vertebrates, primordial germ cell (PGC) specification in birds and reptiles remains controversial. Avians are believed to adopt a predetermination or maternal specification mode of PGC formation, contrary to an inductive mode employed by mammals and, supposedly, reptiles. Here, we revisit and review some key aspects of PGC development that channelled the current subdivision, and challenge the position of birds and reptiles as well as the 'binary' evolutionary model of PGC development in vertebrates. We propose an alternative view on PGC specification where germ plasm plays a role in laying the foundation for the formation of PGC precursors (pPGC), but not necessarily of PGCs. Moreover, inductive mechanisms may be necessary for the transition from pPGCs to PGCs. Within this framework, the implementation of data from birds and reptiles could provide new insights on the evolution of PGC specification in amniotes.

Search for the genes involved in oocyte maturation and early embryo development in the hen

Background

The initial stages of development depend on mRNA and proteins accumulated in the oocyte, and during these stages, certain genes are essential for fertilization, first cleavage and embryonic genome activation. The aim of this study was first to search for avian oocyte-specific genes using an in silico and a microarray approaches, then to investigate the temporal and spatial dynamics of the expression of some of these genes during follicular maturation and early embryogenesis.

Results

The in silico approach allowed us to identify 18 chicken homologs of mouse potential oocyte genes found by digital differential display. Using the chicken Affymetrix microarray, we identified 461 genes overexpressed in granulosa cells (GCs) and 250 genes overexpressed in the germinal disc (GD) of the hen oocyte. Six genes were identified using both in silico and microarray approaches. Based on GO annotations, GC and GD genes were differentially involved in biological processes, reflecting different physiological destinations of these two cell layers. Finally we studied the spatial and temporal dynamics of the expression of 21 chicken genes. According to their expression patterns all these genes are involved in different stages of final follicular maturation and/or early embryogenesis in the chicken. Among them, 8 genes (btg4, chkmos, wee, zpA, dazL, cvh, zar1 and ktfn) were preferentially expressed in the maturing occyte and cvh, zar1 and ktfn were also highly expressed in the early embryo.

Conclusion

We showed that in silico and Affymetrix microarray approaches were relevant and complementary in order to find new avian genes potentially involved in oocyte maturation and/or early embryo development, and allowed the discovery of new potential chicken mature oocyte and chicken granulosa cell markers for future studies. Moreover, detailed study of the expression of some of these genes revealed promising candidates for maternal effect genes in the chicken. Finally, the finding concerning the different state of rRNA compared to that of mRNA during the postovulatory period shed light on some mechanisms through which oocyte to embryo transition occurs in the hen.

Mosaic versus regulation development in avian blastoderms depends on the spatial distribution of Rauber's sickle material

We describe how to prepare unincubated avian eggs to obtain a greater number of clearly visible Rauber's sickles for experimental embryology. After hemi-sectioning of unincubated chicken (Gallus domesticus) blastoderms and cultivating both halves in vitro, two kinds of development can be discerned: (1) when the unincubated blastoderms were hemi-sectioned according to the plane of bilateral symmetry, going through the middle region of Rauber's sickle, we obtained two hemi-embryos (a left and a right one). Each contained a half primitive streak, localized at the cut edge (starting from the most median part of Rauber's sickle) giving rise to a half mesoblast mantle and half area vasculosa, thus indicating mosaic development (each part of the whole fertilized egg would be able to form independently on its own). (2) When the unincubated blastoderm is hemi-sectioned more obliquely, going through a more lateral part of Rauber's sickle (sickle horn), two complete bilaterally symmetrically miniature embryos will form, indicating the so-called regulation phenomena. We demonstrate that these two types of development are in reality due to the different spreading and concentration of Rauber's sickle tissue (containing gamma ooplasm) around the area centralis. Embryonic regulation thus must not be considered as a kind of totipotent regeneration capacity of isolated parts of the unincubated avian blastoderm, but depends on the spatial distribution of a kind of extraembryonic tissue (Rauber's sickle) built up by the oblique uptake of gamma ooplasm (ooplasmic mosaicism) at the moment of bilateral symmetrization (Callebaut [1994] Eur Arch Biol 105:111-123; Callebaut [2005] Dev Dyn 233:1194-1216).

Heart myofibrillogenesis occurs in isolated chick posterior blastoderm: a culture model

Early cardiogenesis including myofibrillogenesis is a critical event during development. ­Recently we showed that prospective cardiomyocytes reside in the posterior lateral blastoderm in the chick embryo. Here we cultured the posterior region of the chick blastoderm in serum-free medium and observed the process of myofibrillogenesis by immunohistochem­istry. After 48 hours, explants expressed sarcomeric proteins (sarcomeric α-actinin, 61%; smooth muscle α-actin, 95%; Z-line titin, 56%; sarcomeric myosin, 48%); however, they did not yet show a mature striation. After 72 hours, more than 92% of explants expressed I-Z-I proteins, which were incorporated into the striation in 75% of explants or more (sarcomeric α-actinin, 75%; smooth muscle α-actin, 81%; Z-line titin, 83%). Sarcomeric myosin was ­expressed in 63% of explants and incorporated into A-bands in 37%. The percentage incidence of expression or striation of I-Z-I proteins was significantly higher than that of sarcomeric myosin. Results suggested that the nascent I-Z-I components appeared to be gener­ated independently of A-bands in the cultured posterior blastoderm, and that the process of myofibrillogenesis observed in our culture model faithfully reflected that in vivo. Our blastoderm culture model appeared to be useful to investigate the mechanisms regulating the early cardiogenesis.

Cell movement during chick primitive streak formation

Gastrulation in amniotes begins with extensive re-arrangements of cells in the epiblast resulting in the formation of the primitive streak. We have developed a transfection method that enables us to transfect randomly distributed epiblast cells in the Stage XI-XIII chick blastoderms with GFP fusion proteins. This allows us to use time-lapse microscopy for detailed analysis of the movements and proliferation of epiblast cells during streak formation. Cells in the posterior two thirds of the embryo move in two striking counter-rotating flows that meet at the site of streak formation at the posterior end of the embryo. Cells divide during this rotational movement with a cell cycle time of 6-7 h. Daughter cells remain together, forming small clusters and as result of the flow patterns line up in the streak. Expression of the cyclin-dependent kinase inhibitor, P21/Waf inhibits cell division and severely limits embryo growth, but does not inhibit streak formation or associated flows. To investigate the role off cell-cell intercalation in streak formation we have inhibited the Wnt planar-polarity signalling pathway by expression of a dominant negative Wnt11 and a Dishevelled mutant Xdd1. Both treatments do not result in an inhibition of streak formation, but both severely affect extension of the embryo in later development. Likewise inhibition of myosin II which as been shown to drive cell-cell intercalation during Drosophila germ band extension, has no effect on streak formation, but also effectively blocks elongation after regression has started. These experiments make it unlikely that streak formation involves known cell-cell intercalation mechanisms. Expression of a dominant negative FGFR1c receptor construct as well as the soluble extracellular domain of the FGFR1c receptor both effectively block the cell movements associated with streak formation and mesoderm differentiation, showing the importance of FGF signalling in these processes.

Early steps in neural development

We studied early neurulation events in vitro by transplanting quail Hensen's node, central prenodal regions (before the nodus as such develops), or upper layer parts of it on the not yet definitively committed upper layer of chicken anti-sickle regions (of unincubated blastoderms), eventually associated with central blastoderm fragments. We could demonstrate by this quail-chicken chimera technique that after the appearance of a pronounced thickening of the chicken upper layer by the early inductive effect of neighboring endophyll, a floor plate forms by insertion of Hensen's node-derived quail cells into the median part of the groove. This favors, at an early stage, the floor plate "allocation" model that postulates a common origin for notochord and median floor plate cells from the vertebrate's secondary major organizer (Hensen's node in this case). A comparison is made with results obtained after transplantation of similar Hensen's nodes in isolated chicken endophyll walls or with previously obtained results after the use of the grafting procedure in the endophyll walls of whole chicken blastoderms.

Primitive streak formation in mice is preceded by localized activation of Brachyury and Wnt3

The prevalent model for the generation of axial polarity in mouse embryos proposes that a radial to a linear transition in the expression of primitive streak markers precedes the formation of the primitive streak on one side of the epiblast. This model contrasts with the models of mesoderm formation in other vertebrates as it suggests that the primitive streak is initially established in a radial pattern rather than a localized region of the epiblast. Here, we examine the proposed correlation between the expression of Brachyury and Wnt3, two genes reported as expressed radially in the proximal epiblast, with the movements of proximal anterior epiblast cells at stages leading to the formation of the primitive streak. Our results reveal that neither Brachyury nor Wnt3 forms a ring of expression in the proximal epiblast as previously thought. In embryos dissected between 5.5 and 6.5 dpc, Brachyury is first expressed in the distal extra-embryonic ectoderm and subsequently on one side of the epiblast. Wnt3 expression is evident first in the posterior visceral endoderm of 5.5 dpc embryos and later in the posterior epiblast. Lineage analysis shows that the movements of the proximal epiblast do not restrict Brachyury expression to the posterior epiblast. Our data suggest a model whereby the localized expression of these genes in the posterior epiblast, and hence the formation of the primitive streak, is the result of local cell-cell interactions in the future posterior portion of the egg cylinder rather than regionalization of a radial pattern of expression in proximal epiblast cells.