Positional information by Rauber's sickle and a new look at the mechanisms of primitive streak initiation in avian blastoderms

Establishment of the Vertebrate Germ Layers

The process of germ layer formation is a universal feature of animal development. The germ layers separate the cells that produce the internal organs and tissues from those that produce the nervous system and outer tissues. Their discovery in the early nineteenth century transformed embryology from a purely descriptive field into a rigorous scientific discipline, in which hypotheses could be tested by observation and experimentation. By systematically addressing the questions of how the germ layers are formed and how they generate overall body plan, scientists have made fundamental contributions to the fields of evolution, cell signaling, morphogenesis, and stem cell biology. At each step, this work was advanced by the development of innovative methods of observing cell behavior in vivo and in culture. Here, we take an historical approach to describe our current understanding of vertebrate germ layer formation as it relates to the long-standing questions of developmental biology. By comparing how germ layers form in distantly related vertebrate species, we find that highly conserved molecular pathways can be adapted to perform the same function in dramatically different embryonic environments.

Buckling along boundaries of elastic contrast as a mechanism for early vertebrate morphogenesis

We have investigated the mechanism of formation of the body of a typical vertebrate, the chicken. We find that the body forms initially by folding at boundaries of stiffness contrast. These boundaries are dynamic lines, separating domains of different cell sizes, that are advected in a deterministic thin-film visco-elastic flow. While initially roughly circular, the lines of elastic contrast form large "peanut" shapes evoking a slender figure-8 at the moment of formation of the animal body, due to deformation and flow in a quadrupolar stretch caused by mesoderm migration. Folding of these "peanut" or "figure-8" motives along the lines of stiffness contrast creates the global pattern of the animal, and segregates several important territories. The main result is that the pattern of cell texture in the embryo serves simultaneously two seemingly different purposes: it regionalizes territories that will differentiate to different cell types and it also locks the folds that physically segregate these territories. This explains how the different cellular types segregate in physically separated domains.

The Textural Aspects of Vessel Formation during Embryo Development and Their Relation to Gastrulation Movements

We have investigated the microscopic physical inhomogeneity ("texture") of the avian embryo in vivo by shadowgraph. This noninvasive technique allows one to correlate the shape of blood vessels to the physical, micro-structural, pattern that exists in the embryo prior to vessel appearance. Before any vessel forms, vascular paths are present and are prepatterned, by fields of cellular orientations and lumen anisotropies. We find the origin of this prepattern in the movements of the embryo during gastrulation, and the related deformation and force field, which establish both the animal and vascular pattern.

Clarifying tetrapod embryogenesis by a dorso-ventral analysis of the tissue flows during early stages of chicken development

The formation of an animal body remains largely a mystery. It is still not clear whether anything like an organization plan or an "archetype" as coined by Darwin himself, actually exists, or whether animals are organized by a succession of stop-and-go genetic, non-linear, instructions with no global pattern. Nevertheless, it was recognized long ago that the early stages of amniote development consist of large scale rotatory movements over a discoidal blastula (Wetzel, 1924). Such rotatory movements reshuffle a mass inside a finite volume, and thus may have to bear physical conservation laws which contribute to establish the plan of animals in a global fashion. In this article I use dual dorso-ventral imaging of the chicken blastula, to show experimentally that the global movement of early vertebrate embryogenesis is organized with a very simple topology, around and away of a series of hyperbolic points in the vector flow of movement. At the first hyperbolic point, a layer of tissue (the mesoderm) ingresses and moves as a viscous sheet radially. It is found that the sheet flows away with a scaling law for the radius R(t)∼exp(t/τ). Also, the movement of this mesoderm changes the flow on the other layer (the ectoderm) by the principle of action and reaction. By mesoderm wetting the ectoderm, the first hyperbolic point migrates from the anal region, to the umbilical region. The final location of the hyperbolic point defines eventually the central part of the body (the umbilical region). Thus, the formation of the vertebrate body is fixed, as a global movement, by the dynamics of singular points in the visco-elastic flow, governed by mechanical forces within the tissue.

Mouse primitive streak forms in situ by initiation of epithelial to mesenchymal transition without migration of a cell population

BACKGROUND

During gastrulation, an embryo acquires the three primordial germ layers that will give rise to all of the tissues in the body. In amniote embryos, this process occurs via an epithelial to mesenchymal transition (EMT) of epiblast cells at the primitive streak. Although the primitive streak is vital to development, many aspects of how it forms and functions remain poorly understood.

RESULTS

Using live, 4 dimensional imaging and immunohistochemistry, we have shown that the posterior epiblast of the pre-streak murine embryo does not display convergence and extension behavior or large scale migration or rearrangement of a cell population. Instead, the primitive streak develops in situ and elongates by progressive initiation EMT in the posterior epiblast. Loss of basal lamina (BL) is the first step of this EMT, and is strictly correlated with ingression of nascent mesoderm. Once the BL is lost in a given region, cells leave the epiblast by apical constriction in order to enter the primitive streak.

CONCLUSIONS

This is the first description of dynamic cell behavior during primitive streak formation in the mouse embryo, and reveals mechanisms that are quite distinct from those observed in other amniote model systems. Unlike chick and rabbit, the murine primitive streak arises in situ by progressive initiation of EMT beginning in the posterior epiblast, without large-scale movement or convergence and extension of epiblast cells.

A change in boundary conditions induces a discontinuity of tissue flow in chicken embryos and the formation of the cephalic fold

The morphogenesis of vertebrate body parts remains an open question. It is not clear whether the existence of different structures, such as a head, can be addressed by fundamental laws of tissue movement and deformation, or whether they are only a sequence of stop-and-go genetic instructions. I have filmed by time-lapse microscopy the formation of the presumptive head territory in chicken embryos. I show that the early lateral evagination of the eye cups and of the mesencephalic plate is a consequence of a sudden change in boundary conditions of the initial cell flow occurring in these embryos. Due to tissue flow, and collision of the two halves of the embryo, the tissue sheet movement is first dipolar, and next quadrupolar. In vivo air puff tonometry reveals a simple visco-elastic behaviour of the living material. The jump from a dipolar to a quadrupolar flow changes the topology of the early morphogenetic field which is observed towards a complex vortex winding with a trail (the eye cups and brain folds). The hydrodynamical model accounts for the discontinuity of the vector field at the moment of collision of the left and right halves of the embryo, at a quantitative level. This suggests a possible mechanism for the morphogenesis of the head of amniotes, as compared to cephalochordates and anamniotes.

Embryonic expression of the chicken Krüppel-like (KLF) transcription factor gene family

The Krüppel-like transcription factors (KLF) are zinc finger proteins that activate and suppress target gene transcription. Although KLF factors have been implicated in regulating many developmental processes, a comprehensive gene expression analysis has not been reported. Here we present the chicken KLF gene family and expression during the first five days of embryonic development. Fourteen chicken KLF genes or expressed sequences have been previously identified. Through synteny analysis and cDNA mapping, we have identified the KLF9 gene and determined that the gene presently named KLF1 is the true ortholog of KLF17 in other species. In situ hybridization expression analyses show that in general KLFs are broadly expressed in multiple cell and tissue types. Expression of KLFs 3, 7, 8, and 9, is widespread at all stages examined. KLFs 2, 4, 5, 6, 10, 11, 15, and 17 show more restricted patterns that suggest multiple functions during early stages of embryonic development.

Introducing the scanning air puff tonometer for biological studies

It is getting increasingly evident that physical properties such as elastoviscoplastic properties of living materials are quite important for the process of tissue development, including regulation of genetic pathways. Measuring such properties in vivo is a complicated and challenging task. In this paper, we present an instrument, a scanning air puff tonometer, which is able to map point by point the viscoelastic properties of flat or gently curved soft materials. This instrument is an improved version of the air puff tonometer used by optometrists, with important modifications. The instrument allows one to obtain a direct insight into gradients of material properties in vivo. The instrument capabilities are demonstrated on substances with known elastoviscoplastic properties and several biological objects. On the basis of the results obtained, the role of the gradients of elastoviscoplastic properties is outlined for the process of angiogenesis, limb development, bacterial colonies expansion, etc. which is important for bridging the gaps in the theory of the tissue development and highlighting new possibilities for tissue engineering, based on a clarification of the role of physical features in developing biological material.

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).

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.

Morphological and molecular analysis of the early developing chick requires an expanded series of primitive streak stages

A detailed analysis of the gastrulating chick embryo was performed using three methods : time-lapse videotaping of embryos in culture, histological semithin sections, and in situ hybridization with 10 mRNA signals expressed during gastrulation. The results suggest that the gene expression pattern of Goosecoid, Hex, Crescent, and Bmp7 may be involved in the axial establishment of the temporal and spatial arrangement of cells forming the prechordal plate endoderm, and that Chordin, cNot1, Noggin, and Brachyury are precocious markers of cells coming from Hensen's node, which contribute to the rostralmost tip of the notochord, its arrowhead, the head process, and, later, the elongating notochord. These results explain several earlier descriptions based only on morphological analyses of the axial mesodermal structures characteristic of the gastrulation stages. The data, carefully observed and compared with the whole-mount observation in time-lapse video, show that the changes in cell populations, movements, and cell differentiation occur step-by-step over a precise temporal range, which requires the establishment of a subdivision of the stages usually employed. Knowledge of new aspects of avian gastrulation, including gene expression patterns, immunocytochemical analyses, and the great number of recent experiments based on microinjections or transplants of groups of cells to analyze processes of induction or regulation, need the support of a precisely defined scheme of primitive streak stages (PS-stages), and a correlation of these stages with other approaches to provide a finer resolution of the staging steps, and thus to facilitate a better understanding of the initial gastrulation period.

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

In the avian oocytal germ disc region, at the end of oogenesis, we discerned four ooplasms (alpha, beta, gamma, delta) presenting an onion-peel distribution (from peripheral and superficial to central and deep. Their fate was followed during early embryonic development. The most superficial and peripheral alpha ooplasm plays a fundamental role during cleavage. The beta ooplasm, originally localized in the peripheral region of the blastodisc, becomes mainly concentrated in the primitive streak. At the moment of bilateral symmetrization, a spatially oblique, sickle-shaped uptake of gamma and delta ooplasms occurs so that gamma and delta ooplasms become incorporated into the deeper part of the avian blastoderm. These ooplasms seem to contain ooplasmic determinants that initiate either early neurulation or gastrulation events. The early neural plate-inducing structure that forms a deep part of the blastoderm is the delta ooplasm-containing endophyll (primary hypoblast). Together with the primordial germ cells, it is derived from the superficial centrocaudal part of the nucleus of Pander, which also contains delta ooplasm. The other structure (gamma ooplasm) that is incorporated into the caudolateral deep part of the blastoderm forms Rauber's sickle. It induces gastrulation in the concavity of Rauber's sickle and blood island formation exterior to Rauber's sickle. Rauber's sickle develops by ingrowth of blastodermal cells into the gamma ooplasm, which surrounds the nucleus of Pander. Rauber's sickle constitutes the primary major organizer of the avian blastoderm and generates only extraembryonic tissues (junctional and sickle endoblast). By imparting positional information, it organizes and dominates the whole blastoderm (controlling gastrulation, neurulation, and coelom and cardiovascular system formation). Fragments of the horns of Rauber's sickle extend far cranially into the lateral quadrants of the unincubated blastoderm, so that often Rauber's sickle material forms three quarters of a circle. This finding explains the regulative capacities of isolated blastoderm parts, with the exception of the anti-sickle region and central blastoderm region, where no Rauber's sickle material is present. In avian blastoderms, there exists a competitive inhibition by Rauber's sickle on the primitive streak and neural plate-inducing effects of sickle endoblast. Avian primordial germ cells contain delta ooplasm derived from the superficial part of the nucleus of Pander. Their original deep and central ooplasmic localization has been confirmed by the use of a chicken vasa homologue. We conclude that the unincubated blastoderm consists of three elementary tissues: upper layer mainly containing beta ooplasm, endophyll containing delta ooplasm, and Rauber's sickle containing gamma ooplasm). These elementary tissues form before the three classic germ layers have developed.

Hypoblast controls mesoderm generation and axial patterning in the gastrulating rabbit embryo

Gastrulation in higher vertebrate species classically commences with the generation of mesoderm cells in the primitive streak by epithelio-mesenchymal transformation of epiblast cells. However, the primitive streak also marks, with its longitudinal orientation in the posterior part of the conceptus, the anterior-posterior (or head-tail) axis of the embryo. Results obtained in chick and mouse suggest that signals secreted by the hypoblast (or visceral endoderm), the extraembryonic tissue covering the epiblast ventrally, antagonise the mesoderm induction cascade in the anterior part of the epiblast and thereby restrict streak development to the posterior pole (and possibly initiate head development anteriorly). In this paper we took advantage of the disc-shape morphology of the rabbit gastrula for defining the expression compartments of the signalling molecules Cerberus and Dickkopf at pre-gastrulation and early gastrulation stages in a mammal other than the mouse. The two molecules are expressed in novel expression compartments in a complementary fashion both in the hypoblast and in the emerging primitive streak. In loss-of-function experiments, carried out in a New-type culturing system, hypoblast was removed prior to culture at defined stages before and at the beginning of gastrulation. The epiblast shows a stage-dependent and topographically restricted susceptibility to express Brachyury, a T-box gene pivotal for mesoderm formation, and to transform into (histologically proven) mesoderm. These results confirm for the mammalian embryo that the anterior-posterior axis of the conceptus is formed first as a molecular prepattern in the hypoblast and then irrevocably fixed, under the control of signals secreted from the hypoblast, by epithelio-mesenchymal transformation (primitive streak formation) in the epiblast.

Competitive inhibition by Rauber's sickle of the primitive streak and/or (pre)neural plate inducing effects of sickle endoblast in avian blastoderms

When in unincubated chicken blastoderms the Rauber's sickle is (sub)totally mechanically removed by selective scraping, the further evolution of the blastoderm in culture is often profoundly disturbed, going from only expansion of the upper layer and preneural plate formation to the development of a slowly growing miniature embryo. Our results suggest that the developmental potencies of the embryo are related to the presence or absence of Rauber's sickle material left after its removal. This can be checked after culture by the presence or nonpresence of junctional endoblast (derived from Rauber's sickle) and the concomitant induction of blood islands in the immediate neighborhood. Our study thus indicates that without Rauber's sickle (in the cases of successful total selective removal), an avian blastoderm cannot develop normally, even in the presence of an intact caudal marginal zone. After placing a fragment of quail sickle endoblast on the anti-sickle region of unincubated chicken blastoderms from which the Rauber's sickle was (sub)totally removed, different developmental scenarios were seen, according to the degree of removal, both in the anti-sickle as in the sickle regions. 1) If Rauber's sickle activity is strongly reduced, then besides a centripetally directed miniature embryo, induced by the remnants of the autochthonous Rauber's sickle, an additional centripetally directed embryo or preneural plate (without accompanying blood islands) develops in the anti-sickle region under inductory influence of the apposed quail sickle endoblast. We make a distinction between a neural plate and a preneural plate. The latter consists of a thickening of the upper layer (with the same initial aspect as a neural plate) adjacent to endophyll or sickle endoblast in the absence of chordomesoblast and gastrulation phenomena. 2) If Rauber's sickle activity is totally absent, then the inducing power of the sickle endoblast fragment becomes maximal and, starting from the anti-sickle region, one single embryo (without blood islands) extending over the whole area centralis appears. 3) If much of the Rauber's sickle material has been left in the blastoderm, then the inducing activity of the sickle endoblast, placed on the anti-sickle region, will be totally suppressed (although the sickle endoblast remains intact) and neither a preneural plate nor a primitive streak was induced. After placing a fragment of quail sickle endoblast on the anti-sickle region of an unincubated chicken blastoderm from which the Rauber's sickle and surrounding tissues were completely excised, an embryo was always induced by the sickle endoblast in the adjacent upper layer of this anti-sickle region. In the absence of sickle endoblast, this never occurred. Thus, our experiments demonstrate that in the absence of the Rauber's sickle, a parent tissue (sickle endoblast) induces both gastrulation and neurulation phenomena, while in the full presence of Rauber's sickle these functions are totally suppressed. Moreover, Rauber's sickle not only organizes gastrulation and blood island formation by itself but also influences neurulation at a distance (in space and time) by part of its cell lineage (i.e., sickle endoblast). Our study suggests that the inhibitory effect of Rauber's sickle on its parent tissue (sickle endoblast) represents an early mechanism impairing polyembryony, so that only a single primary major organizer (Rauber's sickle) remains active in the young avian germinal disc.

Induction of the avian coelom with associated vitelline blood circulation by Rauber's sickle derived junctional endoblast and its fundamental role in heart formation

In histological sections through chicken blastoderms of different ages we describe the temporospatial relationship between junctional endoblast, the formation of blood islands (appearing first from a peripherally migrating mesoblastic blastema), and the formation of coelomic vesicles developing later in/and from a more superficially extending mesoblastic blastema (coelomic mesoblast). After unilateral removal of the Rauber's sickle-derived junctional endoblast in early streak blastoderms (stage 2-4; Vakaet [1970] Arch Biol 81:387-426) and culture to stage 11 (Hamburger and Hamilton [1951] J Morphol 88:49-92), we observed that the early formation of the coelomic cavity was locally or totally disturbed in the operated area. Besides the simultaneous absence of blood islands, the coelomic vesicles did not form normally. Instead of regularly aligned coelomic vesicles, progressively forming the coelomic cavity by fusion, some voluminous irregular cavities appeared. Thus, the extent of the coelomic cavity was greatly reduced and the operated side was considerably smaller than the unoperated side. Furthermore, in the youngest operated blastoderms the cranial portion of the involved coelomic cavity (hemipericardial cavity) exhibited rudimentary development and usually did not reach the region of the foregut endoderm. This resulted in the absence of the myoepicardium and associated endocardium at this side. In another experiment, after removal of the junctional endoblast at one side of the chicken blastoderm, a fragment of quail junctional endoblast was placed isotopically. This resulted, after further in vitro culture, in the restoration of the formation of coelomic vesicles and accompanying subjacent blood islands in the immediate neighborhood of the apposed quail junctional endoblast. Also, the pericardium and primary heart tube developed normally. Similarly, by using the quail-chicken chimera technique, we demonstrated that the splanchnic mesoderm cells of the pericardium develop in intimate association with the most cranial part of the junctional endoblast (derived from the Rauber's sickle horns). Our experiments indicate that the coelom and, in particular, the pericardium and primary heart tube form progressively (in time and space) under the inductory influence of Rauber's sickle and junctional endoblast.