Origin and development of the zebrafish endoderm

Abstract

The segregation of cells into germ layers is one of the earliest events in the establishment of cell fate in the embryo. In the zebrafish, endoderm and mesoderm are derived from cells that involute into an internal layer, the hypoblast, whereas ectoderm is derived from cells that remain in the outer layer, the epiblast. In this study, we examine the origin of the zebrafish endoderm and its separation from the mesoderm. By labeling individual cells located at the margin of the blastula, we demonstrate that all structures that are endodermal in origin are derived predominantly from the more dorsal and lateral cells of the blastoderm margin. Frequently marginal cells give rise to both endodermal and mesodermal derivatives, demonstrating that these two lineages have not yet separated. Cells located farther than 4 cell diameters from the margin give rise exclusively to mesoderm, and not to endoderm. Following involution, we see a variety of cellular changes indicating the differentiation of the two germ layers. Endodermal cells gradually flatten and extend filopodial processes forming a noncontiguous inner layer of cells against the yolk. At this time, they also begin to express Forkhead-domain 2 protein. Mesodermal cells form a coherent layer of round cells separating the endoderm and ectoderm. In cyclops-mutant embryos that have reduced mesodermal anlage, we demonstrate that by late gastrulation not only mesodermal but also endodermal cells are fewer in number. This suggests that a common pathway initially specifies germ layers together before a progressive sequence of determinative events segregate endoderm and mesoderm into morphologically distinct germ layers.

Cell-state transitions and collective cell movement generate an endoderm-like region in gastruloids

“…These observations are compatible with what is observed in mouse embryos where the emergence of endodermal progenitors in small islands of cells has been overlooked (see Figure 3 in (Probst et al, 2021)). Furthermore, a similar mode of endoderm specification has been reported in zebrafish, where it is the predominant mode (Montero et al, 2005; Nair and Schilling, 2008; Warga and Nusslein-Volhard, 1999). The clear manifestation in the gastruloids might reflect the suggestion that ancestral modes are emphasized in this model (Steventon et al, 2021).…”

Optical flow on evolving sphere-like surfaces

“…In order to give coordinate expressions for the terms in (38) arising from the regularisation term we locally choose an orthonormal frame {ê 1 (t, ξ), ê2 (t, ξ)} of the tangent space, see (13). As a consequence, the sought tangent vector field v can be written as v = w i êi (39) for some components (w 1 , w 2 ) . The reason for expressing the unknown in an orthonormal frame, rather than the coordinate frame, is to simplify matters with regard to the Hilbert-Schmidt norm (24) of the covariant derivative.…”

“…By virtue of laserscanning microscopy, (volumetric time-lapse) 4D images of these labelled cells can be recorded without capturing the background. It is known that endodermal cells float on a so called monolayer during early embryonic development meaning that they do not stack on top of each other [39]. Figure 1 depicts two frames of the captured sequence, containing only the upper hemisphere of the animal embryo.…”

Optical Flow on Evolving Surfaces with an Application to the Analysis of 4D Microscopy Data

“…For the biological methods such as the fluorescence marker and the embryos used in this work we refer to [14]. The important aspect about endodermal cells is that they are known to form a monolayer during gastrulation [17], meaning that the radial extent is only a single cell. This crucial fact allows for the straightforward extraction of a surface together with a twodimensional image of the stained cells.…”

“…The specific biological motivation for this work is to understand the motion and division behaviour of fluorescently labelled endodermal cells of a zebrafish embryo. The marked cells develop on the surface of the embryo's yolk, where they form a non-contiguous monolayer [17]. Loosely speaking, they only sit next to each other but not on top of each other.…”