gastrulation

Gastrulation is an early stage of embryogenesis when the basic body plan is determined. For mammals, the generation of the three dimensional body axes (anterior-posterior, dorsal-ventral, and left-right) is inherently linked to the creation of different cell lineages (mesoderm, ectoderm, endoderm) during gastrulation. Each of the cell lineages makes what is called a primary germ layer and this lineage commitment is fundamental for all subsequent stages of embryonic development. The molecular mechanisms responsible for coordinating lineage decisions with spatial positioning within the embryo are major avenues of research in the Merrill lab.

Much is known about the patterning of the mouse embryo during gastrulation. Several recent and classic experiments have shown that Wnt and Nodal antagonists expressed in the anterior visceral endoderm (AVE) are necessary to position the formation of mesoderm in the primitive streak (PS) region at the posterior of the embryo. These signaling cues from the AVE (an extra-embryonic layer that will not contribute cells to the fetus) must act on the pluripotent cells of the epiblast to position the site where lineage commitment will occur and the epiblast cells will lose pluripotency.

Based on the requirement for Wnt3 for PS formation and the duplicated axes in TCF3-/- embryos, Tcf factors appear to have important and perhaps complex functions during gastrulation. Understanding how Tcf factors regulate cell fate decisions in embryos during gastrulation will elucidate mechanisms coordinating cell lineage decisions with the formation of the basic body plan.

Figure - Early mouse embryonic development. A-C) Diagram of a sagittal view of mouse embryos of the given days of embryonic development (e4.5-e7.0) C-bottom) Diagram of a transverse section through an e7.0 embryo. A) Prior to implantation in the uterus, the blastocyst consists of three cell lineages. Trophectoderm (orange) and primitive endoderm (green) lineage are extraembryonic. Inner cell mass cells (blue) are pluripotent and will contribute all adult cell types. B) Upon implantation, the number of pluripotent cells expands (from 20-25 ICM cells to ~500 cells in the epiblast (embryonic ectoderm) (blue). The embryonic ectoderm is thought to be pluripotent and no asymmetries have been described for the embryonic ectoderm at the egg cylinder stage. The extraembryonic endoderm (green) is establishing a three dimensional asymmetry through the migration of the distal visceral endoderm (dark green) towards what will become the anterior. C) During gastrulation, the mesoderm (red) germ layer forms in the primitive streak region, defining the posterior of the embryo. Based on lineage tracing and cell transplantation experiments, anterior embryonic ectoderm (brown) and posterior embryonic ectoderm (blue) begin to display different capacities for lineage commitment during gastrulation. Posterior embryonic ectoderm cells (blue) display pluripotent capabilities whereas anterior embryonic ectoderm is transitioning towards ectodermal (epidermal and neural) lineages.

Figure - Whole mount in situ hybridizations of e8.5 TCF3+/+ (WT) and TCF3-/- (KO) embryos, probed with digoxygenin-labeled cRNAs for Brachyury, which marks the primitive streak (ps) and notochord (ncd) (A-B') Anterior is towards the top of each image. The three embryos in each of frames of B showed different representative aberrant patterns of Brachyury expression, reflective of the extent of node/notochord multiplication. Opposing arrowheads denote thickness of notochords; arrows denote splitting of the notochord, often seen in mutant embryos. (C,C') TCF3-/- embryo with a rostral extension probed for Brachyury expression. Anterior is to left for both images. The primary primitive streak (1o PS) and a secondary primitive streak (2o PS) were positive for Brachyury expression. Emerging from the secondary primitive streak were structures similar to a node (arrows) and notochord (arrowheads).