Browsing by Subject "Gastrulation"

Gene regulatory networks (GRNs) provide a systems-level orchestration of an organism’s genome encoded anatomy. As biological networks are revealed, they continue to answer many questions including knowledge of how GRNs control morphogenetic movements and how GRNs evolve. Morphogenesis is a complex orchestration of movements by cells that are specified early in development.

The activation of an upstream GRN is crucial in order to orchestrate downstream morphogenetic events. In the sea urchin, activation of the endomesoderm GRN occurs after the asymmetric 4th cleavage. Embryonic asymmetric cell divisions often are accompanied by differential segregation of fate-determinants into one of two daughter cells. That asymmetric cleavage of the sea urchin micromeres leads to a differential animal-vegetal (A/V) nuclear accumulation of cell fate determinants, β-Catenin and SoxB1. Β-Catenin protein is localized into the nuclei of micromeres and activates the endomesoderm gene regulatory network, while SoxB1 is excluded from micromeres and enters the nucleus of the macromeres, the large progeny of the unequal 4th cleavage. Although nuclear localization of β-Catenin and SoxB1 shows dependence on the asymmetric cleavage, the mechanics behind the asymmetrical division has not been demonstrated. In Chapter 3, we show that micromere formation requires the small RhoGTPase, Cdc42 by directing the apical/basal orientation of the mitotic spindle at the apical cortex. By attenuating or augmenting sea urchin Cdc42 function, micromere divisions became defective and failed to correctly localize asymmetrically distributed determinants. As a consequence, cell fates were altered and multiple A/V axes were produced resulting in a “Siamese-twinning” phenotype that occurred with increasing frequency depending on the quantitative level of perturbation. Our findings show that Cdc42 plays a pivotal role in the asymmetric division of the micromeres, endomesoderm fate-determinant segregation, and A/V axis formation.

This dissertation also characterizes, at high resolution, the repertoire of cellular movements contributing to three different morphogenetic processes of sea urchin development: the elongation of gut, the formation of the primary mouth, and the migration of the small micromeres (the presumptive primordial germ cells) in the sea urchin, Lytechinus variegatus. Descriptive studies of the cellular processes during the different morphogenetic movements allow us to begin investigating their molecular control.

In Chapter 4, we dissected the series of complex events that coordinate gut and mouth morphogenesis. Until now, it was thought that lateral rearrangement of endoderm cells by convergent extension was the main contributor to sea urchin archenteron elongation and that cell divisions were minimal during elongation. We performed cell transplantations to live image and analyze a subset of labeled endoderm cells at high-resolution in the optically clear sea urchin embryo. We found that the endomesoderm cells that initially invaginate into the sea urchin blastocoel remained contiguous throughout extension, so that, if convergent extension were present, it was not a major contributor to elongation. We also found a prevalence of cell divisions throughout archenteron elongation that increased the number of cells within the gut linearly over time; however, we showed that the proliferation did not contribute to growth, and their spindle orientations were randomized during divisions and therefore did not selectively contribute to the final gut length. When cell divisions were inhibited, we saw no difference in the ability of the cells within the gut to migrate in order to elongate. Also in Chapter 4, we describe our observations of the cell biological processes underlying primary mouth formation at the end of gastrulation. Using time-lapse microscopy, photo-convertible Kaede, and an assay of the basement membrane remodeling, we describe a sequential orchestration of events that leads to the fusion of the oral ectoderm and the foregut endoderm. Our work characterizes, at higher resolution than previously recorded, the temporal sequence and repertoire of the cellular movements contributing to the length of the sea urchin larval gut and tissue fusion with the larval primary mouth.

In Chapter 5, the migration of the small micromeres to the coelomic pouches in the sea urchin embryo provides an exceptional model for understanding the genomic regulatory control of morphogenesis. An assay using the robust homing potential of these cells reveals a “coherent feed-forward” transcriptional subcircuit composed of Pax6, Six3, Eya, and Dach1 that is responsible for the directed homing mechanism of these multipotent progenitors. The linkages of that circuit are strikingly similar to a circuit involved in retinal specification in Drosophila suggesting that systems-level tasks can be highly conserved even though the tasks drive unrelated processes in different animals.

The sea urchin gene regulatory network (GRN) describes the cell fate specification of the developing embryo; however, the GRN does not describe specific cell biological events driving the three distinct sequences of cell movements. Our ability to connect the GRN to the morphogenetic events of gastrulation, primary mouth formation, and small micromere migration will provide a framework for characterizing these remarkable sequences of cell movements in the simplest of deuterostome models at an unprecedented scale.