The animal body plan is organized during the early stages of embryogenesis, when cells that will form internal tissues become positioned in the interior of the embryo. This reorganization, called gastrulation, occurs through the movement of early embryonic cells. Our laboratory uses the model organism C. elegans to investigate the basic cellular mechanisms of gastrulation. During C. elegans gastrulation, specific cells move ('ingress') from the surface of the embryo into a central cavity called the blastocoel. We are focusing on several questions related to these movements: How do early embryonic cells acquire polarity such that proteins needed for blastocoel formation and ingression become properly localized? How does the blastocoel cavity form from an initially adherent group of cells? What are the mechanisms of cell ingression? How are cell ingressions triggered and coordinated during embryogenesis? The C. elegans embryo is ideally suited for such studies because it contains relatively few cells, the movements of these cells can be followed using time-lapse microscopy, and because the function of specific genes can be determined using genetic analysis.

We have found that a group of conserved cell polarity proteins called PAR proteins is required for the polarization of early embryonic cells. Certain PAR proteins such as PAR-3 are initially found around the entire cell perimeter. As cells begin to contact one another, PAR-3 disappears from sites of cell-cell contact, establishing an apical/basolateral (contact-free/contact) asymmetry. When PAR-3 is removed from the embryo at this stage, cells develop defects in their pattern of adhesion to one another. Cells in normal embryos show an asymmetry in adhesion to their neighbors; separations form between cells on opposite sides of the embryo to produce the central blastocoel. In PAR-3-depleted embryos, the pattern of cell adhesions is abnormal and the blastocoel becomes mispositioned. We are currently investigating how cell-cell contact promotes the apical/basolateral asymmetry of PAR proteins, and how PAR asymmetry regulates cell-cell adhesion.

Apicobasal asymmetries in early embryonic cells. PAR-3 (red) is found on apical surfaces, while other proteins such as HMR-1/E-cadherin (green) are found on basolateral surfaces. Nuclei are stained blue.

Ingressing cells change their shape by constricting their apical surfaces as they enter the embryo during gastrulation. Apical constriction appears to result from a local contraction of the actomyosin cytoskeleton, as non-muscle myosin progressively accumulates at apical surfaces of ingressing cells. When PAR proteins are removed from the embryo, cells ingress very slowly, their apical surfaces fail to accumulate non-muscle myosin, and these surfaces do not appear to constrict. We are interested in understanding how PAR proteins control cytoskeletal changes at the apical surface and in identifying PAR-independent mechanisms that cells use to ingress.

Ingressing cells moving into the embryo (surface view). The two endodermal precursors (green) are ingressing into the interior of the embryo. Neighboring cells (outlined in red) are spreading over the surfaces of the ingressing cells. Nuclei are stained blue.

Finally, we have shown that the ingression of different types of cells can be regulated by their fates. The fates of early embryonic cells are determined by the expression of specific transcription factors. When the fates of cells that normally ingress are altered by preventing the expression of cell fate regulators, ingressions can be prevented. We are performing experiments to identify targets of specific cell fate regulators that are needed to initiate cell ingressions during gastrulation.