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Cell polarity and gastrulation in C. elegans

Jeremy Nance1,2 and James R. Priess1,2,*

1 Division of Basic Sciences, Fred Hutchinson Cancer Research Center, Seattle, Washington 98109, USA
2 Howard Hughes Medical Institute, Seattle, Washington 98109, USA



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  		Fig. 1. Blastocoel formation. All images represent sections through the center of embryos. (A) 26-cell embryo indicating the apical (a), lateral (l) and basal (b) surfaces of a cell lining the blastocoel (arrowheads). The E daughters are indicated by asterisks. (B) 4-cell embryo showing a small basal separation (arrowhead); cell names are listed. (C) Electron micrograph of a 28-cell embryo where the E daughters (dashed outline) have partially ingressed. Prominent spaces between cells are indicated by arrowheads. (D,E) Membrane contacts at the same, high magnification showing examples of (D) a small space (arrow) between lateral surfaces and (E) a large space associated with basal surfaces; arrows in E illustrate close apposition of membranes around the space. (F) 16-cell partial embryo derived from an isolated AB blastomere; note the large central cavity (arrowhead). (G) 16-cell par-3 mutant embryo showing abnormally large separations (arrowheads) between lateral membranes. (H) Double embryo after fixation showing a cavity (arrowhead) in the center of AB descendants; a subset of the eight AB descendants present are visible and labeled 1 or 2 according to origin. The P1 descendants (MS, E, P3, and C) from each half of the double embryo are labeled. N, nucleus. Intact embryos (A-C,G) are 50 µm. Bar (for D,E),1 µm.

 


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  		Fig. 2. PAR localization. (A,B) PAR-2 in 4-cell (A) and 7-cell (B) embryos. (C,D) PAR-3 in early 4-cell (C) and 8-cell (D) embryos. (E) 28-cell embryo showing PAR-2 (green), PAR-3 (red) and DNA (blue); the ingressing E daughters are indicated with yellow asterisks. The embryo is oriented as in Fig. 4D. (F) Nomarski micrograph of 50-cell embryo indicating cell surfaces facing the blastocoel (arrowheads). (F') PAR-3 localization in embryo shown in F. PAR-3 is localized to surfaces facing the blastocoel (arrowheads) in addition to localization at the apical surface (arrow). (G) PAR-3 in an 8-cell ama-1(RNAi) embryo. (H) PAR-2 in an 8-cell par-3(it71) embryo; PAR-2 is localized to all cell surfaces (arrows). In A-E and G the germline precursor is indicated with a cyan asterisk.

 


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  		Fig. 3. PAR localization in double embryos. Double embryos were immunostained for the PAR protein listed above each panel. The subset of AB descendants visible are indicated by 1 or 2, according to origin, and names of some of the P1 descendants are indicated. Both germline blastomeres (P2) are indicated with asterisks in A. (C,D) Upper and lower focal planes of the same double embryo. Note the blastocoel visible at the center of the 1 and 2 cells in C. At the time of fixation, double embryos were equivalent to the following single embryo stages: (A) 4-cell plus 4-cell, (B) 8-cell plus 8-cell, (C,D) 7-cell plus 8-cell. Bar, 10 µm.

 


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  		Fig. 4. Cell ingression. (A) Schematic lineage diagram of early blastomeres indicating AB and P1 descendants. (B) Lateral view of a three-dimensional model of nuclei at the 26-cell stage; anterior is left. The subset of descendants of the early blastomeres that ingress, or give rise to ingressing cells, are indicated by the color scheme in A; non-ingressing cells (a subset of AB descendants) are shown in grey. (C) Two diagrams of the ventral surfaces of embryos outlining the regions where various cells ingress and listing the periods of cell ingressions in minutes from the 2-cell stage. The left diagram indicates ingression of the E daughters and the MS ‘wishbone’ descendants (red), and the right diagram indicates later ingressions including the MS ‘central’ descendants (red oval). (D-G) Nomarski light micrographs of representative stages of gastrulation; times as indicated. (D) 28-cell embryo; lateral view as in B. An MS descendant (red arrowhead; MSap) and P4 (cyan arrowhead) are shown spreading across the apical surfaces of the E daughters (yellow asterisks). (E) Ventral view showing MS ‘central’ descendants (red asterisks) and a subset of C descendants (green asterisks) prior to ingression. The arrow indicates the cleft created by ingression of the D descendants. (F) Ventral view showing part of the cleft created by ingression of the MS descendants; neighboring cells (arrows) will eventually move to cover the cleft. (G) Ventral view showing epiboly of the skin cells (arrows) to cover remaining cells on the ventral surface.

 


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  		Fig. 5. Cell extensions over ingressing cells. (A) Scanning electron micrograph of the ventral surface of a 28-cell embryo, anterior is to the left. (B) Tracing of A to indicate cell names. The exposed apical surfaces of the ingressing E daughters are tinted. (C) High magnification of the region corresponding to the boxed area in B. The arrow indicates the thin, leading process from an MS descendant.

 


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  		Fig. 6. Control of MS ingression. All panels are ventral views of embryos after ingression by the MS wishbone descendants; the MS central descendants are indicated by asterisks. The central descendants remain on the surface in (A) a normal embryo, (B) an embryo after ablation of the E blastomere (dotted outline) and (C) a temperature-shifted lit-1 mutant embryo. (D) The central descendants ingress (arrow) along with the wishbone descendants in a temperature-shifted lit-1 mutant embryo after ablation of the E blastomere (dotted outline). The lit-1 chromosome was marked with an unc-32 mutation; the embryo shown in B is a temperature-shifted unc-32 control.

 


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  		Fig. 7. Cell-shape changes in ingressing cells. Images are of living embryos (A,B,E) or are confocal sections of fixed embryos after immunostaining for NMY-2 (C,D,F). (C,D,F) Confocal sections. All embryos are oriented as in Fig. 4D, and arrowheads indicate apical surfaces. (A-D) The E daughters are indicated (yellow asterisks) before ingression (A,C) and during ingression (B,D). (E-F) Ingression of MS descendants (red asterisks). MSapaa is shown in (E) and two of the MS ‘wishbone’ descendants are shown in (F), in which NMY-2 is green and DNA blue. A,B,E and F are shown at twice the magnification of C and D.

 

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© The Company of Biologists Ltd 2002