Cell cycle inhibition leads to somite hyperepithelialization. (A-C) Integrin α5-GFP (red) labels the cell cortex and clusters along somite borders in (A) wild-type (wt), (B) emi1-/- and (C) aphidicolin-hydroxyurea-treated embryos. Embryos were at the 12-somite stage. Somites 2-4, 3-5 and 3-5 are shown, respectively. (D,E) 12-somite-stage emi1-/- embryo labeled with membrane-localized cherry (red) to visualize the cell cortex. The same somites at the beginning (SI and SII) and end (SIV and SV, respectively) of a timelapse are shown. The somite borders are indicated by dashed lines. Somites initially have internal mesenchymal cells, but after three somite cycles have passed, the internal mesenchymal cells (asterisk in D) have moved to the surface of the somite (asterisk in E). (F,G) Fibronectin (Fn) matrix (red) forms along the borders in (F) wild type and (G) emi1-/-. Embryos were at the 12-somite stage. Somites 6-7 and 3-5 are pictured. (H,I) Ephrin B2 expression (green) shows a graded, segmental distribution in (H) wild type and (I) emi1-/-. The lateral membranes of the posterior (p) somite border cells show higher levels of Ephrin B2 than do the lateral membranes of anterior (a) border cells. Embryos were at the 12-somite stage. Shown are somites 5-6 and 4-5. (J,K) Segmental expression of myod (blue) in (J) wild type, somites 3-8, and (K) emi1-/-, somites 4-9, in 10-somite-stage embryos. (L,M) Expression of deltaC (green) in (L) wild type, somites 4-9, is aberrant, but segmental in (M) emi1-/-, somites 4-9, in 10-somite-stage embryos. Nuclei are labeled with DAPI (blue) in A-C,F-I. In D and E, nuclei are labeled with nuclear-GFP (green). β-catenin labels the cell cortex in J,K (yellow) and L,M (red). In all panels, anterior is up.
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