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Fig. 1. Furrow canals are established as discrete cortical compartments at the onset of cellularization. (A) Illustration showing somatic buds over each nucleus (N) extending their margins to form cellularization furrows. At early cellularization, F-actin and Myosin 2 (Myo-2) furrow canals assemble at the furrow tips and E-cadherin (E-cad) and β-catenin (Arm) coalesce into adjacent basal junctions. Basal junctions mark the boundary between furrow canal compartments and the growing lateral plasma membrane (PM). Furrow canals and basal junctions travel in register at the tip of the ingressing furrow until late cellularization, when furrow canals contract to close off the basal ends of the cells. (B,D,E) Confocal images of cellularizing embryos. (B) Cross-sections showing overlap between the PM component Dlg (red) and F-actin (green) as furrows first form. These markers are resolved once furrows reach a length of 5 µm (arrow in B) with Dlg restricted to the lateral PM and F-actin concentrating in the furrow canals. This compartmentalization is maintained throughout late cellularization. (C) TEM image of an early cellularization furrow with arrows indicating the apical (top) and basal (bottom) boundary of the lateral PM. The furrow canal compartment appears as a broadening at the furrow tip. (D) Cross-sections showing Nullo first concentrating with F-actin in early furrow canals and then concentrating at the basal junction regions slightly later. (E) Cross-sections showing Arm accumulating in basal junctions at the basal-most region of the lateral PM (Nrt, red) in wild-type embryos as compared with spreading along the PM in nulloX embryos. Scale bars: 5 µm in B,D,E and 500 nm in C.
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