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First published online July 13, 2004
doi: 10.1242/10.1242/jcs.01312

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Junctions and vesicular trafficking during Drosophila cellularization

Laboratoire de Génétique et de Physiologie du Développement, UMR 6545 CNRS-Université de la Méditerranée, Institut de Biologie du Développement de Marseille (IBDM), Campus de Luminy, case 907, 13288 Marseille CEDEX 09, France

View larger version (59K): [in a new window]   Fig. 1. Cellularization links embryonic cleavage to the formation of an epithelial layer. The plasma membrane (PM, green) invaginates between the nuclei (N, black) located at the cortex of the syncytial embryo shown at the onset (A) and at the end (B) of cellularization. When cellularization is completed, 5000 columnar epithelial cells are formed. The apical adherens junctions (AAJ, red in B) have assembled and ensure the cohesion of the epithelial layer. Cellularization is thus associated with the growth and polarization of the cell surface. (C,D) Confocal sections of a cellularizing embryo during slow phase (C) and at the end of fast phase (D) corresponding to the boxed areas shown in A and B. The embryos are stained with antibodies to the integral membrane protein neurotactin (Nrt, green) that marks the entire cell surface, to the junctional protein PaTJ (turquoise) that marks the membrane front called the furrow canal (FC), to the small GTPase Rab11 (red), a marker of apical recycling endosomes (see Fig. 2), and with the DNA dye Hoechst (Hoe, dark blue) to show the nuclei. Bar, 5 µm.

View larger version (64K): [in a new window]   Fig. 2. Cell-surface remodelling and vesicular transport pathways underlying cellularization. Transmission electron micrographs (A,C,C') and schematic representations of embryos at the onset (A,B,E) and 20 minutes after the beginning (C,C',D,F) of cellularization. (A) Above each nucleus (N), the plasma membrane (PM) forms a dome called the somatic bud that is rich in villous projections (VP). Adjacent somatic buds are separated by a flat depression of the plasma membrane (arrowhead). (B) This membrane domain contains specific sets of junctional proteins such as E-cadherin/ß-catenin complexes, myosin II, PaTJ and Slam, as well as DLG/SAP97 and the multi-PDZ-containing protein Scribbles (Scb). (C,C') As the plasma membrane folds back and forms the furrow canal (FC), (D) three distinct and adjacent membrane domains assemble containing MyoII/PaTJ/Slam in the FC (red), E-cadherin/ß-catenin (green) at the basal adherens junction (BAJ) and DLG/Scb (blue) just apical to the BAJ. The two apposed cell surfaces (C, black arrowheads) grow progressively during the slow phase. Several vesicular organelles can be detected in the vicinity of the cell surface (C, white arrowheads). The formation of the BAJ as shown in D is dependent on Slam (Lecuit et al., 2002). Junctional stabilization requires Nullo, which localizes more broadly in the FC, at the BAJ and apical to the BAJ (Hunter and Wieschaus, 2000). Failure to form or stabilize the BAJ is always associated with an inhibition of membrane invagination in slow phase. (E,F) Trafficking through sub-apical perinuclear recycling endosomes (RE, pink) is required for membrane invagination. This process is controlled by the small GTPase Rab11 and Dynamin/Shibire. Vesicular transport from the Golgi apparatus to RE as well as from the apical plasma membrane to RE by Rab5 might also contribute to this trafficking pathway. Bar, 2 µm.