MT nucleation occurs at discrete foci along the cortex but is absent from the posterior. Also see Fig. S5 and Videos 6 and 7). (A) Tau-GFP–expressing oocyte treated with colcemid (see Materials and methods) showing the reduction of Tau-GFP MT labeling to small discrete foci; (right, first image) subregion of A shown enlarged; (second image) the same region after 60 s of UV inactivation protocol showing the initiation of MTs from the small foci; (third image) 15 min after UV inactivation showing extensive MT regrowth. (B) Oocyte expressing EB1-GFP similarly treated with colcemid revealing similar foci, which can be seen to distribute in a gradient of density from the anterior to posterior but appear to be absent from the extreme posterior (the extreme posterior is indicated by a star. (B′–B′′′′) Selected regions shown at increased magnification (highlighted in B and along a transect between the white asterisk and star). (C) Regrowth of MT after UV inactivation of colcemid assessed by the reappearance of EB1-GFP–labeled tracks (Videos 6 and 7) shows a lack of MT initiation from the posterior cortex. Images were foreground extracted (see Materials and methods) to identify actively extending EB1 tracks. Three time points are shown from the time sequence of UV inactivation of colcemid. For each image, a plot of pixel intensity from anterior to posterior is presented (corresponding to the region highlighted in the dashed box), relating to regrowth of MTs and revealing the restriction of MT initiation to the anterior regions. A, anterior; P, posterior; N, oocyte nucleus. Bars: (A–C) 20 µm; (A, right; and B′) 5 µm; (B′′′′) 10 µm.