PM folds and tubules at centrosomal MTs during actin cap formation. (A) Schematics show a nascent actin cap forming above each centrosome of a telophase cortical compartment of the syncytial embryo. Subsequently, each cap expands above each daughter nucleus and grows into a dome-like compartment by metaphase of the next cycle. (B) Coexpressed markers of PM (PH-domain probe for PIP₂) and mitotic spindle (Jupiter-GFP) shown from nuclear cycle 11–12 as 3D renderings of maximum intensity projections. Color-coded from apical surface (0 µm) to 10 µm below. White bracket shows PM folds across the full apical surface at prometaphase and metaphase 11 (basal pseudocleavage furrows are red and yellow). Corner brackets show PM folds restricted to apical surface domains above astral centrosomal MTs (pink arrows) at poles of anaphase and telophase 11 compartments. By prometaphase 12, each pole becomes a new compartment. (C) Z-sections show apical PM surface at 0 µm, PM infoldings at −1.5 µm above centrosomal MTs (corner brackets), and cross-sections of PM tubules at −5.8 µm amongst centrosomal MTs (curly brackets). (D) Closely spaced Z sections through one nascent cap show PM folds continuous with PM tubules (white arrows), which extend into centrosomal MTs (pink arrows). Features described in B–D seen in 10/10 embryos. (E) Z sections show the entry of extracellular dye (Dextran) in between folds and into connected tubules (arrows). Seen in 8/8 embryos. (F) Schematic of PM tubule extending from PM fold into astral centrosomal array and between ER membranes (ER imaging in Fig. 2).