Figure 4

Proacrosomal vesicle fusion is impaired in AU040320-null mice. TEM images of developing spermatids in seminiferous tubules of wild-type and mutant mice. Wild-type sperm exhibit the characteristic developmental path of the acrosome, with proacrosomal vesicles fusing together to form a large acrosomal granule (A) which becomes flattened along the acroplaxome and the nuclear face in the cap phase (C), extends further over the elongating nucleus in the acrosome phase (E), and matures into a structure covering nearly all the now heavily condensed nucleus in the maturation phase (G). A couple of spermatocytes in cap phase appearing alongside those in maturation phase are shown in panel G. In mutant spermatids, vesicles containing proacrosomal material (B’, white arrows) are observed approaching the nucleus and forming multiple points of indentations on it (B’, black arrows) but fail to coalesce into a single acrosomal granule. As they develop, mutant round spermatids continue to show no acrosomal structure despite the continuous arrival of proacrosomal vesicles (D’, arrows); the curvatures observed at the edge of the acroplaxome in these cells (D, arrowheads) may indicate the presence of the manchette. A similar pattern appears in mutant spermatids in the acrosome phase (F) which display no developing acrosome, despite the more elongated cell morphology and the apparent presence of the acroplaxome revealed by the flattened nuclear surface (F’, arrows), while larger proacrosomic vesicles can be observed aligning to the acroplaxome. At maturation phase, mutant spermatids show a condensed globular, not elongated, nucleus but no acrosome (H), although an apparently normal developing tail (arrow) can be detected. While mutant spermatids do not develop an acrosome, they appear on occasions, at the cap/acrosome phases, to contain a pseudoacrosome-like structure (I,J) consisting of saccules organised in a layered fashion (black arrows) in close proximity to the acroplaxome (white arrows). n, nucleus; g, Golgi; ac, acrosome; apx, acroplaxome; ag, acrosomal granule; v, proacrosomal vesicles; M, midpiece of developing sperm tail. For each condition, n = 5. Scale bars: (A–H) 2 μm, insets 1 μm; (I,J) 1 μm, insets 200 nm.