Fig. 1. (A) Schematic representation of three spermatids undergoing individualization. Spermatogenesis produces 64 elongated spermatids interconnected by cytoplasmic bridges. During individualization, syncytial membrane (black) of the bundled spermatids (far left) is remodeled to individual membranes encasing each of the spermatids separately (far right). The individualization complex is responsible for membrane remodeling, and its central constituent is actin (green; one actin cone is associated with each spermatid in the complex) (Fabrizio et al., 1998). At the beginning of individualization (far left) the actin cones assemble around the spermatid nuclei (blue), and myosin VI (red dots) coats the actin cones in a particulate fashion. Later, the actin cones progress away from the nuclei and down the length of the spermatid axonemes (orange), extruding the cytoplasm between the spermatid tails, resolving the cytoplasmic bridges (area between ovals of membrane between the spermatid axonemes) and remodeling the syncytial membrane into individual membranes that encase each spermatid (middle). As soon as the actin cones move away from the nuclei, myosin VI localizes to an intense band at the front of the actin cones (red band). A bulge of plasma membrane, syncytial cytoplasm, vesicles and organelles (the cystic bulge) develops. When the actin cones reach the apical end of the testis, membrane and cytoplasm collected are pinched off, leaving the spermatids completely encased in their own membranes. The length of the spermatids is approximately 2 mM, and the cones take an estimated 18 hours to travel this distance. (B-M) Laser scanning confocal images of myosin VI staining at different stages of individualization complex progression. Early (B-E) in the process of individualization, actin cones (B) have assembled around the nuclei (D) of spermatids near the basal end of the testis, and myosin VI (C) coated the surface of actin cones in a particulate fashion. A little later (F-I), the actin cones (F) have moved away from the nuclei (H), myosin VI (G) was concentrated at the front of the actin cones. Much later (J-M), myosin VI accumulated and concentrated in a tight band (K) at the front of the actin cones as the individualization complex moves down the testis. Each image shows a single plane through one group of 64 bundled spermatids with 64 associated actin cones.

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