Figure 1.

Schematic diagram of individualization. See Figure 5A for schematic diagram of testes and the relationship of this diagram to overall testes organization. Individualization is depicted in this schematic diagram as proceeding chronologically from left to right. A single cyst (cell membrane shown in black) with two representative spermatids is depicted at three different time points. The cysts shown are not drawn to scale and would measure ∼2 mm long inside the wild-type testis. The first cyst depicts the initial assembly of actin around the nuclei (stippled black) and the first appearance of organized 95F myosin. Actin (red) assembles around each spermatid nucleus in the shape of a cone with the advancing edge of the cone toward the tail end of the spermatids. The tail end of the spermatid is toward the apical end of the testes. 95F myosin (blue) localizes in punctate structures along the actin filaments. The actin cones progress from the nuclear region to the end of the tail, in the direction of the red arrows. The spermatid membranes (green) are punctuated by cytoplasmic bridges. The second cyst depicts the actin cones, the spermatid membranes (green) and 95F myosin after progression of the cones away from the nuclei. 95F myosin (blue) forms in a ring at the base of the actin filaments. As the actin–95F myosin complex moves toward the tail end of the spermatids, many vesicles and organelles (green circles) are present in front of the complex. In the wake of the actin–95F myosin complex each sperm is invested in its own membrane (green) and is devoid of vesicles and organelles (with the exception of a single mitochondrion). The axonemal sheath (purple) is also lost during IC progression. Finally, in the third cyst the complex is depicted reaching the end of the axoneme (black-striped structure) just before formation of the waste bag (not drawn to scale), a structure containing the waste organelles and membrane that is pinched off from the base of the cyst. The actin–95F myosin complex, in addition to any other proteins and organelles not incorporated into the mature sperm, is contained in the waste bag and eventually degrades.