Meiosis I progression in a wild type spermatocyte. At the stage of late prophase the asters are located at opposite positions of the nuclear membrane, and the nucleus is decorated by a phase-dense system of parafusorial membranes. Short after this stage the cell enters prometaphase, the chromosomes suddenly condense and the nuclear region begins to elongate due to spindle assembly. As prometaphase proceeds rapid movements of bivalent chromosomes can be observed between the spindle poles. The bivalents eventually align at the metaphase plate, while the phase dense material (pdm) that decorates the nucleus tends to accumulate at the equatorial region of the bipolar spindle. At anaphase A homologues migrate to opposite poles. Chromosome segregation is followed by anaphase B, during which the spindle and the associated pdm elongates further. At telophase the cleavage furrow constricts leading to the separation of the daughter nuclei and the completion of cytokinesis.

Late spindle collapse in a dd4^S primary spermatocyte. At late prophase I the asters have already segregated to opposite positions outside the nuclear membrane. At the onset of prometaphase chromosomes condense and the nucleus starts to elongate, indicating that spindle assembly is taking place. As bioriented homologues congress to the metaphase plate the pdm accumulates at the equatorial region of the bipolar spindle. Before the onset of chromosome segregation the spindle poles start to approach each other and the pdm material becomes the centre of two nascent conical structures. This process continues until the spindle fully collapses, as shown by the proximity of the asters. While chromosomes decondense the two phase-dense cones elongate further, as if attempting to undergo anaphase B. Finally the cell becomes disorganized, the chromatin individualizes in nuclear-like vesicles, and cytokinesis fails