Localized, Reactive F-Actin Dynamics Prevents Abnormal Somatic Cell Penetration by Mature Spermatids.

Spermatogenesis occurs inside a somatic cell enclosure. Sperm release, the most important final step and a target for contraceptives, has been extensively studied in fixed tissue preparations. Here, we provide a time-lapse description of the release process in Drosophila testis ex vivo. We show that the spermatid tails exit the somatic enclosure and enter the testicular duct first, followed by the spermatid heads. Prior to this, individual spermatid heads attempt to invade the head cyst cell, and on each occasion they are repelled by a rapid and local F-actin polymerization response from the head cyst cell. The F-actin assembly involves N-WASp, D-WIP, and Arp2/3 complex and dissipates once the spermatid head retreats back into the fold. These findings revise the existing spermiation model in Drosophila and suggest that somatic cells can actively oppose mechanical cell invasion attempts using calibrated F-actin dynamics in situ.