Circadian rhythm of sperm release in the gypsy moth,Lymantria dispar: ultrastructural study of transepithelial penetration of sperm bundles

Release of mature bundles of spermatozoa from the testis into the vas deferens is a critical but poorly understood step in male insect reproduction. In moths, the release of sperm bundles is controlled by a circadian clock which imposes a temporal gate on the daily exit of bundles through the terminal epithelium-a layer of specialized epithelial cells separating testis follicles from the vas deferens. The sequence of cellular events associated with the daily cycle of sperm release was investigated by scanning and transmission electron microscopy. In the hours preceding sperm release, there is a solid barrier between the testis and the vas deferens formed by the interdigitation of cytoplasmic processes of adjacent terminal epithelial cells. At the beginning of the sperm release cycle, sperm bundles protrude through this barrier while the terminal epithelial cells change their shape and position relative to the bundles. Subsequently, the cyst cells enveloping the sperm bundles break down and spermatozoa move out of the testis through the exit channels formed between the epithelial cells. Afterwards, cyst cell remnants and other cellular debris are released into the vas deferens lumen, and the epithelial barrier is reconstructed due to phagocytic activity of its cells. These data provide a foundation on which to build an understanding of the cellular mechanisms of clock-controlled sperm release in insects.