Sperm gatekeeping: 3D imaging reveals a constricted entrance to zebra finch sperm storage tubules

Females across many internally fertilising taxa store sperm, often in specialised storage organs in their reproductive tracts. In birds, several hundred sperm storage tubules exist in the utero-vaginal junction of the oviduct and there is growing evidence that sperm storage in these tubules is selective. The nature of the mechanisms underlying female sperm storage in birds remains unknown due to our limited ability to make three dimensional, live observations inside the large, muscular avian oviduct. Here, we describe a new application of fluorescence selective plane illumination microscopy to optically section oviduct tissue from zebra finch Taeniopygia guttata females label-free, by harnessing tissue autofluorescence. Our data provide the first description of the 3D structure of sperm storage tubules in any bird, and reveal the presence of constricted openings to tubules, suggesting active entrapment and release of sperm.