Transmission electron microscopy images of transverse sections of mature sperm heads inside a cyst at the base of a wild-type testis (magnification 28,000×). (A) Cross-section view of mature and individualized sperm nuclei (arrows) inside the head cyst cell (HC, arrowheads). The stage is determined according to an earlier description . Inset shows enlarged view of the nuclei (N) tightly invested with plasma membrane (white arrowhead). The nuclei are embedded in an electron dense material (fine arrows) and membranous projections filled the interstitial space (marked with *). (B) Nuclei (arrows) of post-individualized spermatids inside the head cyst cell are tightly packed with electron dense material around them (fine arrows). They were also placed at one side of the HC. Inset shows 10× enlarged view of part of the bundle. The interstitial space (*) is packed with electron dense material.