Fig. 1. The rCD2 protein marks the sperm plasma membrane. The panels show sperm produced by males homozygous for snky¹ and a transgene expressing rat CD2 gene. The rCD2 protein (green) was detected by immunolocalization and nuclei (red) by PI or DAPI staining. In a cyst of elongating spermatids (A) and mature sperm (B), rCD2 is located on the plasma membrane. After entry into the egg, sperm retain a condensed nucleus (C) and rCD2 (D). The merged image (E) shows overlapping rCD2 and nuclear staining consistent with retention of the plasma membrane surrounding the head. Scale bar: 10 µm.