Figure 4: Localization of the BPI protein within cauda epididymal spermatozoa. (a) Negative control using preimmune rabbit serum. (b-d) BPI attached to the spermatozoa plasma membrane before the acrosome reaction (white arrow); the BPI localized in the acrosome matrix became exposed after the acrosome reaction (blue arrow) and showed a tendency to gather in the equatorial segment (yellow arrow). BPI: bactericidal/permeability-increasing protein. Bar = 10 μm.