Plasma membrane cytoskeletal complex of the mammalian spermatozoa.

Human and ram sperm were incubated in vitro in Tris buffer with (1) calcium ions, (2) ionophore A23187, (3) calcium plus A23187, (4) without any substance, as described in [13], and were reincubated in situ with the protein S-1 of heavy meromyosin (1 mg/ml). These spermatozoa were examined with the electron microscope to study and characterize the cytoskeletal structures after negative staining, freeze fracturing, and thin sectioning. The formation of the cytoskeletal complex, associated with the plasma membrane in the postacrosome region, appeared to be triggered by S-1 in those sperm that were earlier treated with the ionophore. The dynamic nature of actin present in the postacrosomal dense matrix becomes evident with the formation of a cytoskeletal basket in that region. The cytoskeletal complex may play an important role during the exocytotic acrosome reaction, helping to retain the shape of the sperm head and protect the genetic material until fertilization.