Ultrastructural studies of the nuclear envelope in human sperm

In freeze fracture preparations of normal human spermatozoa, nuclear pores were revealed in hexagonal arrangement over the sperm nucleus. The pores were 85 nm in diameter and were studded with numerous particles. The membrane of normal spermatozoa incubated without ionophore or calcium ions appeared continuous and enclosed the condensed chromatin closely. When the sperm were incubated with ionophore A23187 plus calcium ions to induce the acrosome reaction the nuclear envelope appeared to undergo dramatic topographical and morphological changes. Stretching, expanding, and loosening of the nuclear envelope and breaking down in certain regions was seen in thin sections. The importance of this phenomenon is discussed in relation to similar events occurring when the sperm penetrate the ovum.