A mammalian female gamete is retrieved from the reproductive tract and observed under a microscope. It is found to have exactly one polar body in its perivitelline space, and its chromosomes are aligned at the equator of the spindle. Identify the current stage of this cell and the specific physiological event required to break this arrest.

Step-by-step solution

The presence of exactly one polar body in the perivitelline space indicates that the cell has completed meiosis I. The alignment of chromosomes at the equator indicates that the cell is currently arrested at metaphase of meiosis II. Therefore, the cell is a secondary oocyte. In human oogenesis, the secondary oocyte remains arrested at metaphase II until fertilization begins. The specific physiological trigger required to break this arrest and complete the second meiotic division (extruding the second polar body) is