The secondary oocyte completes meiosis II primarily in response to fertilization. When a sperm successfully penetrates the secondary oocyte, it triggers a series of biochemical signals that activate the oocyte’s metabolic processes. This activation includes a rise in intracellular calcium levels, which stimulates the oocyte to resume and complete meiosis II.
During fetal development, female germ cells in the ovaries enter meiosis and are arrested in prophase of meiosis I until ovulation. Upon ovulation, a primary oocyte completes meiosis I, resulting in a secondary oocyte and a polar body. The secondary oocyte then halts at metaphase II and is only completed if fertilization occurs. If no fertilization takes place, the secondary oocyte will disintegrate.
This mechanism ensures that meiosis II is only completed when the conditions are right for potential conception, effectively managing reproductive resources and timing within the female reproductive system.