In human females, oocytes are held in the second metaphase until _.

Explanation

The secondary oocyte commences meiosis 2 which arrests at metaphase and will not continue without fertilization. Oocytes, also known as egg cells or ovum, are the female reproductive cells. They are produced within the ovaries through a process called oogenesis. Oocytes are non-motile and larger in size compared to sperm cells. They contain half the number of chromosomes (23) as compared to other body cells, as they undergo meiosis to prepare for fertilization. During a woman's reproductive years, several oocytes begin to mature each month. However, only one oocyte is typically released during each menstrual cycle in a process known as ovulation. Oocytes are surrounded by supportive cells called follicular cells within the ovarian follicles. Once an oocyte is released during ovulation, it travels through the fallopian tube towards the uterus. It remains in a state of arrested development in the second metaphase of meiosis until fertilization occurs. If the oocyte is fertilized by a sperm cell, it completes meiosis, forming the female pronucleus, and becomes a zygote, which develops into an embryo. If fertilization does not occur, the oocyte eventually disintegrates and is shed along with the uterine lining during menstruation. The process of oocyte development, release, and potential fertilization is an essential part of the female reproductive system and plays a central role in human reproduction.