Final answer:
During each nuclear division in oocyte meiosis, polar bodies are released. A polar body is a small cell that contains genetic material but less cytoplasm, and it typically disintegrates. Only upon fertilization does the secondary oocyte complete meiosis II, eject a second polar body, and form a diploid zygote.
Step-by-step explanation:
Oocyte Meiosis and Polar Bodies
Upon each nuclear division of oocyte meiosis, specific cells known as polar bodies are released. Oocyte meiosis is a special type of cell division necessary for sexual reproduction in females. The initial cell called the primary oocyte, begins meiosis and becomes arrested during prophase I until adolescence. Triggered by hormones, it resumes and completes the first meiotic division to become a secondary oocyte, which is arrested again at metaphase II. Most of the cellular material stays with the secondary oocyte, while the other resulting cell, the polar body, receives minimal cytoplasm and typically disintegrates. If fertilization occurs, the secondary oocyte completes meiosis II, creating another polar body and the fertilized cell that becomes a zygote with a full set of chromosomes.
Prior to ovulation, a primary oocyte resumes meiosis due to a surge in luteinizing hormone, making the transition to a secondary oocyte. During this division, cell material is divided unequally, resulting in a larger, secondary oocyte and a much smaller first polar body. Ovulation leads to the release of the secondary oocyte, which will only develop into an ovum upon fertilization. Otherwise, the polar bodies produced during these stages eventually degrade.
At fertilization, the oocyte completes meiosis II, and the unneeded genetic material is expelled through the second polar body. The result is one single-celled diploid zygote, ready to develop into a human. This intricate process ensures that the resulting embryo has the correct number of chromosomes and genetic diversity required for healthy development.