ovocytes

An ovocyte, also called an oocyte, is the female germ cell involved in reproduction. In humans, it develops within the ovary and, after meiotic divisions, can become an ovum (egg) capable of combining its genetic material with a sperm.

Development and stages: The ovarian reserve begins with primordial germ cells that migrate to the developing gonads and form oogonia. Oogonia proliferate and enter meiosis to become primary oocytes, which are halted in prophase I before birth. During childhood, the pool of primary oocytes is maintained in this arrested state. At puberty, hormonal changes trigger follicle growth. A cohort of primordial follicles begins development; typically, one becomes dominant each cycle. The primary oocyte resumes meiosis I to form a secondary oocyte and the first polar body. The secondary oocyte proceeds to meiosis II but is arrested at metaphase II until fertilization. If fertilization occurs, meiosis II completes, producing an ovum and a second polar body.

Structure and surroundings: Oocytes are large cells rich in cytoplasm and maternal factors, with mitochondria and stored transcripts contributing to early embryonic development. They reside within ovarian follicles, surrounded by granulosa and theca cells, and are encased by the zona pellucida. During ovulation, a mature oocyte is released with the cumulus oophorus and surrounding cumulus cells.

Clinical and biological relevance: The number and quality of oocytes decline with age, affecting fertility. In assisted reproduction, mature oocytes are retrieved after hormonal stimulation and may be fertilized in vitro. Oocyte cryopreservation is used for fertility preservation, and oocytes can be donated for assisted reproduction.