oöcyte

An oöcyte is the female gamete produced during oogenesis. In mammals, it develops within an ovarian follicle and is the cell that can give rise to a zygote after fertilization. The term is an alternative spelling of oocyte and is found in some texts and languages.

Development and maturation follow a defined sequence. Oogonia in the fetal ovary divide by mitosis to form primary oocytes, which enter meiosis I and arrest in prophase I before birth. At puberty, cohorts of primary oocytes resume meiosis I each menstrual cycle, and typically one completes meiosis I to form a secondary oocyte and the first polar body. Meiosis II begins in the secondary oocyte but is arrested at metaphase II and only completes if fertilization occurs, yielding a mature ovum and a second polar body. The oöcyte is surrounded by cumulus cells and a zona pellucida, and its cytoplasm contains maternal mRNA and proteins essential for early development.

Structural and functional features vary by species. In humans, oocytes are relatively large cells, about 100 micrometers in diameter, with cytoplasm rich in nutrients and mitochondria inherited from the mother. The amount of yolk and reserves differs across vertebrates, reflecting divergent reproductive strategies, from yolk-rich oocytes in birds to relatively yolk-poor oocytes in many mammals.

Clinical relevance and applications. Oocytes are central to assisted reproductive technologies, such as in vitro fertilization, where mature oocytes are retrieved after hormonal stimulation. Oocyte cryopreservation preserves female fertility by freezing immature or mature oocytes for later use.