oocyta

An oocyte, also called an egg cell, is the female gamete involved in sexual reproduction. Oocytes are produced in the ovaries and, after fertilization, contribute half of the genetic material of the embryo. In English, the term oocyte is standard; in some languages the form oocyta is used.

Oocyte development begins with oogonia, which proliferate by mitosis during fetal life to form primary oocytes. These enter meiosis and become arrested in prophase I until puberty. At each menstrual cycle, a cohort of follicles matures and one primary oocyte resumes meiosis I, producing a secondary oocyte and the first polar body. The secondary oocyte is arrested at metaphase II until fertilization.

Maturation and fertilization involve the release of the oocyte during ovulation. If sperm penetrates the oocyte, meiosis II resumes, yielding a mature ovum and a second polar body. The fusion of paternal and maternal pronuclei then forms a zygote. The oocyte cytoplasm contains the materials needed for early development, and its surrounding follicular cells regulate its growth and release.

Structureally, the oocyte resides within the ovarian follicle, surrounded by cumulus cells and the zona pellucida, with a specialized outer layer called the corona radiata. The oocyte's cortex contains cortical granules involved in blocking polyspermy after fertilization. Oocytes are large, metabolically active cells rich in organelles and stored nutrients.

In clinical contexts, oocytes are central to assisted reproductive technologies; aging and environmental factors can affect oocyte quality, and techniques such as in vitro maturation and egg retrieval are used in fertility treatments.