Oocytu

Oocytu, or oocyte, is the female gamete involved in sexual reproduction. It is the cell that carries half the genetic material and contributes cytoplasmic factors required for early embryonic development. In most animals, oogenesis (the formation of oocytes) occurs in the ovaries and begins with oogonia transforming into primary oocytes.

In mammals, primary oocytes form during fetal life and arrest in prophase I of meiosis. After puberty, hormonal cycles recruit developing follicles. In each cycle, a primary oocyte completes meiosis I to produce a secondary oocyte and a first polar body. The secondary oocyte begins meiosis II but is arrested at metaphase II and is released during ovulation. If fertilization occurs, meiosis II resumes, yielding a mature ovum and a second polar body; the haploid pronuclei from sperm and oocyte then fuse to form a zygote.

Structure-wise, an oocyte is a relatively large cell surrounded by extracellular layers, such as the zona pellucida and cumulus cells in mammals. The oolemma is the plasma membrane, and the cytoplasm contains maternal mRNA, proteins, mitochondria, and various determinants essential for early development.

Role and development vary among species, but the core function is to contribute maternal genetic material and developmental resources for the embryo. The terms oocyte, ovum, and zygote describe consecutive stages: the oocyte is before fertilization, the ovum after meiosis II is complete and fertilization has occurred, and the zygote is the initial diploid cell formed after fertilization. Etymology traces to Greek roots meaning egg cell.