Oocyte

Primary oocytes form before birth and remain arrested in prophase I (diplotene/dictyate) within primordial follicles for up to five decades. During follicle growth the oocyte enlarges from ≈30 µm to ≈100–120 µm, accumulating mitochondria, ribosomes, stored maternal mRNAs and proteins, and cortical granules beneath its plasma membrane; it secretes the zona pellucida, a thick glycoprotein coat (ZP1–ZP4) that binds sperm in a species-specific manner. Granulosa cells communicate with it through gap junctions and transzonal processes, keeping meiosis arrested (via cGMP/cAMP).

The LH surge breaks the arrest: the oocyte completes meiosis I, extruding the first polar body (unequal division keeps nearly all cytoplasm in the oocyte), and becomes a secondary oocyte that arrests again in metaphase II. It is ovulated with its zona pellucida and corona radiata. If a sperm fuses with it, Ca²⁺ oscillations trigger the cortical reaction and the completion of meiosis II, extruding the second polar body and forming the female pronucleus. Strictly, a mature "ovum" exists only briefly at this point.

The oocyte contributes the entire cytoplasm of the zygote — including all mitochondria (maternal inheritance of mitochondrial DNA) and the maternal mRNAs that drive the first cleavages.