By Rhiannon Clouse | Updated Aug 30, 2022
Egg cells, or ova, are the female gametes that carry half of a future organism’s genetic material. In humans, a mature ovum measures roughly 0.0039 inches (≈100 µm) in diameter and is packed with proteins, ribosomes, and a dense network of mitochondria. These organelles provide the ATP needed for the rapid cell divisions that follow fertilization. As women age, mitochondrial function declines, which can reduce fertility and complicate conception attempts.
Women are born with the full complement of their lifetime ova, but these cells remain dormant until puberty. During each menstrual cycle, a developing egg resides in a follicle—a fluid‑filled sac in the ovary. Rising estrogen levels stimulate the follicle to enlarge, and the woman may experience heightened libido and a thinning cervical mucus film. Ovulation occurs when the dominant follicle ruptures, releasing the ovum into the fallopian tube.
Once in the fallopian tube, the ovum has a narrow window of viability—about 48 hours. If sperm does not fertilize the egg within this period, the cell undergoes apoptosis. The ruptured follicle transforms into the corpus luteum, secreting progesterone for roughly two weeks to prepare the uterine lining for implantation. Should fertilization fail, the corpus luteum regresses, progesterone drops, and menstruation begins.
Fertilization takes place when a sperm penetrates the ovum’s zona pellucida, a glycoprotein shell. Once inside, cortical granules release enzymes that harden the zona, preventing polyspermy. The sperm’s flagellum is shed, and its DNA merges with the oocyte’s nucleus, creating a diploid zygote that will begin the first mitotic divisions.
Ovum’s robust protein‑synthesis machinery and abundant mitochondria make it an ideal vehicle for pharmaceutical production. Scientists can introduce therapeutic genes into the egg, which then produces the desired proteins. Moreover, somatic cell nuclear transfer (SCNT) utilizes the ovum’s cytoplasm to reprogram a donor nucleus, enabling the creation of genetically identical embryos—a technique foundational to cloning research.
