An oocyte is an immature egg cell, or gamete, that resides within the ovaries. As the female reproductive cell, it contains the genetic material that, when combined with sperm, holds the instructions for a potential embryo. The term specifically refers to the cell before it has fully matured into an ovum, or egg, ready for fertilization. All of the oocytes a female will ever have are produced in the ovaries while she is still a developing fetus.
Oocyte Development and Maturation
The creation of oocytes, a process called oogenesis, begins before birth. During fetal development, millions of precursor cells called oogonia form in the ovaries and enter a specialized division process called meiosis. This process is quickly halted, leaving the cells as primary oocytes in a state of suspended development. Each primary oocyte is encased in supportive cells, forming a structure called a primordial follicle.
From birth through puberty, most of these follicles degrade in a process known as atresia. After puberty, a small group of primordial follicles is recruited each menstrual cycle to resume development. These follicles grow, adding more cell layers and developing a fluid-filled cavity. This growth phase can take more than 120 days, with the supportive cells guiding the oocyte’s maturation.
Typically, only one follicle becomes dominant while the others cease to grow. Triggered by a surge of luteinizing hormone before ovulation, the primary oocyte inside completes its first meiotic division. This unequal division produces one large secondary oocyte, which retains most of the cytoplasm, and a small, non-functional polar body. The secondary oocyte is then released from the ovary during ovulation, ready for potential fertilization.
The Role of Oocytes in Fertilization
The mature oocyte released at ovulation is encased in a protective layer called the zona pellucida, which acts as a barrier and docking station for sperm. Its cytoplasm is dense with the nutrients and instructions needed to support early embryonic development after fertilization. The oocyte’s primary function is to be fertilized by sperm and orchestrate the events of conception.
For fertilization to occur, a sperm must bind to the zona pellucida. This binding triggers the sperm to release enzymes that digest a path through this protective coat. Once a single sperm fuses with the oocyte’s plasma membrane, it sets off a chain reaction within the oocyte.
This fusion causes a release of calcium, which prompts granules beneath the oocyte’s surface to release their contents. This hardens the zona pellucida and destroys its sperm receptors, making the oocyte impenetrable to other sperm. The calcium signal also prompts the oocyte to complete its second meiotic division. This final division results in a mature ovum and a second polar body, allowing the genetic material from the sperm and egg to fuse into a zygote.
Oocyte Quantity and Quality Over a Lifetime
A female is born with her entire lifetime supply of oocytes, a concept known as the ovarian reserve. At birth, the ovaries contain approximately one to two million primary oocytes, and no new ones are created. This reserve naturally and continuously declines throughout life via atresia, the programmed degeneration of follicles. This steady loss accounts for over 99% of all oocytes.
The ovarian reserve diminishes with age, and the decline accelerates around age 37. Alongside this decrease in quantity, the quality of the remaining oocytes also declines. Oocyte quality refers to the cell’s genetic health and its ability to develop into a healthy embryo after fertilization.
A primary factor in declining oocyte quality is the increased risk of chromosomal errors. Oocytes are arrested in meiosis for decades, from fetal life until ovulation, and the cellular machinery that organizes chromosomes can become less efficient over time. This can lead to an incorrect number of chromosomes (aneuploidy) in the oocyte. Aneuploidy is a major cause of age-related infertility, failed implantation, and miscarriage.
Oocyte Cryopreservation
Oocyte cryopreservation, or egg freezing, is a procedure to preserve fertility potential by retrieving, freezing, and storing oocytes for future use. People may choose this option for medical reasons, like before cancer treatments that can harm the ovaries, or for social reasons, such as delaying childbearing.
The procedure begins with hormone medications to stimulate the ovaries to mature multiple eggs at once. These mature oocytes are then collected from the ovaries in a minor surgical procedure called an egg retrieval. In the laboratory, the collected oocytes are then prepared for freezing.
The standard freezing technique is vitrification. This method uses cryoprotectant solutions to remove water from the oocyte before flash-freezing it in liquid nitrogen. This rapid cooling process turns the cell’s interior into a glass-like state, preventing the formation of damaging ice crystals. When an individual is ready to attempt pregnancy, the oocytes can be thawed, fertilized in a lab, and the resulting embryo transferred to the uterus.