An oocyte is an immature ovum (an immature ovum). Oocytes develop to maturity from within a follicle. These follicles are found in the outer layer of the ovaries. During each reproductive cycle, several follicles begin to develop. Usually, only one oocyte in each cycle will develop into a mature egg and ovulate from its follicle. This process is known as ovulation. A woman is born with all the oocytes she will ever have. This number naturally decreases with age. Age also reduces the quality and genetic stability of oocytes. This is why it is more difficult to get pregnant after 35.
The full maturation of the Follicule and Oocyte ovum is visible to the human eye and measures 0.1 mm. It is about the size of the period at the end of this sentence. Medications known as fertility drugs can stimulate the ovaries to release multiple oocytes during a menstrual cycle. used to stimulate the ovaries to produce multiple oocytes instead of ovulating as mature eggs. This is the cause of the increased risk of multiple pregnancies when taking fertility drugs. For every ovulated egg, there is a chance that it will be fertilized by a sperm. These fertilized eggs can develop into embryos (and eventually, if all goes well, babies).
During fertility treatments, the doctor will perform ultrasound scans to monitor the growth of the follicles. Oocyte maturation is also taking place, but oocyte maturation is not visible on ultrasound. This is why the growth of follicles and not of oocytes is observed. If too many follicles grow, your treatment cycle may be cancelled to prevent the risk of multiple pregnancies or ovarian hyperstimulation syndrome (OHSS). During in vitro fertilization (IVF), if ultrasound monitoring does not show sufficient follicle growth, meaning not enough oocytes are maturing, the cycle can be cancelled to prevent treatment failure.
Oogenesis is what an oocyte goes through as it develops into a mature egg cell. You can assume that oogenesis occurs over the course of a month since that is how often you ovulate. But you would be wrong! While it is true that any egg that ovulates completes the process of oogenesis in the month that it is released from the ovary, oocyte development began long before you were born. In fact, it started when you were a very young embryo.
Primordial germ cell
The “seed” cell of each oocyte is the primordial germ cell. These are embryonic cells that will eventually become sperm or oocytes. In the developing embryo, these cells move into the area that will eventually become the testicles or ovaries (also known as the gonads).
(Interesting side note: Research published in 2012 found that some of these stem cells from early oocytes are present in the ovaries of adult women. There may be away in the future to take these stem cells and create new oocytes. This would mean that women will no longer be limited to the eggs they were born with).
Once the primordial germ cell reaches the gonads, it is influenced by surrounding cells to become an oogonium (plural, oogonia). Oogonia are diploid cells. This means that they have two (di) complete sets of chromosomes. In the human cell, this is 23 pairs or a total of 46. This is important to know because the oocyte will eventually have only half or 23 chromosomes. (During fertilization, you’ll get the other 23 of the sperm to have a complete set once again.)
During the first five months of prenatal development, the oogonium increases in number through a process known as mitotic cell division. Meiosis is exclusive to germ cells. It only occurs in young eggs and sperm. In a more typical cell division, known as meiosis, cells duplicate by creating clones of themselves, each with a complete set of chromosomes. For example, a skin cell that goes through mitosis will eventually lead to two skin cells, with similar genetic codes.
During mitotic cell division, the oogonium divides into two separate cells containing:
- Only half of a chromosome set: In other words, they only have 23 chromosomes. (These are known as haploid cells).
- Unique Chromosomal Sets: Each oogonium that divides creates unique sister cells. This means that no oogonium shares the same chromosome composition as another.
This mitotic division is the reason that each new life has a unique genetic makeup that is unlike any other. However, it is not completely random. Everything is based on the original genetic material that the embryo received from its father and mother. These cells continue to multiply until they reach their peak. The peak occurs when the developing fetus is about five months old. At this point, the girl fetus has 7 million oocytes. This number will start to decrease after this point. At birth, a girl has only 2 million oocytes left.
Each oocyte will go through two separate meiotic cell divisions before becoming a mature ovum. Meiotic cell division leads to the growth and maturity of the oocyte, not additional oocytes. Towards the end of prenatal development, the oocytes stop multiplying and begin to mature individually. At this stage, they go through the first meiotic cell division. This cell division leads to the growth of oocytes, not more oocytes, as occurs with the oogonium. But they don’t just accelerate development to maturity right now. Primary oocytes are frozen in their development and remain frozen until reproductive hormones trigger the next stage. Oogenesis will continue at the age of puberty.
Puberty initiates the next stage of oocyte maturity. Of course, not all oocytes will go through these later stages of oocyte development together. They more or less take turns during a woman’s reproductive years. Each month, a new set of primary oocytes begins to mature.
Once a primary oocyte is affected by reproductive hormones, it completes Stage I of meiotic cell division. This is known as oocyte maturation. At the end of this first stage of meiotic cell division, the cell divides into two separate cells: a small polar body and a large secondary oocyte. The small polar body eventually decays. The secondary oocyte begins the next stage of maturation.
The oocyte now begins the second phase of meiotic cell division. Eventually, the secondary oocyte will divide again into two separate cells: another small polar body cell and a larger mature cell. This larger mature cell is known as the ootide. As before, the smaller polar body cell will eventually deteriorate. Ovulation occurs when the oocyte has reached the ootid stage of development.
At the time of ovulation, and outside is released from the follicle. Human eggs cannot move on their own. Instead, the finger-like projections draw the oocyte toward and into the fallopian tube. Once inside the fallopian tube, small hair-like projections known as cilia continue to drag the ootide.
In the fallopian tube, if a pregnancy occurs, the egg is fertilized by a sperm. Once this fertilization takes place, the ovum goes through its final stage of maturation and becomes an ovum, a fully mature human ovum. So is; the oocyte cannot actually complete its full development without fertilization.
From oocyte to ovum to zygote
During fertilization, the egg and sperm combine, each with 23 chromosomes. Quite quickly (but not at the exact time of fertilization), these chromosomes fuse together, creating a new cell with a complete set of chromosomes. This new cell is called a zygote. The zygote will develop into an embryo and, about nine months later, into a newborn baby.