The female gamete, known as the ovum or egg cell, is the female reproductive cell. Its purpose is to fuse with a male gamete during sexual reproduction to form a new organism. As one of the largest cells in the human body, its size is related to its function of nourishing the developing embryo in its earliest stages. The ovum contains the cellular machinery and half of the genetic material required to initiate development.
Formation and Maturation of the Ovum
The creation of the female gamete, a process called oogenesis, begins during fetal development. Precursor cells called oogonia multiply and develop into primary oocytes. These primary oocytes initiate the first stage of meiotic division, a type of cell division that reduces the chromosome number by half. The process is then arrested in prophase I and remains paused for years.
At birth, a female’s ovaries contain all the primary oocytes she will ever have, numbering around one to two million, each housed within a follicle. This finite supply of gametes contrasts with the continuous production of sperm in males. From this large initial pool, only 400 to 500 will ever mature and be released for potential fertilization. The vast majority of oocytes will undergo a process of degeneration called atresia.
With the onset of puberty and monthly menstrual cycles, the paused process of oogenesis resumes. Each month, hormonal signals, primarily follicle-stimulating hormone (FSH) and luteinizing hormone (LH), prompt a small group of follicles to grow. One dominant follicle outpaces the others, and the primary oocyte inside it completes its first meiotic division. This division is unequal, producing one large secondary oocyte and a much smaller polar body that eventually degenerates.
The newly formed secondary oocyte is the cell released during ovulation. It immediately begins the second meiotic division but halts again in metaphase II. This second pause is only broken by fertilization. If a sperm cell penetrates the secondary oocyte, it is stimulated to complete meiosis II, resulting in a mature, fertilized egg and another small polar body. If fertilization does not occur within 24 hours, the secondary oocyte degenerates.
Anatomy of the Female Gamete
The mature female gamete, a secondary oocyte, is a large, spherical, and non-motile cell, measuring approximately 120 micrometers in diameter. Its structure is composed of several layers with specific purposes. At its center is the nucleus, which holds the cell’s genetic material in a haploid set of chromosomes.
Surrounding the nucleus is a vast expanse of cytoplasm, also known as ooplasm. This substance is rich in nutritive materials, often referred to as yolk, which includes proteins and lipids. These stored nutrients are designed to sustain the cell and provide energy for the early embryo’s growth after fertilization. The cytoplasm also contains hundreds of thousands of mitochondria for energy production.
The ovum is protected by multiple external layers. Directly surrounding the cell’s plasma membrane is the zona pellucida, a thick envelope made of glycoproteins. This layer acts as a protective barrier and contains receptors that help sperm bind to the egg. Adhering to the outside of the zona pellucida is the corona radiata, the outermost layer of follicle cells that remain attached after ovulation.
The Role in Fertilization
Following its release from the ovary during ovulation, the ovum is swept into the nearby fallopian tube, where fertilization takes place. The ovum is non-motile and relies on the contractions of the fallopian tube and the sweeping motion of its cilia to move toward the uterus. It remains viable for about 12 to 24 hours, awaiting the potential arrival of sperm.
For fertilization to occur, a sperm cell must penetrate the ovum’s protective layers. The sperm first pushes through the corona radiata. Upon reaching the zona pellucida, the sperm’s acrosome releases digestive enzymes that break down a path through this coat. This process, known as the acrosome reaction, allows one sperm to reach the ovum’s plasma membrane.
Once a single sperm fuses with the egg’s membrane, it triggers an immediate response called the cortical reaction. In this process, cortical granules just beneath the egg’s membrane release their contents outside the cell. These enzymes alter the structure of the zona pellucida, causing it to harden and destroying the sperm-binding receptors. This modification makes the layer impenetrable to other sperm, blocking polyspermy.
With the sperm’s nucleus now inside the cytoplasm, the ovum is prompted to complete its final stage of meiotic division. The fusion of the sperm’s haploid nucleus with the ovum’s haploid nucleus creates a single diploid cell known as a zygote. This event marks the beginning of a new individual’s development.
Genetic Significance
The ovum’s role extends beyond its physical function in fertilization; it holds significant genetic importance. Its primary contribution is providing half of the nuclear DNA. The 23 chromosomes in its nucleus combine with the 23 from the sperm to restore the diploid number of 46, establishing the genetic blueprint for a new individual.
A unique contribution of the female gamete is the inheritance of all mitochondrial DNA (mtDNA). Mitochondria, the cell’s energy-producing organelles, contain their own small chromosome. While sperm have mitochondria, these are destroyed after fertilization, meaning an embryo’s mitochondria are derived exclusively from the mother’s egg. This pattern of maternal inheritance is used to trace ancestral lineages and is how certain genetic conditions are passed down.
The process of meiosis that produces the ovum is also a source of genetic variation. During meiosis I, homologous chromosomes exchange genetic material in a process called crossing over. This shuffling of genes ensures that each ovum produced is genetically unique. This variation is important to evolution by creating new combinations of traits.