An ovum is the female reproductive cell, central to human reproduction. It contains half the genetic information needed to create a new organism.
What Exactly is an Ovum?
An ovum, also known as an egg cell, is the female gamete produced within the ovaries. It is among the largest cells in the human body, measuring approximately 120 micrometers in diameter, making it typically visible without magnification. The ovum is spherical and non-motile, meaning it cannot move on its own.
Each ovum contains a nucleus with 23 chromosomes, representing half of the genetic material for a new individual. Surrounding the nucleus is cytoplasm, or ooplasm, which provides nutrients for the ovum’s development and early embryonic growth. The ovum is protected by layers, including the zona pellucida, a transparent membrane that ensures only one sperm can penetrate for fertilization.
How an Ovum Develops
The development of an ovum, a process known as oogenesis, begins remarkably early in a female’s life. Immature egg cells, called primary oocytes, are present in the ovaries from birth. A female fetus produces all the egg cells for her entire life during gestation, with approximately 1 to 2 million eggs present at birth. These primary oocytes remain in a dormant state until puberty.
During each menstrual cycle, typically one primary oocyte matures into a secondary oocyte, preparing for release. This maturation occurs within a fluid-filled sac in the ovary called a follicle. Hormones like follicle-stimulating hormone (FSH) initiate the growth and development of these ovarian follicles, while luteinizing hormone (LH) triggers the final maturation and release of the ovum.
The Ovum’s Journey and Role in Conception
Ovulation marks the release of the mature ovum from the ovary, typically around day 14 of a 28-day menstrual cycle. Once released, the ovum is swept into the fallopian tube by small finger-like projections called fimbriae. It then begins its journey toward the uterus, a process aided by muscular contractions and cilia within the fallopian tube.
Fertilization usually occurs within the fallopian tube, where a single sperm penetrates the ovum’s outer layers. This fusion combines the genetic material from both the sperm and the ovum, forming a single-celled entity called a zygote. The zygote then undergoes rapid cell division, known as cleavage, as it continues its path to the uterus. This series of divisions transforms the zygote into a blastocyst, which then implants into the uterine lining to establish a pregnancy.
Ovum Versus Egg: Understanding the Terms
While “ovum” and “egg” are often used interchangeably, “ovum” is the more precise scientific term for the mature female reproductive cell ready for fertilization. It derives from the Latin word for “egg.”
“Egg cell” is a broader term that can describe the female gamete at various developmental stages, including immature forms. Both terms ultimately refer to the singular cell released from the ovary that can unite with sperm to initiate new life.