Why is only one egg produced during oogenesis while four sperm are produced during the parallel process of spermatogenesis?
Millions of sperm are needed to have a chance of fertilizing a single ovum.
During oogenesis, only one egg is produced because of the unequal division of cytoplasm. Let me explain the process step-by-step:
1. Oogenesis starts with the production of primary oocytes in the ovaries of females before birth. These primary oocytes are arrested in prophase I of meiosis until puberty.
2. After puberty, each menstrual cycle triggers the completion of meiosis I. However, unlike spermatogenesis, where four haploid cells are produced, only one haploid secondary oocyte is produced in oogenesis.
3. At the time of ovulation, the secondary oocyte is released from the ovary. If it is not fertilized, it will disintegrate.
4. If fertilization occurs, the secondary oocyte completes meiosis II, producing one mature egg (ovum) and a haploid polar body. The polar body contains a minimal amount of cytoplasm and does not contribute to the development of the embryo.
In spermatogenesis, the process is different:
1. Spermatogenesis occurs in the testes of males.
2. It starts with the production of spermatogonia, which undergo mitosis to increase their number.
3. Then, primary spermatocytes are formed, which enter meiosis I.
4. Meiosis I results in the production of two haploid secondary spermatocytes.
5. Each secondary spermatocyte undergoes meiosis II, resulting in the formation of four haploid spermatids.
6. Spermatids further undergo structural and functional changes to develop into spermatozoa or sperm cells.
The difference in the number of cells produced in oogenesis and spermatogenesis can be attributed to the unequal division of cytoplasm. During oogenesis, most of the cytoplasm is retained in the secondary oocyte, which provides necessary nutrients for early embryonic development. In spermatogenesis, the four sperm cells produced are relatively small and contain minimal cytoplasm, enabling them to be motile.
During oogenesis, only one mature egg is produced because the primary goal of oogenesis is to provide a single haploid female gamete (egg) that can be fertilized by a sperm to form a zygote.
The process of oogenesis begins during embryonic development when oogonia, the precursor cells, undergo mitotic division to increase in number. The oogonia then enter the first phase of meiosis called the primary oocyte stage. During this stage, the oocytes begin to grow and accumulate nutrients.
However, unlike spermatogenesis, where all four spermatids derived from one spermatogonium mature into functional sperm, during oogenesis, only one of the four daughter cells produced from each primary oocyte eventually becomes a mature egg.
The other three daughter cells, called polar bodies, do not have the ability to be fertilized and contribute to the formation of an embryo. They instead undergo degeneration. This uneven distribution of cellular material is necessary to preserve the cytoplasmic resources of the developing egg, as well as the genetic material, which will combine with the genetic material of the sperm during fertilization.
The primary reason for this difference between oogenesis and spermatogenesis lies in the unequal division of cytoplasm and the presence of sufficient nutrients and organelles required for the embryo's early development. By producing only one egg, with a large amount of cytoplasm and necessary components, oogenesis ensures the successful maturation and survival of the embryo after fertilization.
In contrast, spermatogenesis focuses on producing an ample number of sperm cells that are optimized for motility and the delivery of genetic material to the egg. The surplus of sperm helps increase the chances of successful fertilization, given the relatively hostile conditions sperm face in the female reproductive tract.