Question

Give a schematic representation of oogenesis in human females.

Hint:

In oogenesis, meiosis is incomplete until fertilization. This ensures that the female gamete (ovum) is haploid, ready to merge with the male gamete during fertilization. In pollination, the main aim is to transfer pollen efficiently between flowers to ensure genetic diversity.

Answer 1

Oogenesis is the process of egg (ovum) formation in human females. It occurs in the ovaries and involves several stages that span from fetal development to the onset of menopause. The process is highly regulated by hormones, and the development of a mature egg is essential for fertilization and reproduction.

Stages of Oogenesis:

Fetal Development (before birth):

Primordial Germ Cells (PGCs): During embryonic development, primordial germ cells migrate to the gonads (ovaries) and become oogonia (germ cells).

Oogonia Proliferation: Oogonia divide by mitosis and multiply. By the 5th month of fetal development, they stop dividing and enter the next stage.

Primary Oocytes Formation: Oogonia differentiate into primary oocytes, which are diploid (2n), meaning they have the full set of chromosomes (46 chromosomes in humans).

Arrest in Prophase I: Primary oocytes begin meiosis but stop at prophase I. This is where they remain, arrested in their development, until puberty. A female is born with all the primary oocytes she will ever have, approximately 1–2 million, but only around 400,000 remain by puberty.

Puberty to Menopause (During the Reproductive Years):

Activation of Oocytes: Each menstrual cycle, a few primary oocytes resume meiosis, but typically only one will complete the process.

Secondary Oocyte Formation: The primary oocyte completes meiosis I, resulting in two cells: a large secondary oocyte and a small polar body. The secondary oocyte is haploid (n) and contains most of the cytoplasm, while the polar body contains little cytoplasm and is nonfunctional.

Arrest in Metaphase II: The secondary oocyte enters meiosis II but arrests at metaphase II. It will remain in this state until fertilization occurs.

Ovulation: Around the 14th day of the menstrual cycle, the secondary oocyte is released from the ovary during ovulation. If not fertilized by sperm, the oocyte degenerates and is absorbed.

Fertilization (If Sperm Meets the Oocyte):

Completion of Meiosis II: If a sperm penetrates the secondary oocyte, it resumes and completes meiosis II, forming a mature ovum and another polar body.

Formation of Zygote: The mature ovum and sperm nuclei combine to form a zygote, which is the first stage of embryonic development.

Oocyte Degeneration (If Not Fertilized):

If fertilization does not occur, the secondary oocyte disintegrates and is reabsorbed by the body. The polar body also degenerates.

Key Points:

Meiosis in Oogenesis is Discontinuous: Unlike males, where spermatogenesis is a continuous process, oogenesis is characterized by long periods of arrest during meiosis.

Unequal Cytoplasmic Division: During both meiosis I and II, the division is unequal, with most of the cytoplasm being retained in the oocyte, ensuring that the ovum has sufficient resources to support early embryonic development.

One Egg Per Menstrual Cycle: Typically, only one egg (secondary oocyte) is ovulated each month; however, other developing oocytes degenerate.

This intricate process ensures that only a single, fully developed ovum is ready for fertilization, while the others do not contribute to reproduction.