Question

How many viable cells are produced by oogenesis? a. four b. three c. one d. two

Short answer

In oogenesis, only one viable cell is produced, which is the mature ovum. The other cells produced, the polar bodies, are not viable. Therefore, the correct answer is c. one.

Step-by-step solution

Understand oogenesis

Oogenesis is the process in which female germ cells develop into mature ova or egg cells. This process has several stages: oogonium, primary oocyte, secondary oocyte, and mature ovum. The primary oocyte undergoes meiosis I to produce one large secondary oocyte and a small polar body. The secondary oocyte goes through meiosis II, producing one large mature ovum and another polar body.

Review the process and count viable cells

During oogenesis, two polar bodies and one mature ovum are produced. The polar bodies are not viable cells, as they are too small and do not have the necessary cellular machinery to be fertilized by a sperm cell. Only the mature ovum is viable. So, there is only one viable cell produced by oogenesis. Answer: c. one

Key concepts

Meiosis

Meiosis is a specialized type of cell division that reduces the chromosome number by half, creating four haploid cells from one diploid parent cell. This process is crucial in the formation of gametes—sperm in males and ova in females—ensuring genetic diversity through recombination and independent assortment. In females, meiosis initiates in the primary oocyte. It involves two successive divisions: meiosis I and meiosis II.

• In meiosis I, homologous chromosomes separate, leading to the formation of two haploid cells, each with one chromosome from the pair.
• In meiosis II, sister chromatids separate, resulting in four unique haploid cells.

For oogenesis, the outcome of meiosis is slightly different, as it produces one large viable ovum and small non-viable polar bodies.

Female Germ Cells

Female germ cells, also known as oogonia, are the precursors to the ova (egg cells) in females. These cells are formed in the ovaries during fetal development and play a fundamental role in reproduction. Oogonia multiply through mitosis during fetal development but do not continue to divide after birth. Instead, they develop into primary oocytes and begin meiosis.
While millions of oogonia are initially present, only a fraction develop into oocytes that will mature further. Each menstrual cycle, certain primary oocytes resume meiosis and transform through various stages, eventually leading to the formation of mature ovum if fertilized.

Primary Oocyte

A primary oocyte is a female germ cell that has started the process of meiosis but pauses at the prophase I stage. It exists within the ovarian follicles and remains in this arrested state until a female reaches puberty. At the onset of each menstrual cycle, hormonal signals kickstart the resumption of meiosis in several primary oocytes. However, only one typically completes the first meiotic division each cycle, resulting in:

• A larger secondary oocyte, which contains most of the cytoplasm and organelles necessary for potential development into an ovum.
• A smaller polar body, which is a by-product containing little cytoplasm and is not viable.

The secondary oocyte then undergoes meiosis II but pauses at metaphase II unless fertilization occurs.

Mature Ovum

The mature ovum, also known as the egg cell, is the final product of oogenesis, created after the completion of meiosis II if fertilization occurs. It is the only viable cell produced from oogenesis and possesses the necessary organelles and cytoplasm to support the initial stages of embryonic development. The transition from primary oocyte to mature ovum involves meiosis I and meiosis II:

• Meiosis I results in a secondary oocyte and a polar body.
• Meiosis II, completing only upon fertilization, results in the formation of the mature ovum and an additional polar body.

Therefore, only one mature ovum is prepared per cycle to meet the sperm cell for fertilization, ensuring a single, viable cell results from oogenesis, capable of potentially developing into an embryo.