Question

Describe the steps of oogenesis when fertilization occurs and when it does not occur. Why are polar bodies produced?

Short answer

In both scenarios, oogenesis begins with an oogonium that creates a primary oocyte. When fertilization does not occur, the primary oocyte becomes a secondary oocyte and the first polar body, ultimately disintegrating. If fertilization occurs, the secondary oocyte completes meiosis II, forming an ovum and a second polar body. The ovum combines with sperm to create a zygote (embryo). Polar bodies are produced to allocate most of the cytoplasm to the ovum, ensuring viability after fertilization.

Step-by-step solution

Introduction to Oogenesis

Oogenesis is the process of the development of mature female gametes, called ova or eggs, in the ovaries. This process involves several stages of cell division and differentiation, which can result in a successful fertilization or not depending on certain conditions.

Oogenesis when Fertilization Does Not Occur

When fertilization does not take place, oogenesis proceeds as follows: 1. Oogonium: It all begins with an oogonium, a diploid cell (2n) that undergoes mitosis to produce more oogonia. 2. Primary oocyte: One of the daughter cells of the oogonium becomes a primary oocyte, which is also diploid (2n) and begins meiosis I but stops at the prophase I stage. This primary oocyte remains in this state until ovulation. 3. Ovulation: During ovulation, the primary oocyte completes meiosis I, producing two haploid (n) cells: a secondary oocyte and the first polar body. 4. Degeneration: If fertilization does not occur, the secondary oocyte does not complete meiosis II, and both the secondary oocyte and the first polar body eventually disintegrate and are eliminated from the body.

Oogenesis when Fertilization Occurs

When fertilization takes place, oogenesis continues after ovulation: 1. Secondary oocyte: Upon fertilization, the secondary oocyte resumes meiosis II and divides into two unequal cells - an ovum and a second polar body. 2. Ovum: The ovum (n) then combines genetically with the sperm (n) to form a diploid zygote (2n), which is the starting point for embryonic development.

Purpose of Polar Bodies

Polar bodies are produced during oogenesis as a result of the unequal division of cytoplasm in meiosis I and II. They are small and non-functional, eventually degenerating. The main purpose of polar bodies is to ensure that the egg or ovum receives the majority of the cytoplasm, providing the necessary nutrients and energy for the developing embryo after fertilization. In conclusion, the steps of oogenesis differ based on whether fertilization occurs or not, and polar bodies are produced to allocate most of the cytoplasm to the ovum, ensuring its viability after fertilization.

Key concepts

Fertilization in Oogenesis

Oogenesis is a critical process in reproduction that leads to the formation of female gametes or ova. When fertilization occurs, the path of oogenesis alters, resulting in the union of male and female gametes. The key steps can be simplified as starting with a secondary oocyte, which only completes the second meiotic division upon fertilization.

This division is triggered by the penetration of a sperm cell and results in an ovum and a second polar body. It is essential to understand that fertilization is not just the union of genetic material but also the culminating stimulus for the oocyte's final maturation step. The resultant ovum fuses its haploid set of chromosomes with that of the sperm, creating a diploid zygote. This zygote then embarks on the journey of embryonic development.

• Oogonium becomes primary oocyte
• Primary oocyte halts at prophase I
• Ovulation triggers meiosis I completion
• Secondary oocyte awaits fertilization
• Fertilization prompts meiosis II and formation of ovum

During this critical window of time, mechanisms are in place to ensure that only a sperm with proper maturation and mobility is successful, indicating a tightly regulated process designed to maximize the chances of a viable offspring.

Polar Bodies Function

During oogenesis, the formation of polar bodies serves a very distinct and important function, ensuring that the resulting ovum is equipped with sufficient resources to support early development. Polar bodies are byproducts of meiotic cell divisions and are significantly smaller than the oocyte.

Their creation is due to the asymmetric cytokinesis that occurs during meiosis, where the cytoplasm is unevenly divided. This division ensures that the ovum retains most of the cytoplasm, which includes vital nutrients and organelles necessary for the embryo's growth post-fertilization.

Although polar bodies do contain genetic material, they serve no reproductive function and ultimately degenerate. They are essential in reducing the chromosome number in the gamete while maintaining the bulk of cytoplasm within the oocyte, hence promoting its survival and capability to undergo further development if fertilization occurs.

Oocyte Maturation

Oocyte maturation is a pivotal developmental process that prepares the oocyte for the potential of fertilization. It involves a pause at prophase I (dictyate arrest) during the female's fetal development, where it remains until the individual reaches sexual maturity. Upon reaching ovulation, hormonal cues trigger the completion of meiosis I, producing a secondary oocyte and the first polar body.

The secondary oocyte then remains in metaphase II arrest until fertilization signals it to carry out the second division of meiosis, ultimately maturing into a fully functional ovum. This intricate control of maturation timings ensures that the oocyte is only fully mature—and thus capable of being fertilized—when it is at the correct stage in the ovary and the female’s menstrual cycle.

This maturation is crucial not only in terms of cellular development but also in ensuring that the oocyte is synchronised with the female's reproductive timeline, facilitating the optimal window for fertilization and subsequent embryonic development.