What is oogenesis? Give a brief account of oogenesis.
Oogenesis is the process of formation of a mature female gamete (ovum). It is markedly different from spermatogenesis and is initiated during the embryonic development stage.
During fetal development, a couple of million gamete mother cells (oogonia) are formed within each fetal ovary. No more oogonia are formed or added after birth. These cells start division and enter into prophase-I of the meiotic division and get temporarily arrested at that stage, called primary oocytes. Each primary oocyte then gets surrounded by a layer of granulosa cells and is called the primary follicle.
A large number of these follicles degenerate during the phase from birth to puberty. Therefore, at puberty only 60,000-80,000 primary follicles are left in each ovary. The primary follicles get surrounded by more layers of granulosa cells and a new theca and are called secondary follicles.
The secondary follicle transforms into a tertiary follicle, characterized by a fluid-filled cavity called antrum. The theca layer is organized into an inner theca interna and an outer theca externa. At this stage, the primary oocyte completes its first meiotic division, which is an unequal division resulting in the formation of a large haploid secondary oocyte and a tiny first polar body. The secondary oocyte retains bulk of the nutrient-rich cytoplasm.
The secondary oocyte forms a new membrane called zona pellucida surrounding it. The tertiary follicle further changes into the mature follicle or Graafian follicle. The Graafian follicle ruptures to release the secondary oocyte from the ovary by the process called ovulation.
Explanation
This answer covers all key aspects of oogenesis as described in NCERT Chapter 3 (Human Reproduction): the definition, initiation during embryonic development, formation and arrest of primary oocytes, follicular development stages (primary → secondary → tertiary → Graafian follicle), the unequal first meiotic division producing secondary oocyte and polar body, and finally ovulation. The answer follows the textbook sequence and uses only the provided context.
Solution Steps
Step 1: Define oogenesis as the formation of mature female gamete
Step 2: Describe the initiation during embryonic development and formation of oogonia/primary oocytes
Step 3: Explain follicular degeneration from birth to puberty and remaining follicle count
Step 4: Detail the transformation through secondary and tertiary follicle stages with meiotic division
Step 5: Describe Graafian follicle formation, zona pellucida, and ovulation