What is Oogenesis?

The formation of the ovum (mature female gamete) from undifferentiated germ cells is called oogenesis. This process takes place in the ovaries (female gonads). Oogenesis consists of three stages known as the multiplication phase, growth phase, and maturation phase. 

Gametogenesis 

Gametogenesis is the process in which the formation of gametes takes place. Gametogenesis in males is known as spermatogenesis, and in females, it is termed oogenesis. Gametogenesis involves the formation of gametes from germ cells by both mitotic cell divisions and meiosis. First, spermatogenesis gets completed in the testes. The mature sperms (forms from male germ cells) are then released from the testes, and oogenesis started in the ovaries and got completed in the Fallopian tube of females. Gametes formed by spermatogenesis (sperms) are minute, motile and yolkless structures, whereas the gametes formed by oogenesis (ovum) are large, non-motile structures and possess yolk. 

Ovaries 

Ovaries are the primary sex organs in a human female body and play a primary role in the process of oogenesis. Human ovaries are paired structures located in the upper pelvic cavity. Adult human ovaries are shaped like unshelled almonds and are about 2-2 cm in length. It encloses ovarian stroma. Ovarian stroma has several oogonia that are formed during fetal life. Oogonia divide by mitosis and form the primary oocyte. A layer of granulosa cells surrounds each primary oocyte, and it is then called a primary follicle. Then the primary follicles get surrounded by more granulosa cells and called secondary follicles. The secondary follicles soon change into tertiary follicles, which are characterized by the presence of a fluid-filled cavity called the follicular antrum. The tertiary follicle is further converted into a mature follicle known as the Graafian follicle. Interspersed throughout the ovarian stroma are many ovarian follicles in different stages of development.

Phases of Oogenesis

1. Multiplication Phase

This phase of oogenesis starts during fetal development. First, in the germinal epithelium of fetal ovaries, some germ cells are larger than others. Then in each fetal ovary, these germ cells divide by mitotic cell divisions and produce millions of oogonia (or egg mother cells). After birth, no more oogonia are formed or added. Instead, oogonia undergo cell divisions (mitosis) and primary oocytes. 

2. Growth Phase 

This is a considerably longer phase of oogenesis. It extends for many years. In this phase, the oogonium grows into the large primary oocyte. It takes nutrition for growth from the surrounding follicle cells. This takes place after the onset of puberty.

3. Maturation Phase

This is the last phase of oogenesis. Two maturation divisions take place in each primary oocyte. It is divided by meiosis- meiosis I and meiosis II. During the first maturation division (meiosis), two unequal haploid cells- a large secondary oocyte and a small polocyte or first polar body are formed from the primary oocyte. In the second division (meiosis), the secondary oocyte gets arrested in the prophase of meiosis II, which gets completed on the entry of sperm into the ovum. Then it divides by meiosis into a large ootid and a very small second polar body. The ootid grows into a functional haploid ovum. Also, during the second maturation division, the first polar body may divide and form two second polar bodies. Thus, from one oogonium, one ovum and three polar bodies are formed. The ovum is the actual female gamete. During oogenesis in human beings, the ovary releases the ovum, and it gets matured in the mother's oviduct (Fallopian tube).

In humans and many vertebrates, the secondary oocyte completes the metaphase stage of division of meiosis II, but the meiosis II divisions do not occur in the first polar body. Then, the oocyte stops developing further, and it waits for the completion of meiosis II that is completed after the entry of sperm. The cell cycle restarts after the entry of sperm, which is accomplished by breaking down the MPF (M-phase promoting factor) and starting APC (anaphase-promoting complex). The secondary oocyte gets converted into a fertilized ovum (egg) or zygote (and a second polar body) after the completion of meiosis II.

Hormonal Regulation of Oogenesis

Various hormones control oogenesis. The hypothalamus secretes GnRH (Gonadotropin-Releasing Hormone); the anterior lobe of the pituitary gland gets stimulated by GnRH to secrete two hormones- LH and FSH. The production of estrogen, development of oocyte within the follicle to complete meiosis I to form secondary oocyte, and Graafian follicles' growth gets stimulated by FSH. The function of LH is to induce the rupture of the mature Graafian follicle; it then facilitates the release of the ovum. Thus, ovulation is caused by LHe5. The rising estrogen levels inhibit the release of GnRH, resulting from which production of FSH and LH gets inhibited.

Significance of Oogenesis 

• One oogonium produces one ovum.
• Polar bodies possess a considerably small amount of cytoplasm. Thus, it helps to retain an adequate amount of cytoplasm in the ovum, which is necessary to develop early embryos. In addition, one set of chromosomes (n) in the ovum is also maintained due to the formation of polar bodies.
• During meiosis, the first crossing-over takes place, which is responsible for the variations in the offspring generation.
• Oogenesis takes place in many organisms. Therefore, the basic association between different organisms is supported. 

Ovulation

The release of a secondary oocyte (ovum) from the ovary is called ovulation. In humans, it is released about fourteen days before the onset of the next menstrual cycle. Secondary oocytes are released by the rupturing of the wall of the ovary. This process is induced by the hormone LH (Luteinizing Hormone) secreted by the anterior lobe of the pituitary. After the entry of sperm into the secondary oocyte (during fertilization), the maturation of the oocyte gets completed.

Ovum

The mature female gamete (Ovum) is spherical and is free of yolk. Such ovum is called alecithal. The cytoplasm of the ovum is called the ooplasm, and it contains a large nucleus, called a germinal vesicle. In addition, it contains a well-defined nucleolus.

Layers surrounding the ovum: During oogenesis, the cytoplasm of the ovum gets covered by the plasma membrane (cell membrane). Outside the plasma membrane, narrow perivitelline space is present. Followed by perivitelline space a non-cellular layer zona pellucida is present. Outer to zona pellucida, a thick cellular corona radiata is present.

Differentiation of Ova during Oogenesis

During differentiation of ova, the following changes occur

• Changes in the nucleus: Due to the production of a large amount of nuclear sap in the nucleus, the oocyte's nucleus becomes enlarged. Lamp brush chromosomes (a special type of chromosome) appear in the oocyte of some animals such as amphibians, reptiles, birds, etc. The nucleolus of a growing oocyte also increases in size greatly.
• Changes in the cytoplasm: the number of changes in the cytoplasm of the ova appears during the differentiation process:
• Golgi bodies: Golgi bodies disappear completely in the mature oocytes. However, these may change to some other structures.
• Endoplasmic reticulum (ER): The membranes of ER in the mature oocytes get perforated by pores and nuts that lack ribosomes.
• Mitochondria: During the growth of oocytes, the mitochondria number increases greatly. 
• Cortical granules: Cortical granules are a special structure formed in mature oocytes. These are mucopolysaccharides containing single membrane-bound organelles.
• Vitello genesis: This is the process of synthesis of yolk in the primary oocytes. In humans and other vertebrates (except fishes and amphibians), the yolk is synthesized in the liver of the females. It is then transported to the follicle cells of the oocytes via blood in a soluble form. In oocytes, the yolk is deposited in the form of yolk platelets or yolk granules.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelor of Science in Biology
• Bachelor of Science in Zoology
• Master of Science in Zoology
• Masters in human health and reproduction

Related Concepts

1. Gametogenesis
2. Spermatogenesis
3. Ovulation
4. Menstrual cycle
5. Meiosis (prophase)
6. Germ cells