Gametogenesis: Reproduction is important for the continuation of life, and reproduction is not possible without gametes which are formed by the process of Gametogenesis. It helps in the production of haploid cells called gametes from undifferentiated diploid germ cells in the gonads. The gametes in animals, including human beings, are formed from germ cells in the embryo. The germ cells are collectively referred to as the germ line, consisting of cells that will or can become the future gametes.

When both the male and female gametes are observed, the male gamete looks small and motile, known as sperm, while the female gamete is large and immotile, known as an egg or an ovum. Each of these gametes contributes a haploid chromosome set, so the zygote contains diploid chromosome sets. Let’s learn about gametogenesis, its definition, types, and sexual reproduction.

Define Gametogenesis

Gametogenesis refers to the production of gametes or germ cells in sexually reproducing animals. Germ cells or gametes are produced by meiosis.

Gametogenesis Diagram

Fig: Gametogenesis- Oogenesis & Spermatogenesis

What is Gametogenesis?

Gametogenesis includes spermatogenesis and oogenesis. Spermatogenesis refers to the formation of sperms or male gametes in testes. Oogenesis refers to the formation of ova in the ovaries.

Gametogenesis- Process (Spermatogenesis and Oogenesis)

Sperm

A typical human sperm consists of the haploid nucleus. Mature sperm becomes very streamlined and DNA is tightly compressed. The acrosomal vesicle or acrosome lies in front of the nucleus in the form of a cap-like structure that digests protein and complex sugars.

This complex is associated with lysing of the outer covering of the egg. The acrosome and nucleus constitute the head of the sperm. In addition, sperm has a fragile tail that helps it to swim all along the female reproductive tract to fertilize the ovum and form a diploid zygote.

Fig: Sperm

Spermatogenesis

1. This process takes place in seminiferous tubules, and these are lined by germinal epithelium, which consists of germ cells.
2. Primary germ cells are formed when the endodermal cells of the yolk sac migrate in testes which enter the process of spermatogenesis. Primary germ cells are formed when the endodermal cells of the yolk sac migrate in testes.
3. This cell has a diploid number of chromosomes like any other cell of the body.
4. The process of formation of haploid functional spermatozoa from diploid germinal cells of seminiferous tubules is called spermatogenesis.
5. This process begins during adolescence in the body and continues even in old age.
6. Spermatogenesis is a continuous process.

The process of spermatogenesis can be divided into four steps:-

1. Multiplication Phase
   a. As the name suggests, each primary germ cell enters the multiplication phase and, by repeated mitotic divisions, gives rise to many of them.
   b. The diploid-formed cells are known as spermatogonia (singular- spermatogonium) which again continues to divide to increase the number.
   c. After this multiplication, each cell enters the growth phase.
2. Growth Phase
   a. In this phase, spermatogonia continue to grow to a considerable size. This happens due to the accumulation of nutrients. The nutrients which are accumulated here are secreted from the germinal epithelium. The enlarged cells are now called primary spermatocytes.
   b. Here only the size has increased and no change in the genetic material, i.e. it remains diploid.
3. Maturation Phase
   a. In this phase, which comes after the growth phase, in which each primary spermatocyte undergoes meiosis I and  II.
   b. After meiosis I, two secondary spermatocytes are formed from one primary spermatocyte. This is reductional division. So, haploid cells are formed.
   c. Each secondary spermatocyte undergoes a second meiotic division and forms two spermatids (in total, four spermatids). Or we can say each primary spermatocyte undergoes meiosis, which includes both meiosis I and meiosis II, and forms four haploid spermatids.
   d. Meiosis II is the equational division.
4. Spermiogenesis
   a. Spermiogenesis refers to the transformation of a circular spermatid to specifically functional cells called motile sperms.
   b. In this process, condensation of the nucleus takes place. It contains only DNA and basic proteins.
   c. The nucleus becomes pointed anteriorly, oval, and flat.
   d. The nucleus of the spermatid becomes the head of the spermatozoan.
   e. The acrosomal cap is formed from the Golgi apparatus of the spermatid, which contains proteolytic enzymes.
   f. The complete structure is covered with plasma membranes.
   g. The cytoplasm is lost but only present in the tail region.
   h. Mitochondria changes to a ribbon-like structure.
   i. Centrioles lie in the neck region of the sperm.
5. Spermiation- At the end of spermatogenesis, spermiation takes place. During which the sperm head is embedded in the Sertoli cells to get nutrition and is released from there.
   a. The entire process of spermatogenesis takes \(74\) days.
   b. Only after passing through the epididymis, spermatozoa acquire some motility.

Factors Affecting Spermatogenesis

1. Gonadotropin-releasing hormone (GnRH) from the hypothalamus stimulates the anterior pituitary to secrete FSH, which stimulates the process of spermatogenesis.
2. GnRH also stimulates the LH (ICSH) secretion, which in turn stimulates the process of spermatogenesis.

Fig: Spermatogenesis

Egg Cell

The egg cell is referred to as the reproductive cell. It carries a set of chromosomes that combine with the chromosomes of sperm during fertilization in the formation of a zygote. All the material or nutrients necessary for the beginning of the growth and development of an embryo are stored inside the mature egg (the ovum).

Oogenesis

1. The production of eggs in females begins before the birth itself. Oogonia are produced during the development of the foetus, undergoing mitosis and forming primary oocytes.
2. The formation of ova in the ovary from the primordial germinal cells is said to be oogenesis.
3. Oogenesis is a discontinuous process.

The process of oogenesis can be divided into four steps:-
1. Multiplication Phase
a. As the name suggests, here the number of cells is increased by the mitosis process. The germinal epithelium cells of the ovary multiply by mitotic divisions to form small clusters of cells known as follicles.
b. Cells from the follicle differentiate to form oogonia (singular oogonium).
c. Millions of oogonia are produced with each fetal ovary. Whatever the oogonia to be used throughout life are produced before the birth of a female child.
2. Growth Phase
a. This is the second phase when oogonium grows in size about 200 times and attains maturity, and now it is called a primary oocyte.
b. It is diploid.
c. These primary oocytes are arrested at the prophase I until puberty.
3. Maturation
a. Oogenesis is arrested in the Diplotene stage of Prophase I of meiosis I and after puberty, only it will continue.
b. During the Maturation stage, the first meiotic division occurs. Primary oocyte undergoes first meiotic division to form secondary oocyte and a polar body. Each has half the number of chromosomes.
c. While cytoplasm is retained with one of the cells because it has to provide nutrition to the developing embryo, so there is the formation of a large ovum with small polar bodies.
d. The egg is released from the ovary at this secondary oocyte stage.
e. This secondary oocyte is ovulated from the Graafian follicle during ovulation.
f. This secondary oocyte is carried along the fallopian tube for fertilization.
g. The secondary oocyte divides (meiosis II) to form a large ovum and a polar body. The Polar body also divides into two to form two polar bodies, which later disintegrate.
h. However, meiosis II is triggered only when sperm enters.

Fig: Oogenesis.

Significance of Gametogenesis

Gametogenesis is a biological process that helps in the production of gametes through the process of meiosis (a reductional division). Gametes are the haploid cells that form a diploid cell called a zygote after fertilization. Zygote forms the first stage in the development of a unique entity and thus helps in the continuation of generations.

Important Points on Gametogenesis

Gametogenesis is a biological process by which diploid or haploid precursor cells undergo cell division and form mature haploid gametes. Depending on the biological life cycle of the organism, gametogenesis occurs by the meiotic division of the diploid gamete into different gametes or mitosis. For example, plants produce gametes through mitosis in gametophytes. Gametophytes grow from haploid spores after sporadic meiosis.

The existence of a multicellular, haploid stage in the life cycle between meiosis and gametogenesis is also known as the alternation of generations. Animals produce gametes through meiosis directly from diploid mother cells in organs called gonads (testes in males and ovaries in females). Males and females of a species that reproduce sexually have different forms of gametogenesis:

• Spermatogenesis (male)
• Oegenesis (female)

Summary

Gametogenesis refers to spermatogenesis and oogenesis. Former is the process of formation of sperms while later is the process of formation of egg cells. These are reproductive cells without which continuation of a species is impossible. So, it can be said that gametogenesis is the preparatory phase for the process of reproduction. It is an important process for sexual reproduction and if this process is hampered, the process of reproduction and offspring generation will be hampered in turn.

FAQs

Q.1. What are the facts of Gametogenesis?
Ans: Gametogenesis refers to the formation of gametes. It includes spermatogenesis that refers to the formation of sperms or male gametes in testes and oogenesis refers to the formation of ova in the ovaries.

Q.2. Explain Gametogenesis with a diagram.
Ans: Gametogenesis refers to the production of gametes or germ cells in sexually reproducing animals.  Germ cells or gametes are produced by meiosis. The spermatogenesis process takes place in seminiferous tubules. The process of formation of haploid functional spermatozoa from diploid germinal cells of seminiferous tubules is called spermatogenesis.

Q.3. What is Gametogenesis and explain its function?
Ans: Gametogenesis refers to the formation of germ cells. It is of two types:- spermatogenesis and oogenesis. Spermatogenesis helps in the formation of sperm while oogenesis refers to the formation of the egg. These germ cells help in the continuation of life.

Q.4. What are the two types of Gametogenesis?
Ans: The two types of gametogenesis are:-
a. Oogenesis- It refers to the process of production of egg cells.
b. Spermatogenesis- It refers to the process of production of sperms.

Q.5. What is the importance of Gametogenesis?
Ans: Gametogenesis is the process of production of gametes or germ cells, i.e. sperm and ova. These are responsible for the production of future babies. So, the continuation of life would not be possible without gametogenesis.

Q.6. Where does Gametogenesis occur?
Ans: Gametogenesis takes place in the gonads.

Q.7. Explain the Gametogenesis process.
Ans: The process of formation of haploid functional spermatozoa from diploid germinal cells of seminiferous tubules is called spermatogenesis. This process begins during adolescence in the body. It takes place in four stages, namely:
a. Multiplication phase
b. Growth phase
c. Maturation
d. Spermiogenesis
The production of eggs in females begins before the birth itself. Oogonia are produced during the development of the foetus, undergoing mitosis and forming primary oocytes. It takes place in the following stages, namely,
a. Multiplication phase
b. Growth phase
c. Maturation

Fig: Spermatogenesis

The production of eggs in females begins before the birth itself. Oogonia are produced during the development of the foetus, undergoing mitosis and forming primary oocytes. The formation of ova in the ovary from the primordial germinal cells is said to be oogenesis.

Primary oocyte undergoes first meiotic division to form secondary oocyte and a polar body. Each has half the number of chromosomes. While cytoplasm is retained with one of the cells because it has to provide nutrition to the developing embryo, so there is the formation of a large ovum with small polar bodies.

Fig: Oogenesis.

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