Gametogenesis – Definition, Types, Steps

What is gametogenesis?

• Gametogenesis, derived from the Greek words “gametos” meaning marriage and “genesis” meaning origin, is the process of creating gametes in sexually reproducing organisms. It involves the formation of specialized cells called gametes, which are responsible for sexual reproduction.
• In animals, the body contains two types of cells: somatic cells and germinal cells. Somatic cells undergo mitotic division and provide support for the development and maturation of germinal cells. The germinal cells, found in the gonads (testes in males and ovaries in females), have the ability to produce gametes.
• The process of gametogenesis involves a combination of mitotic and meiotic divisions. Mitotic divisions allow the germinal cells to proliferate, while meiotic divisions result in the production of haploid gametes. Meiosis is a specialized type of cell division that reduces the number of chromosomes in the resulting cells by half.
• In males, gametogenesis is known as spermatogenesis. It occurs in the testes and leads to the production of spermatozoa or sperm. Spermatogenesis involves multiple mitotic divisions and cell growth of precursor germ cells. These cells then undergo two rounds of meiotic division (meiosis I and II), resulting in the formation of haploid daughter cells. Finally, these daughter cells differentiate into functional spermatozoa.
• In females, the process of gametogenesis is called oogenesis. It takes place in the ovaries and results in the formation of ova or eggs. Oogenesis begins during fetal development, where a pool of germinal cells undergoes mitotic divisions. However, the process is paused at prophase I of meiosis until puberty. After puberty, a small number of these cells resume development and become primary oocytes. Each month, one primary oocyte completes meiosis I, resulting in the formation of a secondary oocyte and a polar body. The secondary oocyte then arrests at metaphase II until fertilization occurs. If fertilization takes place, the secondary oocyte completes meiosis II, resulting in the formation of a mature ovum and another polar body.
• Spermatogenesis and oogenesis are distinct processes that ensure the production of functional gametes in males and females, respectively. These gametes, sperm and eggs, carry half the genetic material of the organism and are crucial for sexual reproduction and the creation of genetically diverse offspring.

Gametogenesis Definition

Gametogenesis is the biological process through which precursor cells undergo cell division and differentiation to form mature sex cells or gametes.

Spermatogenesis

• Spermatogenesis takes place in the male gonads or testes. The testes of vertebrates are formed of many seminiferous tubules bordered by germinal epithelial cells.
• Sperms are produced by the cells of the germinal epithelium during the process of spermatogenesis. But in certain animals, such as mammals and Mollusca, etc., there are somatic cells known as Sertoli cells located between germinal cells.
• The Sertoli cells offer nutrients to the growing sperm and anchor the differentiating cells. The insects are devoid of Sertoli cells.
• Spermatogenesis is a continuous process that can be investigated in two distinct stages for the sake of convenience. 1. spermatid formation; 2. spermiogenesis.

1. Formation of Spermatids 

Primary germinal cells or primordial cells refer to the male germinal cells that create sperm. The primordial cells undergo the following three steps in order to create spermatids:

(i) Multiplication phase

• The primordial cells or undifferentiated germ cells have big, chromatin-rich nuclei.
• These cells multiply by multiple mitotic divisions to form spermatogonia (Greek: sperma = sperm or seed; gone = offspring).
• Each spermatogonium is diploid and contains twice as many chromosomes as a gamete.

(ii) The growth phase

• During the development phase, spermatogonial cells amass an abundance of nutrients and chromatin.
• Currently, every spermatogonial cell is referred to as a primary spermatocyte.

(iii) The maturation phase

• Primary spermatocytes are prepared to undergo their initial meiotic or maturation division. The homologous chromosomes begin pairing (synapsis), each homologous chromosome divides longitudinally, and the exchange of genetic material or crossing over occurs between the chromatids of the homologous chromosomes through the creation of chiasmas.
• The DNA quantity is replicated at the start of the division. Two secondary spermatocytes are created through the first meiotic or homotypic division.
• Each secondary spermatocyte consists of x chromosomes and is haploid. Each secondary spermatocyte undergoes a second maturation, second meiotic division, or heterotypic division and generates two spermatids.
• Thus, a meiotic or maturation division creates four haploid spermatids from a diploid spermatogonium.
• These spermatids cannot directly function as gametes, so they must undergo the subsequent process, spermiogenesis.

2. Spermiogenesis 

Spermiogenesis refers to the transformation or differentiation of spermatids into sperms. Because the sperm or spermatozoon is a highly active and mobile cell, the excess material of developing sperms is eliminated in order to maximise the sperm’s mobility. For the lowering of sperm weight, the spermatids undergo the following changes:

(i) Changes in the nucleus

• In many animals, the nucleus loses water from the nuclear sap, shrinks, and acquires diverse forms.
• In humans and bulls, the sperm nucleus becomes spherical and laterally flattened. The sperm nucleus of rats and amphibians is scimitar-shaped with a pointed tip.
• In birds and mollusks, the nucleus becomes corkscrew-like spirally twisted. In bivalve mollusks, the sperm nucleus is spherical.
• The form of the nucleus also dictates the shape of the sperm head, which is optimised for active propulsion through water.
• The RNA content of the nucleus and nucleolus is drastically diminished.
• The DNA gets more condensed, while the chromatin material shrinks in volume.

(ii) Acrosome formation

• The acrosome is located on the anterior side of the sperm nucleus and includes protease enzymes that facilitate the sperm’s entry of the egg. The Golgi apparatus generates the acrosome.
• To produce the acrosome, the Golgi apparatus is concentrated near the front end of the sperm nucleus.
• One or two vaculoes of the Golgi apparatus grow in size and occupy the space between the tubules. Within the vacuole, a thick granule known as the proacrosomal granule develops shortly after.
• Leblond (1955) discovered granule-rich pro acrosomal mucopolysaccharides. The pro acrosomal granule binds to the front end of the nucleus and grows to become the acrosome.
• The membranes of Golgi vacuoles create the double membrane (unit membrane of lipoprotein) sheath and cap-like structure of spermatozoa around the acrosome.
• During Golgi rest, the remainder of the Golgi apparatus is diminished and eliminated from the sperm.
• Between the acrosome and the nucleus, an acrosomal cone or axial body develops in the sperm of certain animals.

(iii) The centrioles

• Behind the nucleus, the two centrioles of the spermatids are placed one after the other.
• The front one is referred to as the proximal centriole, whereas the posterior one is referred to as the distal centriole.
• The distal centriole transforms into basal bodies and gives rise to the sperm’s axial filament.
• The axial filament or flagellum is composed of two longitudinal fibres in the centre and nine fibres in the periphery.
• In the middle portion of spermatozoa, the distal centriole and the base of the axial filament can be found.
• The mitochondria of the spermatids fuse and wind around the axial filament in a spiral pattern.
• During spermiogenesis, the majority of the cytoplasmic portion of the spermatid, excluding the nucleus, acrosome, centriole, mitochondria, and axial filament, is discarded.

Oogenesis

Oogenesis occurs in the primordial germinal cells of the ovary’s germinal epithelium, where the process of oogenesis occurs. The oogenesis consists of the three sequential stages listed below: 1. phase of multiplication; 2. phase of growth; 3. phase of maturation

1. Multiplication Phase 

• The primordial germinal cells divide repeatedly to generate the oogonia (Gr., oon=egg).
• The oogonia multiply by the mitotic divisions and generate the primary oocytes which transit through the growth phase.

2. Growth Phase 

• The oogenesis growth phase is significantly longer than the spermatogenesis growth phase.
• During the development phase, the primary oocyte expands tremendously. Initially, the primary oocyte of the frog has a diameter of approximately 50 mm, but following the growth phase, the diameter of the mature egg increases to between 1000mm and 2000mm.
• In the main oocyte, a substantial quantity of lipids and proteins accumulate in the form of yolk, which, due to its high weight (or gravity), is often concentrated at the base of the egg to create the vegetal pole.
• Typically, the portion of the cytoplasm holding the egg pronucleus remains distinct from the yolk and is located at the egg’s animal pole.
• The oocyte’s cytoplasm becomes abundant in RNA, DNA, ATP, and enzymes. In addition, the mitochondria, Golgi apparatus, ribosomes, etc., become concentrated in the oocyte’s cytoplasm.
• In certain oocytes (those of amphibians and birds), mitochondria aggregate in the cytoplasm of the oocyte to create mitochondrial clouds.
• During the development phase, the nucleus of the primary oocyte undergoes enormous modifications.
• The enlarged nucleus caused by a rise in nucleoplasm is known as a germinal vesicle.
• Due to the excessive synthesis of ribosomal RNA by rDNA in the nucleolar organiser region of chromosomes, the nucleolus grows enlarged or its number increases.
• Due to the synthesis of ribosomal RNA, the nucleus or germinal vesicle of the primary oocyte of Triturus contains 600 nucleoli, that of Siredon has 1000 nucleoli, and that of Xenopus has 600 to 1200 nucleoli.
• The transformation of the chromosomes into large lampbrush chromosomes is directly associated with an increase in the transcription of mRNA molecules and an active protein synthesis in the cytoplasm.
• When the primary oocyte’s cytoplasm and nucleus have finished growing, it is ready for the maturation phase.

3. Maturation Phase

• The maturation phase is accompanied by meiotic division or maturation. The maturation division of the primary oocyte is drastically different from that of the spermatocyte.
• Here, following the meiotic division of the nucleus, the cytoplasm of the oocyte divides unequally to produce one giant haploid egg and three little haploid polar bodies or polocytes.
• The relevance of this form of uneven split for the egg is significant. If the main oocyte could have undergone equal divisions, the stored nourishment would have been divided evenly to the four daughter cells, which could have been insufficient for the developing embryo.
• Therefore, these unequal divisions allow one of the four daughter cells to contain an adequate amount of cytoplasm and food reserves for the developing embryo.

(i) First maturation division

• During the first maturation division or meiosis, the primary oocyte nucleus’ homologous chromosomes undergo pairing or synapsis, duplication, chiasm formation, and crossing over.
• As soon as the nuclear membrane ruptures, bivalent chromosomes shift towards opposing poles due to contraction of chromosomal fibres.
• The endoplasmic reticulum generates a new nuclear sheath surrounding the daughter chromosomes.
• After karyokinesis, uneven cytokinesis occurs, resulting in the formation of a small haploid polar body or polocyte and a large haploid secondary oocyte or ootid.

(ii) Second meiotic division

• The secondary haploid oocyte and the first polocyte undergo the second meiotic division.
• The secondary oocyte undergoes a second meiotic division, which results in the formation of a mature egg and a second polocyte.
• First polocyte differentiates into two secondary polocytes during the second meiotic division. While the haploid egg cell prepares for fertilisation, these polocytes ooze out of the egg and degenerate.

Structure of Mature Egg 

The mature egg has a cell-like structure and composed of the following parts: 

1. Plasma membrane

• The developed egg is coated by the unit membrane, a plasma membrane.
• It consists of an outer and an inner protein layer.
• Both layers have a thickness of 50Ao.
• Between the protein layers is a 60Ao thick layer of lipodous material.

2. Primary egg membranes

• In addition to the plasma membrane, the eggs of most animals except the sponges and certain coelenterates consist of various other extra egg membranes.
• The first and second membranes of an egg, respectively.
• The major egg membrane is secreted around the plasma membrane by the oocyte itself.
• The principal egg membrane is called the vitelline membrane in insects, mollusks, amphibians, and birds but the chorion in tunicates and fishes.
• The equivalent membrane in mammalian eggs is called the zona pellucida.
• There are mucoproteins and fibrous proteins in the vitelline membrane.
• Later in development, a void, called the perivitelline space, forms between the plasma membrane and the vitelline membrane, despite the vitelline membrane’s typical close adhesion to the plasma membrane.

3. Secondary egg membranes

• Around the primary egg membranes, the ovarian tissues secrete the secondary egg membranes.
• Amphibians’ exoskeletons are made of jelly, while insects’, ascidians’, and cyclostomes’ are made of chitin.

4. Tertiary egg membranes

• The membranes of the tertiary eggs are created by the oviduct or other female reproductive organs.
• Amphibians have jelly coverings, elasmobranch fishes have a hard horny capsule, and birds have albumen, shell membranes, and a calcareous shell.

5. The ooplasm

• Ooplasm refers to the cellular material found inside an egg. The ooplasm stores a lot of energy in the form of yolk, which can be accessed in case of emergency.
• In addition to a lipoprotein and pigment granules, water, RNA, ribosomes, mitochondria, and other cellular inclusions round out its make-up.
• Numerous microvilli and cortical granules are seen in the cortex, the outermost layer of the ooplasm.
• The microvilli, which form when the plasma membrane pushes outward, aid in the transfer of nutrients and other substances from the follicle cells to the developing egg.
• Cortical granules are spherical entities of varying sizes. Comparatively, frog eggs are just 2.0 m in length, while sea urchin eggs are 8.0 m.
• The Golgi apparatus is the source of the granules found in the cortex, which are encased in unit membranes.
• Eggs from sea urchins, frogs, fish, bivalve mollusks, some annelids, and a few mammals have been found to contain homogenous and granular acid mucopolysaccharides.
• Human, rodent, guinea pig, snail, urodele amphibian, insect, and avian ova, on the other hand, do not have them.

Yolk contents of the ooplasm

Ooplasm might include more or less yolk depending on the species. Various types of egg cells have been identified based on the amount of yolk present.

• Microlecithal: Microlecithal refers to eggs that have an extremely small amount of yolk, such as those laid by eutherian mammals like the Amphioxus.
• Mesolecithal: Ova and eggs with a medium amount of yolk, such as those of the Peteromyzon, Dipnoi, and Amphibia, are called mesolecithal.
• Macrolecithal: Myxine, cartilaginous and bony fishes, reptiles, birds, and Monotremata all lay eggs that are considered macrolecithal because they contain a disproportionately large amount of yolk. In addition, the distribution of the yolk inside the ooplasm allows for a further categorization of the eggs into two distinct categories.
  1. Homolecithal: Eggs of echinoderms and other homolecithal animals have yolk contents that are spread uniformly throughout the ooplasm.
  2. Heterolecithal: Heterolecithal eggs have a yolk that is not uniformly distributed throughout the ooplasm. There are a few different forms of heterolecithal eggs.
     1. Telolecithal:  Telolecithal refers to the condition in which the egg’s yolk is concentrated in one half, creating the vegetative pole. Eggs can range from being mildly telolecithal (as in amphibians) to extremely so (e.g., hen and other birds). The yolk of macrolecithal and very telolecithal eggs is extremely abundant, and it takes up almost the whole egg, save for a little disc of cytoplasm. Eggs from fish, reptiles, and birds all include a germinal disc in their cytoplasm that contains the zygote nucleus.
     2. Centrolecithal: Eggs that are centrolecithal have the yolk concentrated in the middle of the ooplasm, such as bug eggs.

What is the major difference between male and female gametogenesis?

• Definition: Male gametogenesis is the creation and development of sperm or sperm cells, whereas female gametogenesis is the production and development of egg cells or gametes. This is therefore the primary distinction between male and female gametogenesis.
• Size of the Gametes: In addition, male gametogenesis generates small cells, whereas female gametogenesis generates huge cells.
• Motility: Motility is a further distinction between male and female gametogenesis, as male gametogenesis typically generates motile cells whereas female gametogenesis generates non-motile cells.
• Food Reservation: In addition, the sperm cells produced by male gametogenesis have food saved, but the egg cells produced by female gametogenesis do not.
• In Animals: Male gametogenesis in animals is known as spermatogenesis, whereas female gametogenesis is known as oogenesis.
• Occurrence in Animals: Male gametogenesis happens in the testis, while female gametogenesis takes place in the ovary. Consequently, this is yet another significant distinction between male and female gametogenesis.
• Precursor Cells in Animals: Spermatogonia are the precursor cells of male gametogenesis, whereas oogonia are the precursor cells of female gametogenesis in animals.
• Continuity : Male gametogenesis in animals is a continuous process, whereas female gametogenesis is not.
• Growth Phase: In animals, male gametogenesis has a brief growth period, whereas female gametogenesis has a lengthy growth phase.
• Total Gametes Produced: In addition, male gametogenesis produces four sperm cells per precursor cell, whereas female gametogenesis produces one ovum per precursor cell.
• Nuclear Condensation: The nucleus of sperm cells undergoes nuclear condensation, whereas the nucleus of egg cells does not.
• Occurrence in Plants: Another distinction between male and female gametogenesis is that male gametogenesis happens in the antheridia of plants whereas female gametogenesis occurs in the archegonia.
• Production of Gametes: In male gametogenesis, gametes are produced within the pollen tube, whereas in female gametogenesis, gametes are produced within the embryo sac of the ovule.
• Number of Gametes Produced: Additionally, male gametogenesis generates two sperm cells, whereas female gametogenesis generates a single egg cell.

Difference between gametogenesis and spermatogenesis

Aspect	Gametogenesis	Spermatogenesis
Definition	The biological process of forming mature sex cells	The specific process of forming mature spermatozoa
Occurrence	Found in both males and females	Exclusive to males
Process	Includes both oogenesis and spermatogenesis	Specifically refers to the process of sperm production
Result	Formation of both ova (eggs) and spermatozoa (sperm)	Formation of spermatozoa (sperm)
Location	Takes place in the gonads (testes and ovaries)	Specifically occurs in the testes
Cell Division	Involves both mitotic and meiotic divisions	Involves mitotic divisions followed by meiotic divisions
Chromosome	Reduces the chromosome number to half (haploid)	Results in the production of haploid sperm cells
Cell Differentiation	Results in the formation of ova (eggs) with cytoplasmic changes	Results in the formation of highly motile spermatozoa
Hormonal Regulation	Regulated by various hormones such as FSH and LH	Primarily regulated by follicle-stimulating hormone (FSH) and testosterone

References

• https://ib.bioninja.com.au/higher-level/topic-11-animal-physiology/114-sexual-reproduction/gametogenesis.html
• https://flexbooks.ck12.org/cbook/ck-12-biology-flexbook-2.0/section/2.37/primary/lesson/gametogenesis-bio/
• https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/43%3A_Animal_Reproduction_and_Development/43.03%3A_Human_Reproductive_Anatomy_and_Gametogenesis/43.3C%3A__Gametogenesis_(Spermatogenesis_and_Oogenesis)
• https://teachmephysiology.com/reproductive-system/embryology/gametogenesis/
• https://courses.lumenlearning.com/wm-biology2/chapter/gametogenesis/
• https://www.onlinebiologynotes.com/gametogenesis-spermatogenesis-and-oogenesis-in-human/