WHAT IS GAMETOGENESIS? PROCESS AND FUNCTIONS - BIOLOGY - 2025

The gametogenesis is the formation of gametes or sex cells in living organisms. This process allows individuals to transform and transmit some of the temporary changes in the expression of their genes, which were "induced" by external signals, to their offspring.

All individuals who have sexual reproduction regularly produce two kinds of germ cells which are called "gametes." These cells cannot develop directly like the spores of fungi, that is, they cannot always give rise, by themselves, to a new individual.

Representative schematic of female and male gametogenesis (Source: Elversberg via Wikimedia Commons)

Both types of cells, the male gamete and the female gamete, must fuse with each other in an event known as "fertilization." Only after fertilization, the cell product of this fusion, the zygote, can give rise to a new individual.

The gametes of a large number of animals are synthesized from the gonads, which are organs strictly specialized in this function. The gonads have a germinative epithelium with cells called "gonias", to which they owe their name. Gonias are morphologically the same in both sexes. However, in males these are called "spermatogonia" and in females they are called "oogonia".

Gametogenesis encompasses both spermatogenesis and oogenesis and both processes are homologous, sharing three fundamental steps for the formation of gametes.

Gametogenesis is distinguished for being a process of cell division through which the chromosomal load of an individual is reduced by half, which is possible thanks to meiotic division, where two consecutive chromosomal segregations occur.

The production of sex cells in an animal or in a plant depends on several factors, among which the differential expression of some genes that encode the "instructions" necessary both for cell divisions to occur and for changes to be triggered stand out. corresponding morphogenetic.

Male gametogenesis

Male gametogenesis is the process by which spermatogonia mature and differentiate into sperm. This is a complex process in which totipotential stem cells divide to produce daughter cells that will become sperm.

In most living things that have male gametogenesis, this does not take place until a certain developmental age. In the case of humans, it begins to occur during puberty and continues for the rest of life.

Male gametogenesis in many animals, including man, is called "spermatogenesis" and consists of three steps: mitotic proliferation, meiotic proliferation, and cell remodeling.

Process

Spermatogenesis begins with a mitosis that increases the number of spermatogonia. Spermatogonia are a population of cells that are in constant mitotic division, since they are in charge of renewing stem cells to originate sperm.

Thus, the mitotic process in male gametogenesis is crucial for the proliferation and maintenance of spermatogonia.

Some of the spermatogonia caused by mitosis increase in size to become primary spermatocytes. Each primary spermatocyte undergoes a reduction in chromosomal load through a first meiotic division (meiosis I) resulting in two secondary spermatocytes.

Secondary spermatocytes enter a second meiotic division (meiosis II), but in this no interface occurs (the chromosomal load is not divided again) so the resulting cells have the same chromosomal load, that is, they are haploid.

The resulting haploid cells are called spermatids and each can contain only the chromosomes of maternal or paternal origin or a mixture of varying proportions of the chromosomes of both parents.

Spermatids enter a process called "spermiogenesis", in which they undergo various morphological changes, condense their chromosomes, lengthen their flagellum, reduce their cytoplasmic content and, finally, become mature sperm (although maturation, in many cases, culminates while these are released in the female reproductive tract).

Features

Despite the fact that spermatogenesis occurs continuously throughout the reproductive life of an adult animal, this process has the sole purpose of producing the cells through which its genetic information will be transmitted to the offspring, which will be possible only to through sexual reproduction with a female of the same species.

In addition, this allows the males of the species to mix the genetic information of their predecessors and their own with that of the female to increase the genetic variability of the offspring.

This ability to mix genetic information helps species acquire important qualities that can help them overcome changes or unfavorable conditions in the environment in which they live.

Female gametogenesis

Female gametogenesis or oogenesis has been one of the most studied processes throughout the history of humanity. Scientists from various areas such as medicine, biology, economics, sociology and public policy, etc. have dedicated themselves to its study.

The English doctor William Harvey formulated a famous phrase about oogenesis that reads: “everything that is alive comes from the egg”.

Over the lifetime of many female animals, about 1 to 2 million oocytes are produced, but only 300 to 400 of the oocytes mature and are "ovulated." In females of many animal species, after puberty, one or more oogonia develop each month, forming mature ovules.

Process

The germ cells of the ovary, called oogonia or oogonia, increase in number through mitosis. Each resulting oogonia has the same number of chromosomes as the other somatic cells. Once the oogonia stop multiplying, they grow in size and become primary oocytes.

Before the first meiotic division occurs, the chromosomes in the primary oocytes are paired with their homologous chromosomes, half inherited from the mother and half from the father.

Representation of the menstrual cycle. 1) Menstruation; 2) Maturation of the follicle; 3) Mature follicle; 4) Corpus luteum and 5) Degradation of the corpus luteum (Source: M.Komorniczak, via Wikimedia Commons)

Then the first “reductional” or maturation division occurs, that is, the first meiosis. This division results in two cells, one cell with half the genetic material, with poor cytoplasmic content and known as the “first polar body”.

The second cell resulting from the first meiosis is large and much richer in cytoplasmic content than the polar body, in addition, this cell has the other half of the genetic content of the primary oocyte that gave rise to it. This second cell is called a “secondary oocyte”.

In the second meiotic division, the secondary oocyte and the first polar body divide, forming one large "ovotid" and three small polar bodies, respectively. The ovotid grows and transforms to give rise to a mature ovum.

The polar bodies are not functional and end up disintegrating, but their formation is necessary to discharge the egg of the “excess” of chromosomes. In turn, the uneven cytoplasmic division allows a large cell to be produced with enough reserve material for the development of a new individual.

Features

Like male gametogenesis, female gametogenesis has the ultimate goal of producing the female gamete. However, this gamete has different characteristics and functions than the male gamete.

As in the synthesis of male gametes, female gametes also mix the genetic information of the parents and the individual that produces them to transmit said information and, at the same time, increase the genetic variability of their offspring.

Despite the fact that in female gametogenesis, the primary oocytes give rise to a single functional ovum (female gamete), they have all the nutritional material to give rise to the new individual once fertilization occurs.

It is noteworthy that, in humans, for example, female gametogenesis is a continuous process from puberty, but it is finite, that is, in the fetus of a female baby, all the primary oocytes that a woman will have are formed in all their lives, which are "lost" with menstruation every month.

Gametogenesis in plants

Only in higher plants does one speak of gametogenesis proper and the process in plants is quite similar to that of animals.

The main difference is that plants have the ability to produce gametes at a late stage of development, which has not been previously predetermined, while, in animals, the formation of gametes is limited to specific regions that have been established during embryonic development.

Gamtogenesis in flowering plants (Source: Pablo damiani2 via Wikimedia Commons)

Another important feature is that although the fertility of gametes can be very frequently affected by genetic mutations, these mutations are rarely lethal to the offspring.

In higher plants the male and female gametes are the pollen grain and the ovule, respectively. Both the ovule and the pollen grain are sessile (immobile) and are found inside each of their corresponding gametophytes (which are analogous to gonads).

Female gametogenesis

In flowering plants, the ovule production sites are known as "megasporangia" and are found within an ovary that contains one or multiple ovules. Each ovule is made up of a megasporangium called a nucela and is surrounded by one or more integuments.

The integuments come together at one end to form the micropyle, an opening through which the pollen tube of the pollen grain will penetrate. Within the megasporangia, a cell known as a “megasporocyte” acts as the mother cell of the megaspore (ovule).

The megasporocyte undergoes meiosis and forms four haploid megaspores. Three of the megaspores usually disintegrate and the one furthest from the micropyle survives and becomes the megagametophyte.

In most angiosperms the developing megagametophyte produces eight nuclei. Four nuclei go to one end of the egg and the other four go to the other. A nucleus from each of the ends migrates towards the center of the ovule, these are known as "polar nuclei".

The remaining nuclei at each end form cells and one of these cells near the micropyle will develop into a mature egg cell.

The mature megagametophyte is made up of 8 nuclei in 7 different cells. This is also known as the "embryo sac", since the embryo develops inside after fertilization occurs.

Male gametogenesis

Pollen grains or microgametophytes are produced and lodged in the stamens of the flower. Each stamen has an anther and each anther usually has four microsporangia, which are known as pollen sacs.

Inside each pollen sac are the stem cells of microspores, that is, of pollen grains. All stem cells undergo a meiotic process, and four haploid microspores are produced from each stem cell.

Microspores grow and develop into an immature pollen grain. These immature pollen grains have a cell from which the "pollen tube" arises and a generative cell, which will produce two sperm cells.

Before pollen is released from the anther, it develops an outer protective shell of a protein called exin and an inner protective shell of another protein, intin. Many species of plants can be identified through the pattern that develops on the inner covering of pollen grains.

The final development of the pollen grain occurs with the “germination” of the pollen tube, this only occurs after the pollen grain is deposited on the stigma of the flower that will subsequently pollinate.

References

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