Gametogenesis

Chromosomas X (red) and chromosomes Y (green) in embryonic stem cells of male mice (X / Y) and females (X / X).

Gametogenesis is the formation of so-called gametes through meiosis from germ cells. Through this process, the genetic content in germ cells is reduced from diploid (2n, double) to haploid (n, single), that is, to half the number of chromosomes contained in a normal cell of the species in question.. In the case of men; the process that aims to produce spermatozoa and is called spermatogenesis, taking place in the testicles and in the case of women, the result is oocytes, called oogenesis and takes place in the ovaries.

This process takes place in two chromosomal and cytoplasmic divisions, called the first and second meiotic divisions or simply meiosis I and meiosis II. Both comprise prophase, metaphase, anaphase, and telophase.

During meiosis I, the members of each homologous pair of chromosomes first unite and then separate with the mitotic spindle and are distributed at different poles of the cell. In meiosis II, the sister chromatids that make up each chromosome separate and are distributed in the nuclei of new cells. Between these two successive phases there is no S phase (DNA replication).

Meiosis is not a perfect process, sometimes errors in mitosis are responsible for the main chromosomal abnormalities. Meiosis manages to keep the number of chromosomes in the cells of the species constant to maintain genetic information.

In general, members of a pair of chromosomes are not in close proximity either in the resting cell or during mitotic division. The only time they come into intimate contact is during meiotic or maturational divisions of germ cells.

Oogenesis

Oogenesis is the process of formation of the female gametes. It takes place in the ovaries. The oogonia are located in the ovarian follicles, they grow and have modifications; these lead to the first meiotic division that results in a primary oocyte (containing most of the cytoplasm) and a first polar body (its role is to carry half of the total chromosomes of the species). The two resulting cells carry out meiosis II, from the oocyte this takes between 1 and 2 weeks, later a large cell (which has most of the cytoplasm) and a second polar corpuscle are formed, these disintegrate rapidly, while the cell large develops into female gametes called ova. The female gamete is stuck in meiosis II, specifically in Metaphase II.

Oogenesis has several and/or different phases that are

• Proliferation: during embryonic development, the germ cells of ovaries suffer from mitosis to originate from ovogones.
• Growth: In puberty they grow to originate the first order oocytes.
• Maduration: the first order ovocyte suffers from meiosis.

Oogenesis begins before birth and is completed during the female's reproductive life, when fertilization occurs.

Spermatogenesis

Spermatogenesis is the process of producing male gametes (spermatozoa) that are produced in the testicles, specifically in the seminiferous tubules. Within this, the following processes stand out:

• Proliferation: the germ cells of the testicles suffer mitosis so that the amount of sperm is wide.
• Growth: At this stage the cell suffers an interphase, growing and doubling the amount of DNA, transforming into sperm1.
• Maduration: Here sperm 1 suffer two consecutive cell divisions. Of the first meiosis are sperm 2, of a haploid cell condition, and of the second, sperms.
• Difference: The sperms, which are already haploids and simple chromosomes, are generated by the scourge and acrosome. These sperms after their transformation are called a wait.

In spermatogenesis, four sperm cells are produced for each germ cell.

Gonads

Also called sexual sexual organs Primary glands function as mixed glands to the extent that hormones and gametes are produced. The secondary sexual organs are those structures that mature at puberty and are essential in the care and transport of gametes, they are traits that are considered sexual attraction.

• Them testicles They are two oval structures that are suspended within the scrotum by spermatic cords, are those that produce semen and testicular fluid; their endocrine function is to release male hormOnas such as testosterone, who participate in maintaining male sexual characters.

• Them ovaries They are two almond-shaped organs, located at the ends of the fallopian tubes, ovaries are formed approximately when the female fetus is 3 months old and when the woman enters puberty the eggs are developing. Its endocrine function is to release hormones such as progesterone and estrogen, which will intervene in the ovarian cycle.

Role of hormones in spermatogenesis

Testosterone is the main male hormone and is synthesized by a group of cells called Leydig cells. This hormone promotes spermatogenesis or, in cases of abundance, inhibits it. The hypothalamus secretes gonadotropin-releasing factor (GnRH), which stimulates the anterior pituitary to release luteinizing hormone in the ovaries that stimulates the formation of the corpus luteum, the ovulation and the synthesis of oestradiol and progesterone. Follicle-stimulating hormone (FSH) in the ovaries stimulates follicular growth and spermatogenesis in the testicles.

Differences between spermatogenesis and oogenesis

Permatogenesis

• It is done in the testicles.
• It happens from sperm.
• Each sperm gives birth to four sperm.
• In meiosis the material is divided equally.
• Sperms occur throughout your life.
• It occurs in man.
• Of a sperm I, 4 functional sperms are formed.

Ovogenesis

• It is done in the ovaries.
• It happens from the ovogonia.
• Each ovogonia gives origin to an ovocyte II which only in the event of being fertilized will be called egg and 2 polar bodies I and a polar body II (only in case of fertilization).

Ovocito during Meiosis II.

• Meiosis I does not divide cytoplasm alike, leaving a daughter cell (ovocyte II) with almost all cytoplasm.
• Women are born with a certain number of follicles, approximately 400,000.
• It occurs in the woman.
• Of some oocytes II, a functional egg is formed.

Similarities between spermatogenesis and oogenesis

• Both are gametogenesis subprocesses.
• They both produce gametes.
• Meiosis occurs in both.
• Both are processes of sexual reproduction in mammals.
• Both processes occur within the gonads.
• Both begin their phases from meiosis.

Comparison between eggs and spermatozoa

Ovules

• Bigger than sperm.
• It has vitelo (nutritive reserve).
• There's no movement.
• Serve only one of each germ cell.
• It occurs in the ovary.

Permatozoid

• Small compared to oocyte II.
• It has no nutritional reserves.
• He moves through his scourge.
• They serve four of each germ cell.
• It occurs in the testicle.

In Vitro Gametogenesis or IVG

In vitro gametogenesis (IVG) is a procedure that allows artificially obtaining functional gametes (sperm and eggs) from differentiated cells of the adult individual, such as skin cells.

This technique consists of taking differentiated cells from the skin of an adult and reprogramming them to behave like pluripotent embryonic stem cells or iPSCs (Induced Pluripotent Stem Cell). Subsequently, they are stimulated to differentiate into ovules and spermatozoa, which, in turn, are used in the in vitro generation of an embryo that will be implanted in a healthy adult uterus.

Already in 1962 John Bertrand Gurdon demonstrated that differentiated cells maintained pluripotency and, in 2006, Shinya Yamanaka discovered the existence of four genes involved in differentiation capacity: Oct3/4, Sox2, Kfl4 and C-Myc. Once the cell has differentiated, the expression of these genes is inhibited (it loses the ability to re-differentiate). However, reactivation of such genes can cause differentiated adult somatic cells to dedifferentiate into induced pluripotent stem cells, or iPSCs.

In recognition of their contribution to science, both scientists received the Nobel Prize in 2012 and, although to date it has only been carried out in mice, it is expected that in the future the technique will be able to be carried out in humans, with all the ethical-legal implications that this entails.

Experts describe this technology as promising and say it has the potential to improve current IVF techniques. In addition, if it can be carried out in humans, the applications would be very numerous. On the one hand, it would facilitate the process of obtaining samples and would make it possible to dispense with surgical interventions in those cases where the extraction of gametes is complicated. On the other hand, it is a good alternative for couples unable to produce gametes but who wish to have their own children. Likewise, it would prevent women from having to expose themselves to high doses of drugs when it comes to retrieving the eggs necessary for in vitro fertilization, something that frequently happens today.

Although the potential benefits of this technique are many, the biggest obstacle it currently faces are ethical committees, since it would also allow same-sex couples to have biological children and single women to conceive without the need for a male donor..