SPERMATOGENESIS: STAGES AND THEIR CHARACTERISTICS - BIOLOGY - 2024

The spermatogenesis is a process of formation of spermatozoa from germ cells (spermatogonia). It occurs in male individuals of eukaryotic organisms with sexual reproduction.

For this process to be carried out efficiently, it needs specific conditions, including: a correct chromosomal division with precise gene expressions and an adequate hormonal medium, to produce a high number of functional cells.

Source: Anchor207

The transformation of spermatogonia to mature gametes occurs during sexual maturation in organisms. This process is triggered due to the accumulation of certain hormones of the pituitary gonadotropin type, such as HCG (human chorionic gonadotropin) that intervenes in the production of testosterone.

What is spermatogenesis?

Spermatogenesis consists of the formation of male gametes: sperm.

The production of these sex cells begins in the seminiferous tubules, located in the testes. These tubules occupy about 85% of the total volume of the gonads and in them are the immature germ cells or spermatogonia that continually divide by mitosis.

Some of these spermatogonia stop reproducing and become primary spermatocytes, which begin the process of meiosis to each produce a pair of secondary spermatocytes with their full chromosomal load.

The latter complete the second stage of meiosis, finally giving rise to four spermatids with half the chromosomal load (haploid).

Later they undergo morphological changes, generating sperm, which go to the epididymis located in the scrotum next to the testicles. In this duct the maturation of the gametes occurs that are ready to transmit the individual's genes.

The spermatogenesis process depends on hormonal and genetic regulation. This process is testosterone-dependent, so specialized cells (Leydig cells) are found in the seminiferous tubules in the production of this hormone.

Genetic elements involved

Some important genes in spermatogenesis are the SF-1 gene, which acts in the differentiation of Leydig cells, and the SRY gene, which intercedes in the differentiation of Sertoli cells and the formation of testicular cords. Other genes are involved in regulating this process: RBMY, DBY, USP9Y, and DAZ.

The latter is found on the Y chromosome. It acts in the coding of RNA binding proteins and its absence is linked to infertility in some individuals.

Stages and their characteristics

Seminiferous tubules with mature sperm. Nephron

The primordial germ cells (gonocytes) are formed in the yolk sac and move to the genital crest, dividing between the Sertoli cells, thus forming the seminiferous tubules. The gonocytes are found inside, from where they migrate towards the basement membrane to give rise to the spermatogonia.

Proliferation of the primordial germ cells and the formation of spermatogonia occur during the embryonic development of the individual. Shortly after birth, the mitotic division of these cells stops.

The process by which mature sperm are produced is divided into three phases: spermatogon, spermatocyte, and sperm.

1. Spermatogon phase

As the period of sexual maturity of individuals approaches, an increase in testosterone levels activates the proliferation of spermatogonia. These germ cells divide to generate a series of spermatogonia that differentiate into primary spermatocytes.

In humans, several morphological types of spermatogonia are distinguished:

Spermatogonia Ad: Located next to the interstitial cells of the seminiferous tubule. They suffer mitotic divisions that generate an Ad-type pair that in turn continues to divide, or an Ap-type pair.

Ap spermatogonia: These follow the differentiation process to generate sperm, dividing consecutively by mitosis.

Spermatogonia B. Product of the mitotic division of Ap spermatogonia. They have a spheroidal nucleus and the peculiarity of being connected to each other by “cytoplasmic bridges”.

They form a kind of syncytium that persists in the subsequent stages, separating in sperm differentiation, as sperm are released into the lumen of the seminiferous tubule.

The cytoplasmic union between these cells allows a synchronized development of each pair of spermatogonia and that each one obtains the complete genetic information necessary for their functioning, since even after meiosis, these cells continue to develop.

2. Spermatocytic phase

In this phase, the B spermatogonia have divided mitotically, forming the I (primary) spermatocytes that duplicate their chromosomes, so that each cell carries two sets of chromosomes, carrying twice the usual amount of genetic information.

Subsequently, meiotic divisions of these spermatocytes are carried out, so the genetic material in them undergoes reductions until reaching the haploid character.

Mitosis I

In the first meiotic division, the chromosomes are condensed in the prophase, resulting, in the case of humans, 44 autosomes and two chromosomes (one X and one Y), each with a set of chromatids.

Homologous chromosomes couple to each other while lining up on the equatorial plate of the metaphase. These arrangements are called tetrads as they contain two pairs of chromatids.

Tetrads exchange genetic material (crossing-over) with the chromatids rearranging into a structure called the synaptonemic complex.

In this process, genetic diversification occurs by exchanging information between the homologous chromosomes inherited from the father and the mother, ensuring that all the spermatids produced from the spermatocytes are different.

At the end of the crossing-over, the chromosomes separate, moving to opposite poles of the meiotic spindle, “dissolving” the structure of the tetrads, the recombined chromatids of each chromosome remaining together.

Another way to guarantee genetic diversity with respect to the parents is by the random distribution of the chromosomes derived from the father and mother towards the poles of the spindle. At the end of this meiotic division, II (secondary) spermatocytes are produced.

Meiosis II

Secondary spermatocytes begin the second meiosis process immediately after being formed, synthesizing new DNA. As a result of this, each spermatocyte has half the chromosome load and each chromosome has a pair of sister chromatids with duplicated DNA.

At metaphase, the chromosomes are distributed and aligned on the equatorial plate, and the chromatids separate by migrating to opposite sides of the meiotic spindle.

After rebuilding the nuclear membranes, haploid spermatids are obtained with half the chromosomes (23 in humans), a chromatid and a copy of the genetic information (DNA).

3. Sperm phase

Spermiogenesis is the last phase of the spermatogenesis process, and cell divisions do not occur in it, but morphological and metabolic changes that allow cell differentiation to haploid mature sperm.

Cellular changes occur while spermatids are attached to the plasma membrane of Sertoli cells, and can be described in four phases:

Golgi phase

It is the process by which the Golgi apparatus gives rise to the acrosome, by the accumulation of proacrosomic granules or PAS (periodic acid-Schiff's reactive) in the Golgi complexes.

These granules lead to an acrosomal vesicle located next to the nucleus and its position determines the anterior portion of the sperm.

The centrioles move towards the posterior portion of the spermatid, aligning themselves perpendicularly with the plasma membrane and make the doublets that will integrate the microtubules of the axoneme at the base of the sperm flagellum.

Cap phase

The acrosomal vesicle grows and extends over the anterior portion of the nucleus, forming the acrosome or acrosomal cap. In this phase, the nuclear content is condensed and the part of the nucleus that remains under the acrosome thickens, losing its pores.

Acrosome phase

The nucleus elongates from round to elliptical, and the flagellum is oriented so that its anterior end attaches to the Sertoli cells pointing toward the basal lamina of the seminiferous tubules, within which the developing flagellum extends.

The cytoplasm moves posterior to the cell and the cytoplasmic microtubules accumulate in a cylindrical sheath (manchette) that runs from the acrosomal cap to the posterior portion of the spermatid.

After developing the flagellum, the centrioles move back towards the nucleus, adhering to a groove in the posterior portion of the nucleus, from where nine thick fibers emerge that reach the microtubules of the axoneme; in this way the nucleus and flagellum are connected. This structure is known as the neck region.

The mitochondria move towards the posterior region of the neck, surrounding the thick fibers and are arranged in a tight helical sheath forming the intermediate region of the sperm tail. The cytoplasm moves to cover the flagellum already formed, and the "manchette" dissolves.

Ripening phase

The excess cytoplasm is phagocytosed by the Sertoli cells, forming the residual body. The cytoplasmic bridge that was formed in the B spermatogonia remains in the residual bodies, so the spermatids are separated.

Finally, the spermatids are released from the Sertoli cells, releasing into the lumen of the seminiferous tubule from where they are transported through the straight tubes, rete testis and efferent canals to the epididymis.

Hormonal regulation

Spermatogenesis is a process finely regulated by hormones, primarily testosterone. In humans, the entire process is triggered in sexual maturation, by the release in the hypothalamus of the hormone GnRH that activates the production and accumulation of pituitary gonodotropins (LH, FSH and HCG).

Sertoli cells synthesize testosterone transporter proteins (ABP) by stimulation of FSH, and together with the testosterone released by Leydig cells (stimulated by LH), they ensure a high concentration of this hormone in the seminiferous tubules.

In Sertoli cells, estradiol is also synthesized, which is involved in the regulation of Leydig cell activity.

Fertilization

The epididymis connects with the vas deferens that flow into the urethra, finally allowing the exit of sperm that later seek an egg to fertilize, completing the cycle of sexual reproduction.

Once released, sperm can die in a matter of minutes or hours, having to find the female gamete before this happens.

In humans, about 300 million sperm are released in each ejaculation during intercourse, but only about 200 survive until they reach the region where they can mate.

Sperm must undergo a training process in the female reproductive tract where they acquire greater mobility of the flagellum and prepare the cell for the acrosome reaction. These characteristics are necessary to fertilize the eggs.

Sperm capacitation

Among the changes that sperm present, biochemical and functional modifications stand out, such as hyperpolarization of the plasma membrane, increased cytosolic pH, changes in lipids and proteins, and the activation of membrane receptors allowing them to be recognized by the zona pellucida. to join this.

This region works as a chemical barrier to avoid the crossing between species, since by not recognizing specific receptors, fertilization does not take place.

Eggs have a layer of granular cells and are surrounded by high concentrations of hyaluronic acid that form an extracellular matrix. To penetrate this layer of cells, sperm have hyaluronidase enzymes.

Upon coming into contact with the zona pellucida, the acrosome reaction is triggered, in which the contents of the acrosomal cap are released (as hydrolytic enzymes), which help the sperm to cross the region and join the plasma membrane of the ovum, releasing within it its cytoplasmic content, organelles and nucleus.

Cortical reaction

In some organisms, a depolarization of the plasma membrane of the ovule occurs when it comes into contact with a sperm, thus preventing more than one from fertilizing it.

Another mechanism to prevent polyspermia is the cortical reaction, where enzymes are released that change the structure of the zona pellucida, inhibiting the glycoprotein ZP3 and activating ZP2, making this region impenetrable for other sperm.

Characteristics of sperm

Male gametes have characteristics that make them very different from female gametes and highly adapted to spread the individual's genes to subsequent generations.

In contrast to ovules, sperm are the smallest cells present in the body and present a flagellum that allows them to move in order to reach the female gamete (which does not have such mobility) to fertilize it. This flagellum consists of a neck, intermediate region, main region and the terminal region.

In the neck are the centrioles, and in the intermediate region the mitochondria are located, which are responsible for providing the energy necessary for their mobility.

In general, sperm production is very high, these being very competitive among them since only around 25% will actually be able to fertilize a female gamete.

Differences between spermatogenesis and oogenesis

Spermatogenesis has characteristics that differentiate it from oogenesis:

-Cells make meiosis continuously since the sexual maturation of the individual, each cell producing four mature gametes instead of one.

-Sperm mature after a complex process that begins after meiosis.

-For the production of a sperm, twice as many cell divisions occur as in the formation of an ovum.

References

1. Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberth, K., & Walter, P. (2008). Molecular Biology of the Cell. Garland Science, Taylor and Francis Group.
2. Creighton, TE (1999). Encyclopedia of Molecular biology. John Wiley and Sons, Inc.
3. Hill, RW, Wyse, GA, & Anderson, M. (2012). Animal Physiology. Sinauer Associates, Inc. Publishers.
4. Kliman, RM (2016). Encyclopedia of Evolutionary Biology. Academic Press.
5. Marina, S. (2003) Advances in the knowledge of Spermatogenesis, Clinical Implications. Ibero-American Fertility Magazine. 20 (4), 213-225.
6. Ross, MH, Pawlina, W. (2006). Histology. Editorial Médica Panamericana.