It is a complex process that involves numerous biochemical pathways and morphological changes.
These sex cells are produced by a specialized type of cell replication known as meiosis.
The following gametes contain half the genetic information as their parent cells, and they are also unique when compared to the parent cells and to each other.
This article will review the gametogenesis process (with an emphasis on sex-specific differences), the neurohormonal pathways involved in the process, and some complications that can arise during the process.
Chromatin, chromosomes and gametes
The genetic code that determines gender, height, eye color, and other variable phenotypic expressions are stored as chromatin molecules. These are a series of deoxyribonucleic acid molecules that are organized in a unique sequence.
All of this genetic information is condensed within the nucleus of each cell by tightly wrapping chromatin around protein structures known as histones. In somatic cell lines, there are twenty-three pairs of chromosomes; giving a total number of 46 chromosomes.
Chromosomes 1 to 22 occur in duplicate; The remaining two chromosomes are known as the X and Y sex chromosomes.
Considering that the formation of a zygote (single cell that forms after fertilization) requires the fusion of two cells; the preceding cells must have half the total number of chromosomes that should be in a somatic cell.
Therefore, specialized sex cells known as gametes are produced through the process of meiosis.
Meiosis is a form of cell division that results in the production of four unique haploid cells (containing 23 chromosomes) from one diploid cell (containing 46 chromosomes).
This is different from the general cell division known as mitosis which produces two identical diploid cells from one diploid cell. Meiosis occurs in two stages (meiosis I and II); each contains a specialized form of prophase, metaphase, anaphase, and telophase. The interface is only seen in the first phase of meiosis.
Before interface, when the cell is in the growth phase, DNA exists as euchromatin or heterochromatin. However, during interphase, there is condensation of the chromatin chains into visible pairs of chromosomes.
Each chromosome contains a pair of sister chromatids joined at a centromere. This phase also represents a period of cell growth where cell constituents are duplicated in preparation for cell division.
Therefore, the original cell has changed from the diploid state (which has 46 chromosomes) to a tetraploid state (which has 92 chromosomes, that is, 46 pairs of chromosomes).
After the interface, the cell enters a complicated yet precisely orchestrated series of events that is categorically referred to as prophase I.
In general, this phase involves the alignment of homologous pairs of chromosomes and the exchange of genetic material. However, there are several steps required to complete this process. Therefore, prophase I has been subdivided into five sub-stages that are explained below:
The leptotene stage marks the beginning of the exchange of genetic information. A maternal and paternal copy of the same chromosome (each has two sister chromatids attached at the centromere) is found within the nucleus.
Additional condensation of chromosomes is also observed. In addition, the telomeres of each chromosome are attached to the nuclear envelope.
Homologous pairs of chromosomes are closely linked together near the telomere region. The point of attachment is known as the synapse; and the general process marks the zygotene stage.
As synapses form, they often cluster on one side of the nucleus. This arrangement is sometimes referred to as a bouquet since the chromosomes are positioned similarly to a bouquet of flowers. The bivalent chromosomes are held together by the synaptonemic complex.
The sex chromosomes (X and Y) are not paired in males. Therefore, the synapse occurs in the pseudoautosomal region; which is an area of DNA sequence shared between these chromosomes.
Hypercondensation of the sex-specific components of the chromosome results in the formation of a sex vesicle.
Synapses are important for genetic recombination; which contributes to the diversification of the gene pool.
At the melting points, genetic information is exchanged between fused chromatids. Here the synapses are known as chiasmata or knots that will join the chromosomes.
Once all the chromosomes have formed their synapses, then the cell has entered the pachytene stage. Although there are four chromatids within each bivalent, at this stage they appear as a single bound structure in the synaptonemic complex. Recombination is complete during this phase.
The dissolution of the synaptonemic complexes (not the chiasmate) marks the beginning of the diplotene stage. Maternal and paternal genetic information exchange has occurred and chromosomes appear shorter than before.
It is noteworthy that this stage occurs already in the fifth gestational week in women. Their gametes then stop at diplotene until the onset of puberty, where a primary oocyte will complete meiosis each month before the start of their menstrual cycle.
Diakinesis is analogous to the prometaphase of mitosis. The bivalents attach to the fibers of the spindle and begin to line up along the equatorial plate of the metaphase.
Metaphase I of meiosis is very similar to the process observed in mitosis. The spindle microtubules bind to pairs of homologous chromosomes and align them along the equator of the spindles.
The centromere associated with the chromatid pair points in the direction of the spindle fiber. The bivalents then separate to opposite poles. The only thing that holds the homologues together is the chiasmate near the telomeres.
The final separation of the bivalent occurs during anaphase I when the chiasmate is removed and the homologous pairs separate at opposite poles of the cell.
Telophase I varies between men and women. Cytoplasmic division in women occurs asymmetrically and produces a small polar body and a much larger primary oocyte.
In men, cell division is incomplete and spermatocytes retain a cytoplasmic bridge. Although the resulting cells are diploid, they are not identical to the precursor cells that produced them.
Meiosis II occurs shortly after the completion of telophase I. DNA replication does not occur during this phase. This ensures that the resulting cells will have half the genetic material as the progenitor cells.
The rest of the division is quite similar to that of mitosis. The sister chromatids are aligned with the centromeres in metaphase II; and in anaphase II, they separate along the spindle fibers to opposite poles of the cell. Therefore, telophase II results in the production of four genetically unique haploid cells.
The development and maturation of spermatocytes (also called sperm) is known as spermatogenesis. Unlike their female counterparts, male gametogenesis only begins at the beginning of puberty.
Under the influence of gonadotropin-releasing hormone from the hypothalamus , the pituitary gland releases both luteinizing and follicle-stimulating hormones. Luteinizing hormone acts on the Leydig cells of the testes which subsequently secrete testosterone .
Testosterone, along with follicle stimulating hormone, will stimulate the sertoli cells of the testes; leading to inhibin production as well as upregulation of testosterone-binding globulin receptors.
Receptor preregulation allows for increased stimulation of cells by testosterone, resulting in activation of spermatogenesis.
It is noteworthy that testosterone (which acts on the hypothalamus and anterior pituitary gland) and inhibin (which acts on the anterior pituitary gland) form a negative feedback loop that produces a reduction in gonadotropin secretion, luteinizing and stimulating follicle. .
Sertoli cell activity results in mitotic activation and proliferation of previously dormant spermatogonia within the seminiferous tubules of the testes. They become primary spermatocytes, which then enter the first meiotic division mentioned above.
The resulting secondary spermatocytes then enter the second meiotic division, which ends with the production of four haploid spermatids.
The maturation of spherical spermatids into tadpole-like spermatocytes is known as spermiogenesis. This process involves lengthening of the cell body and reduction in cytoplasmic volume. Mature spermatocytes are made up of:
- A head that contains the haploid nucleus and the acrosome that contains the proteolytic enzymes necessary for fertilization. Note that the acrosome is a derivative of the Golgi apparatus of the spermatid.
- A neck that forms a bridge between the head and the tail.
- A tail (divided into central, main and final parts) that facilitates motility. It also houses the mitochondria that produce adenosine triphosphate (ATP) for cell motility.
Spermatocytes migrate from the lumen of the seminiferous tubules to the epididymis through peristaltic movements.
Here, they are stored and continue to mature. Genetically, there are two types of spermatocytes. They all contain 22 copies of autosomes (that is, non-sex chromosomes) and one X or Y chromosome (the nomenclature used is 23, X or 23, Y).
The final stage of spermatocyte maturation occurs after ejaculation. Inside the uterus or fallopian tubes, both seminal protein and glycoprotein coatings are shed from the sperm acrosome.
Cells in the female genital tract are believed to facilitate this process. After training, spermatocytes are unable to fertilize a secondary oocyte.
Oogenesis and follicular maturation
During the prenatal period there is mitotic proliferation of oogonia (primordial oocytes). There is a subsequent increase in the size of these cells, at which point they are recognized as primary oocytes.
It was previously mentioned that females begin gametogenesis in the fifth gestational week, but the cells stop in early prophase I.
Primary oocytes are surrounded by a simple squamous layer of follicular cells (granulosa). They secrete the oocyte maturation inhibitor, which prevents the primary oocyte from completing meiosis. Together, they are known as the primordial follicle.
By the time a woman is born, they possess approximately 2 million primordial follicles.
No new primary oocytes will be produced after the females are born. Most of these primordial follicles will degenerate, leaving approximately 40,000 primary oocytes at the beginning of puberty.
Of these cells, only about 400 will mature during their reproductive life (that is, from menarche to menopause).
There is continuous growth of the primary oocyte in the peripubertal period. There is a concurrent evolution of flat follicular cells into first cuboidal cells, then columnar cells.
The cells also produce an amorphous, fenestrated glycoprotein substance called the zona pellucida that surrounds the primary oocyte. In general, only one primordial cell will mature each month during the menstrual cycle (there are some exceptions as seen with maternal twins).
As observed in their male counterparts, the onset of puberty in females heralds the release of gonadotropin-releasing hormone from the hypothalamus. It acts on the anterior pituitary gland, releasing luteinizing and follicle-stimulating hormone in a similar way.
Follicle stimulating hormone acts on granulosa cells, resulting in the production of estrogen hormones. Estrogen continues to act on the granulosa cells, promoting their proliferation and stratification around the oocyte.
As the follicle begins to increase in size, the outer connective tissue cells become more organized and form theca folliculi.
These cells separate into an inner inner theca (vascular layer with glandular function) and an outer outer theca (capsular layer).
Luteinizing hormone acts on theca interna, resulting in the production of androgens. The granulosa cells subsequently convert androgens into more estrogen hormones.
Theca interna produces pockets of follicular fluid that later fuse to form the antrum. These events coincide with morphological changes in the follicle, such that it appears to have a more oval shape and the oocytes move towards a random pole of the follicle (which forms the oophore cluster).
This general structure is now known as a secondary follicle. The primary oocyte would also have increased in size and completed meiosis I.
There is an uneven distribution of the cytoplasm and its constituents between the two resulting cells. As a result, the product of this division is a relatively large secondary oocyte and a redundant first polar body.
After ovulation, the secondary oocyte progresses through meiosis II to the point of metaphase II; at that point it stops until fertilization of the oocyte occurs. Once fertilized, a second polar body will be released and both will be extruded from the mature oocyte.
While spermatocytes have a fifty percent chance of being 23, X or 23, Y, all progeny of oogenesis have a 23, X genome. However, they are still genetically unique when compared to each other and to the cell. parental due to genetic variety and sharing.