Creating embryonic gametes
In men, cells of the spermatic lineage reproduce, differentiate, and progress to eventually produce spermatozoa, spermatocytes, spermatocytes 1, spermatozoa 2, pre-sperm, and spermatozoa.
Origin of gametes
The origin of gametes are primordial sex cells, also known as germ cells. These cells appear very early in the embryo, first in the wall of the yolk sac (around the end of the 3rd week), from the wall of the yolk sac, the germ cells migrate to where the gonads will be generated. around the end of week 4, beginning of week 5). In human embryos of male sex, in testicular germ cells, primitive sex cells have diploid chromosome sets of 2n = 44A + XY. In human embryos with female sex, in the ovarian germ, primitive sex cells carry the diploid set of chromosomes 2n = 44A + XX. In the germinal gonad, the primordial sex cells differentiate to give rise to the progenitors of the sex cell lines. There are two sex cell lines: sperm and ovum.
The process of making gametes
Sperm making process
Figure: The process of spermatogenesis.
In men, sperm cells reproduce, differentiate, and evolve to eventually produce sperm. From the beginning to the end of the sperm line there are cells: spermatogonia, spermatocytes 1, spermatozoa 2, pre-sperm and spermatozoa.
In the seminiferous tubules of foetuses and children, spermatogonia cells contain only spermatozoa and are called squamous cells, which are the progenitor cells of the lineage with diploid chromatin 2n=46=44A +XY. formed by differentiation of primordial sex cells. The spermatozoa reproduce by mitosis to rapidly increase their numbers.
It is only from puberty to the end of sexual life that the differentiation and progression of spermatozoa always continue to produce sperm. During each mitosis, a spermatogonium produces two daughter cells: one retains the properties of the spermatozoon and serves as a lifelong reserve for spermatogenesis. One will differentiate into dusty squamous spermatozoa, and then into squamous cell spermatozoa. All spermatozoa have diploid chromosomes 2n=44A +XY.
Sperm cell 1:
Squamous cell spermatocytes differentiate into spermatozoa 1 with diploid chromosomes. Sperm 1 undergoes meiosis to produce sperm. Therefore, meiosis is also known as a division to maturity.
The process of meiosis consists of 2 divisions. The result of the first division: one sperm cell 1 gives rise to two sperm cells 2, each sperm cell 2 has only haploid chromosomes n= 23. There are 2 types of sperm cells 2: one with an X chromosome and the other has a Y chromosome.
2 spermatozoa are produced during the second division of meiosis. Each sperm cell 2 produces 2 pro-sperm having a haploid set of n=23 like spermatozoon 2 and also has 2 types of pro-sperm: X-carrying and Y-carrying.
Pre-sperm are not fertile, they differentiate into sperm through a complex process. Thus, each sperm also has a haploid set of chromosomes and has 2 types of sperm: X-carrying type and Y-carrying type. Thus, during gametogenesis, a sperm cell with diploid chromosome set 2n= 44A + XY through meiosis
produces 4 sperms, each with a haploid set of n= 23, with 2 types of sperm: 22 + X and 22 + Y. The ratio between the two types is 1/1. Sperm are highly differentiated cells that are no longer able to reproduce and have a complex structure.
In females, oocytes reproduce, differentiate, and progress to eventually produce mature (mature) ovum capable of fertilization. The oocyte lineage cells from beginning to end include: oocyte, oocyte 1, oocyte 2 and mature oocyte.
Figure: Ovulation process.
In the foetal ovary, oocytes with diploid chromosomes 2n = 44A + XX are surrounded by epithelial cells that later differentiate into follicular cells and form oocyte-containing sacs called follicles. . Among these epithelial cells, oocytes reproduce many times in a mitotic fashion to rapidly increase their number. Eventually, the oocysts will differentiate into oocyte 1. The oocyte is only found in the ovaries of the foetus because before the birth of a girl, the entire oocyte has differentiated into oocyte 1. So, At birth, the ovaries no longer have a reserve of oocysts to differentiate into oocyte 1, so a woman's fertility is limited.
Oocyte 1 has a chromosome set of 2n = 46A + XX, is contained in the primordial follicle. Oocyte 1 grows because the cytoplasm accumulates the nutrients necessary for their development. Oocyte 1 undergoes meiosis to produce a mature ovum, but it is not until the end of the first division (prophase) of the first division that it stops dividing. When a girl is born, the entire oocyte 1 has undergone meiotic division and has stopped dividing at this stage. The time to stop division is long or short depending on the oocyte 1.
From puberty to menopause, a monthly number of oocytes in the ovaries 1 continue the first division of the stopped meiosis process. The result of this division is the production of 2 daughter cells with the same haploid chromosome set n=23 = 22A + X but with different sizes and effects: one cell is large because the cytoplasm contains many substances. a reserve nutrient called oocyte 2, which has a sexual function, and a small cell called pole 1, which has no sex function.
The newly produced oocyte begins the second division of meiosis. As a result, an oocyte 2 will give rise to 2 daughter cells, both with haploid sets of chromosomes n=23 = 22A + X but with different sizes and effects: a large cell contains many stored nutrients. and can fertilize called a mature ovum, and a small cell called pole 2. While pole 1 also produces 2 poles 2, the poles are not able to fertilize.
Thus, unlike spermatogenesis, during oocyte formation, oocyte 1 also produces 4 cells, but only one cell can fertilize the mature ovum.
Morphologically abnormal gametes
Abnormal sperm can be classified into several main categories:
Sperm are just irregular in shape: big or small, round or pointed.
Immature sperm: head and neck contain many cytoplasms.
Old sperm: pitted head, with or without pigment.
Figure: Normal and abnormal sperm.
(A. normal sperm; B. immature sperm; C. abnormally shaped sperm; D. old sperm; E. degenerated sperm).
Degenerative sperm: head atrophied or deformed, with 2 heads or 2 tails.
Abnormal ovum usually degenerates before maturation:
It is possible to encounter a follicle containing 2-3 oocytes 1 or an oocyte 1 containing 2 - 3 nuclei but very rarely.
Chromosome aberrations in gametes
Here only the deviation in the number of chromosomes is mentioned. During gametogenesis, due to the non-disjunction of chromosomes during meiosis, there are 1 extra gamete, and 1 gamete missing. The extra or missing chromosome can be an autosome or an X or Y sex chromosome.
Image: Abnormal ovum.
(A. the primordial follicle has two oocytes; B. oocyte contains three nuclei).