Please note: This article contains sexual health content and it is purely made for educational purposes.
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Sex cells |
The female reproductive system is a type of system which has the capability of producing offspring that can develop into a fully-formed human being. The female reproductive system is located in the lower abdominal region of a woman, where its main job is to produce an ovum; also called as an unfertilized egg (a female sex cell) for fertilization. Similar to the female reproductive system, the male reproductive system also produces its sex cells called sperms. When both the sex cell (ovum and sperm) meets, fertilization happens in the female reproductive system and could develop into a baby. Fertilization is the process of fusion of two different types of sex cells to form a single cell. We can discuss the process of the male reproductive system and the process of fertilization briefly, in our next to next post. Now let’s see how the female reproductive system works by mainly considering the process of ovum production at a chromosome level.
The female reproductive system is a complex structure that consists of the following parts as shown in the below figure. The main parts that need to be considered here are; the ovaries, fallopian tube, uterus, and vagina. Women have two ovaries for storing their ova (plural for ovum) for their entire lifetime purpose. A woman after her puberty stage produces a mature ovum every month owing to be fertilized by a sperm. When the sperm meets the ovum in the woman's reproductive system, the ovum becomes fertilized (which will be now called as an egg) and will develop into a baby. The baby develops in the mother’s uterus path for 9 months and gets out through the vagina.
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A women’s ovary is a basket for thousands of ova made for producing babies. Every single ovum can develop into a baby when it is fertilized by a sperm. The ova start growing in the woman's ovary as oogonia (plural for oogonium) cells when she was in her mother’s womb. These oogonia cells will be in a diploid manner (2N2C) at starting, which means it contains 23 pairs of chromosomes. This chromosome amount is present in every cell of an adult human body except the sex cells (sperm and egg). The opposite of diploid is haploid (1N1C) which contains only 23 chromosomes and is present only in the sex cells of a human body. The reason why the sex cells have only 23 chromosomes and not 23 pairs of chromosomes is that; during fertilization, the fusion of two cells should make 23 pairs of chromosomes. If the sex cells have 23 chromosomes each, then during fertilization it can form 23 pair of chromosomes and can duplicate further to develop into a human body. Chromosomes are a collection of genetic material called DNA wound around proteins called histones.
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Fertilization Process |
To know more about the chromosomes please visit my blog.
https://sciencetopic03.blogspot.com/2021/11/detailed-explanation-of-mitosis-and.html
The oogonia cells develop into a fully mature ovum when the girl reaches puberty stage; through the process of oogenesis. Now let’s talk about how these ova are produced in a girl’s ovary from her fetal development.
At first, during her development in her mother’s womb, these diploid oogonia cells start to grow by mitotic division, which gets up to 1 to 2 million cells per ovary. The number of chromosomes in the oogonia cells, after the mitotic division, will still be 23 pairs of chromosomes (2N2C). The mitotic division is a process of duplicating a cell, which gets the oogonia cells to million counts. These mitotic divisions get up to 7 months of the baby's development. After 7 months, the produced oogonia cells proceeds into meiotic division and becomes primary oocyte. Only a certain amount of oogonia survives till the end of 7 months, and most of them die over the developing stage. The primary oocyte that goes through the meiotic division doesn’t complete the full meiotic division and it stops at the prophase-1. This stoppage will be before the birth or shortly after the birth. At this stage, the number of chromosomes in each oocyte will be (2N4C), which means there are 23 pairs of chromosomes or 92 chromatids. Let’s see what the alphanumeric character 2N2C and 2N4C describes,
For example, let’s take a single diploid chromosome which is said to be 2N2C; also known as diploid in number. This diploid chromosome consists of two chromosomes attached together. Here one chromosome is from the mother and one from the father (also called a homologous chromosome). It is said as 2N because the chromosome is made from two persons (2N), through whom there are two chromosomes, and those two acts as a separate chromatid (2C). If there is one chromosome that will be from one person (1N) and has its own chromatid (1C), it is said as the haploid chromosome (1N1C). If the diploid chromosome is replicated once (i.e., the father is replicated once and the mother is replicated once) it will be said as 2N 4C, because there is a copy of the same chromosome, which means there are totally four chromatids (4C). If it is replicated for another time it will be said as 2N 6C. The C value represents the number of chromatids where the N value gives the number of types of chromosomes. In the case of the primary oocyte, for a single chromosome, there is one father and one mother; so 2N, and replicated once to four chromatids; thus 4C. So a simple oogonia cell that has 23pairs of chromosomes (2N 2C), has replicated in the DNA replication of the interphase cycle and has got to 46 pairs of chromosomes (2N 4C). This interphase cycle is mandatory for every cell that is going to get divided by mitosis or meiosis after.
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Difference between chromosomes and chromatids |
Please visit my below mentioned blogs for a detailed understanding of the DNA replication and mitotic and meiotic division stages:
https://sciencetopic03.blogspot.com/2021/11/detailed-explanation-of-mitosis-and.html
Usually, when the girl was born from her mother’s womb, the primary oocyte with the chromosome amount (2N 4C) will be in the meiotic arrest and will stay in this position for several years. When the girl gets to the age of 10 to 14, she reaches the puberty age, where the primary oocyte completes the meiotic 1 phase fully and becomes a secondary oocyte. Now the secondary oocyte takes over the ovulation and takes over the second phase of meiotic 2 division which halts at metaphase II. Puberty is a period, during which one of the stored primary oocytes with the completion of the meiotic first phase is ejected out from the ovary owing to be fertilized. This is the period where the first ovum is released by the ovary. At this stage, the ovum is ready to be fertilized by a sperm to make the fertilization process. If the ovum is not fertilized, then the ovum will get out from the body through the vagina. This process of releasing an ovum from the ovary is called ovulation. During ovulation, the number of chromosomes in the ovum will be 23 chromosomes, 46chromatids (1n2C).
All the primary oocyte does not complete the meiotic 1 phase. Most of them die during the meiotic arrest, thus a million counts of oogonium cells, get reduced to a considerable amount. So when the primary oocyte completes its first phase of meiotic division and divides a single diploid cell into two haploid cells, it gets transformed into a secondary oocyte (1N2C). But when diving, both the haploid cells do not share the same amount of cytoplasm. One gets a large number of cytoplasm, and the other gets less number of cytoplasm. The cell which gets less cytoplasm becomes a polar body, which later dies off. When a matured ovum (secondary oocyte) with a halt at metaphase II is fertilized by a sperm, the ovum will complete the second meiotic division and create a perfect haploid cell for fertilization to occur. Thus the chromosome amount reduces from (1N 2C) to (1N 1C), which means 23 pairs of chromosomes to 23 chromosomes. It also creates a second polar body that has a minimum cytoplasm. The process of mitotic and meiotic division of the oogonia cells right from birth can be clearly understood from the below figure.
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Oogenesis Process |
Even males too produce sperms with more over the same process of oogenesis. They also go through the same mitotic and meiotic division and produce the sperms with 23 chromosomes. Similar to the process of production of ovum which is called as oogenesis; the production of sperm is called as spermatogenesis. Let’s see the process of spermatogenesis in my next post. Now let’s talk about how a single oocyte of a woman is processed and ejected out from her ovary, which is also called as an ovarian cycle.
Inside the ovary, all the oogonia cells are surrounded by a primary follicle made up of granulosa cells. These cells provide the necessary hormones for various purposes, particularly for making the primary oocyte to stop at the meiotic 1 phase. As the day moves on, by each month right after puberty, the granulosa cells get built up around the oogonium and a layer of zona pellucida is formed in the inner side of granulosa cells as a protective layer. During the development of the follicles, an antrum starts to form inside the granulosa cells. As the oocyte begins to maturate, the cell begins to bulge the ovary, owing to come out. When the cell becomes too big, it punctures the ovary and ejects it outside the ovary. This ovum may be destined to be fertilized by a sperm. An ovary contains thousands of such follicles with its oocyte. Some of them together get developed and mature. But at the final stage, only one cell becomes dominant and is ejected. The other cells which had reached their final maturity, began to degenerate and die. That’s why women usually release only one ovum per month owing to be fertilized. In some cases, rarely two ova are ovulated at the same time, which makes the two ova to get fertilized by the sperm.
After the oocyte was ejected, the follicle remains in the ovary which further turns into corpus luteum. This corpus luteum is necessary for generating necessary hormones to prepare the lining (a type of tissue) in the uterus path for the ovum to embed on the path; if in case it was fertilized. As the ovum moves through the uterus path, even if it is fertilized or not, the corpus luteum sends signals to the hypothalamus in the brain to produce the hormones namely Estrogens and progesterone, to prepare the lining in the uterus path for the ovum. If the ovum was not fertilized by the sperm, then it will be discarded along with the lining out of the vagina. This comes out during the periods as blood. It is nothing to be scared of; it is a normal natural cycle. As the ovum is continuously ejected from the ovary it does not mean, that the ovary will run out of oocytes. The ovary is made with millions of oogonium at the beginning, for this purpose.
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