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What is Ovum?

It is the single-cell released from either of the female reproductive organs like ovaries. Which is capable of developing into new organisms when fused with the sperm cell is known as an ovum. The word ovum meaning in Latin is ‘egg’ and the plural of the ovum is ova. Ovum is the name given to the haploid female reproductive gamete. Ova are produced by both animals and land plants like embryophytes. In 1672, the ovum meaning was first defined.

 

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Structure of Ovum

The structure of mature ovum is generаlly sрheriсаl, nоn-mоtile gаmete with yоlky сytорlаsm аnd enсlоsed in оne оr mоre egg envelopes. Size varies in different animals and depends uроn the amount оf yolk. The size of the ovum varies from ten microns to a few сm, so the structure of the ovum depends on the size. 

The largest egg is оf оstriсh аnd is аbоut 170 x 135 mm. Egg size аnd yоlk аmоunt аre interdeрendent. It is аbоut 50 micron in many роly сhatte worms, 150 micron in tuniсаtes but very large in birds аnd reрtiles. In mаmmаls, it is generаlly miсrоleсithаl аnd аbоut 100 micron.

 

Structure of Human Ovum

Humаn ovum anatomy is miсrоleсithаl with a large amount of сytорlаsm. Сytорlаsm is differentiated into  оuter, smaller and trаnsраrent exорlаsm оr egg cortex and inner, lаrger аnd ораque endорlаsm оr  оорlаsm. Egg cortex is with sоme cytoskeletal structures like microtubules аnd miсrоfilаments (Bаlinsky,  1981), рigment grаnules аnd соrtiсаl grаnules оf muсороlysассhаrides. Endорlаsm is with cell-organelles, infоrmоsоmes, tRNАs, histоnes, enzymes etс.

Nuсleus оf оvum is lаrge, blоаted with nuсleорlаsm аnd is саlled germinаl vesiсle. Nuсleus is exсentriс  in роsitiоn sо humаn оvum hаs а роlаrity. The side of the ovum with the nucleus and роlаr body is called animal  роle, while the side of the орро site is called vegetal pole.

The structure of the egg envelope is surrounded by a number of egg envelopes like a vitelline membrane, zona pellucida, corona radiata. The vitelline membrane is inner, thin, and transparent. And it is secreted by the ovum itself.  Zone pellucid is the middle, thin, transparent, and non-cellular. They are partly secreted by the follicular cells and partly by the oocytes. 

 

Ovum Function

The main function of the ovum is to carry the set of chromosomes contributed by the female gamete. It creates the right environment to occur fertilization with the help of sperm. And it also provides nutrients to the growing embryo until it sinks into the uterus and further, the placenta takes over. 

Ovum has the central nucleus which contains the female's genetic material. The female genetic material with the sperm cell determines the characteristics of the child inherited.  The nucleus is surrounded by the cell plasma or yolk, which contains the nutritional elements essential for developing the egg cell. 

If the egg does not get fertilized within 24 hours of its eruption, eggs start to degenerate. Once the ovum is fertilized it undergoes a series of cell divisions. And if the fertilized egg splits into two in the early stage of development it results in identical twins. If the division of the cell is incomplete it results in the formation of Siamese twins who are born physically joined. When two separate eggs are released and fertilized independently, it results in fraternal twins. 

 

Working of the Female Reproductive System 

The female reproductive system is mainly controlled by the hormones, released from the brain and the ovaries. The hormones produced by the brain and ovaries are GnRH, FSH, LH, estrogen, and progesterone. All these hormones combine together to form the women's reproductive cycle.

The length of the reproductive cycle ranges between 24-35 days, during which duration the ovum cell is developed and matured. At the same period, the lining of the uterus is prepared to receive the egg fertilized from the sperm. If somehow, the fertilized egg is not implanted into the uterus. The lining of the uterus is shed and expelled from the body. This leads to the bleeding known to have the period of menstruation. Traditionally the first day of the bleeding is known as the first day of the reproductive cycle, the main event of the cycle is evolution. One of the ovaries releases the mature egg, which takes place around the 14th day of the reproductive cycle. 

 

Did You Know?

Human females are born with all the eggs they will ever have. Generally. Most of the cells in the body regenerate or get replaced by the younger and healthier ones. In humans, all one to two million eggs are born along with the birth and they are the enduring element.  

 

Brief on Ovum

Ovum can be defined as the structure that occurs during oogenesis or egg formation and is the secondary oocyte. Here, the second maturation division hasn’t occurred yet. In such a stage, the mature ovum or egg is released from the female reproductive system or ovary which then enters the uterus and fuses to the male gamete (sperm) leading to fertilization. The process of expelling the egg or ovum from the Graffian follicle along with a polar body is called ovulation. 

The size of the ovum is very small and is around 0.15 mm in humans. It is round in shape and non-motile in nature. The ovum is surrounded by a layer of liquidy substance called the cytoplasm which is named ooplasm and has a small amount of yolk in humans and is called alecithal. In other female animals where there is a large amount of yolk present, the egg cytoplasm or ooplasm is filled with lipoproteins, water, pigment granules as well as cytoplasmic organelles. The ooplasm has a cortex which is its peripheral layer and it has cortical granules and microvilli. Microvilli are tubular structures which assist in the transportation of substances in and out of the ooplasm. 

On the other hand, the cortical granules are composed of mucopolysaccharides which are covered by membranes of the golgi. These are not present in mammalian eggs. The nucleus is large in size and is placed centrally in mammalian eggs but it is mostly towards the animal pole. This absorbs the cytoplasmic fluid and then swells into a germinal vesicle. In humans, the vitelline membrane is the layer which covers the egg followed by another primary covering or membrane called the zona pellucida. The two membranes have a narrow space between them which is called the perivitelline space. 


Ovum in Female Reproductive System

When the ovum is discharged from the Graafian follicle, the many layers of epithelial cells stick to the outer surface of the zona pellucida layer and are radially formed. This gives rise to a structure called the corona radiata. The hormones that are crucial for the functions of the female reproductive system are stimulated by the hypothalamus. These are secreted by the ovary. Some of the important hormones are progesterone and estrogen. The gametes of the reproducing organs are haploid in nature i.e. they contain only a set of chromosomes(n) which is the half amount of the genetic material found in a species. In humans, the gametes have 23 chromosomes each which means the ovum only has a set of chromosomes. Thus, the ovum makes sure the set of chromosomes present is contributed to the process of fertilisation and it creates the right environment for it to occur. 


Conclusion

Studying about Ovum structure and its function is vital to know how the system works and to maintain the same.

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FAQs on Ovum

1. What is the meaning of Ovum?

Ovum is the mature female germ cell. Which develops into the new member after the process of fertilization. Female gametes are also known as eggs generally, the shape of the egg is oval.

2. Can you see a Human Ovum?

The human ovum is one of the largest cells in the body. The size of each ovum is nearly about 0.1 mm, Which seems quite smaller than the assumption. But it is visible through the human naked eyes.

3. What are the Six Steps of Fertilization?

The six steps which occur in fertilization are sperm Capacitation, Sperm-Zona Pellucida Binding, The Acrosome Reaction, Penetration of the Zona Pellucida, Sperm-Oocyte Binding, Egg Activation and the Cortical Reaction, The Zona Reaction, and later on the post-fertilization events. 

4. What is the importance of ovum?

Ova or oocytes are the egg cells which are the female gametes produced by the female reproductive organ ovaries. These eggs are then transported to the fallopian tube of the female reproductive system where it can undergo fertilization with the male gamete i.e. sperm. When fertilized, the egg moves to the uterus whose lining thickens due to normal hormonal responses of the reproductive cycle. Thus, the ovum makes the environment suitable for fertilization to occur. Furthermore, it carries the set of chromosomes which is required for the process of fertilization. The ovum also plays an important role during the growth of the embryo by providing it with nutrients and nourishments until when the placenta starts doing it.

5. What is fertilization and how does it take place?

Fertilization can be defined as the fusion of female gametes which is the ova and the male gamete called sperm. This process takes place in the fallopian tube which connects the uterus to the ovaries from where the eggs are released. After the fertilization process occurs, the newly formed cell is called the zygote which moves to the uterus from the fallopian tube. The zygote then starts burrowing into the uterine lining which thickens due to hormonal changes and this process is called implantation. After this the zygote is called the blastocyst which contains more cells than the zygote and is nourished by the uterine lining.

6. What are reproductive cycles?

Reproductive cycles are the cycles that occur in mammals which is crucial for fertilization to occur. There are two types of reproductive cycles – menstrual cycle which occurs in primates and the oestrus cycle which occurs in lower mammals. In the menstrual cycle, the cycle occurs monthly and occurs when the fertilization process does not occur. This is because the ovum is not fertilized which in turn leads to the shedding of the uterine lining. This is called menstruation. On the other hand, the oestrous cycle occurs either seasonally or during a year. Here, the females undergo various physiological changes induced by the reproductive hormones. This cycle occurs in animals like dogs, cows etc. 

7. Why does menstruation occur?

Menstruation occurs when the ovum is not fertilized by a male gamete or sperm. This cycle occurs monthly and is called the menstrual cycle. There are four different phases of the menstrual cycle that occur on different days.

They are :

  1. Secretory Phase: Here, the thickened lining of the uterus called the endometrium is shed through vagina if no fertilization occurs whereas the lining is maintained if pregnancy occurs.

  2. Follicular Phase: This is the time between the first day of period and the process of ovulation (release of an egg mid-cycle)

  3. Proliferative Phase: Uterine lining starts building after the end of cycle.

  4. Luteal Phase: This is the phase between ovulation and the start of menses. The luteinising hormone level peaks here. 

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