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.
FAQs on Ovum
1. What is an ovum?
An ovum is the mature female reproductive cell, also known as the egg cell. It is a haploid cell, meaning it contains half the number of chromosomes of a normal body cell. Its primary role is to fuse with a male gamete (sperm) during fertilization to form a zygote, which develops into a new organism.
2. What is the difference between an egg cell and an ovum?
While often used interchangeably, 'egg cell' or oocyte is a broader term for the female gamete at any stage of its development. The term 'ovum' specifically refers to the final, mature stage of the egg cell, which is ready for fertilization after completing meiosis.
3. What are the main parts of a human ovum?
A human ovum is a complex cell with several key parts designed to support fertilization and early development. These include:
- Nucleus: Contains the female's genetic material (23 chromosomes).
- Cytoplasm (Ooplasm): Rich in nutrients (yolk) to nourish the embryo after fertilization.
- Zona Pellucida: A thick, jelly-like outer layer that the sperm must penetrate. It plays a crucial role in preventing multiple sperm from fertilizing the egg.
- Corona Radiata: An outermost layer of follicle cells that surrounds the ovum when it is released from the ovary.
4. What is the main function of the ovum in reproduction?
The ovum has two critical functions in reproduction. First, it carries the mother's genetic information (DNA) on its 23 chromosomes. Second, it provides all the necessary nutrients and cellular machinery to support the growth of the embryo in the first few days after fertilization, before it implants in the uterus.
5. How does an ovum compare to a sperm cell in terms of size and mobility?
The ovum and sperm are very different. The ovum is one of the largest cells in the human body, is spherical, and is non-motile, meaning it cannot move on its own. In contrast, the sperm is one of the smallest cells, has a distinct head and tail, and is highly motile, allowing it to swim towards the egg.
6. Is the human ovum visible to the naked eye?
Yes, just barely. The human ovum is about 0.1 millimetres in diameter, which is roughly the size of a small grain of salt or the dot of a pen. It is one of the very few human cells that can be seen without the aid of a microscope.
7. What happens to the ovum if fertilization does not occur?
If an ovum is not fertilized by a sperm within about 12 to 24 hours of ovulation, it begins to disintegrate and is reabsorbed by the body. The drop in hormone levels that follows causes the uterine lining, which had thickened to prepare for a pregnancy, to break down and shed. This process results in menstruation.
8. Why is the ovum so much larger than other cells in the body?
The ovum's large size is due to its vast amount of cytoplasm, which is packed with nutrients like proteins, lipids, and ribosomes. This nutrient-rich store, known as yolk, is essential for providing energy and building blocks for the developing embryo during its first few days, before it implants in the uterine wall and can receive nutrients from the mother.
9. How does the ovum prevent more than one sperm from fertilising it?
The ovum has a clever mechanism to prevent fertilization by multiple sperm (a condition called polyspermy). When the first sperm successfully penetrates the zona pellucida, it triggers an immediate chemical reaction. The ovum releases substances that instantly harden the zona pellucida, creating a barrier that blocks any other sperm from entering.
10. Where is the ovum produced and how does it mature?
An ovum is produced in the ovaries through a process called oogenesis. A female is born with millions of immature egg cells called primary oocytes. During each menstrual cycle, hormones stimulate one of these oocytes to resume its development inside a fluid-filled sac called a follicle. It then undergoes cell division (meiosis) to become a mature ovum, which is released from the ovary during ovulation.
