Structure Of Ovum
The ovum is the female gamete, produced in the ovaries through oogenesis. It is the largest human cell and plays a key role in fertilisation and embryonic development. This guide explains ovum structure, types, functions, disorders, and NEET exam questions.
This Story also Contains
- What is Ovum?
- Role of Ovum in Reproduction
- Structure of Ovum
- Types of Ova
- Ovum Formation (Oogenesis)
- Functions of Ovum
- Ovum Associated Disorders
- Ovum NEET MCQs
- FAQs on Ovum
- Recommended Video On 'The Structure Of Ovum'
Ovum is the single egg cell released from either of the ovaries in a single menstrual cycle. During human reproduction, the ovum fuses with sperm and forms a zygote. The outer surface of the ovary is surrounded by a layer of the germinal epithelium which gives rise to the ovarian follicles. The follicles contain the ovum, which then gradually matures during the process of oogenesis and takes around 4 months to develop completely. During the reproductive age of the female, about 300-400 follicles mature and release eggs. These eggs are capable of being fertilised. At the time, when a woman reaches menopause, the remaining follicles in the ovary degenerate.
What is Ovum?
The ovum is the female gamete in the majority of organisms, including humans. It is a big, non-motile cell that plays a fundamental role in reproduction through the fusion of the male gamete, sperm to form a zygote, eventually developing into a new organism.
Ova are produced in the ovaries through the process of oogenesis. Each ovum contains half the quantity of genetic material required to make an organism, contributing 23 chromosomes in humans that will merge with the 23 chromosomes from the sperm to form a complete set of 46 chromosomes.
Role of Ovum in Reproduction
The ovum has been designed for human reproduction. It carries the female's genetic material and provides the appropriate environment for fertilisation and implantation. Released from the ovary, it travels down the fallopian tube, where it may meet with sperm. If fertilised, the changes that take place in the ovum result in the formation of a zygote. This begins embryonic development.
The oocyte also contains all the cytoplasmic determinants necessary for early development, including certain nutrients and organelles that provide for cell division and growth until the embryo can implant in the uterus and initiate nutrient uptake from the mother.
Structure of Ovum
It is the largest cell in the female body. It usually measures between 100 to 200 micrometres in diameter. It has a round shape. It is pale yellow or white in colour
The detailed structure of the ovum reveals several important components:
Plasma Membrane
It consists of a lipid bilayer in which proteins are embedded.
Helps fusion with sperm and initiates fertilisation.
Cytoplasm
Present organelles are mitochondria, ribosomes, and endoplasmic reticulum.
Yolk: Supplies nutrients to the developing embryo.
Cytoplasmic Organelles: Essential for cellular activity and early development.
Nucleus
The central organelle contains the genetic material of the ovum.
Contains DNA that combines with sperm DNA to make up the genome of the zygote.
Zona Pellucida
A glycoprotein layer covers the plasma membrane.
Protects the ovum from mechanical injury.
Provides a site for sperm fusion with the ovum.
Corona Radiata
A layer of granulosa cells surrounds the zona pellucida.
Provides nutrition and support to the ovum.
Types of Ova
Ova can be differentiated based on yolk content and the presence or absence of a shell. The types, based on yolk content and the presence or absence of a shell, are summarised in the table below-
Ovum Formation (Oogenesis)
Oogenesis is the process of ovum (egg) formation in the female reproductive system, occurring within the ovaries. It involves stages like multiplication, growth, and maturation, resulting in a mature haploid ovum ready for fertilisation.
Multiplication Phase: Mitotic division of oogonia.
Growth Phase: Increase in size of the primary oocyte.
Maturation Phase: The Immature ovum goes through various stages of meiosis, resulting in a mature ovum.
Commonly Asked Questions
A polar body is a small cell produced during meiosis in oogenesis. It contains excess chromosomes and very little cytoplasm. Polar bodies ensure that the ovum retains most of its nutrient-rich cytoplasm while still reducing its chromosome number to haploid.
Oogenesis involves several key stages: formation of primordial germ cells, their transformation into oogonia, primary oocyte formation through mitosis, meiosis I to form secondary oocytes, and completion of meiosis II upon fertilization. This process spans from fetal development to adulthood.
The first polar body is formed during meiosis I of oogenesis. It contains half of the original chromosomes but very little cytoplasm. Its primary role is to allow the oocyte to reduce its chromosome number to haploid while retaining most of its cytoplasmic resources for future embryo development.
In oogenesis, meiosis is asymmetric and produces one large ovum and three small polar bodies. It's also interrupted, pausing at metaphase II until fertilization. In contrast, spermatogenesis produces four equal-sized, functional sperm cells and completes without interruption.
The ovum maintains its arrested state at metaphase II through the action of cytostatic factor (CSF). CSF inhibits the anaphase-promoting complex, preventing the cell from progressing through meiosis. This arrest is only relieved when a sperm fertilizes the egg, triggering calcium release and CSF degradation.
Functions of Ovum
The ovum provides half of the genetic material needed for reproduction and supports early embryo development after fertilisation. It also plays a crucial role in attracting and fusing with the sperm during fertilisation.
Nutrient Storage: Supplies all of the nutrients necessary during the early phases of embryonic growth and development.
Contribute Genetic Material: Provides half of the genetic material necessary to make a new individual.
Trigger Development: Activates the process of development upon fertilisation.
Ovum Associated Disorders
Disorders related to the ovum include anovulation (absence of ovulation), poor egg quality, and premature ovarian failure. These conditions can lead to infertility or hormonal imbalances, affecting reproductive health.
Anovulation: The failure of the release of ova from the ovary is a cause of infertility.
Polycystic Ovary Syndrome (PCOS): A hormonal disorder that causes the ovaries to enlarge with small cysts that could affect the production of ova.
Premature Ovarian Failure (POF): Premature loss of ovarian function leads to infertility.
Commonly Asked Questions
Mitochondria in the ovum are of particular importance because they are inherited exclusively from the mother (maternal inheritance). The large number of mitochondria in the ovum ensures that sufficient mitochondrial DNA is passed on to support the energy needs of the developing embryo.
The ovum's structure supports genomic imprinting through the maintenance of specific epigenetic marks on certain genes. These imprints, established during oogenesis, ensure that some genes are expressed in a parent-of-origin specific manner in the developing embryo, which is crucial for normal development.
Calcium ions play a pivotal role in egg activation following fertilization. The sperm entry triggers a series of calcium waves within the egg, which initiate various activation events including the cortical reaction, completion of meiosis II, and the start of embryonic gene expression.
The ovum's ability to complete meiosis upon fertilization is crucial for maintaining the correct chromosome number in the offspring. This process reduces the egg's chromosome number to haploid, allowing it to combine with the haploid sperm nucleus to form a diploid zygote.
The ovum protects its genetic material through several mechanisms: the zona pellucida provides a physical barrier, the arrest at metaphase II keeps chromosomes condensed and stable, and various cellular checkpoints ensure genomic integrity. After fertilization, the egg activates DNA repair mechanisms to fix any damages.
Ovum NEET MCQs
Q1. Which of the following situations correctly describe the similarity between an angiosperm egg and a human egg?
i. Eggs of both are formed only once in a lifetime
ii. Both the angiosperm egg and human egg are stationary
iii. Both the angiosperm egg and human egg are mobile
iv. Syngamy in both results in the formation of zygote
Choose the correct answer from the options given below:
ii and iv
iv only
iii and iv
i and ii
Correct answer: 2) iv only.
Explanation:
Eggs are formed several times during the lifetime of angiosperms. Option (ii) is incorrect as human eggs travel from the ovaries to the fallopian tube. Option (iii) is also incorrect as angiosperm eggs are stationary. In angiosperms, the egg cell is located within the embryo sac of the ovule and remains fixed in place. Fertilization occurs when the pollen tube delivers sperm cells to the egg. Unlike in animals, angiosperm eggs do not migrate but depend on external agents like wind or pollinators for fertilization.
Hence, the correct answer is option (2) iv only.
Q2. Ooplasm
contains centrioles, nucleolus and other cell organelles
lacks centrioles but contains nucleolus and other cell organelles
contains centrioles and nucleolus but lacks any other cell organelles
lacks centrioles but contain nucleus and other cell organelles
Correct answer: 2) lacks centrioles but contains nucleolus and other cell organelles
Explanation:
Ooplasm, or the cytoplasm of an ovum, is the specialized substance within the egg cell that surrounds the nucleus and plays a critical role in early embryonic development. The ooplasm also contains stored nutrients, including yolk granules and lipids, to sustain the developing embryo until external nourishment becomes available.
Hence, the correct answer is option 2) lacks centrioles but contains nucleolus and other cell organelles.
Q3. Study the following statement and interpret a correct option given below about follicle cells of corona radiata :
Statement 1: Surrounds the ovum and remains attached to it following ovulation
Statement 2: Provide nutrients for the oocyte and regulates maturation of the oocyte
Statement 1 is correct but Statement 2 is incorrect
Statement 1 is incorrect but Statement 2 is correct.
Both statements are correct.
Both statements are incorrect.
Correct answer: (3) Both statements are correct.
Explanation:
There are several protective layers to the ovum's structure. A perivitelline space is a narrow area between the plasma membrane and zona pellucida, a thick, acellular layer secreted by follicular cells. This corona radiata found outside the zona pellucida contains essential nutrients and is formed by the elongated follicular cells, while the most important role of this ovum is through the animal pole where polar bodies extrude as it has its opposite side in the vegetal pole.
Hence, the correct answer is Option (3) Both statements are correct.
Also Read:
FAQs on Ovum
What is the role of the ovum in reproduction?
The ovum plays a crucial role in sexual reproduction. It provides the genetic material from the mother which combines with the sperm to form a zygote. The ovum also supplies nutrients, mitochondria, and cytoplasmic determinants that guide early embryonic development. The ovum’s cytoplasm contains essential factors for cell division and differentiation. Its protective layers also prevent polyspermy, ensuring only one sperm fertilizes it. Thus, the ovum helps in the formation of the new organism.
What is oogenesis?
Oogenesis is the process of formation and development of the female gamete, ovum. It occurs in the ovary. It begins before birth when oogonia multiplies by mitosis and enters meiosis, forming primary oocytes. These remain arrested in prophase I until puberty. After puberty, selected oocytes undergo meiosis during each menstrual cycle. The process involves three phases: multiplication, growth, and maturation. It involves formation of one haploid ovum and polar bodies.
What are the types of ova?
Ova can be classified based on yolk content and shell structure.
Based on yolk:
Microlecithal: small yolk, seen in mammals.
Mesolecithal: moderate yolk, e.g., amphibians.
Macrolecithal: large yolk, e.g., reptiles and birds.
Based on shell:
Cleidoic eggs: enclosed in a hard shell, e.g., birds, reptiles).
Non-cleidoic eggs: without hard shell, develop inside the mother's body, e.g., mammals.
What disorders are associated with ovum?
Several disorders affect ovum development and release, leading to fertility issues. Anovulation refers to the absence of ovulation, resulting in infertility. Polycystic Ovary Syndrome (PCOS) is caused by hormonal imbalance resulting in irregular ovulation, cyst formation, and poor ovum quality. Premature Ovarian Failure (POF) occurs when ovaries lose function before the age of 40, leading to fewer or no ova being released. These conditions can reduce chances of fertilization and pregnancy, and can be treated using hormonal therapy or assisted reproductive techniques.
Recommended Video On 'The Structure Of Ovum'
Frequently Asked Questions (FAQs)
The corona radiata supplies nourishment and gives support to the ovum.
The oogenesis has the multiplication phase, growth phase and maturation phase.
The oocyte is larger and non-motile and provides nutrition, while the sperm is small and motile and provides the genetic material.
Zona pellucida protects the oocyte and allows sperm binding at the time of fertilization.