Structure Of Ovum

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.

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.

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:

1. ii and iv

2. iv only

3. iii and iv

4. 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

1. contains centrioles, nucleolus and other cell organelles

2. lacks centrioles but contains nucleolus and other cell organelles

3. contains centrioles and nucleolus but lacks any other cell organelles

4. 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

1. Statement 1 is correct but Statement 2 is incorrect

2. Statement 1 is incorrect but Statement 2 is correct.

3. Both statements are correct.

4. 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:

• MCQs on Structure of Ovum

• Male accessory gland

• Implantation

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:

1. Microlecithal: small yolk, seen in mammals.

2. Mesolecithal: moderate yolk, e.g., amphibians.

3. Macrolecithal: large yolk, e.g., reptiles and birds.

• Based on shell:

1. Cleidoic eggs: enclosed in a hard shell, e.g., birds, reptiles).

2. 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.

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