Structure Of Ovum

Ovum is the single egg cell released from either of the ovaries in a single menstrual cycle. The cell is capable of forming a whole organism when fertilised with a sperm cell. The outer surface of the ovary is surrounded by a layer of the germinal epithelium. The germinal epithelium gives rise to the ovarian follicles.

This Story also Contains

1. What is Ovum?
2. Role of Ovum in Reproduction
3. Structure of the Ova
4. Types of Ova
5. Ovum Formation (Oogenesis)
6. Functions of the Ovum
7. Ovum Associated Disorders
8. MCQs on The Structure of the Ovum
9. Recommended Video On 'The Structure Of Ovum'

The follicles contain the ovum, which then gradually matures during oogenesis. Once activated, it takes around 4 months for a follicle 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 have degenerated. The structure of the ovum is an important topic in the biology subject.

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—in this case, 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 implanation. 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 the Ova

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 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 the Ovum

The ovum provides half of the genetic material needed for reproduction and supports early embryonic 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.

• Contributor of Genetic Material: Provides half of the genetic material necessary to make a new individual.

• Trigger of 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.

MCQs on The Structure of the Ovum

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:

Option 1: ii and iv

Option 2: iv only

Option 3: iii and iv

Option 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

Option 1: contains centrioles, nucleolus and other cell organelles

Option 2: lacks centrioles but contains nucleolus and other cell organelles

Option 3: contains centrioles and nucleolus but lacks any other cell organelles

Option 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

Option 1: Statement 1 is correct but Statement 2 is incorrect

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

Option 3: Both statements are correct.

Option 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.

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