Structure Of Ovum: Definition, Function, Structure, Types, Functions And Oogenesis

Definition Of The 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 reproduction—it carries the female's genetic material and provides the appropriate environment for fertilization. Released from the ovary, it travels down the fallopian tube, where it may meet with sperm. If fertilized, 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 Ovum

The general structure of the oocyte is shown by its general features:

General Features

• Size: Normally the largest cell in the female body; it usually measures between 100 to 200 micrometers in diameter.

• Shape: Round.

• Color: Pale yellow or white.

Advanced Structure of the Ovum

The detailed structure of the ovum reveals several important components:

Plasma Membrane

• It consists of a lipid bilayer in which proteins are embedded.

• Provides fusion with sperm and initiates fertilization.

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

• Central and 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 attachment to the ovum.

Corona Radiata

• A layer of granulosa cells that surround 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:

Concerning Yolk Content

• Microlecithal: A very small amount of yolk is present. Example mammals.

• Mesolecithal: A moderate amount of yolk is present. Example: amphibians.

• Macrolecithal: Abundant yolk is present. Examples: birds, and reptiles.

Concerning Shell

• Cleidoic: Ova covered with a shell. Example: Bird's egg.

• Non-cleidoic: Ova without a shell covering. Example: Mammalian ova.

Table: Types Of Ova

Ovum Formation (Oogenesis)

Oogenesis is a process through which ova are formed:

Stages of Oogenesis

• Multiplication Phase: Mitotic division of oogonia.

• Growth Phase: Increase in size of the primary oocyte.

• Maturation Phase: Meiosis resulting in a mature ovum.

Functions Of The Ovum

There are several key roles played by the ovum:

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 fertilization.

Process Of Fertilization

Several key interactions and changes occur during fertilization:

Interaction with Sperm

• The sperm then binds to the zona pellucida and gains access through the plasma membrane.

Post-Fertilization Changes

• Cortical Reaction: Prevents polyspermy by hardening the zona pellucida

• Zygote Formation: The sperm and ovum nuclei fuse to form a diploid zygote.

Ovum Associated Disorders

There are several disorders associated with the ova :

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.

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