Yolk in Biology Structure Types and Role in Development

• In contrast to the white, the yolk is the yellow and primary substance of an egg.

• In certain animals that produce eggs, the yolk (also known as the vitellus) is the nutrient-bearing portion of the egg whose main purpose is to supply food for the embryo's development.

• Some eggs have no yolk, for example, because they are laid in scenarios where the food supply is sufficient (such as in the body of a parasitoid's host) or because the embryo grows in the parent's body, which supplies the food, usually via a placenta.

• Matrotrophic reproductive systems can be defined as those systems in which the mother's body directly supplies the embryo; lecithotrophic reproductive systems are those in which the embryo is supplied by yolk.

Yolk in Different Species

• In many species, including all birds and most reptiles and insects, the yolk comes in the form of a special storage organ built in the mother's reproductive tract.

• Many other animals, particularly very small species such as some fish and invertebrates, have the yolk material inside the egg cell rather than in a special organ (ovum).

• Yolks, as a stored food, are frequently high in vitamins, minerals, lipids, and proteins. Proteins serve as food in and of themselves, as well as regulating the storage and supply of other nutrients. In some species, for example, the quantity of yolk in an egg cell influences the developmental processes that occur after fertilisation.

• The yolk is largely passive material, i.e. deutoplasm, rather than living cell material, such as protoplasm. During oogenesis, the food material and associated control structures are supplemented.

• Some of the material is stored more or less in the form in which the maternal body supplied it,  which is partially processed by dedicated non-germ tissues in the egg. While a part of the biosynthetic processing into its final form is known to happen in the oocyte itself.

• Other organisms, such as algae, especially those that are oogamous, can accumulate resources in their female gametes in addition to other animals. The remains of the female gametophyte serve as the food supply in gymnosperms and the endosperm in flowering plants.

Uses of Yolk

[Image will be uploaded soon]   

1. The yolk provides nourishment to the developing embryo inside the egg.

2. The yolk is sometimes separated from the egg white for cooking as well as is commonly used as an emulsifier in mayonnaise, custard, hollandaise sauce, crème brûlée, avgolemono, and ovos moles.

3. It is used as a component of traditional egg tempera in painting.

4. It is used to make an egg yolk agar plate medium, which is used to test for the presence of Clostridium perfringens.

5. Antiglobulin (IgY) is an antibody found in egg yolk. Passive immunity allows the antibody to transfer from the laying hen to the egg yolk, protecting both the embryo and the hatchling from microorganism invasion.

6. Egg yolk can be used to make liqueurs like Advocaat and eggnog.

7. Egg yolk is used to extract egg oil, which has cosmetic, nutritional, and medicinal applications.

Egg Yolk Meaning

The vitelline membrane, which differs from a cell membrane in structure, encloses the yolk mass and the ovum proper (after fertilisation, the embryo).

The yolk is mostly extracellular to the oolemma, being not accumulated inside the cytoplasm of the egg cell (as occurs in frogs), contrary to the assertion that the avian ovum and its yolk are a single large cell.

Following fertilisation, the embryo cleaves, resulting in the formation of the germinal disc. The chicken egg yolk is a good source of vitamins and minerals as a food. It contains all of the fat and cholesterol in an egg, as well as nearly half of the protein. Mixing the two components that is the white as well as the yellow part together before cooking results in a pale yellow mass, as in omelettes and scrambled eggs.

Double Yolk

[Image will be uploaded soon]

A double yolk occurs when a chicken releases two yolks into the same shell and is typically produced by young chickens with immature reproductive systems. Double yolks can also be produced by older chickens nearing the end of their egg-laying season. Eggs with two yolks are perfectly safe to consume. 

The odds of getting a double yolk are said to be one in a thousand, but this depends on the age of the flock. More than one double yolk egg can be found in the same dozen.

Types of Eggs According to Distribution of Yolk

• Homolecithal: In this type of egg, the yolk is distributed equally throughout the cytoplasm. As an example, consider Amphioxus.

• Centrolecithal: The yolk forms a net-like structure around the nucleus.

• Telolecithal: The yolk that's been deposited in the vegetal pole. The telolecithal type is light, with a moderate amount of yolk.