Yolk

Yolk is the nutrient-rich part of an egg. It provides food for the developing embryo. In many animals, such as birds, reptiles, and insects, the yolk is a special storage organ made in the mother's reproductive tract. In smaller animals, like some fish and invertebrates, the yolk is inside the egg cell itself.

Yolks are rich in vitamins, minerals, lipids, and proteins. These nutrients help the embryo grow and develop. The amount of yolk can affect how the embryo develops after fertilization.

Yolks are not living material like protoplasm but are mostly passive material called deutoplasm. The food and control structures are supplied during the formation of the egg. Even some plants, like algae, gymnosperms, and flowering plants, store resources in their female gametes.

Avian egg yolk

In bird eggs, the yolk is usually yellow and round. It floats in the egg white, also called albumen, thanks to special spiral bands of tissue named chalazae.

The yolk, along with the egg’s center where the embryo forms, is protected by a special layer called the vitelline membrane. The yolk is a big part of the egg’s inner material, called cytoplasm, and is made of fats, proteins, and enzymes. After an egg is fertilized, the embryo starts to form in a spot called the germinal disc.

Chicken egg yolks are a great source of vitamins, minerals, and nutrients. They contain all the egg’s fat and cholesterol, plus almost half of its protein. When you fry an egg sunny-side up, the yolk stays in the middle, surrounded by egg white. If you mix the yolk and white together before cooking, like in omelets or scrambled eggs, the whole mixture turns yellow.

The yolk gives food to the growing embryo inside the egg. People sometimes separate yolks from egg whites for cooking. Yolks are used to make foods like mayonnaise, custard, and hollandaise sauce. They are also used in paintings called tempera and in making special paints for old books. Yolks have a special ingredient called IgY that helps protect the embryo from harmful germs. They are also used to make sweet drinks like advocaat and eggnog, and to get egg oil for skincare and health products.

The yolk makes up about one-third of an egg’s weight and has three times the energy of the egg white because of its fat content. It contains important vitamins like A, D, E, and K, and many minerals such as calcium, iron, and zinc. The yellow color of yolks comes from special pigments in the chicken’s food, like carrots or green plants, but this color does not tell us how nutritious the egg is.

Eggs with two yolks, called double-yolk eggs, happen when a hen’s body makes two yolks at once. These eggs are rare and usually cannot hatch on their own because the embryos would get in each other’s way.

Eggs without yolks, called yolkless or wind eggs, are usually laid by young hens who are learning to lay eggs. These eggs cannot hatch because they lack a yolk.

The color of an egg yolk depends on what the hen eats. Feeds with yellow pigments, like green plants, can make yolks more yellow. Even red or green yolks can happen depending on the hen’s diet, but this does not change how healthy the egg is.

In fish

All bony fish, some sharks, and rays have yolk sacs during part of their growth. Fish that lay eggs keep their yolk sacs after hatching. Some special sharks called Lamniform sharks develop inside their mother’s body. Their eggs hatch inside her, and the developing sharks eat unfertilized eggs for food.

In crustaceans

The yolk in crustacean eggs is very important for helping the baby animals grow. It gives them the food they need. In animals called decapod crustaceans, the main protein that helps make this yolk is called apolipocrustacein (apoCr). This protein is different from the ones usually found in other animals that lay eggs.

ApoCr is a big protein made of about 2,600 tiny building blocks called amino acids. It has special parts that help it do its job, like areas that can move lipids and others that are only found in certain proteins. Scientists have found that apoCr is more similar to proteins in insects and vertebrates than to the usual yolk proteins in most animals. In decapods, apoCr is made in both the ovary and another organ called the hepatopancreas, helping with both fat handling and yolk creation. In some species, the gene for apoCr has copied itself, creating different versions that work in specific tissues.