Yolk

In many species, such as all birds, and most reptiles and insects, the yolk takes the form of a special storage organ constructed in the reproductive tract of the mother.

In many other animals, especially very small species such as some fish and invertebrates, the yolk material is not in a special organ, but inside the egg cell.

It is spherical and is suspended in the egg white (known alternatively as albumen or glair/glaire) by one or two spiral bands of tissue called the chalazae.

Phosvitins are one of the most phosphorylated (10%) proteins in nature; the high concentration of phosphate groups provides efficient metal-binding sites in clusters.

[16] Double-yolked eggs seldom lead to successful hatchlings without human intervention, as the chicks interfere with each other's hatching process and do not survive.

Mature hens rarely lay a yolkless egg, but sometimes a piece of reproductive tissue breaks away and passes down the tube.

Such a scrap of tissue may stimulate the egg-producing glands to react as though it were a yolk and wrap it in albumen, membranes, and a shell as it travels through the egg tube.

[21] This type of egg occurs in many varieties of fowl and has been found in chickens, both standard and bantams, guineas, and coturnix quail.

For example, some of the natural pigments that produce a rich yolk color are xanthophylls without much nutritional value, rather than the carotenoids that act as provitamin A in the body.

In decapod crustaceans, the primary yolk precursor protein is apolipocrustacein (apoCr), which differs from the traditional vitellogenins (Vtgs) found in most oviparous animals.

ApoCr shares greater structural and evolutionary similarity with insect apolipophorin II/I (apoLp-II/I) and vertebrate apolipoprotein B (apoB), distinguishing it from other members of the large lipid transfer protein (LLTP) superfamily.

[28] ApoCr is a large glycolipoprotein, approximately 2,600 amino acids long, with conserved structural domains characteristic of LLTPs.

In decapods, apoCr is typically expressed in both the ovary and hepatopancreas, supporting its dual roles in lipid metabolism and yolk formation.