Vitellogenesis

Vitellogenesis is a phase of oogenesis in many animals during which yolk, a nutrient-rich mass, is formed and deposited in developing oocytes. The process centers on vitellogenin, a precursor of yolk proteins that is synthesized in extra-ovarian tissues and taken up by oocytes, where it is processed into yolk proteins such as vitellin and associated lipids.

In most vertebrates, the liver (or hepatic equivalent) synthesizes vitellogenin under the influence of estrogen and related hormones. The protein is secreted into the bloodstream and delivered to the ovary, where oocyte membranes express vitellogenin receptors that mediate receptor-mediated endocytosis. Accumulation of yolk materials enlarges the oocyte and provides nutrients during embryogenesis. Yolk composition includes major yolk proteins, lipovitellin, phosvitin, as well as lipids, carbohydrates, and minerals.

In many invertebrates, such as insects, vitellogenin is produced in a somatic tissue such as the fat body under hormonal control by ecdysteroids and juvenile hormone. It is released into hemolymph and transported to developing oocytes, where it is internalized by vitellogenin receptors and sequestered as yolk within yolk bodies.

Vitellogenesis typically follows previtellogenic oocyte growth and precedes final oocyte maturation. It can be divided into early and late vitellogenesis, and is coordinated with reproductive cycles and hormonal regulation. The onset and duration vary across species.

Proper vitellogenesis is essential for embryo development in oviparous species; disruptions can affect fertility. Comparative studies illuminate reproductive strategies and nutrient allocation.