Endocrinology of Vitellogenesis

Abstract

The ovum (female germ cell) is truly the most remarkable of all animal cells: once activated, it can give rise to a complete new individual and it is the only cell, in higher animals, that is capable of doing so. The most distinguished feature of the ovum is its large size, which is required for the storage of reserves for the development of the embryo. During the early part of the developmental program, very rapid rates of cleavage take place in the embryo. This requires a high rate of synthetic activities, which is made possible by storage of large reserves of messenger RNAs, ribosomes, enzymes and other precursors of macromolecular synthesis in the ovum prior to fertilization. The other requirement for development and growth is metabolic energy, the reserve form of which is stored in the oocytes as “yolk. ” In most vertebrate oocytes, yolk is not synthesized in situ but derived from a precursor protein known as vitellogenin, synthesized in the liver of the animal. A sequence of complex but well-coordinated processes is involved in the production of yolk and its accumulation in vertebrate eggs. This sequence of events, collectively referred to as vitellogenesis, includes the following steps: 1) the induction of vitellogenin synthesis in the liver and its subsequent release into blood circulation, 2) the transport of vitellogenin in the blood stream, 3) the uptake of vitellogenin by the growing oocytes and 4) the conversion of vitellogenin into storage forms. Studies of vitellogenesis have recently been reviewed by Tata,115 Tata & Smith,6 Wahli,125 Wallace & Selman,136 Wiegand,146 Ho et al.,64 and Wallace.131,132 The scope of these reviews is limited to the cellular and molecular events of hepatic vitellogenesis, and the vitellogenin uptake and yolk formation process. In this article, I will focus on reviewing literature about the involvement of hormones in the regulation of vitellogenesis. As will be evident from my discussion, vitellogenesis is under multihormonal control. Upon activation of the hypothalamo-pituitary axis, gonadotropins are released from the pituitary and stimulate estrogen production in the ovaries. Rising levels of estrogen, in turn, induce vitellogenin production and release by the liver. At the same time, under the stimulation of gonadotropins, ovarian follicles are capable of sequestering circulating vitellogenin at high rate. At various points in the process of vitellogenesis, hormones other than estrogens and gonadotropins are exerting their influences in “fine-tuning” the vitellogenic response.