Chapter 9 - Endocrine Regulation of Vitellogenesis

Abstract

Although the process of vitellogenesis in Crustacea is fairly understood, hormonal mechanisms controlling the synthesis as well as uptake of the yolk proteins are still under intensive investigation. The available information, however, posits a bihormonal regulation, by the existence of an inhibitory neuropeptide in the optic ganglia as well as a variety of stimulatory hormonal factors, originating from diverse sources. Two major inhibitory neuropeptides, molt-inhibiting hormone (MIH) and gonad-inhibiting hormone (GIH), both negatively controlling molting and reproduction, are the pivotal physiological regulators in crustaceans. Several studies have been made to elucidate the mechanisms underlying the inhibitory action of GIH/vitellogenesis-inhibiting hormone (VIH) on vitellogenin synthesis in different crustacean species. Uniquely, VIH exerts its influence directly on both ovary and hepatopancreas, the two organs purportedly involved in vitellogenin synthesis. In contrast, different gonad stimulatory hormones are implicated in the positive control of vitellogenin synthesis in different crustacean taxa. They are gonad stimulatory neurosecretory hormone, secreted from brain and thoracic ganglia, biogenic amines of both stimulatory and inhibitory nature, emanating from central nervous system, methyl farnesoate, originating from mandibular organ, the molting hormone, ecdysone synthesized by Y-organ, and the vertebrate sex steroids, estradiol and progesterone, secreted from as yet undetermined sources. While discordant opinions have been expressed on their regulatory role in vitellogenin synthesis in different crustacean species, a multihormonal control of these stimulatory factors is proposed in accomplishing vitellogenic processes. Nevertheless, our present understanding on the hormonal control of vitellogenesis would bolster development of protocols to improve egg maturation in the commercially significant species, reared under controlled conditions.