Control of reproductive cycles in metazoa: Exogenous and endogenous factors

The reproductive activity in most Metazoa is cyclic. Such cyclicity is synchronized by different types of environmental factors, in particular by temperature (heterotherms) and photoperiod (homeotherms). Environmental information is gathered through particular sense organs, and in the absence of these, directly by the neurosecretory system. This is the only modulator of reproductive activity in lower invertebrates and echinoderms. In most molluscs and arthropods, the neurosecretory system exerts a trophic influence on epithelial endocrine glands, which in turn control the development and physiology of the reproductive system. Lastly, in vertebrates there is another endocrine component, the endocrine gonad. It is interesting to note that the feed-back mechanism well-documented in vertebrates is still only suspected in invertebrates. Finally, one of the main unsolved aspects of reproduction is refractoriness. i.e. the impossibility of inducing a new cycle in the post-reproductive period of many animals, and this may have vast adaptive implications.