Invertebrate Oogenesis - A Review And Synthesis: Comparative Ovarian Morphology, Accessory Cell Function And The Origins Of Yolk Precursors

Oogenesis is a fundamental biological process observed in most animals. Animals employ many different mechanisms to make their eggs and oogenesis cannot be completed without the support of other intraovarian and/or extraovarian cells and tissues. Invertebrates account for 99% of all animal life on earth and their complex life histories are a reflection of the structural heterogeneity of their ovaries and the diversity of their vitellogenic mechanisms. The process of oogenesis in different invertebrates differs because it is constrained by limits imposed by their respective body design, the architecture of their ovaries and their collective phylogenetic history. Oogenesis in the majority of invertebrates has yet to be investigated and current knowledge is limited to relatively superficial descriptions of a fraction of 1% of described species in each of the major animal phyla. It has been over 30 years since the last published review of invertebrate oogenesis and during the intervening years considerable progress has been made in our understanding of the ultrastructural, biochemical and molecular aspects of the process. This review aims to identify some unifying themes by collating and synthesizing the widely dispersed literature on invertebrate oogenesis covering 30 metazoan phyla for which basic information is available.