Abstract
Much of the scientific knowledge on oocyte maturation, fertilization, and embryonic development has come from the experiments using gametes of marine organisms that reproduce by external fertilization. In particular, echinoderm eggs have enabled the study of structural and biochemical changes related to meiotic maturation and fertilization owing to the abundant availability of large and transparent oocytes and eggs. Thus, in vitro studies of oocyte maturation and sperm-induced egg activation in starfish are carried out under experimental conditions that resemble those occurring in nature. During the maturation process, immature oocytes of starfish are released from the prophase of the first meiotic division, and acquire the competence to be fertilized through a highly programmed sequence of morphological and physiological changes at the oocyte surface. In addition, the changes in the cortical and nuclear regions are essential for normal and monospermic fertilization. This review summarizes the current state of research on the cortical actin cytoskeleton in mediating structural and physiological changes during oocyte maturation and sperm and egg activation in starfish and sea urchin. The common denominator in these studies with echinoderms is that exquisite rearrangements of the egg cortical actin filaments play pivotal roles in gamete interactions, Ca2+ signaling, exocytosis of cortical granules, and control of monospermic fertilization. In this review, we also compare findings from studies using invertebrate eggs with what is known about the contributions made by the actin cytoskeleton in mammalian eggs. Since the cortical actin cytoskeleton affects microvillar morphology, movement, and positioning of organelles and vesicles, and the topography of the egg surface, these changes have impacts on the fertilization process, as has been suggested by recent morphological studies on starfish oocytes and eggs using scanning electron microscopy. Drawing the parallelism between vitelline layer of echinoderm eggs and the zona pellucida of mammalian eggs, we also discuss the importance of the egg surface in mediating monospermic fertilization.Graphical abstract
Highlights
During gametogenesis, primordial germ-cells, which produce either eggs or spermatozoa in sexually-reproducing animals, undergo mitosis to increase in number and generate oogonia and spermatogonia
The altered pattern of Ca2+ release suppressed embryonic development, probably due to the failure of proper oocyte activation or to the polyspermy induced by the absence of adequate Ca2+-linked block mechanisms against the entry of supernumerary sperm [144]. These results suggest that the existence and a role of a PLCζ-independent Ca2+ release mechanism(s) at the oocyte surface are yet to be understood in mammalian oocytes, and raise some concerns in that, despite the essential role of PLCζ in inducing Ca2+ oscillations to ensure monospermic fertilization in mice, the intracytoplasmic sperm injection (ICSI) protocol bypasses what normally occurs at the oocyte cortex following spermegg fusion
An increasing body of evidence suggests that the structural organization of the actin cytoskeleton and its physical changes induced by external signals dynamically regulate a large number of events occurring during oocyte meiotic maturation and fertilization
Summary
Primordial germ-cells, which produce either eggs or spermatozoa in sexually-reproducing animals, undergo mitosis to increase in number and generate oogonia (in females) and spermatogonia (in males). Sperm can penetrate immature oocytes of starfish before GVBD, cortical events that block the entry of supernumerary spermatozoa and ensure normal egg activation and cleavage take place within a precise time frame only after 1-MA stimulation.
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