Cellular and Molecular Aspects of Oocyte
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Santella et al. Zoological Letters (2020) 6:5 https://doi.org/10.1186/s40851-020-00157-5 REVIEW Open Access Cellular and molecular aspects of oocyte maturation and fertilization: a perspective from the actin cytoskeleton Luigia Santella1*, Nunzia Limatola1 and Jong Tai Chun2 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. Keywords: Actin, Calcium, Oocyte maturation, Polyspermy, Sea urchin, Starfish, Ageing, Fertilization, Vitelline layer, Zona pellucida * Correspondence: [email protected] 1Department of Research Infrastructures for Marine Biological Resources, Stazione Zoologica Anton Dohrn, Villa Comunale, Napoli 80121, Italy Full list of author information is available at the end of the article © The Author(s). 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Santella et al. Zoological Letters (2020) 6:5 Page 2 of 21 Introduction remain blocked at that stage until spawning (Fig. 1). The During gametogenesis, primordial germ-cells, which pro- release from meiotic arrest, i.e. the maturation process, duce either eggs or spermatozoa in sexually-reproducing is triggered by the hormone 1-methyladenine (1-MA), animals, undergo mitosis to increase in number and gen- which is secreted by the follicles cells surrounding the erate oogonia (in females) and spermatogonia (in males). oocytes, while the fully grown immature oocytes are still With the onset of meiosis, these diploid cells are trans- in the ovary [6, 7]. The hormone acts on a yet- formed into primary oocytes and spermatocytes. The unidentified receptor on the cell surface, and thereby in- meiotic (maturation) divisions that follow are markedly duces the breakdown of the envelope of the large nu- different in the two gametes. In male, each primary cleus known as germinal vesicle (GV). The germinal spermatocyte divides to produce four haploid sperma- vesicle breakdown (GVBD) allows the intermixing of the tids, which differentiate into spermatozoa capable of fer- nucleoplasm with the cytoplasm and is followed by for- tilizing the eggs. In female, unequal partition of the mation of the polar bodies (Fig. 1e arrows). The spawned cytoplasm results from two meiotic divisions that are maturing oocytes at sea are at the stage of their GVBD asymmetric in size and lead to the formation of one (Fig. 1c), and are to be fertilized before the extrusion of large haploid oocyte and three polar bodies, the latter the first polar body [8, 9]. At variance with oocytes of of which eventually degenerate. Another striking distinc- other vertebrate species in which a second meiotic arrest tion between oocytes and spermatozoa is that the latter may occur at different cell cycle stages, starfish eggs ma- fertilize the eggs at the end of the meiotic divisions, tured in vitro can complete meiotic divisions without whereas oocytes reach the period of fertilizability at dif- further maturation arrest. ferent stages of the maturation process, depending on Although sperm can penetrate immature oocytes of animal species. Indeed, marine invertebrate oocytes can starfish before GVBD, cortical events that block the be grouped into four different classes based on the mei- entry of supernumerary spermatozoa and ensure normal otic stage at which fertilization takes place. Oocytes of egg activation and cleavage take place within a precise various annelids and molluscs are fertilized at the GV time frame only after 1-MA stimulation. Indeed, it is stage, and the breakdown of the nuclear envelope is a well known that starfish eggs lose their ability to prevent sign that the oocytes have been activated. Eggs of ne- polyspermic fertilization when inseminated after being merteans, ascidians, some annelids, and molluscs are treated with 1-MA for several hours (overripe eggs). blocked at the metaphase of the first meiotic division These results indicate that the competence of the egg until fertilization triggers the completion of meiosis. The cytoplasm to be successfully fertilized is achieved at a eggs of frogs and mammals are arrested and fertilized at precise maturation stage but is lost soon after that. Stud- metaphase II, i.e., after the second maturation spindle ies of oocyte maturation using Patiria pectinifera (a.k.a. has formed and one polar body has been extruded [1–4]. Asterina pectinifera, Pacific Ocean) and Astropecten ara- Thus, as highlighted by Ernest Everett Just, “the fertiliz- nciacus (Mediterranean Sea) have made interesting ob- ability of all animal eggs hangs together with some condi- servations about the time frame and other requirements tion in the cytoplasm of the egg and is independent of its for eggs’ optimal fertilizability and successful develop- nuclear state, as germinal vesicle, as first or second ment [10–12]. maturation-nucleus or as a completely matured nucleus” Recent studies have provided evidence that the cortical ([5], page 185), and may be related to the structural actin cytoskeleton is a key player in the development of organization of the cortex of the female gametes. mature and competent eggs manifesting normal Starfish and sea urchin eggs, which respectively are fertilization responses. It is well established that actin, naturally fertilized before the extrusion of the first polar which is one the most abundant and highly conserved body and at the end of the two meiotic divisions, are proteins in eukaryotic cells, participates in the mainten- useful animal models for the study of in vitro oocyte ance of cell shape, as well as in many cellular functions maturation, fertilization, and embryonic development. such as cell migration, growth, motility, organelle move- This is due to the fact that they offer advantages not ment, polarization, and exocytosis/endocytosis. Together only in size and abundance but also in transparency of with myosin, actin can drive not only muscle contrac- their cytoplasm and extracellular coats. These unique tion, but also regulation of genes in the nucleus [13]. properties of the echinoderm eggs allow us to perform Actin molecules undergo transition between monomeric time-lapse imaging to follow the early structural and globular (G-actin) and filamentous (F-actin) states under ionic events regulating the maturation process, during the control of its own concentration and by the action of which the oocytes acquire the full cytoplasmic compe- numerous actin-binding proteins (ABPs) that affect their tence to be fertilized, as well as egg activation. polymerization