Publisher Summary This chapter describes some of the principal morphological and cellular features of fertilization in fishes, from the initial encounter between sperm and egg to their fusion to form the zygote. The pathway leading to the fusion between a single sperm and a previously quiescent egg and the subsequent union of male and female pronuclei consists of a predictable, highly ordered sequence of events. The chapter describes the events and changes that take place during the sperm–egg interaction and their temporal relationships to each other. The eggs of fishes, particularly those of teleosts, appear to be particularly useful and advantageous for investigations of fertilization. Teleost eggs are generally large and possess the built-in advantage of having the location of sperm entry topographically restricted to a predetermined, identifiable site in the animal pole. The direct interaction of sperm and egg is mediated by complementary receptors located on their plasma membranes. Such receptors may function in sperm–egg recognition, binding, fusion, and even the activation of the egg in some species. An important functional component of these receptors is carbohydrate based on studies of the mouse sperm receptor (ZP3) and the sea urchin egg-binding protein (bindin).

Fertilization in teleost fishes: mechanisms of sperm-egg interactions

In Xenopus laevis eggs, cisternae shells which surround cortical granules (CG) are part of a cortical endoplasmic reticulum (ER) network. In this paper the origin of such ER shells has been studied in full-grown, progesterone-exposed Xenopus oocytes. Furthermore, the possible role of the cortical ER in the activation process has been investigated by pricking maturing oocytes. It has been shown that in full-grown ovarian oocytes ER CG shells are absent and ER cisternae are extensively and randomly distributed throughout the peripheral cytoplasm, where they appear to be continuous with annulate lamellae (AL). Following hormone treatment, the AL completely disaggregate and the ER cisternae gradually migrate to the cortex where they surround the CG constituting the typical cortical network described in uterine eggs. Furthermore, it has been found that 8 h after progesterone treatment (before the first polar body extrusion) the response to pricking (CG exocytosis) occurs only at the animal half; there is no observable response in the vegetal half. At this time ER shells surround CG only in the animal hemisphere. A complete CG exocytosis occurs following the first polar body emission, when the cortical ER is well organized in the whole oocyte cortex. The correlation between the differentiation of the cortical ER and the arousal in the oocyte of the ability to respond to a pricking stimulus is discussed in the light of an involvement of the cortical ER in the propagation of CG exocytosis.

The modifications of cortical endoplasmic reticulum during in vitro maturation of Xenopus laevis oocytes and its involvement in cortical granule exocytosis.

Publisher Summary Amphibians exhibit both styles of fertilization: monospermy and polyspermy. Monospermy is confined to tailless frogs and toads that comprise the order Anura, while physiological polyspermy is found in the tailed newts and salamanders of the order Urodela. This chapter reviews the process of fertilization in amphibians, particularly as it relates to polyspermy. The chapter discusses ways to explore the nature of the cytological ancestry. Amphibians exhibit a great diversity of reproductive patterns. This diversity makes the amphibians ideal animals for examining the cellular evolution of developmental processes. Fertilization in anurans and urodeles is so different that it represents an important character distinguishing the orders. Comparison between fertilization in anurans and urodeles shows a mirror image dichotomy with respect to polyspermy. The anuran egg has a fast and slow block to polyspermy, and it has no protection when more than one sperm enters. On the other hand, the urodele egg lacks both external blocks to polyspermy but can deal with accessory nuclei within the egg. The arrangement of components in the membranes and in the cortical granules is considered important for the polyspermy-blocking reactions.

Fertilization in amphibians: the ancestry of the block to polyspermy.

The wave of activation current in the Xenopus egg

The development of mouse oocyte cortical reaction competence is accompanied by major changes in cortical vesicles and not cortical granule depth.

In the egg of Xenopus laevis a cortical network of smooth endoplasmic reticulum (SER) surrounds and interconnects each cortical granule (CG) (Campanella and Andreuccetti, '77). This network is a possible intracellular site of calcium storage to be called into action for CG exocytosis. In our experiments, Xenopus eggs, unfertilized or activated by pricking or by calcium ionophore A 23187, have been fixed in osmium-pyroantimonate for calcium localization.



Our data show that deposits can be detected only in activated egs. The calcium chelator edetate (EGTA) and x-ray microprobe analysis demonstrate that they contain calcium. Deposits are found on liposomes and on all intraovular cytomembranes, which therefore appear to be possible sites of calcium sequestration. In the case of ionophore-activated eggs, deposits are detectable independently of the presence of extracellular calcium. These data show that in Xenopus at activation an intracellular liberation of calcium occurs similar to that described in other species.



Furthermore, the fact that antimony deposits are observed only after activation makes Xenopus eggs appropriate material in which to follow the temporal and spatial sequence of appearance of the deposits during the early stages of activation. Our results show that antimony deposits appear first in SER vesicles between the plasma membrane and CGs and then spread to the rest of the egg cytomembranes. These data corroborate our hypothesis that in Xenopus the cortical SER network is the first intracellular site where calcium is released at activation. The possible mechanism of calcium release and propagation along the egg cortex is discussed.

S. Denis-Donini

Papers

Calcium ultrastructural localization in Xenopus laevis eggs following activation by pricking or by calcium ionophore A 23187.