Showing papers in "Current Topics in Developmental Biology in 1995"

Regulation of oocyte growth and maturation in fish

Abstract: Publisher Summary This chapter discusses the current status of the investigations on the hormonal regulation of oocyte growth and maturation in fish. Pituitary gonadotropins are of primary importance in triggering these processes in fish oocytes. In both cases, however, the actions of gonadotropins are not direct but are mediated by the follicular production of steroidal mediators, estradiol-17β (oocyte growth), and 17α,20β-dihydroxy-4-pregnene-3-one (DP) or 4-pregnen-17,20β,21-triol-3-one (20β-S) (oocyte maturation). It has been established that both estradiol- 17β and 17α,20β-DP are biosynthesized by salmonid ovarian follicles through an interaction of two cell layers—namely, the thecal and the granulosa cell layers (two-cell-type model). The granulosa cell layers are the sites of production of these two steroidal mediators, but their production depends on the provision of the precursor steroids by the thecal cell layers.

Molecular basis of mammalian egg activation.

Abstract: Publisher Summary The chapter discusses signal transduction mechanisms in the activation of mammalian eggs, primarily in the mouse, and explains the sequence of events that accompanies fertilization in the mouse. This information provides a framework within which to examine known and proposed signaling mechanisms that regulate egg activation, helps understand the role of several possible second messengers in egg activation, and helps describe the sperm–egg interaction that may result in the production of second messengers. In the mammal, the first interaction between the sperm and egg is at the level of the zona pellucida (ZP), an extracellular coat that surrounds all mammalian eggs. In the mouse, the ZP is composed of three glycoproteins: ZP1, ZP2, and ZP3. Although acrosome-intact sperm establishes primary binding with ZP3 through an O-linked carbohydrate moiety, the identity of this moiety is controversial. Following sperm binding, ZP3 induces the acrosome reaction of the bound sperm.

Sperm--egg recognition mechanisms in mammals.

Abstract: Publisher Summary This chapter describes gamete surface components that support tight species-specific binding of the free-swimming mammalian sperm to unfertilized eggs. Such binding is mediated in part, by carbohydrate. The zona pellucida glycoprotein, “ZP3,” serves as sperm receptor during fertilization in mammals. Close inspection of the characteristics of this glycoprotein, coupled with some speculation, may shed light on sperm–egg recognition mechanisms in mammals. ZP3 consists of at least two different functional domains, perhaps separated from each other, by a hinge region of the polypeptide. One of the domains is thought to include the C-terminal third of the ZP3 polypeptide, which possesses the O-linked oligosaccharides that constitute, at least in part, the combining site for the sperm.

Confocal immunofluorescence microscopy of microtubules, microtubule-associated proteins, and microtubule-organizing centers during amphibian oogenesis and early development.

Abstract: Publisher Summary During the past 5 years, the picture of the microtubule (MT) cytoskeleton of amphibian oocytes has changed dramatically, due in large part to refinements in the techniques for preserving and visualizing the oocyte cytoskeleton. Individual MTs are first identified in the cortex of Xenopus oocytes in samples prepared by rapid freezing. Subsequently, whole-mount immunocytochemistry and confocal immunofluorescence microscopy has revealed an extensive network of MTs extending throughout the cytoplasm of oocytes from both X. laevis and Rana pipiens . In addition, these techniques have revealed that dramatic changes in MT assembly, organization, and dynamics accompany distinct stages in oocyte differentiation, maturation, and early embryonic development. This chapter presents a view of the assembly and organization of the MT cytoskeleton during oogenesis and early development in the African frog, X. laevis , which is based on results obtained in laboratories around the world. Results obtained from other amphibian species are discussed when informative. In addition, some of the mechanisms that might serve to regulate MT organization and dynamics during amphibian oogenesis and early development are addressed, focusing on the potential contributions of microtubule organizing centers (MTOCs) and microtubule-associated proteins (MAPS).

3 Mechanisms of Calcium Regulation in Sea Urchin Eggs and their Activities during Fertilization

Abstract: Publisher Summary This chapter discusses the current evidence of Ca 2+ regulation in sea-urchin eggs during fertilization. Studies on calcium regulation during the fertilization of mammalian eggs and comparative changes in deuterostome eggs during fertilization are also discussed in the chapter, along with other events associated with egg membranes during fertilization. The regulation of intracellular Ca 2+ activity ([Ca 2+ ] i ) in a wide variety of cells is well established as an important event during signal transduction of numerous extrinsic stimuli. Different sources, including influx of external Ca 2+ and release from the intracellular stores, contribute to the rise in [Ca 2+ ] i . Irrespective of the cell type, resting [Ca 2+ ] is generally in the range of 50–200 n M and accounts for only a minute fraction of the total cellular Ca 2+ content, therefore the bulk of the cellular Ca 2+ is bound and may reside within the membrane-bound organelles.

Transgenic fish in aquaculture and developmental biology.

Abstract: Publisher Summary The techniques for generating transgenic fish have been established for numerous species, including some species suitable for developmental studies—such as the zebrafish and medaka—and many commercially important species Transgenic fish models are useful for testing the promoter activity and for analyzing the pattern of gene expression Compared with many well established transgenic systems such as transgenic mice, the transgenic fish system is still in its infancy in developmental biology Many advanced transgenic techniques developed in mice—such as gene targeting and cell lineage ablation—are likely to be applicable to fish, but a big gap between mice and fish, in terms of the availability of gene resources, must be filled before a thorough analysis of the molecular mechanism of fish development becomes possible

5 Nuclear Transplantation in Mammalian Eggs and Embryos

Abstract: Publisher Summary This chapter discusses the progress of nuclear transplantation studies in mammals, the important biological factors influencing the development of eggs and embryos reconstituted by nuclear transplantation, and the prospects for future studies in this field. The transplantation of a nucleus to an appropriate cytoplast and the subsequent analysis of its embryonic development provide the most vigorous and direct tests of genomic totipotency of a cell nucleus. The most advanced studies are those carried out in amphibia. Totipotency, the capacity to direct the formation of fertile frogs, has been demonstrated for the nuclei of many kinds of embryonic cells; pluripotency, as judged by the development of heart-beating larvae, has been proved for all the cell nuclei tested to date.

Specifying the dorsoanterior axis in frogs: 70 years since Spemann and Mangold.

Abstract: Publisher Summary Many molecules have been identified to generate mesoderm and axial structures in the Xenopus embryo, but that does not guarantee that all the important molecules have been discovered. Other ligand–receptor combinations and molecules that are yet to be uncovered are probably involved in signal transduction. The absence of genetics makes it difficult to know when most of the major molecular players have been detected. A further problem presented by the absence of genetics is that as more and more molecules are found, assessing the molecular interactions and causal relationships in axis formation becomes increasingly difficult. The assays described earlier explain what a molecule can do but not necessarily what it normally does in the embryo. The assays lack the precision of genetic mutants because the time and place of expression relative to normal events are not well controlled.

Axis formation during amphibian oogenesis: reevaluating the role of the cytoskeleton.

Abstract: Publisher Summary This chapter provides substantial support for the hypothesis that cytoskeletal networks composed of filamentous actin (F-actin) and microtubules (MTs), and perhaps cytokeratin (CK) filaments, play significant roles in establishing and maintaining the polarized architecture of amphibian oocytes and eggs, including features such as nuclear position, localization of maternal messenger ribonucleic acid (mRNA), and the location and orientation of the meiotic spindles. The examples in the chapter describe that the cytoskeleton itself undergoes a remarkable reorganization and polarization during the formation of the animal–vegetal (A–V) axis, and suggest that polarization of the oocyte cortex contributes to the structural and functional polarization of the oocyte.

Intermediate filament organization, reorganization, and function in the clawed frog Xenopus.

Abstract: Publisher Summary Cytoplasmic intermediate filaments (IFs) are arguably the most enigmatic component of the eukaryotic cytoskeleton. IF subunit proteins (IFPs) are a highly conserved common feature of vertebrate cells, where, they often form a substantial and largely insoluble network. The disruption of IF organization in cultured cells has little, if any, effect on cellular morphology or behavior. Even in the context of the intact organism, the absence of normally abundant IF networks have been seen to produce surprisingly subtle effects. It is also clear that IFs play critical roles in the maintenance of epidermal integrity, neuronal axon diameter, and muscle-extracellular matrix attachment sites. This chapter chose the African clawed frog Xenopus laevis for the studies of IF organization and function based on its economy (particularly when compared to mice) and the fact that Xenopus has been the focus of many recent studies on the cell biology of axis formation and cellular differentiation during embryogenesis. Xenopus offers unique experimental opportunities to study; (1) the cellular control of IF organization, (2) the processes of IF assembly/disassembly during the cell cycle, and (3) the role of IFs and associated structures in complex cellular functions.

Remodeling of the specialized intermediate filament network in mammalian eggs and embryos during development: regulation by protein kinase C and protein kinase M.

Abstract: The sheets serve as an maternal supply of assembled, cytokeratin, intermediate filaments. They are remodeled at each major developmental transition in mammalian early development, that is fertilization, embryonic compaction, blastocyst formation, and formation of the primitive ectoderm and primitive endoderm during implantation into the uterine wall. Our results indicate that the sheets exist as specialization for placental development as they have a major role in the maintenance of epithelial integrity at the time the embryo is implanting into the uterine wall. They also contribute intermediate filaments to the junctional complexes required for embryonic compaction. Our analyses demonstrate the they are regulated at the time of fertilization by the action of PKC/PKM, a kinase that acts as a cellular chronometer with both temporal and spatial precision that remodels the egg into the zygote.

Cytoskeleton and ctenophore development.

Abstract: Publisher Summary Ctenophores are gelatinous marine organisms characterized by rows of beating “combs” of cilia. They are related only very distantly to other animals. Although their precise evolutionary position is controversial, it is clear that they diverged very early from other metazoans. The ctenophore egg is a beautiful eukaryotic cell containing dynamic microtubule and actin filament networks with organelles shuffling around them. During the cleavage stages of development, a stereotypic asymmetric cleavage pattern produces cells with distinct shapes, sizes, and composition. Associated with these cleavages are dramatic cytoplasmic reorganizations, which segregate components between different cells. Distinct types of blastomere from the early embryo develop autonomously to give particular adult structures, implying that different “developmental potentials” become segregated at each cleavage. The challenge is to understand the relationship between the cytoplasmic events and the attribution of developmental potential to different cells. This chapter shows that Beroe ovatu eggs and embryos are ideal for detailed microscopy, and investigates the cell biology of early developmental events. The relationship between the behavior of the microtubule network and cytoplasmic relocation is examined and the findings and outlines the possibilities for future study in the context of embryological knowledge are summarized.