ISSN:0741-0581    

DOI:10.1002/jemt.1060160202

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1990

数据来源: WILEY

摘要:

AbstractThis review of the anatomical, histological, biochemical, and molecular biological literature on echinoderm oogenesis includes the entire developmental history of oocytes; from their inception to the time they become ova. This is done from a comparative perspective, with reference to members of the five extant echinoderm classes; crinoids, holothurians, asteroids, ophiuroids, and echinoids. I describe the anatomy and fine structure of the echinoderm ovary, with emphasis on both the cellular relationships of the germ line cells to the somatic cells of the inner epithelium, and on the neuromuscular systems. I review the literature on the growth of oogonia into fully formed oocytes, including the process of vitellogenesis, presenting an ultrastructural analysis of the organelles and extracellular structures found in fully formed echinoderm oocytes. Echinoderm oocyte maturation is reviewed and a description of the ultrastructural, biochemical and molecular biological changes thought to occur during this process is presented. Finally, I discuss oocyte ovulation, the severing of cellular connections between the oocyte and its surrounding somatic epithelial cells.

ISSN:0741-0581    

DOI:10.1002/jemt.1060160203

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1990

数据来源: WILEY

摘要:

AbstractThis review discusses the ultrastructure of sperm with reference to their development, the surface morphology of the egg, and the processes of sperm binding and penetration during fertilization. These topics are treated for selected invertebrates and lower vertebrates which live in aquatic environments and fertilize their eggs externally. Specifically, sperm and eggs from cnidarians, echinoderms, decapod crustaceans, ascidians, lampreys, bony fishes, and amphibians are discussed. Sperm from the majority of these groups exhibit the classical head‐midregion‐tail configuration characteristic of primitive sperm. Specific variations within this general morphology have been described. The notable exceptions to the primitive‐sperm paradigm are the sperm of decapod crustaceans and amphibians. Eggs from all of the animals considered are covered by complex vitelline envelopes except those of cnidarians. In general, the ultrastructural analysis of these egg envelopes shows that they are composed of fibrous subunits. Sperm bind to the vitelline envelope and then penetrate through it to fertilize the egg in all groups reviewed except fishes. In fishes, the sperm reaches the egg surface via micropyles. Finally, we discuss the specific changes in sperm ultrastructure which occur during penetration of the egg envelopes in both flagellated and non‐flagellated sperm. These changes, which involve membrane fusion and reorganization as well as movement of membranous organelles, aid the sperm in reaching the actual site of gamete

ISSN:0741-0581    

DOI:10.1002/jemt.1060160204

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1990

数据来源: WILEY

摘要:

AbstractAn accurate analytical procedure for phase identification for electron diffractionists has been developed. The method opens new frontiers in the identification of solid‐state materials, as crystalline samples in the size range 10 μm to 10 Å can be accurately characterized. Research with NIST CRYSTAL DATA (a large database with chemical, physical, and crystallographic data on solid‐state materials) has proved that a material can be uniquely characterized on the basis of its lattice and chemical composition. To characterize a material, it is sufficient to determine any primitive cell of the lattice and the element types present. Using a modern analytical electron microscope (AEM), the experimentalist can collect the required data on an unknown sample. The lattice information is obtained by rotation of the sample to obtain two or more planes of data. From these planes, a unit cell defining the lattice can be deduced. The chemical data are determined by energy‐dispersive spectroscopy (EDS). Once the experimental data are measured, the unknown is identified against the database of knowns using lattice/element‐type matching techniques. The basic strategy consists of three conceptual steps. First, the unknown lattice is searched against the database to find all lattices that are the same or related; the results are kept in set 1. Second, the unknown is searched against the database to find all materials with the same or similar element types; the results are kept in set 2. Finally, the results in sets 1 and 2 are combined to obtain the answer set. Experience has proved that the procedure is highly selective and

ISSN:0741-0581    

DOI:10.1002/jemt.1060160205

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1990

数据来源: WILEY

摘要:

AbstractWe have developed an instrument capable of freezing transient intermediates in rapid biochemical reactions for subsequent freeze‐fracturing, replication, and viewing by transmission electron microscopy. The machine combines a rapid mixing unit similar to one widely used in chemical kinetics (Johnson, 1986) with a propane jet freezing unit previously used to prepare static samples for freeze‐fracturing (Gilkey and Staehelin, 1986). The key element in the system is a unique thin‐walled flow cell of copper that allows for injection and aging of the sample, followed by rapid freezing. During freeze‐fracturing, a tangential cut is made along the wall of the flow cell to expose the sample for etching and replication. The dead time required for mixing and injection of the reactants into the flow cell is less than 5 ms. Electronic controls allow one to specify, on a millisecond time scale, any time above 5 ms between initiation of the reaction and quenching by rapid f

ISSN:0741-0581    

DOI:10.1002/jemt.1060160206

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1990

数据来源: WILEY

摘要:

AbstractA detailed design for a simple and inexpensive variable‐speed (1.0–5.8 m s−1) pneumatic plunge‐freezing device is presented. Cultured cells, grown on Formvar‐coated 75‐mesh gold finder grids, are pneumatically driven into a stirring mixture of propane/isopentane (3:1) cooled by liquid nitrogen (LN2). Premature freezing of the sample in the cryogenic vapors above the cryogen is prevented by plunging through an entry tube into an insulating box, to which a partial vacuum is applied. The cryogenic vapors are drafted into the box at the level of the liquid cryogen by the vacuum, thereby preventing a layer of cold gas from collecting above the cryogen. To prevent the sample from thawing during transfer from the cryogen to the substitution medium, the box top is removed and compressed air is forced through a corrugated tube running the length of the box. The resulting boiling LN2creates an atmosphere below −120°C in which the transfer can b

ISSN:0741-0581    

DOI:10.1002/jemt.1060160207

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1990

数据来源: WILEY

ISSN:0741-0581    

DOI:10.1002/jemt.1060160201

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1990

数据来源: WILEY