Actin-like filaments in the acrosomal apparatus of spermatozoa of a sea urchin

The Effect of Synthetic Polyamine BPA-C8 on the Fertilization Process of Intact and Denuded Sea Urchin Eggs

Sea urchin eggs are covered with layers of extracellular matrix, namely, the vitelline layer (VL) and jelly coat (JC). It has been shown that sea urchin eggs' JC components serve as chemoattractants or ligands for the receptor on the fertilizing sperm to promote the acrosome reaction. Moreover, the egg's VL provides receptors for conspecific sperm to bind, and, to date, at least two sperm receptors have been identified on the surface of sea urchin eggs. Interestingly, however, according to our previous work, denuded sea urchin eggs devoid of the JC and VL do not fail to become fertilized by sperm. Instead, they are bound and penetratedby multiple sperm, raising the possibility that an alternative pathway independent of the VL-residing sperm receptor may be at work. In this research, we studied the roles of the JC and VL using intact and denuded eggs and the synthetic polyamine BPA-C8. BPA-C8 is known to bind to the negatively charged macromolecular complexes in the cells, such as the JC, VL, and the plasma membrane of echinoderm eggs, as well as to the actin filaments in fibroblasts. Our results showed that, when added to seawater, BPA-C8 significantly repressed the Ca2+ wave in the intact P. lividus eggs at fertilization. In eggs deprived of the VL and JC, BPA-C8 binds to the plasma membrane and increases fibrous structures connecting microvilli, thereby allowing the denuded eggs to revert towards monospermy at fertilization. However, the reduced Ca2+ signal in denuded eggs was nullified compared to the intact eggs because removing the JC and VL already decreased the Ca2+ wave. BPA-C8 does not cross the VL and the cell membrane of unfertilized sea urchin eggs to diffuse into the cytoplasm at variance with the fibroblasts. Indeed, the jasplakinolide-induced polymerization of subplasmalemmal actin filaments was inhibited in the eggs microinjected with BPA-C8, but not in the ones bath-incubated with the same dose of BPA-C8.

The Chlamydomonas mating type plus fertilization tubule, a prototypic cell fusion organelle: isolation, characterization, and in vitro adhesion to mating type minus gametes

In the biflagellated alga Chlamydomonas, adhesion and fusion of the plasma membranes of gametes during fertilization occurs via an actin-filled, microvillus-like cell protrusion. Formation of this approximately 3-microm-long fusion organelle, the Chlamydomonas fertilization tubule, is induced in mating type plus (mt+) gametes during flagellar adhesion with mating type minus (mt-) gametes. Subsequent adhesion between the tip of the mt+ fertilization tubule and the apex of a mating structure on mt- gametes is followed rapidly by fusion of the plasma membranes and zygote formation. In this report, we describe the isolation and characterization of fertilization tubules from mt+ gametes activated for cell fusion. Fertilization tubules were detached by homogenization of activated mt+ gametes in an EGTA-containing buffer and purified by differential centrifugation followed by fractionation on sucrose and Percoll gradients. As determined by fluorescence microscopy of samples stained with a fluorescent probe for filamentous actin, the method yielded 2-3 x 10(6) fertilization tubules/microg protein, representing up to a 360-fold enrichment of these organelles. Examination by negative stain electron microscopy demonstrated that the purified fertilization tubules were morphologically indistinguishable from fertilization tubules on intact, activated mt+ gametes, retaining both the extracellular fringe and the internal array of actin filaments. Several proteins, including actin as well as two surface proteins identified by biotinylation studies, copurified with the fertilization tubules. Most importantly, the isolated mt+ fertilization tubules bound to the apical ends of activated mt- gametes between the two flagella, the site of the mt- mating structure; a single fertilization tubule bound per cell, binding was specific for gametes, and fertilization tubules isolated from trypsin-treated, activated mt+ gametes did not bind to activated mt- gametes.

Cytochalasin B does not block sperm penetration into denuded starfish oocytes

During fertilisation in starfish oocytes, the fertilisation cone develops temporarily beneath the penetrating sperm. The role of the fertilisation cone in sperm incorporation in the starfish Asterias amurensis was examined using cytochalasin B (CB). CB (2 microM) allowed sperm acrosomal process-egg plasma membrane fusion and egg activation, but inhibited the development of the fertilisation cone containing actin microfilaments. When sperm were added to intact oocytes (with the jelly coat and vitelline coat) in seawater containing CB, the sperm head did not penetrate the fertilisation membrane. Although the acrosomal process fused with egg plasma membrane, the sperm head remained outside the fertilisation membrane. On the other hand, denuded oocytes without the jelly coat and vitelline coat allowed sperm penetration even in the presence of 2 microM CB. Electron microscopy revealed that sperm organelles, including the acrosomal process, nucleus, mitochondrion and tail, were incorporated into the slightly electron-dense cytoplasm, which was similar to the cytoplasm of the fertilisation cone. These results show that the development of the fertilisation cone/actin filament complex is not essential for incorporation of the sperm, since incorporation can occur in denuded oocytes. However, the cone is required for fertilisation of intact oocytes, suggesting that this actin-filament-containing structure is necessary for getting the sperm through the outer egg coats.

Mouse testes contain two size classes of actin mRNA that are differentially expressed during spermatogenesis

Using several actin isotype-specific cDNA probes, we found actin mRNA of two size classes, 2.1 and 1.5 kilobases (kb), in extracts of polyadenylated and nonpolyadenylated RNA from sexually mature CD-1 mouse testes. Although the 2.1-kb sequence was present in both meiotic and postmeiotic testicular cell types, it decreased manyfold in late haploid cells. The 1.5-kb actin sequence was not detectable in meiotic pachytene spermatocytes (or in liver or kidney cells), but was present in round and elongating spermatids and residual bodies. To differentiate between the beta- and gamma-actin mRNAs, we isolated a cDNA, pMGA, containing the 3' untranslated region of a mouse cytoplasmic actin that has homology to the 3' untranslated region of a human gamma-actin cDNA but not to the 3' untranslated regions of human alpha-, beta-, or cardiac actins. Dot blot hybridizations with pMGA detected high levels of presumptive gamma-actin mRNA in pachytene spermatocytes and round spermatids, with lower amounts found in elongating spermatids. Hybridization with the 3' untranslated region of a rat beta-actin probe revealed that round spermatids contained higher levels of beta-actin mRNA than did pachytene spermatocytes or residual bodies. Both probes hybridized to the 2.1-kb actin mRNA but failed to hybridize to the 1.5-kb mRNA.

Mouse testes contain two size classes of actin mRNA that are differentially expressed during spermatogenesis

Using several actin isotype-specific cDNA probes, we found actin mRNA of two size classes, 2.1 and 1.5 kilobases (kb), in extracts of polyadenylated and nonpolyadenylated RNA from sexually mature CD-1 mouse testes. Although the 2.1-kb sequence was present in both meiotic and postmeiotic testicular cell types, it decreased manyfold in late haploid cells. The 1.5-kb actin sequence was not detectable in meiotic pachytene spermatocytes (or in liver or kidney cells), but was present in round and elongating spermatids and residual bodies. To differentiate between the beta- and gamma-actin mRNAs, we isolated a cDNA, pMGA, containing the 3' untranslated region of a mouse cytoplasmic actin that has homology to the 3' untranslated region of a human gamma-actin cDNA but not to the 3' untranslated regions of human alpha-, beta-, or cardiac actins. Dot blot hybridizations with pMGA detected high levels of presumptive gamma-actin mRNA in pachytene spermatocytes and round spermatids, with lower amounts found in elongating spermatids. Hybridization with the 3' untranslated region of a rat beta-actin probe revealed that round spermatids contained higher levels of beta-actin mRNA than did pachytene spermatocytes or residual bodies. Both probes hybridized to the 2.1-kb actin mRNA but failed to hybridize to the 1.5-kb mRNA.

Interactions between sperm and sea urchin egg jelly

The addition of egg jelly to sea urchin sperm induces multiple changes in morphology and behavior. When jelly is added to sperm diluted in seawater, the acrosome reaction is triggered, the mitochondrion rounds up, the internal pH is transiently alkalinized and then reacidified, and respiration becomes uncoupled and rapidly decreases. Sperm also become unable to fertilize eggs within a few minutes after jelly addition. In order to explore in more detail the effect of egg jelly on sperm, we have studied the response to jelly in the presence of inhibitors of the acrosome reaction. When jelly is added to sperm under conditions which are inhibitory for the acrosome reaction, an alkalinization takes place without the subsequent reacidification, the mitochondria remain coupled, and respiration and intracellular ATP levels remain high. Sperm viability is prolonged by some of these conditions, but not others. The addition of jelly to sperm in the absence of calcium elicits an internal alkalinization but no other rapid change in sperm physiology. The capacity of egg jelly to alter sperm physiology even when the overall acrosome reaction is inhibited indicates that some of the physiological changes either are early events in the triggering pathway that happen before the inhibitory step or are unrelated to the acrosomal reaction itself. The reacidification of the internal pH, the uncoupling and decrease of the respiration, and the decrease of the ATP levels might be linked together by the large influx of calcium that occurs after the acrosome reaction.

An ultrastructural analysis of Dentalium vulgare (Mollusca, Scaphopoda) gametes with special reference to early events at fertilization

The ultrastructure of Dentalium gametes, and their fate at fertilization, were investigated. Unexpectedly, fixation carried out in 2% osmium tetroxide, at 4 degrees C, although destroying most cytoplasmic organelles, reveals the presence of bundles of "stress fibers" in the microvilli and the fertilization cone of the eggs. Anti-actin labeling supports the view that the bundles are made of actin. Fertilization, as in other molluscs, does not cause any cortical granule exocytosis, at least during the first 15 min following insemination. Ultrastructure of the unreacted and reacted spermatozoon is presented and a tentative model for the morphological interpretation of the acrosomal reaction is proposed. A reevaluation of cytoplasmic heterogeneity, e.g., the so-called cytoplasmic prelocalization, as observed after the germinal vesicle breakdown is also provided as well as an analysis of concomitant surface changes.

Actin in mammalian sperm heads

An actin-like substance has been detected in the postacrosomal region of mammalian spermatozoa by indirect immunofluorescence. The antigen was localized using an anti-actin antiserum from a patient with active chronic hepatitis. The actin-like contractile proteins may be important in sperm function and spermegg interactions during fertilization.

Is P-protein actin-like?-not yet

Microfilaments associated with cytoplasmic streaming in Nitella flexilis internodes can be decorated with heavy meromyosin (HMM) from rabbit, both in vitro in cytoplasmic suspensions, and in situ in glycerinated cell segments. The bound HMM consists of clearly discernible, polarized arrowheads in a regular repeat of 360-380 Å that are similar to those produced on F-actin. In contrast, similar arrowheads or decorations are not evident on P-protein filaments in sieve elements of glycerinated hypocotyl segments of Phaseolus vulgaris L. treated with HMM. Thus, these results contradict a recent claim that P-protein binds HMM and is actin-like. The mass of other evidence now available from diverse studies indicating that P-protein does not consist of actin or tubulin is discussed.

Observations on the functional organization of the ovarian balls of Moniliformis and Polymorphus (Acanthocephala)

By a variety of microscopical techniques, an initial survey has been made of the functional organization of ovarian balls from mature, inseminated femaleMoniliformis dubiusandPolymorphus minutus.On the basis of evidence from observations made with the transmission electron microscope, we have concluded that the ovarian ball consists of three components; these are two separate, multinucleate syncytia and a cellular zone. The inner region is considered to be an oogonial syncytium from which the germ-line cells arise to form the cellular zone. The oogonial syncytium and the cellular elements are embedded in a supporting syncytium which also forms the boundary of the ovarian ball. Details of the ultrastructure of these components, observations on fertilization and a hypothesis to account for the main events occurring in an acanthocephalan ovarian ball are also presented.

Acanthocephalans are dioecious parasites which become sexually mature in the alimentary tract of vertebrates. After insemination, individual female worms begin to release embryonated eggs at rates which have been estimated to vary, on average, from 2000 per day in the case ofPolymorphus minutus(Crompton & Whitfield, 1968) to 260000 per day in the case ofMacracanthorhynchus hirudinaceus(Kates, 1944). Irrespective, however, of the species of acanthocephalan and the number of eggs produced, a most interesting mode of ovarian development and multiplication is believed to exist (see Bullock, 1969). Within the female worm, the primordial ovarian tissue gives rise to separate ovaries which are often termed ovarian balls. These in turn give rise to more ovarian balls and the process continues until large numbers have been formed. The ovarian balls of a mature female have no permanent attachment to the tissues of the worm. They are contained in the fluid of the body cavity either freely, or loosely constrained in large membranous chambers known as ligament sacs.

Few investigations have been made into the structure and cytology of the ovarian balls of the Acanthocephala. Early observations on the histology of the germ-line constituents of ovarian balls were made by Hamann (1891), Kaiser (1893) and Meyer (1928), each of whom suggested that some of the developing oogonial stages were arranged in a syncytial manner. In a study of the embryology ofP. minutus.Nicholas & Hynes (1963) interpreted preparations of ovarian balls, which had been subjected to the Feulgen reaction, as indicative of syncytial tissue in the central region of the ball. They also recorded their uncertainty about whether an ovarian ball has a bounding membrane or not. More recently, Robinson (1964, 1965) described mitotic and meiotic divisions in oogonia and oocytes from ovarian balls ofMac. hirudinaceusandMoniliformis dubiusand Stranack (1972) included an electron micrograph of part of an ovarian ball ofPomphorhynchus laevisin a description of the nature of the egg envelopes of that species. In this paper, we present a general survey, based largely on evidence obtained with the electron microscope, of the functional organization of the ovarian balls of matureM. dubiusandP. minutus.