Incorporation and dispersal of sperm surface antigens in plasma membranes of inseminated sea urchin (Arbacia punctulata) eggs and oocytes

Paternal contributions to the mammalian zygote: fertilization after sperm-egg fusion

Mammalian fertilization has traditionally been regarded as a simple blending of two gametes, during which the haploid genome of the fertilizing spermatozoon constitutes the primary paternal contribution to the resulting embryo. In contrast to this view, new research provides evidence of important cytoplasmic contributions made by the fertilizing spermatozoon to the zygotic makeup, to the organization of preimplantation development, and even reproductive success of new forms of assisted fertilization. The central role of the sperm-contributed centriole in the reconstitution of zygotic centrosome has been established in most mammalian species and is put in contrast with strictly maternal centrosomal inheritance in rodents. The complementary reduction or multiplication of sperm and oocyte organelles during gametogenesis, exemplified by the differences in the biogenesis of centrosome in sperm and oocytes, represents an intriguing mechanism for avoiding their redundancy during early embryogenesis. New studies on perinuclear theca of sperm revealed its importance for both spermatogenesis and fertilization. Remodeling of the sperm chromatin into a male pronucleus is guided by oocyte-produced, reducing peptide glutathione and a number of molecules required for the reconstitution of the functional nuclear envelope and nuclear skeleton. Although some of the sperm structures are transformed into zygotic components, the elimination of others is vital to early stages of embryonic development. Sperm mitochondria, carrying potentially harmful paternal mtDNA, appear to be eliminated by a ubiquitin-dependent mechanism. Other accessory structures of the sperm axoneme, including fibrous sheath, microtubule doublets, outer dense fibers, and the striated columns of connecting piece, are discarded in an orderly fashion. The new methods of assisted fertilization, represented by intracytoplasmic sperm injection and round spermatid injection, bypass multiple steps of natural fertilization by introducing an intact spermatozoon or spermatogenic cell into oocyte cytoplasm. Consequently, the carryover of sperm accessory structures that would normally be eliminated before or during the entry of sperm into oocyte cytoplasm persist therein and may interfere with early embryonic development, thus decreasing the success rate of assisted fertilization and possibly causing severe embryonic anomalies. Similarly, foreign organelles, proteins, messenger RNAs, and mitochondrial DNAs, which may have a profound impact on the embryonic development, are propagated by the nuclear transfer of embryonic blastomeres and somatic cell nuclei. This aspect of assisted fertilization is yet to be explored by a focused effort.

Stages leading to and following fusion of sperm and egg plasma membranes

The site of gamete interaction of electrophysiologically recorded Lytechinus variegatus eggs, fixed with osmium tetroxide (OsO4) and/or glutaraldehyde (GTA) at varying intervals after the onset of the increase in membrane conductance induced by an attached sperm, has been examined by high-voltage and conventional transmission electron microscopy. Although GTA and a GTA-OsO4 mixture induced different electrical responses, specimens prepared with the two fixatives were ultrastructurally similar. In specimens observed within 5 s of the change in conductance, the acrosomal process projected through the vitelline layer and abutted the egg plasma membrane. A conspicuous layer of bindin surrounded the acrosomal process and connected the sperm to the egg's vitelline layer. In a fortuitous specimen fixed within 4 s following the change in conductance, the area of contact between the gamete plasma membranes possessed a trilaminar structure that separated the egg's and sperm's cytoplasms. The morphology of this area of contact was consistent with previously proposed intermediates of membrane fusion. Five to six seconds after the change in conductance, the sperm was connected to the egg via a narrow cytoplasmic bridge that consisted of the former acrosomal process and a projection of the egg cortex. The region of the bridge midway between the fused gametes was encircled by dense material that marked the site of sperm-egg fusion. Gamete interactions in which the activation potential was recorded (unclamped egg) were comparable in time and ultrastructure to events taking place in voltage-clamped eggs except for one major difference. Intact cortical granules (one to three) were observed beneath the tip of the incorporating sperm in unclamped eggs fixed following the onset of the activation potential, whereas all cortical granules dehisced in clamped eggs.

Fertilization-induced changes in the vitelline envelope of echinoderm and amphibian eggs: self-assembly of an extracellular matrix

The surface of the unfertilized sea urchin egg is covered by the vitelline layer (VL), a fibrous extracellular matrix that contains receptors for sperm. At fertilization, cortical granule exocytosis releases enzymes and structural proteins that cause the VL to elevate and become remodelled into the mechanically and chemically tough fertilization envelope. This envelope prevents further penetration of sperm and protects the embryo during early development. A thicker, more complex vitelline envelope surrounds the Xenopus laevis egg. This fibrous coat is also restructured at fertilization to produce an impenetrable barrier to sperm. The biochemical steps that occur during self-assembly of these fertilization envelopes are reviewed, and the ultrastructural changes that occur, as seen in platinum replicas of quick-frozen, deep-etched, and rotary-shadowed eggs, are illustrated.

Gamete interactions and the fate of sperm organelles in fertilized echinoderm eggs

Investigations of gamete fusion, sperm entry and the fate of the sperm nucleus, plasma membrane, mitochondrion, and axonemal complex in fertilized echinoderm eggs are reviewed. The timing of gamete fusion with respect to the onset of electrical activity characteristic of the activated egg and the affects of fixation conditions on the stability of fusing membranes are discussed. Observations from investigations using cationized ferritin labeled gametes and immunogold cytochemistry to demonstrate the mixing of sperm plasma membrane components within the egg plasma membrane, in particular along the surface of the fertilization cone, are compared with results from studies in somatic cells. Transformations of the sperm nucleus into a male pronucleus, consisting of sperm nuclear envelope breakdown, chromatin dispersion, and formation of a pronuclear envelope, are correlated with recent biochemical observation of similar processes in other cellular systems. Fates of the sperm mitochondrion and axonemal complex are examined.

A hyaline layer protein that becomes localized to the oral ectoderm and foregut of sea urchin embryos

An antigen is described which is a marker for the oral ectoderm and foregut of the sea urchin embryo. In Lytechinus variegatus, the antigen is first detectable by immunofluorescence on the surface of fertilized eggs, and remains globally distributed through the early stages of gastrulation. Thereafter the antigen is localized to the oral ectoderm and foregut, coincident with the morphogenesis of these regions. The antigen is a large, detergent-insoluble, filamentous glycoprotein associated with the tips of the microvilli in the hyaline layer. This glycoprotein is present in two forms, a approximately 350-kDa form that is maternally synthesized and a much larger form which is synthesized at late gastrula stage as a 350-kDa precursor before becoming modified and assembled into the hyaline layer. The timing of synthesis of the zygotic form of the molecule correlates precisely with the localized expression of the antigen. The antigen copurifies with intact hyaline layers and cosediments with hyalin in the presence of calcium, suggesting that it is a structural component of the hyaline layer.

Incorporation and fate of specific sperm plasma membrane components following insemination as revealed by ultrastructural immunocytochemistry

Studies have been carried out to 1) further characterize sperm specific plasma membrane polypeptides (33 and 35 kDa) that are recognized by a monoclonal antibody previously described (Longo, 1989) and 2) follow the incorporation and dispersal of these proteins within plasmalemmae of monospermic and polyspermic sea urchin (Arbacia punctulata) eggs and oocytes utilizing immunocytochemical methods at the ultrastructural level of observation. Only sperm labeled when incubated with monoclonal antibody to the 33 and 35 kDa proteins followed by colloidal gold-tagged second antibody. Colloidal gold label was observed on the egg plasma membrane immediately after gamete membrane fusion; the amount and extent of label, i.e., the distance from the site of sperm incorporation, increased with time postinsemination. By 20 min postinsemination approximately one hemisphere of the inseminated egg/oocyte was associated with label. The expanding distribution of colloidal gold label on inseminated eggs and oocytes vs. time reflects the free diffusion of 33 and 35 kDa sperm surface proteins among egg/oocyte plasma membrane components. Label was also found in forming endocytotic vesicles, suggesting that sperm plasma membrane proteins may be internalized.