In some animals, such as fish, insects, and cephalopods, the thick egg coat has a narrow canal-a micropyle-through which spermatozoa enter the eggs. In fish, there is no indication that spermatozoa are attracted by eggs from a distance, but once spermatozoa come near the outer opening of the micropyle, they exhibit directed movement toward it, suggesting that a substance exists in this defined region to attract spermatozoa. Since Coomassie Blue (CB) binds preferentially to the micropyle region in flounder, herring, steelhead, and other fish, it probably stains this sperm guidance substance. This substance-a glycoprotein based on lectin staining-is bound tightly to the surface of the chorion, but can be removed readily by protease treatment. Although fertilization in fish (flounder) is possible after removal of this substance, its absence makes fertilization inefficient, as reflected by a drastic reduction in fertilization rate. The sperm "attraction" to the micropyle opening is species specific and is dependent on extracellular Ca(2+). Eggs of some insects, including Drosophila, have distinct micropyle caps with CB affinity, which also may prove to assist sperm entry. Our attempts to fertilize fly eggs in vitro were not successful.

When the micropyle area of salmonid (trout and salmon) eggs was observed continuously from the moment of insemination, spermatozoa were seen moving along the surface of the chorion and entering the micropyle one by one in a directed fashion. The ability of spermatozoa to enter the micropyle was reduced after the treatment of chorions with pronase; this reduction in sperm entry was observed even before the outer opening of the micropyle channel was narrowed due to gradual swelling of the chorion by pronase treatment. Herring spermatozoa, unlike spermatozoa of most other marine fishes, were motionless in seawater. However, they became vigorously motile on contact with the micropyle area of the herring egg chorion and entered the micropyle rapidly and efficiently. Motility initiation of herring spermatozoa in the micropyle area was dependent on extracellular calcium and potassium. Sodium also appears to be intricately involved in this process as demonstrated by the initiation of sperm movement in sodium-free seawater. When herring eggs were treated with acidic seawater, organic solvents, or glutaraldehyde, spermatozoa did not initiate movement in the micropyle area, and sperm entry was not observed. Herring spermatozoa did not initiate movement in the micropyle area of salmonid eggs. These and other observations suggest that the micropyle areas of salmonid and herring eggs possess some sperm guidance factors which facilitate entry of homologous spermatozoa into the micropyle.

Sperm of the Pacific herring, Clupea pallasi, are unique in that they are immotile upon spawning in the environment. Herring sperm have evolved to remain motionless for up to several days after spawning, yet are still capable of fertilizing eggs. An egg chorion ligand termed ''sperm motility initiation factor'' (SMIF) induces motility in herring sperm and is required for fertilization. In this study, we show that SMIF induces calcium influx, sodium efflux, and a membrane depolarization in herring sperm. Sperm motility initiation by SMIF depended on decreased extracellular sodium (<350 mM) and could be induced in the absence of SMIF in very low sodium seawater. Motility initiation depended on > 1 mM extracellular calcium. Calcium influx caused by SMIF involved both the opening of voltage-gated calcium channels and reverse sodiumcalcium (Na ؉ ͞Ca 2؉ ) exchange. Membrane depolarization was slightly inhibited by a calcium channel blocker and markedly inhibited by a Na ؉ ͞Ca 2؉ exchange inhibitor. Sodium efflux caused by SMIF-initiated motility was observed when using both extracellular and intracellular sodium probes. A Na ؉ ͞Ca 2؉ exchange antigen was shown to be present on the surface of the sperm, primarily over the midpiece, by using an antibody to the canine Na ؉ ͞Ca 2؉ exchanger. This antibody recognized a 120-kDa protein that comigrated with the canine myocyte Na ؉ ͞Ca 2؉ exchanger. Sperm of Pacific herring are now shown to use reverse Na ؉ ͞Ca 2؉ exchange in motility initiation. This mechanism of regulation of motility initiation may have evolved for both maintenance of immotility after spawning as well as ligand-induced motility initiation.T eleost fish sperm are quiescent within the testes and seminal plasma before spawning, but most initiate motility after dilution into the external medium (freshwater or seawater for most species) in which spawning occurs (1). In salmonids, motility initiation occurs with dilution in freshwater, specifically from a reduction in extracellular potassium that drives a membrane hyperpolarization and an increase in intracellular calcium ([Ca 2ϩ ] i ) (2-6). Changes in the concentrations of specific ions (Ca 2ϩ , K ϩ , and possibly Na ϩ and Cl Ϫ ) also have been linked to motility initiation in Atlantic croaker sperm (7). A hyperpolarization of the sperm membrane also has been documented in carp sperm, reportedly linked to the opening of voltage-gated Ca 2ϩ channels and an increase in [Ca 2ϩ ] i (8). In other freshwater teleost sperm (goldfish, zebra fish), as well as in marine teleost sperm (puffer, flounder), motility is believed to occur as a result of nonspecific hypo-or hyperosmotic changes that drive changes in intracellular ion concentrations (5, 9, 10). Thus, in all systems studied to date, a membrane hyperpolarization leads to an increase in [Ca 2ϩ ] i and the initiation of motility.Although the stimulation of sperm motility in the vicinity of eggs has been reported in some teleost fish, only herring sperm have been shown to require an egg chorion-derived ligand for ...