48 SEPTEMBER 2013 • WORLD AQUACULTURE • WWW.WAS.ORG various physiological conditions, leading to comparative information describing swimming in relation to their ability to fertilize and/or to be cryopreserved and the sensitivity of swimming traits to trace concentrations of potent chemical pollutants (Cosson et al. 2008a). Why Study Fish Spermatozoa? Several advantages favor studies on fish spermatozoa: • Broodfish, especially of farmed species, can be made available all year. • In many fish species, spermatogenesis can be induced by hormonal injection, which facilitates access to sperm samples for experiments. • Fish sperm is easy to store under laboratory conditions. • Milt can be collected in large quantities, both in volume and density. For example, in rainbow trout Oncorhyncus mykiss, sperm can be collected up to one year ahead by manipulation of environmental conditions such as photo- or thermoperiod and stored under laboratory condition for 8-12 h in plastic tubes on ice at 4 C. Up to 50 mL of milt containing 4x1010 sperm/mL per male can be collected, allowing biochemical purification of components, such as flagella, dyneins or proteasomes. • Fish sperm is usually immotile in seminal fluid and it is easy to fully trigger sperm motility by transfer to an activation (swimming) medium. • Fish spermatozoa present homogenous behavior, meaning that all spermatozoa swim in a very similar manner at a certain time after activation. In many fish species, the average length of spermatozoon is 50-60 µm and up to 6 curvatures (3 full sine waves) are observed along the length while swimming. • Symmetry or asymmetry of flagellar beating is dependent on species, but also on advancement in the motility period. As an example, turbot spermatozoa follow tracks which are quasilinear; flagellar bending is symmetrical because of the absence of sensitivity to Ca2+ ion concentration. • In most fish species, attenuation or “damping” occurs along the length of the flagellum, an event that is fully reversible. Damping is an attenuation of wave amplitude appearing in the distal portion of the flagellum as the motility period progresses (especially at the end). Signals to Activate Motility Received by Fish Sperm Cells from the Environment Signals coming from the environment are mainly osmolarity, ionic and gaseous components of external media and, in some cases, egg-derived substances. In most freshwater species, spermatozoa usually move for less than 2 min and, in some cases, are highly active for less than 30 sec. So, in conditions of natural reproduction, motility is induced after release of spermatozoa from the male genital tract into the aqueous environment, where spermatozoa must cope with water-soluble components, mostly ions, of the external milieu. Osmolarity of the environment is a ubiquitous factor involved in motility activation. Osmolality of seminal plasma differs greatly in freshwater and seawater fish and slightly among fish species (Alavi and Cosson 2005, Alavi and Cosson 2006). The nature of external signal triggering motility initiation is highly dependent on the fish reproduction environment (freshwater or seawater) and peculiarities of reproductive behavior. For example, freshwater fish sperm cells, when released into the surrounding water, can increase their cytoplasmic volume in response to osmotic stress. Carp spermatozoa can increase their volume several times as a result of the influx of water. An opposite effect occurs in seawater species: hypertonicity induces the motility of spermatozoa in many species of marine teleosts. Additional factors can control sperm motility in specific species. In herring, for instance, activation needs the contact of sperm with egg-derived substances. In seminal fluid of tilapia, a sperm motility inhibiting factor (Mochida et al. 1999) is responsible for immotility before contact with external fluid. Osmotic stress (hypotonicity or hypertonicity) is not the only factor triggering sperm motility, but osmolarity effects on sperm cells are difficult to separate from the effects of ionic and gaseous composition of the surrounding media. There is much published information concerning the specific effects of K+, Ca2+, Mg2+ and other cations on sperm motility activation in particular fish species. As for the effects of different gaseous components of external media, dissolved CO2 in equilibrium with HCO3 - contributes to the ion concentration and, therefore, to osmolarity. This is a very efficient way to control motility in spermatozoa of flatfish species (Inaba et al. 2003). Nitric oxide gas enhances motility of fathead minnow spermatozoa (Creech et al. 1998) at a very low active concentration range. Outside Signal Perception by the Sperm Membrane Various ion channels are present in the sperm plasma membrane (Inaba 2003). The involvement of K+ and Ca2+ transport through ion channels at the plasma membrane of spermatozoa in triggering motility initiation has been demonstrated for rainbow trout spermatozoa. Carp sperm motility is inhibited by Ca2+ channel blockers and recovery of motility occurs after removal of these blockers by washing, strongly suggesting that Ca2+ influx through Ca2+ channel and the resulting increase in intracellular Ca2+ are major events triggering carp sperm motility (Krasznai et al. 2000). Cytosolic pH (protons) is considered as another component of signaling pathways that influence sperm motility. The decrease of internal pH in sperm directly inhibits dynein-motor activity. The nature of osmotic signal perception at the membrane level is less understood. One aspect to consider is the mechanosensitivity of flagella. Gadolinium, a specific and reversible inhibitor of stretch activated channels (SAC), is active on carp spermatozoa (Krasznai et al. 2003a, 2003b) and more generally on spermatozoa of several fish species, including marine species such as seabass, turbot and tuna (Cosson et al. 2008b), but inactive on sperm of species other than fishes. The SACs are mechanosensitive channels that increase membrane conductivity to ions, such as Ca2+ or K+, when distortion of the membrane occurs by mechanical constraints. Mechano-sensitivity is of biological importance (Ingber 2006); flagella act as mechano-sensitive detectors (Kambara et al. 2011). Aquaporins play a potent role in fish sperm activation. The putative presence of aquaporins in fish spermatozoa is confirmed because sperm motility in several marine fish species is sensitive to specific inhibitors of aquaporins (e.g. HgCl2) at very low concentrations (Cosson et al. 1999, Abascal et al. 2007). Low concentrations of HgCl2 reduce motility of turbot and seabass
RkJQdWJsaXNoZXIy MjExNDY=