It especially focuses on the latter, reviewing the wide variety of actinobacterial bioactive molecules and their benefits for diverse industrial applications such as agriculture, aquaculture, biofuel production and food technology. SearchWorks Catalog Sperm morphology and biology in sturgeon. Please select Ok if you would like to proceed with this sperrmatology anyway. Alpha Science International Ltd.
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Introduction The main function of a spermatozoon is to convey the male genome remotely to the female one, which occurs in case of fish by swimming in the external milieu, marine or freshwater. Spermatozoa must access, bind, and penetrate an egg, for successful fertilization. Therefore, most of the physiological activity of fish spermatozoa is motility oriented. These processes include as a prerequisite the activation of spermatozoon motility.
In case of fish species, spermatozoa stored in the seminal plasma are immotile during transit through the genital tract of most externally fertilizing teleost and chondrostean. Motility is induced immediately following the release of spermatozoa from the male genital tract into the aqueous environment.
Triggering signals include osmotic pressure, ionic and gaseous components of the external media, and, in some cases, egg-derived substances used for sperm guidance. Environmental factors influencing fish spermatozoon motility have received a large attention: these extensive studies led to several mechanisms of activation for freshwater and marine fish spermatozoa. However, after reception of the signal, a transduction pathway initiated by these mechanisms must lead the information to the flagellar motility apparatus axoneme.
For each step in this signaling process, quantitative methods were developed to evaluate the quality of the sperm samples that are used for aquaculture propagation of many fish species. These methods as well as examples of their usefulness for application to fish artificial reproduction are presented in the last part of this chapter. In teleost fish, spermatozoa generally have no acrosome in contrast to chondrostean such as sturgeon , and the impenetrable chorion presents a micropyle that gives access to the membrane of the oocyte.
Spermatozoa often show a spherical nucleus with homogenous, highly condensed chromatin, a nuclear fossa, a midpiece of variable size with or without a cytoplasmic channel, and one or two long flagella [ 2 ].
Moreover, fish spermatozoa can be classified into two forms, aqua sperm and intro-sperm, according the external or internal mode of fertilization, respectively [ 2 ].
The main components present in a fish spermatozoon [ 3 , 4 ] are: the head is occupied mostly by the nucleus with paternal DNA material. In a mature spermatozoon, because the protein synthesis machinery is absent, no gene expression occurs. The centriolar complex of midpiece consists of the proximal and the distal centrioles, which forms the basal body of the flagellum, used for anchoring the flagellum to the head of the sperm cell.
The flagellum is a highly conserved organelle during evolution, and there are very few differences between molecular composition of sperm flagella relatively to that in protists [ 1 , 10 ].
Flagellar bending is generated by a highly organized cylindrical system of microtubules, called the axoneme, emanating from the basal body [ 11 ]. This structural arrangement [ 12 ] is illustrated in Figure 1. Such axonemal pattern is highly conserved and almost identical among eukaryotic cilia and flagella from protozoans to human. Figure 1. Schematic structure of the head-tail junction of a model spermatozoon: the artistic view applies to a teleost such as trout  as an example of sperm cell.
In many fish species, the mitochondria not shown for simplification are localized in this head-tail junction zone, and two centrioles are orthogonally assembled to form the centriolar complex. The structural connections between the nine peripheral outer doublets and the sheath surrounding the central pair occur through the radial spokes. The central pair of singlets is enclosed in this sheath of proteins forming a series of projections that are well positioned to interact with each of the spoke heads and regulate the wave propagation [ 14 ].
Each of the outer doublets is connected to adjacent pairs of doublets by nexin links, presenting elastic properties allowing to resist the free sliding of the microtubules; nexin is a dynein regulatory protein [ 15 ].
The peripheral doublets are strung with two rows of dynein arms along the entire length of microtubules. These dynein arms consist of macromolecular ATPase complex [ 16 , 17 ] and represent the basic motor actuating the whole axoneme; they extend from an outer doublet toward an adjacent doublet [ 18 ].
Both the spokes and the dynein complex contain different calcium-binding proteins so as for flagella to be able to respond to regulation by free calcium concentration through altering their beating pattern [ 19 , 20 ]. As briefly described above, axonemes are complex structures composed of at least different protein components [ 21 ].
The bending process in an axoneme is caused by sliding between two adjacent doublets of outer microtubules that slide relatively to each other due to the motive force, generated by molecular dynein motor activity [ 16 ]. Due to enzymatic hydrolysis of ATP by the latter, which induces force generation of the power stroke of individual dynein molecules, the dynein arms interact with tubulin of the B tubule from the adjacent doublet, causing a process of active sliding in a cooperative way [ 22 ].
Several local bending processes occur because this sliding activity is present in only some segments of the axoneme at a given time, while other segments remain inactive [ 4 ]. Wave propagation from head to tip provokes the translation of the whole spermatozoon in the opposite forward direction.
Spermatogenesis in fish For obtainment of full efficiency of motility, all the elements of the flagellum must have been, during spermatogenesis, correctly assembled mostly as an elongated structure called axoneme playing the role of propelling engine, surrounded by the plasma membrane, and this device must be provided in energy in terms of ATP [ 8 ], the fuel common to many cell types that is mostly generated by mitochondrial respiration in case of fish sperm as detailed in paragraph 4.
Spermatogenesis is an important phase in case of fish spermatozoa because it is the ATP store that constitutes the main source of energy that will sustain the short but highly energy-demanding motility period [ 8 ]. A detailed description of fish spermatogenesis is well documented in many species. Briefly, spermatogenesis is a developmental process during which a small number of diploid stem cells spermatogonia produce a large number of highly differentiated spermatozoa carrying a haploid, recombined genome, and a structurally complete flagellum.
Survival and development of those germ cells depend on their close contact with specific cells called Sertoli cells. Fishes represent the largest and most diverse group of vertebrates.
In an amniote vertebrates fishes and amphibians , one observes a cystic type of spermatogenesis, which presents two main differences compared to higher vertebrates [ 23 ]. First, within the spermatogenic tubules, cytoplasmic extensions of Sertoli cells form cysts that envelope a single, clonally and hence synchronously developing group of germ cells deriving from a single spermatogonium.
Second, the cyst-forming Sertoli cells retain their capacity to proliferate also in the adult fish. Sertoli cells are surrounding and nursing one synchronously developing germ cell clone. Different clones being in different stages of development generate a tubular compartment containing differently sized groups of germ cells in different stages of spermatogenesis. So as for the distribution of spermatogonia in the germinal compartment, one can observe either a first type of restricted spermatogonial distribution, which is found in the higher teleost groups, such as in the order Atheriniformes, Cyprinodontiformes, and Beloniformes, where the distal regions of the germinal compartment are occupied by Sertoli cells surrounding early, undifferentiated spermatogonia.
While the cells divide and enter in meiosis and the cysts migrate toward the region of the spermatic ducts located centrally in the testis, this is where spermiation occurs, i. In a second type, where an unrestricted spermatogonial distribution that is considered a more primitive pattern found in less evolved taxonomic groups, such as in the order Cypriniformes, Characiformes, and Salmoniformes, occurs, spermatogonia are spread along the germinal compartment throughout the testis.
The cysts do not migrate during their development. In addition, intermediate forms also exist between restricted and unrestricted spermatogonial distribution, such as in Perciformes, tilapia, Pleuronectiformes, or Gadiformes.
Therefore, the development of spermatogenic cells strictly depends on their interaction with the somatic elements of the testis, among which Sertoli cells play a crucial role. During fish spermatogenesis, Sertoli cells are formed by mitosis just in time and in exactly the number required. This tailored Sertoli cell proliferation was first described in the guppy [ 24 ]. So far, we observe that spermatogenesis is a highly organized and coordinated process, in which diploid spermatogonia proliferate and differentiate to form spermatozoa in their final morphology.
The duration of this process is usually shorter in fish than in mammals. In principle, this process can be divided, from a morpho-functional point of view, in three different phases: i the mitotic or spermatogonial phase with the different generations of spermatogonia, ii the meiotic phase with the primary and secondary spermatocytes, and iii the spermiogenic phase with the haploid spermatids emerging from meiosis and differentiating, without further proliferation, into flagellated spermatozoa.
Analysis of the role of hormones reveals a complex process. Three steps at which reproductive hormones play a critical regulatory role are i the balance between self-renewal and differentiation of spermatogonial stem cells, ii the transition from type A spermatogonia to rapidly proliferating type B spermatogonia, and iii the entry into meiosis. During later developmental stages, on the other hand, the endocrine system seems to ensure a permissive rather than stimulatory role, enabling Sertoli cells and possibly other somatic cells to generate a microenvironment that germ cells require to proceed through meiosis and spermiogenesis [ 25 ].
In case of fish, three types of spermiogenesis have been described, based on the orientation of the flagellum relatively to the nucleus and on whether or not a nuclear rotation occurs. Flagellum genesis The main organelle in the flagellum, the axoneme, is resulting from the progressive assembly of groups of elements synthesized in the cell body and then transported to the flagellar compartment and delivered to the flagellar tip to elongate it thanks to the motility of specific transporters called intra-flagellar transport IFT along the internal side of the flagellar membrane [ 21 ].
The trafficking is ensured by two important molecular motors, belonging to the dynein group retrograde, meaning from tip to base of the flagellum and to the kinesin group anterograde, meaning from the cell body to the flagellum tip [ 26 ].
Successive steps leading to fish sperm motility 3. The maturation step Maturation is the step following the end of spermatogenesis and that provides to the spermatozoon its ability to respond to motility-activating factors [ 27 ].
Signals for maturation are quite various among fish species. Sperm maturation is also regulated by the endocrine system. Examples of sperm maturation have been studied in details in different fish species: salmonids, cyprinids, and sturgeons. In salmonids, the group of Morisawa, in Japan, demonstrated, in particular in case of salmon, that maturation is mainly under control of cAMP and pH of the water where fish are transiting during migration [ 28 ].
In case of sturgeon species, it was shown that spermatozoa are not able to become activated at simple contact with external water [ 30 ]; sperm cells need a transient contact with urine, the latter getting mixed with milt prior to ejaculation [ 31 ] which is enough to render spermatozoa fully motile. The activation step per se This is a very brief event lasting a fraction of a second [ 32 , 33 ]. The osmolarity signal depends on the external medium where fish delivers its sperm: marine fish spermatozoa activate when coming in contact with sea water, a salty solution of high osmolarity, while freshwater fish species shed their sperm in a very low-osmolarity medium.
The environmental osmotic pressure appears consistently to be the main factor involved in fish motility activation among species [ 35 , 36 ]. In a few fish species, osmolarity is acting in synergy with another factor such as specific ions [ 35 ]. In some marine species such as herring, activation of spermatozoa requires egg-derived substances. Two types of sperm-activating factors have been identified in Pacific herring, Clupea pallasii, eggs: a water-soluble protein released into the surrounding water [ 37 ] and a water-insoluble sperm motility-initiating factor localized in the vicinity of the micropylar opening of eggs [ 38 ].
The perception of the signal by the sperm membrane Freshwater fish sperm cells when released into the surrounding water can increase their cytoplasmic volume in response to osmotic stress. In case of carp spermatozoa, the cell volume increases several times as a result of the influx of water [ 39 ].
However, the volume changes measured by the authors are of low amplitude: the engendered volume difference is so low that it cannot be responsible of a physiological role in motility control. In many marine teleost species, hypertonicity induces the motility of spermatozoa. Nevertheless, an increase in external osmolality is sometimes not the only condition for motility activation of marine fish spermatozoa: in case of herring, this activation also needs the contact of sperm with egg-derived substances, facilitating fertilization [ 37 , 38 , 43 , 44 ].
Sperm-activating factors are two types of in the Pacific herring, Clupea pallasii: a water-soluble herring egg protein [ 37 , 43 , 44 ] and water-insoluble initiating factor, from the vicinity of the micropylar opening of the egg [ 38 ]. In another group of marine fish species collectively named flatfishes, the main signal perceived by the sperm membrane is the CO2 concentration [ 34 ] that is high in the genital tract but very low in sea water.
The intracellular equilibrium between CO2 and bicarbonate constitutes the second step in the control of intracellular ionic concentration that leads to regulation of flagellar motility [ 33 ].
As mentioned above, it is clear that water transport itself is not a main process involved in fish sperm motility activation. Nevertheless, in the presence of aquaporins in the head and flagella plasma membrane of the seawater fish, gilthead sea bream Sparus aurata spermatozoa and their involvement in cAMP-mediated phosphorylation of axonemal proteins were established [ 45 , 46 ].
In this species, the water efflux via aquaporins would determine a reduction in the cell volume, which would raise the intracellular concentration of ions. This would lead to the activation of adenylyl cyclase and motility initiation by cyclic AMP-dependent protein phosphorylation and dephosphorylation [ 46 ].
Such cascade of events remains hypothetical because of the timing of such process compared to the extreme briefness less than 0. The presence of different ion channels was described in sperm plasma membrane [ 34 ]. Cytosolic pH could be considered as another participant of signaling pathways, as it is known to be one of the parameters influencing sperm motility [ 47 , 48 ].
Environmental conditions that inhibit spermatozoa motility can decrease the intracellular pH, resulting in a more acidic cytoplasm in nonmotile spermatozoa than in motile spermatozoa [ 49 ]. The decrease of internal pH in sperm would directly affect flagellar movement through inhibition of dynein activity.
The former authors suggest that the regulation of the exchangers depends on osmolality conditions [ 52 ]. Altogether, the precise mechanisms of regulation of ion channel activity and their participation in the hyperpolarization of the spermatozoon membrane, which is associated with the activation of sperm motility [ 35 , 53 ], remain poorly understood.
What are the next-step processes occurring at the level of membrane leading the subsequent activation of the axoneme? Transduction and reception of the signal at the axoneme level Among other processes, an increase of intracellular concentration of ions could lead to the activation of adenylyl cyclase, which in turn would determine the motility initiation by a cAMP-dependent protein phosphorylation and dephosphorylation mechanism [ 46 ].
It is known that, in mammals, protein tyrosine phosphorylation of several proteins is upregulated by reactive oxygen species ROS. Cyclic AMP is an important factor in the activation process of fish spermatozoa.
Fish Sperm Physiology: Structure, Factors Regulating Motility, and Motility Evaluation
FISH SPERMATOLOGY PDF