Rentabilidad en la Crianza de Caracoles

sperm are fundamentally excitable cells (Nelson, 1975), but for a considerable proportion of their lives are in a state of dormancy or quiescence and undiluted semen taken directly from the body cavity in nearly all animal groups contains sperm which is immotile. For example, the sperm of sea urchins (see Trimmer & Vacquier, 1987), horseshoe crab Limulus polyphemus (Clapper & Brown, 1980), salmonid fishes, and amphibians (see Morisawa, 1987) become motile only when they are spawned. Of these, the sperm of the horseshoe crab and also that of the herring ClupeaharengusÇYangxmanchi, 1957) remain immotile in sea water for a variable time following their release. In some animals, however.

notably the mammals (Morton etal, 1974) and in the lugworm Arenicola wanna, the subject of this study, (see Howie, 1984) sperm become motile in the body cavity prior to release . In the case of mammals this occurs in the genital tract and in Arenicola it occurs in the coélomic cavity.

The biochemistry of quiescent spermatozoa is little understood, although it is probably dependent upon the specific sperm activation mechanism employed by a particular species. For convenience however, the mechanisms which result in the maintenance of sperm quiescence can be divided into two groups.

In some species, sperm dormancy appears to be maintained by the osmotic or ionic conditions of the fluids in the body cavity or testes. Upon release, there is an accompanying change in these conditions and it is at this point that sperm motility is initiated. It is for no group other than sea urchins however, that the intracellular mechanism for the maintenance of dormancy is understood. In sea urchins, a low intracellular pH (maintained by a high CO2 tension) keeps both respiration and motility suppressed (see Trimmer & Vacquier, 1986). Because the utilization of energy by the flagellum is strictly coupled to its production (Tombes & Shapiro,

1985), then the control over mitochondrial activity is dependent upon whether or not the flagellum is actively using ATP, which in turn depends upon the intracellular pH of the cell.

In other animal groups however, sperm become activated only in response to a specific trigger. The sperm of horseshoe crabs and the herring become motile following interaction with an egg-derived substance. In both mammals, and the lugworm Arenicola marina, an internal stimulus is required for sperm activation. The specific details of these activation mechanisms will be discussed in section 1.3.2, but it appears that it is the denial of such a stimulus which is responsible for maintaining sperm dormancy in these animals.

In some species, heavy metals appear to have a role in maintaining the quiescence of sperm, and sperm motility of both starfish and the horseshoe crab is inhibited by the addition of zinc (see Morisawa & Morisawa, 1990).

In mammalian systems, the maintenance of sperm quiescence appears to be more complex. Mammalian sperm acquire the capacity to swim during their transit through the epididymis, but become motile only when they are removed or when they are mixed with seminal fluids at ejaculation. On considering sperm motility in the mammals it is unclear whether the important factor in maintaining quiescence is denial of a trigger stimulus prior to the mixing of sperm in seminal fluid (see section 1.3.2) or whether mechanical immobilisation in the testes is the factor which maintains quiescence. In the bovine spermatozoon, it certainly appears that motility is inhibited by a factor in caudal epididymal fluid. - Sperm from the bovine caudal epididymis (CE), when diluted in physiological buffers become motile over a period of about ten minutes, if they are placed back in caudal epididymal fluid however, they quickly become quiescent (Carr & Acott, 1984). The effect is pH dependent, and sperm motility is inhibited at pH 5.5 but not at pH 7.6 (Acott & Carr, 1984). This pH effect occurs not directly on the sperm, but is mediated by an unidentified factor or factors present in the CE fluid and it has been speculated that these interact at the level of the sperm membrane to modulate an ion transport event. Factors from the epididymis of a number of mammals have been implicated in Inhibiting sperm motility and include the presence of glycerylphosphorylcholine (Turner etal,

1978), carnitine (Brooks et al, 1974) or proteinaceous factors (Turner & Giles, 1982) in the CE fluid. In the rat, a mucin-like glycoprotein ‘immobilln’ has been described which imobilizes CE sperm physically (Usselman & Cone, 1983). That immobilin is capable of inhibiting bovine CE sperm motility demonstrates the importance of viscoelasticity, but that the kinetics of this method are unlike that of natural inhibition, suggest that it does not play a major role in this system (Carr &

Acott, 1984). The spermatozoa of the rabbit appears to be an exception to the general rule however, and are motile whilst being stored in the epididymis (Turner & Reich, 1985).

In some species, studies using demembranated spermatozoa have provided interesting suggestions as to the intracellular mechanisms which are involved in the maintenance of dormancy and activation. In many systems it is unclear whether quiescence is maintained by the lack of available ATP, or whether other factors are required before ATP can be utilised. Until very recently, it was thought that because demembranated sperm of many species could be reactivated by the addition of ATP alone (see Brokaw, 1984; see also Morisawa & Morisawa, 1990), it was the lack of available energy that was maintaining sperm quiescence. However, it now seems apparent that membrane debris and soluble cell fractions found following demembranation are capable of synthesising cAMP. In ‘naked sperm’ preparations in which these have been removed, it is now known that cAMP is required for the initiation of motility (see Morisawa, 1987; Morisawa & Morisawa, 1990; Tash, 1990). The role of cAMP in cellular function will be discussed in greater detail in section 1.3.2.