PROBLEMÁTICA DE LOS SUELOS TROPICALES

Pirlot (1933) observed that ova mature in the nephromixia during oviposition. He concluded that oocytes could not be fertilized until they had been shed. Howie (1961b) found that oocytes are not physiologically mature while in the coelom, but contradictory to Pirlot (1933), maturation begins within the coelomic cavity and that maturation precedes spawning. At the time of spawning, oocytes have reached a

diameter of 180 pm and fi*om histological examination all oocytes undergo maturation

simultaneously followed immediately by spawning (Howie, 1961b).

Only mature oocytes are accepted automatically by the ciliated funnels of the nephromixia so that maturation of the oocytes in the body cavity is the immediate cause of their shedding. The mechanism by which the nephromixia distinguish between mature and immature gametes is unknown but may be due to the change in shape (becoming biconvex) of the matured oocyte (Howie, 1961b).

Howie (1963) showed that decerebrated worms did not spawn during the noimal spawning season and maturation of the oocytes had not occurred. Injection of homogenised female prostomium induced the decerebrated animals to spawn and oocytes to mature normally and be feitilizable. However, male, or mixtures of male and female prostomia and implantation of intact whole prostomia failed to induce spawning in decerebrated females. (Howie, 1963). Howie concluded from these experiments that a substance within the prostomium of the females induces matuiation of the oocytes and therefore initiates spawning of the oocytes. However, subsequent work by Howie (1966) showed that both male and female prostomia were capable of inducing spawning in females, although Auckland (1993) failed to induce spawning in females with male prostomia. Pacey and Bentley (1992a) confirmed the difference between males and females. Injection of the putative spawning hormone in males, 8,11,14, eicosatrienoic acid, failed to induce spawning in females. This suggests that male prostomia contain a substance that induces females to spawn but this is different from 8,11,14, eicosatrienoic acid.

The substance from female prostomia which induces spawning, was only synthesised and secreted during the breeding season and from examination of the brains, secretory cells that may be a potential source of maturation hormone were scarce. When these cells did occur, they were concentrated in the mid brain, the dorsal, vertical and oblique fibres temiinating at the ventral pericapsular membrane, and in the posterior lobes. Experiments indicated that the posterior lobes alone could cause spawning when homogenised and injected into decerebrated females but other parts of the brain such as the anterior lobes had no hormone or very little as in the case of the midbrain (Howie, 1966).

Meijer and Durchon, working in France (1977), found that if oocytes are incubated with prostomia, in concentrations from 0.1 to 0.01 prostomium per ml, 100% of the oocytes will undergo germinal vesicle breakdown (GVBD). The time for GVBD to occur is dependent on the concentration of prostomia and ranges from 0.5 to

3 hours. This indicates a direct action of the hormone upon the oocyte and not involving any intermediates. However, it has recently been shown that maturation in

oocytes of Arenicola marina will not take place following direct incubation in

prostomial homogenate (Auckland, 1993). This is in contradiction to the work of Meijer and Durchon (1977). This anomaly can be explained because Meijer and

Durchon almost certainly worked on Arenicola defodiens from Northern France

(Bentley, pers. comm.) which unlike A. marina does not require a second endocrine

step for maturation. Although the evidence is not conclusive, for ease of understanding in all future chapters the worms worked on by Meijer (1979a,b; 1980)

are referred to as Arenicola defodiens and the worms used in this present study are

referred to as Arenicola marina..

In Arenicola defodiens, Meijer (1980) found that a number of chemicals, known to affect calcium, affect oocyte maturation. These include propranolol, oxprenolol, tetracaine, procaine and lanthanum chloride. All these chemicals induce oocyte maturation suggesting a link between maturation and the intiacellular free calcium concentration. However, the ionophore A23187, which is known to increase free calcium levels (Pressman, 1973), fails to induce maturation complicating the role of calcium in the maturation process (Meijer, 1980). The ability of oocytes to mature in calcium free sea water and the fact that maturation is not affected by the blocking of calcium transport through the oocyte membrane suggests that calcium may be released from internal stores to increase the free calcium concentiation within the oocyte (Meijer, 1980). Unpublished results of Meijer found that the Triton X-100 fraction of oocyte cortices releases calcium if incubated with calcium free brain extract, as detected by the injection of the calcium sensitive fluorescent protein, aequorin. He therefore suggested that the brain hormone may have a direct impact on the calcium equilibrium of the oocyte cortex and that, as in other maturation systems, calcium may play a fundamental role in the activation of maturation.

Oocyte maturation can also be induced in Arenicola defodiens by two disulphide reducing agents dithiothreitol (DTT) and 2,3-dimercapto-propanol (BAL, British Anti-Lewisite) and this points to the possible involvement of -SH groups during maturation (Meijer, 1980).

The chemical nature of the maturation factor from the prostomium, of both

species, and the second matuiation factor in Arenicola marina coelomic fluid, their

precise site of production and nature of action still remain unknown.