Sperm-egg Association and its Relationship to the Activation of the Egg in Salmonid Fishes

ABSTRACT A study has been made of the initial stage of fertilization in Salmonids, using the lake trout Salmo trutta L. morpha lacustris Linné and the salmon S. salar L. morpha sebago Girard. Lake trout spermatozoa are activated by coelomic fluid and by Ringer solution; in these media they retain their motility and fertilizing capacity much longer than in water.At the moment when the spermatozoa kept in coelomic fluid or Ringer solution lose their fertilizing capacity, they are not reactivated by the addition of water. After a period of ‘rest’, however, a proportion of them acquires motility upon the addition of water which leads to a partial restoration of the fertilizing capacity of the sperm suspension.When trout and salmon eggs are inseminated in coelomic fluid or Ringer solution, the union of the gametes is achieved without egg activation. A cytological investigation shows that the spermatozoon reaches the cortical egg layer along the micropylar canal and the anterior part of its head gets embedded in the cytoplasm.When trout spermatozoa are placed into egg water or other media containing products of egg secretion no changes resembling the extrusion of the acrosome filament are seen with the electron microscope. This supports previous data on the absence of the acrosome in salmonid spermatozoa.The development of eggs inseminated in coelomic fluid or Ringer solution is initiated by immersion into water. The involvement of the fertilizing spermatozoon in development is possible upon egg activation for up to several days after insemination. To achieve such an effect, a brief exposure to water is necessary (this varies from 10 to 70 sec. for different eggs).Normal development of the egg may be achieved upon its activation either after or before insemination (the only limitation being the fact that the penetration of the spermatozoon in the activated egg is possible only before the expulsion of the contents of the cortical alveoli in the micropylar region).When water pretreated eggs are inseminated in calcium-free medium, no involvement of the spermatozoon in development takes place.The latent period of the cortical reaction on activation of trout eggs by water (from the moment of appearance of changes setting the process in train until the beginning of the expulsion of the contents of the cortical alveoli) in the bulk of the eggs takes 40–50 sec. at 3 ·0°C and 30–40 sec. at 8 · 9°C.The phenomena of union of the gametes and egg activation in salmonid fishes are to a considerable degree autonomous : they can easily be separated in time and combined in any sequence. This is related to the peculiarities of the mechanism of sperm-egg interaction.