The fertilization and development of the sea urchin egg, Arbacia lixula, under the influence of fractions of egg homogenate

Abstract

Unfertilized eggs of Arbacia lixula were homogenized in the presence of isotonic NaCl solution. The microscopic granules were spun down by centrifugation for 30 minutes at 8 000 g. The granular fraction was dissolved in distilled water and dialyzed against sea water. The supernatant was also dialyzed against sea water. Before being tested both the extract from the granular fraction (I) and the supernatant (II) have been diluted with sea water, usually 1:10. I contains a membrane elevating factor, MbElF(E); II contains antifertilizin or according to the terminology adopted above the jelly precipitating factor, JePptF(E). (E) indicates that the factors are obtained from the eggs. Upon insemination of the Arbacia eggs in normal sea water the eggs are fertilized but in most batches of eggs the membrane adheres closely to the egg surface and has not the optical properties of the normal membrane. In the presence of MbElF(E) a membrane elevation occurs. The membrane becomes more refractile and more solidified. The retardation is higher than in the adherent membranes. Heating of the extract from the granular fraction to 45 ° or 60 °C for 30 minutes decreases or abolishes the MbElF-effect. Eggs with jelly precipitated under the action of JePptF(E) can be fertilized and they exhibit an elevated refractile fertilization membrane. Even the heated II has this effect, although no precipitation of the jelly coat occurs. In eggs deprived of their jelly coats an adherent membrane is formed upon fertilization in the presence of II. JePptF thus causes no elevation of the membrane in absence of the jelly coat. Some kind of interaction between JePptF and the jelly coat is thus necessary to induce the membrane elevation. If jellyless fertilized eggs remain in contact with II a sort of tanning of the membrane occurs which remains adherent to egg surface. This observation speaks in favour of the presence of a membrane toughening factor in II. MbElF(E) favours the membrane elevation also when unfertilized eggs are subjected to a hypertonic solution. In certain experiments the homogenization was carried out in a mixture of NaCl and CaCl 2 (concentrations as in sea water). It was evident from the results that in the presence of calcium MbElF(E) is partly released from the granules. After previous homogenization in the presence of Ca the MbElF-action is thus also found in II. Under a certain season the control almost invariably gave radially symmetrical or slightly animalized larvae. This tendency was counteracted when the eggs were fertilized in and reared 5–6 hours after fertilization in sea water containing extract from the granular fraction. Like MbElF(E) the anti-radialization factor was inactivated by warming to 45 ° or 60 °C for 30 minutes. These results suggest that a release of substances from the microscopic granules in the egg may play a role for the balance between ventral and dorsal tendencies in the early sea urchin development.