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Some of the important theories concerning the activation of egg by sperm are as follows: 1. Bataillon’s Theory 2. Sensitization to Calcium Theory 3. Loeb’s Theory 4. Lillie’s Theory 5. Repressor Theory of Activation.
1. Bataillon’s Theory:
Bataillon (1910, 11, 12 and 16) suggested that the exudation or the excretion of substances into the perivitelline space and elevation of the fertilization membrane activate the egg. He believed that the unfertilized egg was inhibited because of accumulation of metabolic products and the activation or fertilization led to release of these substances to the exterior of the egg.
2. Sensitization to Calcium Theory:
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The importance of calcium in egg activation has been emphasised in two independently developed theories, namely “sensitization to calcium” theory of Dlacq, Pasteels and Brachet and the “Calcium release-protoplasmic clotting” theory of Heilbrunn.
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These Theories Revealed That:
(i) Isotonic calcium chloride solutions or calcium rich solutions are effective activators in many species of animals;
(ii) Activation of other chemical agents is dependent upon the presence of calcium ions; and
(iii) Depriving the eggs of calcium will sensitize them to subsequent action of calcium solution.
3. Loeb’s Theory:
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Loeb believed that in normal fertilization the sperm brings in a lytic principle which brings about cortical cytolysis and a second substance called “corrective factor” which regulates oxidation. Lovtrup has supported Loeb’s theory of activation.
According to him there may be some cytolytic agents which may function as activators by causing a partial dissolution and perforation of plasma membrane opening of cortical granules and subsequent swelling of perivitelline space and further activation of fertilized egg.
4. Lillie’s Theory:
R.S. Lillie suggested that the cortical changes are the main aspects of activation of egg. The decrease in viscosity permits the interaction of various cytoplasmic substances which normally are kept separated in the unactivated egg. The activating substance ‘X’, formed in the egg is comparable to the growth hormone. This substance is formed by the union of two substances ‘Y’ and ‘Z’ that may be initially present in low concentration in the egg.
Substance ‘Y’ is considered to be a product of hydrolytic process in the egg and its formation to be stimulated by the action of such agents as heat and acids, which can act under anaerobic conditions. Substance ‘Z’ is considered to be formed by synthetic processes that may be stimulated by agents like hypertonicity acting only in aerobic conditions. Since some substance of each type is present, the threshold concentration of ‘X’ can be reached by increase in either ‘Y’ or ‘Z’.
5. Repressor Theory of Activation:
Monroy (1965) has given some convincing arguments about activation of egg by sperm in molecular terms. According to him, during maturation of egg, the energy yielding systems are blocked, so all the reactions requiring a large amount of energy are also inhibited.
The accumulation of inhibitory substances during maturation of egg seems to be the most likely explanation and indeed, he recognised one such inhibitor. However, how such inhibitions are produced during the final phase of maturation and how they are inactivated or disposed off by the egg following fertilization still remain to be explained.
Monroy’s repressor theory has been supported by many embryologists such as Tomkins, et. al. Metafora, et. al., D. Epel, Berrill and Karp, and Balinsky. Now there are large number of evidences in favour of the fact that the repressor substances manufactured by the egg in the later phase of maturation inhibit both the metabolic activities residing in the cytoplasm and the genetic activities of the nucleus.
The key reaction of fertilization is, thus the removal of the repressors, releasing at the same time the cytoplasmic metabolic activities and the activity of the nuclear genetic system.
