Question

Egg activation marks a key point in development. In your answers to the following questions, discuss specific experiments to support the key facts you cite.

i. What molecular and morphological events occur in animals where an acrosomal reaction and/or sperm entry is required for egg activation, and what is the purpose of these events?

ii. What molecular and morphological events occur in animals where sperm entry and/or an acrosomal reaction is not required for egg activation, and what is the purpose of these events?

iii. What is the evolutionary significance of the events that are shared between the two types of egg activation discussed above? Specifically, discuss what types of signals are used and how they are set up.

Answer 1

Although fertilization is often depicted as merely the means to merge two haploid nuclei, it has an equally important role in initiating the processes that begin development. These events happen in the cytoplasm and occur without the involvement of the nuclei.*

The mature sea urchin egg is a metabolically sluggish cell that is activated by the sperm. This activation is merely a stimulus, however; it sets into action a preprogrammed set of metabolic events. The responses of the egg to the sperm can be divided into “early” responses, which occur within seconds of the cortical reaction, and “late” responses, which take place several minutes after fertilization begins.

The first clues to the process of gamete activation date back to nearly 60 years ago. The mutual activation of gametes is a crucial event during fertilization. In the testis and ovaries, spermatozoa and oocytes are in a state of meiotic and metabolic quiescence and require reciprocal signals in order to undergo functional changes that lead to competence for fertilization. First, the oocyte activates sperm by triggering motility, chemoattraction, binding and the acrosome reaction, culminating with the fusion of the two plasma membranes. At the end of this cascade of events, collectively known as sperm capacitation, sperm-induced oocyte activation occurs, generating electrical, morphological and metabolic modifications in the oocyte.