Fertilization


Polyspermy block


Polyspermy, the fertilization of an egg by more than one sperm, is a common problem faced by the eggs of all sexually reproducing species and causes severe chromosomal defects and embryonic mortality.  The effects of polyspermy are evident in the development of cleavage furrows shortly after fertilization.


 
In normal fertilization, the embryonic cleavage furrows develop symmetrically and transverse the entire embryo (center photo at 8 cell stage).  By contrast, the cleavage furrows develop irregularly following fertilization by more than one sperm (right photo) (Wozniak  & Carlson, unpublished).

In normal fertilization, the embryonic cleavage furrows develop symmetrically and transverse the entire embryo (center photo at 8 cell stage).  By contrast, the cleavage furrows develop irregularly following fertilization by more than one sperm (right photo) (Wozniak  & Carlson, unpublished).

 

Our lab strives to uncover the crucial signaling events ensuring that only one sperm enters the egg and thereby allowing for normal embryonic development.  Eggs can possess two mechanisms to prevent more than one sperm from entering at fertilization: the fast block and the slow block.  Whereas the slow block to polyspermy is well understood, many questions remain regarding the molecular mechanisms underlying the fast block to polyspermy.  The fast block exists in many species in the form of a prolonged membrane depolarization of the egg.


Fertilization-evoked depolarization in X. laevis eggs (recording by Wozniak, unpublished).  Line denotes 0 mV.

Fertilization-evoked depolarization in X. laevis eggs (recording by Wozniak, unpublished).  Line denotes 0 mV.

 
 

The channels that mediate this depolarization and the signaling pathways that regulate it are unknown.  To uncover the earliest signaling events evoked by fertilization, we use the African clawed frog Xenopus laevis as the model system.  Frogs naturally fertilize externally, facilitating the study of their fertilization in the lab.  Furthermore, X. laevis are particularly well-suited for the experiments proposed here due to the large size of their eggs (~1.4 mm diameter) and ease of exogenous protein expression.

Our lab is interested in elucidating the signaling events that occur within the first minutes of sperm entry into the egg.  We use various techniques including electrophysiology and fluorescence imaging to establish the timeline of events.