Sperm Entry in Sea Urchin Eggs: Recent Inferences Concerning its Mechanism

Abstract

The mechanism by which a sperm enters the cytoplasm of an egg during fertilization is not known; but, inn the sea urchin egg, membrane potentials in two different ranges are capable of precluding sperm entry. we have attempted to determine what steps of sperm entry are regulated by membrane potential in eggs and oocytes of Lytechinus variegatus with the objective of obtaining insight into the sperm entry process. Early sperm-egg interactions include: sperm-egg attachment or binding, sperm-egg fusion, and an abrupt increase of membrane conductance which leds to a depolarization of the egg’s membrane potential. The positive-going activation potentail of the sea urchun egg following attachment of the first sperm decreases the probability of entry for sperm which attach subsequently (rapid, voltage block to polyspermy). A sperm fails to enter the egg or oocyte unless it first causes a conductance increase. In voltage-clamped eggs and oocytes, the probability that an attached sperm causes a conductance increase is maximal at potentials more negati ve than -10m V and decreases to 0 near +20 m V. It is possible that the lack of a conductance increase at potentials more positive than +20 m V results from a voltage-induced failure of sperm to fuse with the egg following attachment, hence precluding their entry. When sperm entry occurs at less positive potentials, the sperm’s nucleus slowly migrates into the cytoplasm of the egg beginning roughly 60s after the initial conductance increase. However, if an egg or oocyte is voltage clamped at a potential more negative than -30 mV (their resting potentials are -75 m V), it is unlikely that a sperm which attaches and causes a conductance increase will be incorporated. A characteristic “cutoff” of the conductance increase occurs in association with the failure of these sperm to enter. In oocytes which are not voltage clamped, depolarizations associated with attachment of a single sperm also cut off prematurely and are too small topermit sperm incorporation. Sperm entry occurs in oocytes only when many sperm attach and involves a cooperative summation of their depolarizations. Cooperativity is not seen for eggs. In oocytes, sperm incorporation, fertilization cone size, and the duration of the electrophysiological response are statistically associated and exhibit voltage dependences which are indistinguishable, suggesting that the three events are interrelated. We conclude that sperm entry involves two voltage-dependent, separable steps: sperm-egg fusion and nuclear incorporation. One common, voltage dependent mechanism may regulate nuclear incorporation and fertilization cone formation.