Changes of intracellular free calcium during intracytoplasmic sperm injection

The present experiments were undertaken to investigate whether the procedure of intracytoplasmic sperm injection (ICSI) is associated with changes in the intracellular free calcium concentration ([Ca2+]i). [Ca2+]i was measured, using the calcium-sensitive dye fura-2, during and after impalement of mouse oocytes with an ICSI pipette and injection of a small amount of medium alone or of medium containing a normal human spermatozoon. Forty-five oocytes were injected with medium. Two different responses were observed: 20 of these cells showed a large increase of [Ca2+]i upon impalement; the other 25 cells did not show any change of [Ca2+]i, neither in the acute period nor in a late period 4 hr after impalement. All the cells that responded with an increase of [Ca2+]i subsequently lysed within the first 30 min following impalement, while all the cells with no [Ca2+]i change remained intact. This observation suggests that only traumatic impalement is associated with an increase of [Ca2+]i. Thirty-one oocytes were successfully, i.e., without subsequent cell lysis, injected with a normal mouse or human spermatozoon. In none of these cells could any acute or late change of [Ca2+]i be observed. The experiments illustrate that successful performance of the ICSI procedure, i.e., ICSI not followed by cell lysis, is not associated with changes of [Ca2+]i in mouse oocytes. This suggests that the ICSI technique, by itself, does not help in activating the oocyte via manipulation-induced changes of [Ca2+]i.