Expression of the zebrafish intermediate neurofilament Nestin in the developing nervous system and in neural proliferation zones at postembryonic stages

Background

At fertilisation, mammalian oocytes are activated by oscillations of intracellular Ca²⁺ (Ca²⁺]_i). Phospholipase Cζ, which is introduced by fertilising spermatozoon, triggers Ca²⁺]_i oscillations through the generation of inositol 1,4,5-triphosphate (IP₃), which causes Ca²⁺ release by binding to IP₃ receptors located on the endoplasmic reticulum (ER) of the oocyte. Ability to respond to this activating stimulus develops during meiotic maturation of the oocyte. Here we examine how the development of this ability is perturbed when a single spermatozoon is introduced into the oocyte prematurely, i.e. during oocyte maturation.

Results

Mouse oocytes during maturation in vitro were fertilised by ICSI (intracytoplasmic sperm injection) 1 – 4 h after germinal vesicle break-down (GVBD) and were subsequently cultured until they reached metaphase II (MII) stage. At MII stage they were fertilised in vitro for the second time (refertilisation). We observed that refertilised oocytes underwent activation with similar frequency as control oocytes, which also went through maturation in vitro, but were fertilised only once at MII stage (87% and 93%, respectively). Refertilised MII oocytes were able to develop Ca²⁺]_i oscillations in response to penetration by spermatozoa. We found however, that they generated a lower number of transients than control oocytes. We also showed that the oocytes, which were fertilised during maturation had a similar level of MPF activity as control oocytes, which were not subjected to ICSI during maturation, but had reduced level of IP₃ receptors.

Conclusion

Mouse oocytes, which were experimentally fertilised during maturation retain the ability to generate repetitive Ca²⁺]_i transients, and to be activated after completion of maturation.