Objective: The study of zona free oocytes fertilization using intracytoplasmic sperm injection (ICSI) with the aim to increase the pregnancy rate in the assisted reproduction. Design: Pilot study. Setting: Assisted Reproduction Centre, Gynecology Obstetrics Department of the 1st Medical Faculty and General Faculty Hospital, Charles University, Prague. Methods: Zona free oocytes were fertilized using ICSI and cultured in standard conditions in vitro for 5 days until the blastocyst stage. Blastocysts were cryopreserved using 8% glycerol. Results: The fertilization process of zona free oocyte was physiological, oocytes expulsed the polar body and in standard time, i.e. 18 hour post insemination, two distinct, morphologically normal pronuclei were apparent in cytoplasm. During early cleavage, from the first to the third day of in vitro culture, the dynamics of cell division was normal, the three-dimensional arrangement of cells was more flattened than in normal, zona intact embryo. The fourth and fifth day of culture, the morphological appearance of embryo corresponded to normal development. Conclusion: The fertilization of zona free oocytes using ICSI can give morphologically normal zygote and after 5 days culture in vitro these zygotes can develop into blastocyst stage. This method can increase the pregnancy rate in patients with repeatedly low number of oocytes or with defects of zona pellucida.

        Key words: zona free oocyte, ICSI, blastocyst development, differentiation, early embryo