Functional assessment of centrosomes of spermatozoa and spermatids microinjected into rabbit oocytes

Although intracytoplasmic sperm injection (ICSI) is a widely used assisted reproductive technique, the fertilization rates and pregnancy rates of immature spermatids especially in round spermatid injection (ROSI) remain very low. During mammalian fertilization, the sperm typically introduces its own centrosome which then acts as a microtubule organizing center (MTOC) and is essential for the male and female genome union. In order to evaluate the function of immature germ cell centrosomes, we used the rabbit gamete model because rabbit fertilization follows paternal pattern of centrosome inheritance. First, rabbit spermatids and spermatozoa were injected into oocytes using a piezo-micromanipulator. Next, the centrosomal function to form a sperm aster was determined. Furthermore, two functional centrosome proteins (gamma-tubulin and centrin) of the rabbit spermatogenic cells were examined. Our results show that the oocyte activation rates by spermatozoa, elongated spermatids, and round spermatids were 86% (30/35), 30% (11/36), and 5% (1/22), respectively. Sperm aster formation rates after spermatozoa, elongated spermatids, and round spermatids injections were 47% (14/30), 27% (3/11), and 0% (0/1), respectively. The aster formation rate of the injected elongating/elongated spermatids was significantly lower than that of the mature spermatozoa (P = 0.0242). Moreover, sperm asters were not observed in round spermatid injection even after artificial activation. These data suggest that poor centrosomal function, as measured by diminished aster formation rates, is related to the poor fertilization rates when immature spermatogenic cells are injected.