D. Gook et al., Fluorescent study of chromatin and tubulin in apparently unfertilized human oocytes following ICSI, MOL HUM REP, 4(12), 1998, pp. 1130-1135
In this study we examined 138 oocytes which were meiotically mature and, on
light microscopic examination, contained either no or one pronucleus follo
wing intracytoplasmic sperm injection (ICSI). Oocytes were fixed and simult
aneously stained for chromatin (Hoechst 33258) and the spindle (alpha-tubul
in antibody). In nine oocytes, no sperm nucleus was observed. The remaining
oocytes were separated into two groups following staining; (i) oocytes whi
ch had remained at metaphase II after ICSI (n = 74); and (ii) oocytes in wh
ich resumption of meiosis was observed after ICSI (n = 55). In all oocytes
in which sperm chromatin was absent no resumption of meiosis had occurred a
nd therefore parthenogenetic activation by the process of ICSI seems to be
a rare event. In 17 out of 74 (23%) oocytes which remained at metaphase II,
staining identified premature chromosome condensation (PCC) of the sperm c
hromatin (G(1)-PCC). Sperm nuclear decondensation or further transformation
of the sperm chromatin was observed in 56 out of 74 (76%) oocytes which re
mained at metaphase II after ICSI and in 46 out of 55 (84%) oocytes which h
ad resumed meiosis, indicating that initiation of sperm decondensation is i
ndependent of the resumption of meiosis in the oocyte. In contrast, transit
ion of the sperm nucleus beyond the decondensed stage only occurred in asso
ciation with resumption of meiosis in the oocyte (no pronuclei in metaphase
II oocytes). The presence of both male and female pronuclei in 53% of oocy
tes which had resumed meiosis indicates that changes in sperm chromatin bey
ond the initial decondensation stage are dependent on cytoplasmic condition
s which also permit female pronuclear formation.