**15. Oocyte morphology**

Poor oocyte morphology is a major determinant of failed or impaired fertilization. Normal features of a healthy mature oocyte at Metaphase-II (MII) include presence of a polar body, a round even shape, light colour cytoplasm with homogenous granularity, a small perivitelline space without debris and a colourless zona pellucida. In denuded oocytes, it is possible to assess the morphology and the nuclear maturity but not the cytoplasmic maturity. The MII oocytes with apparently normal cytoplasmic organization may exhibit extra-cytoplasmic characteristics, such as increased perivitelline space, perivitelline debris and/or fragmentation of the first polar body, which may reduce developmental competence of the oocyte [152]. It is common that extra-cytoplasmic and cytoplasmic dysmorphism occur together in the same oocyte **(Figure 4 and 5)**. The dysmorphic phenotypes, which arise early in meiotic maturation, may be associated with failed fertilization and aneuploidy, while those occurring later in maturation may cause a higher incidence of developmental failure [153, 154].

**Figure 4.** Oocytes in both rows show extra-cytoplasmic and cytoplasmic dysmorphism.

Decreased fertilization rates due to some oocyte dysmorphisms have been reported [152], while others failed to observe that association [155, 156, 157, 158, 159]. Lower pregnancy and implantation rates result when the transferred embryos originate from cycles with >50% dysmorphic oocytes and the same dysmorphism repeats from cycle to cycle [155]. The repetitive organelle clustering is associated with an underlying adverse factor affecting the entire follicular cohort. The presence of a dark cytoplasm decreases by 83% the likelihood of obtaining good quality embryos [160]. However, another study did not find any adverse impact of dark colour of the oocytes on fertilization, embryo development and pregnancy rate [161]. In human oocytes, the cytoplasmic granularity can be homogeneous affecting the whole cytoplasm, or concentrated in the centre with a clear peripheral ring giving a darkened appearance to the cytoplasm [162]. The abnormal changes in the cytoplasm of MII oocytes may be a reflection of delayed cytoplasmic maturation that is unsynchronized with nuclear maturity [163].

**Figure 5.** Oocytes in first row represent different degrees of vacuoles in cytoplasm. Each oocytes in second row has increased central granularity.

Normal fertilization, embryo development and live birth are possible after ICSI in oocytes with thick zonae, abnormal morphology or repeated polyspermia following conventional IVF. The oocytes with extreme morphological abnormalities should not be discarded as ICSI can result in fertilization, cleavage and normal embryonic development [164, 161]. The zonafree oocytes may be fertilized normally after ICSI and develop to the blastocyst stage [165]. Pregnancy in human [166] and live birth in mouse [167] and pig [168] have been obtained after transfer of embryos resulting from zona-free oocytes.
