[Ultrastructure of unfertilized human oocytes and undivided human zygoytes]

To study the ultrastructure of unfertilized human oocytes and undivided human zygoytes.

Unfertilized human oocytes and undivided human zygoytes were collected during in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI), and then observed under transmission electron microscope (TEM) after fixation, dehydration, embedding, and ultrashing.

The unfertilized oocytes from conventional IVF showed (1) normal zona pellucida (ZP) but without cortical granules (CG) beneath oolemma, condensed sperm nuclei, and disappearance of mitochondria and smooth endoplasmic reticulum (SER), (2) abnormal dense in ZP, lack of other organelles [SER tubuli (SER-T),SER vesicles (SER-V)] besides CG and mitochondria, and intact acrosome that penetrated the ZP. The unfertilized oocytes from ICSI showed (1) normal cortex containing one row of dense cortical granules beneath oolemma, SER-T and SER-V associated with mitochondria, some injected spermatozoon with lost nuclear envelope, and some decondensed chromatin, (2) little or scattered cortical granules, which dispersed beneath oolemma, scarce mitochondria and SER-T or isolated mitochondria. The undivided zygotes showed cortical granules contents free in the perivitelline space, little mitochondria, some second lysosomes filled with droplets, and large clumps of annulate lamellae (AL).

Appearance of organelles in abnormal position is related to oocytes cytoplasmic maturation, which leads to failure in fertilization. Absence of aerosome reaction and abnormal dense of zona pellucida are associated with failed fertilization in conventional IVF. Absence of oocyte activation is associated with failed fertilization in ICSI. The presence of lipofuscin body and abnormal assembly of annulate lamellae are associated with fertilization arrest at the pronuclear stage of human zygotic development.