Table 1 Overview of the alterations of nuclear and cytoplasm during oocyte maturation
From: In vitro maturation (IVM) of human immature oocytes: is it still relevant?
Nuclear maturation Stage | GV (Germinal Vesicle) | GVBD (Germinal Vesicle Breakdown) | Metaphase-I | Metaphase-II |
|---|---|---|---|---|
Cytoplasmic Organelles | Prophase-I | Nucleus of the oocyte breaks down and the nuclear envelope disintegrates | The chromatin within the nucleus condenses (Chromosome condensation) | The first polar body extrusion (chromosome segregation) and the chromosomes align at the metaphase plate |
Mitochondria | Aggregated around the GV Predominantly spherical to oval with dense matrices and few arch-like or transverse cristae presenting an inert appearance Usually absent from the cortical part of the cytoplasm | Move away from the perinuclear region Upon GVBD, most mitochondrial clusters are larger. Later, a large proportion form smaller clusters throughout the cytoplasm | More numerous and dispersed in the ooplasm | Occupy most of the egg volume Voluminous aggregates with SER tubules and vesicles (human) |
Golgi apparatus | Dispersed throughout the ooplasm in the form of a continuous membranous system, but it is slightly more concentrated in the interior than at the cortex | Fragmented and aggregated in the central part of the oocyte | Further fragmented and dispersed throughout the oocyte | The distribution is maintained |
Endoplasmic reticulum (ER) | Uniformly distributed throughout the cytoplasm | Localizes in cortical regions | Concentrated in the vegetal half of the mature egg | ER clusters are present throughout the entire oocyte |
Cortical granules | Uniformly distributed throughout the cytoplasm | Uniformly distributed throughout the cytoplasm | Migrate towards the cortical cytoplasm and arrest in the cortex | Arrest in the cortex Present over the spindle region (human) |
Cytoskeleton: | ||||
- Intermediate Filament | Confined to large cortical aggregates | Confined to large cortical aggregates | Disperse into multiple small spots | Evenly distributed throughout the cytoplasm |
- Microtubules | Uniformly distributed throughout the cytoplasm | Condensed around the chromosomes | Observed as fully organized meiotic spindles | Observed as fully organized meiotic spindles |
- Microfilament | Uniformly distributed throughout the cytoplasm | Densely accumulated in the subcortical region of the oocytes | Densely accumulated in the subcortical region of the oocytes | Mainly accumulate in the cortical cytoplasmic region |