Oocyte Morphology Notes

Embryologists spend considerable time examining oocytes under a microscope.
Initial evaluation involves assessing oocyte morphology at retrieval.
Maturity assessment was previously based on cumulus-coronal cell expansion during retrieval.
Currently, maturity is commonly assessed post cumulus cell removal, before ICSI.

Cumulus cells surround the oocyte and support:
- Maturation
- Metabolic and signaling support
- Oocyte transport and fertilization
Corona radiata cells (innermost layer of cumulus cells):
- Crucial in nutrient transfer and protection.
- Mediate sperm binding during fertilization.
Cumulus complex: Combination of cumulus and coronal cells.
Removed prior to Intra-Cytoplasmic Sperm Injection (ICSI).

Meiosis I:
- Reductional division during fetal development, arrested in prophase I until ovulation.
- Resumes before ovulation; separates homologous chromosomes.
- Results in a haploid secondary oocyte and the first polar body.
Oocyte then enters Meiosis II and arrests at Metaphase II.
Meiosis II:
- Equational division (like mitosis), begins after ovulation, arrests at metaphase II until fertilization.
- Fertilization triggers completion of Meiosis II (calcium surge).
- Sister chromatids separate; oocyte extrudes the second polar body.
- Haploid maternal pronucleus merges with haploid paternal pronucleus from sperm forming a diploid zygote.

It is difficult to identify oocyte maturity without removing cumulus and coronal cells.
Maturity becomes apparent under microscopic observation after cell removal.

Immature (Metaphase I of Meiosis 1) human oocyte, also referred to as M1 or Intermediate.
Meiosis is not yet complete, contains 46 chromosomes (diploid, $2n$ ), each with two sister chromatids, therefore, 92 total sister chromatids.
Does not have a polar body yet, but no longer has a GV.

Oocyte has completed the first round of meiosis.
Mature, good quality oocyte characteristics:
- 1 visible Polar Body (PB) with no visible GV, PB containing half the chromosomes.
- Smooth, compact zona.
- Good sized PVS (Perivitelline Space).
- Uniform cytoplasm.
This is what mature ova look like at ovulation and fertilization.

Abnormal characteristics:
- Abnormally large PVS
- Dark cytoplasm
- Abnormal structures in the PVS
- Abnormal appearing polar body

Thick Zona pellucida:
- Originates from altered follicular development, often in older patients.
- May impair sperm penetration and embryo hatching.
- Potentially reducing fertilization and implantation success.
Thin Zona:
- Originates from premature oocyte activation, suboptimal follicular development, or enzymatic or mechanical damage during retrieval/handling.
- Compromises protective functions; increases susceptibility to polyspermy.
- Negatively affects embryo development and cryo survival.
Zonae with a dark appearance:
- Originates from protein aggregation, glycoprotein cross-linking, or oxidative changes during follicular growth or oocyte handling.
- Indicates poor oocyte quality and reduced fertilization/developmental potential.

Enlarged and Oval shaped zonae:
- Develop in oocytes retrieved after controlled ovarian hyperstimulation (COH) due to altered granulosa cell function or dysregulated secretion of zona pellucida glycoproteins (ZP1, ZP2, ZP3).
- Disrupt sperm binding and penetration, especially in conventional IVF, due to altered zona receptor presentation or physical barrier effects.
Zonae with a septum:
- Appear oval shaped and contain a dividing line, partitioning off a portion of the zona from the rest.
- Uncommon.
- Form due to irregular deposition or organization of glycoproteins, similar to enlarged Zona.

Vacuoles:
- Clear, round, membrane-bound structures within the cytoplasm, varying in size and number.
- Presence, especially when large or numerous, is considered a dysmorphic feature, reducing developmental potential.
- Appear due to disruptions in cytoplasmic homeostasis, linked to oxidative stress, suboptimal follicular environment, or mechanical stress during retrieval and handling; also influenced by intrinsic oocyte quality and maturation defects.

Dark clusters:
- Dense, irregularly shaped aggregations within the cytoplasm, often composed of granular or organelle-rich material.
- Aggregation of mitochondria and other organelles.
- Typically, the bordering cytoplasm appears smooth around the cluster.
- Indication of oocyte stress, related to sub-optimal environmental conditions or overall oocyte quality.

Refractile bodies:
- Cytoplasmic inclusions, often associated with lipofuscin accumulation.
- Lipofuscin: yellow-brown, autofluorescent pigment composed of lipid-containing residues of lysosomal digestion (wear-and-tear or aging pigment).
- Accumulates in long-lived cells (neurons, muscle cells, and oocytes) as a byproduct of oxidative stress and cellular aging.
- Vary in size and are sometimes linked to reduced developmental potential.

Smooth endoplasmic reticulum (SER):
- Involved in calcium storage and regulation, critical for oocyte maturation and activation at fertilization.
- SER aggregates are visible in some oocytes but not others due to variability in cytoplasmic maturation, oocyte quality and developmental stage, and differences in how organelles reorganize during in vitro maturation or stimulation protocols.

Which abnormalities negatively impact outcome?
- A, B, C, D - Yes
- E - No
- F - Maybe

The spindle apparatus functions to accurately align and segregate chromosomes during meiosis, ensuring that each resulting cell (either the oocyte or the polar body) receives the correct number of chromosomes.
This is critical for maintaining genomic integrity and preventing aneuploidy in the resulting embryo.

The spindle apparatus can be visualized using polarized light.
In human metaphase II (MII) oocytes, the spindle apparatus is located just beneath the oolemma, somewhat adjacent to the first polar body.

In some cases, spindles may be located offset from the PB.
Spindles are temperature dependent. When the temperature drops below roughly $37℃$ , the proteins can begin to disassociate.
There is a safety net of about 2-3℃.
Temperatures above $37°C$ lead to spindle disorganization, chromosome misalignment, and increased risk of aneuploidy, so it’s better to be on the cooler side, than hotter.