Postovulatory ageing induces structural changes in the mouse zona pellucida.

Area of Science:

• Reproductive Biology
• Cell Biology
• Developmental Biology

Background:

• The zona pellucida (ZP) is a crucial glycoprotein layer surrounding mammalian oocytes, essential for fertilization.
• Understanding ZP structural integrity is vital for assessing gamete viability and reproductive potential.

Purpose of the Study:

• To investigate the structural changes in the zona pellucida (ZP) of aged mouse oocytes.
• To correlate ZP structural alterations with fertilization capacity in vitro.

Main Methods:

• Analysis of mouse oocytes collected immediately post-ovulation and 20 hours post-ovulation.
• Application of conventional electron microscopy and alcoholic phosphotungstic acid (PTA) staining.
• In vitro fertilization assays to assess the developmental potential of aged oocytes.

Main Results:

• Aged oocytes (82.14%) exhibited a ZP with two distinct regions: a dense inner layer and a low-density outer layer.
• The ZP displayed a fibrillar banded structure with parallel fibrillar threads in both regions.
• In vitro fertilization resulted in only 16.85% of aged oocytes reaching the two-cell embryo stage, compared to 66.93% for fresh oocytes.
• No sperm penetration was observed in non-fertilized aged oocytes, indicating a barrier to fertilization.

Conclusions:

• Aging induces significant structural modifications in the mouse zona pellucida, characterized by regional differentiation and altered fibrillar organization.
• These ZP structural changes in aged oocytes impede sperm penetration and drastically reduce in vitro fertilization rates.
• The findings highlight the impact of oocyte aging on the zona pellucida's structural integrity and its functional consequences for fertilization.