Fading Fertility: Using Confocal Microscopy to Understand Oocyte Quality
Introduction

Fading Fertility: Using Confocal Microscopy to Understand Oocyte Quality

Confocal microscopy used to investigate disease models and the aging process.

A high quality oocyte is a precondition for successful fertilization and subsequent embryonic development. In most mammals, female reproductive aging is marked by a prominent decline in the quantity and quality of oocytes. Low quality of oocytes is a common and insurmountable problem for women with higher maternal age.

Dr. Bo Xiong is a reproductive biologist at the College of Animal Science and Technology, Nanjing Agricultural University, China. He has been working in this field for over 20 years. His lab has published work using confocal microscopy to uncover new roles of proteins involved in producing fertilization-competent eggs in mice and pigs.

Dr. Bo Xiong, Nanjing Agricultural University, China

Our work aims to identify the molecular biomarkers for oocyte quality and develop effective approaches and strategies to protect the oocytes from deterioration induced by maternal aging, contributing to improved fertility and enhanced efficiency of assisted reproductive technology.

Dr. Bo Xiong

College of Animal Science and Technology, Nanjing Agricultural University, China

Spindle assembly and chromosome alignment in a mouse oocyte in metaphase stage
Spindle assembly and chromosome alignment in a mouse oocyte in metaphase stage

Spindle assembly and chromosome alignment in a mouse oocyte in metaphase stage I, immunostained with α-tubulin-FITC antibody to show the spindle morphology (green), with CREST to display the kinetochores (red), and counterstained with Hoechst to visualize the chromosomes (blue). The proper organization of spindle/chromosome structure drives the oocyte nuclear maturation. Image acquired with the ZEISS LSM 900 confocal microscope.

Spindle assembly and chromosome alignment in a mouse oocyte in metaphase stage I, immunostained with α-tubulin-FITC antibody to show the spindle morphology (green), with CREST to display the kinetochores (red), and counterstained with Hoechst to visualize the chromosomes (blue). The proper organization of spindle/chromosome structure drives the oocyte nuclear maturation. Image acquired with the ZEISS LSM 900 confocal microscope.

A New Protein in Oocyte Maturation

Cohesin is a well-studied protein complex important for sister chromatid cohesion, chromosome segregation, and many other DNA-related activities. Sororin is an accessory protein to cohesin and acts as a stabilizer for cohesin during cell division.

In C. Zhou et al., they found that sororin also has a non-canonical role during oocyte meiotic progression into mature eggs, acting as a regulator of mammalian ooctyes as they begin cell division and creation of the meiotic spindle that will separate homologous chromosomes. These findings not only uncover a novel function and an unknown downstream effector of sororin during female egg development, but also extend our understanding of the molecular basis underlying the etiology of oocyte maturational arrest in humans.

Confocal Microscopy in This Research

Critical for Accurate Visualization of Organelle Dynamics

The binding of mouse sperm to the mouse egg
The binding of mouse sperm to the mouse egg

The binding of mouse sperm to the mouse egg. Mouse sperm binding to the zona pellucida of a mouse egg were stained with Hoechst to show the sperm head (blue). The number of sperm binding to the zona pellucida surrounding the whole egg is a critical indicator for evaluating fertilization ability. Image acquired with the ZEISS LSM 900 confocal microscope.

The binding of mouse sperm to the mouse egg. Mouse sperm binding to the zona pellucida of a mouse egg were stained with Hoechst to show the sperm head (blue). The number of sperm binding to the zona pellucida surrounding the whole egg is a critical indicator for evaluating fertilization ability. Image acquired with the ZEISS LSM 900 confocal microscope.

3D Imaging of Oocytes with Confocal Microscopy

For these studies with mammalian oocytes, clear visualization is critical to understand the dynamics of organelles, such as the microtubule organization of the meiotic spindle, chromosome alignment, kinetochore-microtubule attachment, actin polymerization, mitochondrial distribution and cortical granule translocation. The ZEISS LSM 900 confocal microscope allowed elegant investigation of protein localization, co-localization and fluorescent signal quantification. In particular, 3D imaging is necessary when counting the number of sperm binding to the zona pellucida surrounding the whole egg, a critical indicator for evaluating the fertilization ability.

Dr. Bo Xiong, Nanjing Agricultural University, China

What's Next

In spite of its great clinical success, IVF is mainly associated with abnormal polyspermy, due to the dispermic oocyte penetration resulting in tripronuclear (3PN) oocytes. Thus, how only one sperm fertilizes each egg becomes one of the central questions about mammalian fertilization.

We now understand that to prevent polyspermy, the egg develops the plasma membrane and zona pellucida to block post-fertilization fusion, penetration and binding of additional sperm. The molecular basis and underlying mechanism for the post-fertilization zona pellucida block to sperm penetration has not been uncovered. The next goal of our lab is dedicated to address this scientific question.


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