Importance of the Microtubule Cytoskeleton in Oocyte Competence
Condition: ART
Study type: Observational

Summary: 
      This study will assess if there are microtubule cytoplasmic features specifically associated
      with oocyte vitrification.
    

Detailed description: 
      Successful fertilization is heavily dependent upon inherent qualities of the oocytes, and
      thus reliant upon the fidelity of oocyte maturation. Reduced oocyte developmental competence
      is one of the main reasons for the decreased potential of in-vitro-produced embryos.
      Approximately 8,6% to 15% of all infertility patients produce at least one meiotically
      incompetent oocyte. There are also extreme cases of women which generate oocytes with
      complete or predominant failure to complete meiosis. These oocytes arrest and do not complete
      meiosis in response to exogenous final maturation and ovulation triggering. The incidence of
      this is unknown, and given the complexity of the processes involved in oocyte maturation, it
      has been difficult to define the key events or morphological features underlying oocyte
      competence.

      An increasing amount of evidence in the past years has suggested that the microtubule (MT)
      cytoskeleton may have an important role in providing the oocyte with competence potential.
      Oocyte maturation involves major rearrangements of the MT cytoskeleton. It is well known the
      importance of forming a proper microtubule meiotic spindle for chromosome segregation upon
      meiosis. Moreover, MTs seem to be important for the redistribution of different organelles
      upon maturation. However, the exact functional significance of this reorganization and how
      MTs influence competence through the repositioning of these organelles is largely unknown.

      The main MT organizing center in eukaryotic cells is the centrosome. Indeed, centrosomes are
      essential for the formation of the mitotic MT bipolar spindle. Interestingly, in the vast
      majority of female eggs these structures are eliminated, and the meiotic spindle does not
      contain centrosomes. A recent study in the lab organism Drosophila melanogaster (fruit fly),
      showed that interrupting this elimination, forcing the maintenance of centrosomes up to
      meiosis, led to female infertility. Both meiosis and the first nuclear divisions upon
      fertilization showed abnormal MT spindles with abnormal chromosome congression. This shows
      that proper spaciotemporal MT organization is likely to have important implications on oocyte
      competence.

      The long-term goal of this research group is to analyze the MT cytoskeleton of oocytes from
      women which, upon assisted reproductive technologies, generate a high percentage of arrested
      embryos. In order to do this, the MT features of already-available surplus and unused oocytes
      which are vitrified and available for research purposes will be compared. These oocytes were
      donated from cycles in which a low (<20% the fertilized embryos) and high (>20% the
      fertilized embryos) incidence of arrested embryos was seen. Previous studies have posited
      that oocyte vitrification and warming may expose the oocyte to a variety of physical and
      chemical processes which may impact on their structural and genomic integrity. Therefore,
      this initial feasibility pilot study will investigate potential structural consequences for
      the oocytes of the vitrification procedure used in house.

      Thus, it will be assess whether the Cryotop® - Open System induces specific MT features which
      may be absent in fresh oocytes. In case that both fresh and vitrified oocytes (derived from
      the same patient - sibling oocytes study) show a similar MT cytoskeleton organization, a
      collection of cryopreserved oocytes donated for research purposes will be used in the planned
      future studies of this research group.
    

Intervention name(s): Vitrification and thawing
Intervention type(s): Device
Intervention description(s): Vitrification using the Cryotop® - Open System followed by thawing
Observational model: Cohort
Time perspective: Prospective
Sampling method: Non-Probability Sample
Study population: 
        Healthy women who have consented to a cycle of fresh oocyte donation and who have a total
        antral-follicle count (AFC) above 20 at the onset of ovarian stimulation.
      
Biospecimen description: 
      Consenting subjects will donate supernumerary mature oocytes for the study following
      exogenous ovarian stimulation. However, no genetic analyses will be performed.
    
Keywords: Oocyte competence; Oocyte vitrification; Cryobiology; Microtubule cytoskeleton