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What Causes Embryo Arrest in IVF?

Many factors affect embryo growth in IVF. Embryonic arrest may be due to extrinsic and intrinsic embryonic factors. Great care is taken to minimize the potential extrinsic environmental factors that may cause arrest in the ART laboratory. Extrinsic factors include culture media composition, oxidative metabolites and others. Factors intrinsic to the embryo from genes, cytoplasmic factors and chromosome rearrangements may lead to arrested development. The laboratory should be under positive pressure (i.e. 25 Pa) and have both charcoal (remove VOCs) and HEPA filtration to remove particulate matter. Many compounds found in the office or hospital setting (solvents, styrene, etc.) may cause the embryo to arrest. Growing the embryo under oil may protect the embryo from such environmental contaminants, but it could act as a sink for them as well. The water used to produce the culture should be pyrogen free. Extremes of media osmolality may inhibit growth. Labs should check the osmolality of media used. Most commercially prepared media have different ionic concentrations, and the embryos essentially must thrive in an environment which may not be optimal for a given embryo. High concentrations (125 mM) of NaCl may be detrimental (Gardner and Lane, 2000), Ca++ and phosphate levels are also important. Human embryos are generally phosphate and lactate dependent during the 1st 3 cell divisions. Glucose may be more important later. The embryo has a block in glycolysis during the 1st 3 divisions and pyruvate is required. Glucose in the presence of phosphate may retard growth (1) at the zygote stage, but is required for blastocyst development. Thus, alterations in sugars may affect growth. Embryos produce de novo nucleotides, so the presence of nucleotides ( i.e. hypoxanthine, adenosine) in the culture media may inhibit their growth. Chelators (EDTA) enhance cleavage stage but not blastocyst stage development. Another factor which may inhibit embryo growth is oxidative stress. The generation of oxygen radicals may inhibit growth. This may be a rationale to add anti-oxidants to the culture media or protocol for patient stimulation.

Several genes have been identified that are important for embryo growth. Zygote Arrest 1 (ZAR 1) is a maternal effect gene that is essential for transition from oocyte to embryo. ZAR 1 null mice fail to progress past the zygote phase. MATER (maternal antigen that embryos require) appears to be required for embryo development beyond the 2 cell stage. Interestingly, BRCA2 deficiency in mice leads to meiotic arrest in spermatogenesis and in oocytes produce impaired early division (2). Robertsonian and reciprocal translocation have been reportedly associated with embryonic developmental arrest (3). It is well known that cleavage arrest at the 2-cell stage can be overcome by direct injection of ooplasm from healthy oocytes. This technique may correct unbalanced anti- and pro-apoptotic factors or mitochondrial defects but clearly has many uncertainties related to mitochondrial or genetic imprinting (4).

From the above, it is clear that as we begin to understand what makes embryos grow we will be able to devise better methods to help them in vitro.

References:

1. Gardner DK, Lane M. Embryo culture systems. In Handbook of In Vitro Fertilization, 2nd ed. Eds. Trounson AO, Gardner DK. CRC Press, Boca Raton. Pp205-264.

2. Sharan SK, Pyle A, Coppola V, Babus, et al, BRCA2 deficiency in mice leads to meiotic impairment and infertility. 2003. Development; 131:131-142.

3. Findikli N, Kahraman S, Kumtepe Y, Donmez E, Birick A, Sertyel S, Berkil H, Melil S. Embryo development characteristics in Robertsonian and reciprocal translocations: ac comparison of results with non-translocation cases. 2003 RBM online. 7(5):563-571.

4. Levy R, Elder K, Menezo Y. Cytoplasmic transfer in oocytes: biochemical aspects. 2004. Human Reprod. Updates. 10(3):241-250.
 

 

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