Submitted by Dr. Hesham Al-Inany, M.D. Lecturer, Gynaecology & Obstetrics dept. Kasr El-Aini hospital, Cairo University, Egypt.
Microspermic repair of polyspermic human zygotes
The human oocyte possesses an efficient block to polyspermy, as 20%
of aborted fetuses are triploid. With the introduction of microsurgical
fertilization, especially zona drilling and subzonal insertion, incorporation
of two or more spermatozoa in the oocyte has become very common .
After using Acidic Tyrode's for dissolution of human ZP glycoprotiens, approximately 50% of zygotes were polyspermic when the spermatozoa were from oligospermic males.
Partial zona dissection, which uses mechanical force to make an incision in the ZP, apparently inhibits the incidence of polyspermy. This inhibition probably is due to the effect of sucrose, primarily applied to reduce the cytoplasmic volume, on the activation process (Malter and Cohen, 1989).
Studies for correcting polyspermy using microsurgical extraction of extramale pronuclei are, therefore, warranted
Correction of human polyspermy should be applied only after it is firmly
established that female and male pronuclei can be reliably distinguished
from each other. Moreover, it should be determined with certainly that
pronucleus removal does not lead to partial fragmentation of the pronucleus
with possible aneuploid incorporation of viable chromosomes.
Microsurgically repaired embryos are not allowed to be replaced until appropriate investigation of the genetic consequences of pronucleus extraction has occurred. It should be considered that repeated insertions of the enucleation pipette, reducing the chances of embryo survival, or an excessive loss of cytoplasm (with resulting developmental arrest) may occur after multiple extractions.A study was done by Malter and Cohen,1989, for correction of polyspermy through pronuclear extraction. The study was applied to 25 polyspermic human zygotes.
Nine zygotes survived the procedure, and seven cleaved normally (two of which were fixed for chromosome analysis). Eighteen of the polyspermic zygotes (14 with three pronuclei and 4 with four pronuclei ) were obtained from ZP intact oocytes, and seven (1 five pronuclei, 3 four pronuclei, 3 three pronuclei) from previously zona drilled oocytes. Survival and cleavage occurred in all groups except in four and five pronuclei zona drilled zygotes.
Criteria used to identify male pronuclei were: (1) pronucleus-associated
sperm tail. (2) increased pronucleus size. (3) greater distance (relative
to female pronuclei) from the second polar body.
Sperm tails were never seen and pronucleus size usually was identical. Therefore, the criterion was used, although its reliability should be further evaluated.
Until complete pronucleus removal techniques and reliable pronucleus selection criteria are perfected, embryo replacement after polyspermy correction could result in aneuploid and molar pregnancy.
Continued efforts in this direction should delineate the role such a procedure may play in clinical management of ART programs.
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