title: Symmetrical division of mouse oocytes during meiotic maturation can lead to the development of twin embryos that amalgamate to form a chimeric hermaphrodite.
abstract: BACKGROUND
  Gentle compression of mouse oocytes during meiosis-1 prevented the usual extrusion of a small polar body and resulted in the symmetrical division of the ooplasm into two cells of similar size within the zona pellucida. The purpose of our study was to determine whether such cells, equivalent to two small oocytes, were capable of embryonic development and would result in birth following transfer to the uterus.
  
  
  METHODS
  IVF of the 2-celled oocytes was performed and the twin intra-zonal embryos were observed. In each case, the two embryos that originated from fertilized cells with two pronuclei were observed to amalgamate and form a single morula and subsequent blastocyst that was transferred to the uterus of a recipient of a different mouse strain. FISH analysis was performed on sectioned paraffin-embedded tissue of the offspring.
  
  
  RESULTS
  In symmetrically divided oocytes each cell contained a metaphase II spindle. Both cells were fertilizable and cleaved to form twin embryos within the same zona pellucida. Most twin embryos amalgamated to form a single compacted morula, which progressed to hatched blastocysts that contained a single inner cell mass. In total, 104 of these blastocysts were transferred to 19 mice, two of which became pregnant, resulting in the birth of three offspring. FISH analysis showed that one newborn contained both XX and XY cells.
  
  
  CONCLUSIONS
  We found that two small oocytes fertilized within the same zona pellucida to form twin embryos that amalgamate to establish a single chimeric embryo. This may be one mechanism that leads to the formation of a chimeric hermaphrodite when an embryo containing XX cells mixes with its intra-zonal twin containing XY cells.
authors.affiliation: Department of Obstetrics and Gynecology, Nagai Clinic, 607-1 Kamihikona Misato, Saitama 341-0004, Japan. otsuki.midori.junko@gmail.com
authors.forename: Junko
authors.initials: J
authors.lastname: Otsuki
authors.forename: Yasushi
authors.initials: Y
authors.lastname: Nagai
authors.forename: Alexander
authors.initials: A
authors.lastname: Lopata
authors.forename: Kazuyoshi
authors.initials: K
authors.lastname: Chiba
authors.forename: Lubna
authors.initials: L
authors.lastname: Yasmin
authors.forename: Tadashi
authors.initials: T
authors.lastname: Sankai
mesh_terms: D000818:Animals; D053624:Blastocyst Inner Cell Mass / Q000166:cytology; D048429:Cell Size; D002678:Chimera / Q000196:embryology* / Q000235:genetics; D003433:Crosses, Genetic; D012734:Disorders of Sex Development / Q000196:embryology* / Q000235:genetics; D004624:Embryo Transfer; D047108:Embryonic Development; D005260:Female; D005307:Fertilization in Vitro; D017404:In Situ Hybridization, Fluorescence; D059471:In Vitro Oocyte Maturation Techniques; D049469:Meiotic Prophase I; D051379:Mice; D009028:Morula / Q000166:cytology; D009865:Oocytes / Q000166:cytology*; D009866:Oogenesis; D011247:Pregnancy; D011312:Pressure / Q000009:adverse effects; D008941:Spindle Apparatus / Q000378:metabolism