Birth of Rhesus Macaque (Macaca mulatta) Infants After In Vitro Fertilization and Gestation in Female Rhesus or Pigtailed (Macaca nemestrina) Macaques

A study was conducted to assess the possibility of using pigtailed macaques (Macaca nemestrina) as recipients for rhesus macaque (Macaca mulatta) embryos. A total of 250 oocytes were collected from 11 rhesus monkeys during 12 follicular aspirations. We performed 15 embryo transfers with two embryos each into rhesus recipients, which resulted in eight pregnancies, of which two were lost during the second trimester. Among the remaining six pregnant rhesus macaques, two were carrying twins, resulting in the birth of eight infants. Twelve transfers of rhesus embryos into pigtailed macaques resulted in one pregnancy and the birth of one infant. Fetal growth and development were monitored by monthly ultrasound examinations, during which biparietal measurements were taken and compared with those derived from 22 pregnant control monkeys. In vitro fertilization-derived singletons tended to develop faster than did twins and naturally conceived control singletons during the initial months of pregnancy and weighed more at birth than did twins. There were pronounced morphologic changes in the placenta of the rhesus that developed in the female pigtailed macaque. These included an irregular shape, elevated placenta-to-birth-weight ratio, and an abnormal length and diameter of the umbilical cord. Histologic analyses of the rhesus–pigtailed placenta showed evidence of maternal–placental floor infarction and thrombosis of the spiral artery with resulting infarction of the villi. These results demonstrate that pigtailed macaques can carry rhesus fetuses to term, but further studies are necessary to determine the cause of the decreased pregnancy rates and observed placental abnormalities.