A longitudinal observation of the postnatal craniofacial growth in artificial monozygotic twin mice.

An artificial monozygotic twin mouse produced by bisecting a 16-cell stage embryo with a fine glass-needle and then transferring a pair of demi-embryos together into the right uterus of a pregnant recipient female mouse after a 24 hr in-vitro cultivation allowed us to examine the postnatal growth changes in monozygotic twin mice. With regard to the preimplantational growth process of the embryo, we investigated the success rate of bisecting an embryo into paired demi-embryos, the developmental ability of the bisected demi-embryo up to the early blastocyst, and the production rate of such monozygotic twin mice. The items examined in the postnatal growth process of the monozygotic twin mouse were the growth curve of the twin weight and the craniofacial size of the monozygotic twin mouse by cephalometric observations at the 3rd, 10th, 21st, 42nd, 70th, and 100th postnatal days, respectively. The following results were thus obtained. 1) The success rate of bisecting the 16-cell stage embryos into a paired demi-embryos was 86.1% (2,528/2,936). As a result, 2,301 of 2,528 paired demi-embryos could develop up to blastocysts after 24 hr of in-vitro cultivation (91.0%). 2) In addition 42 of the 394 (10.7%) recipient female mice, which had been transferred as paired demi-embryos and delivered a litter, resulted in 31 singleton mice (7.9%) or 11 paired monozygotic twin mice (2.8%), respectively. 3) The weight of each twin increased rapidly up to the 42nd day but thereafter only increased gradually up to the 100th day. The within-pair difference among each paired monozygotic twin mouse could be observed from after the 21st day until the 100th day. 4) Both the dorsoventral craniofacial size of each twin mouse and the lateral craniofacial size of each monozygotic twin mouse increased rapidly up to the 42nd day and thereafter gradually reached a plateau by the 100th day. Almost no within-pair difference was observed among each paired monozygotic twin mouse after weaning up to the 100th day. Based on these results, it is thus concluded that the craniofacial growth of each paired monozygotic twin mouse more closely resembled the growth pattern than that of the somatic growth, which thus indicated that the craniofacial growth was more greatly controlled by the genetic effect than the somatic growth. Furthermore, these findings thus suggest that an embryo bisection is an essential for producing artificial monozygotic twin mice from paired demi-embryos. Therefore, artificial monozygotic twin mice are considered to be a useful animal model for examining the effect of genetic-environment interaction on either the prenatal dento-craniofacial morphogenesis or the postnatal craniofacial growth in mice.