Low fertility in vivo resulting from female factors causes small litter size in 129 inbred mice
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RESEARCH NOTE
Low fertility in vivo resulting from female factors causes small litter size in 129 inbred mice Toshiaki Hino Æ Kanako Oda Æ Kenji Nakamura Æ Yutaka Toyoda Æ Minesuke Yokoyama
Received: 5 February 2009 / Accepted: 22 June 2009 / Published online: 22 July 2009 Ó Japan Society for Reproductive Medicine 2009
Abstract Purpose 129 inbred mice show poor reproductive ability, as evidenced by small litters; however, the exact cause of this is unknown. In the present in vivo study we examined fertility and subsequent post-implantation development in an attempt to clarify the cause of small litter size in 129 mice. Methods 129 or C57BL/6J females that displayed vaginal plugs 1 day after mating with males of the same strain were examined for the presence of fertilized eggs. Reciprocal matings were also performed between 129 and C57BL/6J mice. Subsequent post-implantation development of fertilized eggs was examined by dissecting females 18–19 days after the vaginal plugs were found. Results Mean numbers of recovered eggs were 7.9 and 8.0 in 129 and C57BL/6J mice, respectively. Half of the recovered eggs were unfertilized in 129 mice, whereas all were fertilized in C57BL/6J mice. Mean numbers of live fetuses 18–19 days after mating were significantly lower in 129 mice (4.7) than in C57BL/6J mice (7.3). In different types of pairings using both strains of mice, the
T. Hino K. Oda K. Nakamura M. Yokoyama Mitsubishi Kagaku Institute of Life Sciences (MITILS), Tokyo, Japan Y. Toyoda National Research Center for Protozoan Diseases, Obihiro University of Agriculture and Veterinary Medicine, Hokkaido, Japan M. Yokoyama (&) Brain Research Institute, Niigata University, 1-757 Asahimachi dori, Niigata 951-8585, Japan e-mail: [email protected]
fertility was significantly lower whenever 129 females were used. Conclusions The small litter size in 129 mice is caused by low fertility resulting from female factors. Keywords Female factors Fertility in vivo 129 mice Poor reproduction Small litter size
Introduction The 129 inbred mouse strain has been used for the study of testicular teratoma [1, 2] and for the derivation of embryonic stem (ES) cells [3, 4]. According to the 129 nomenclature on the Jackson Laboratory website (http:// jaxmice.jax.org/jaxnotes/archive/483i.html), 129 mice are divided into four substrains: ‘‘Parental’’, ‘‘Steel’’, ‘‘Ter’’, and a genetically contaminated ‘‘X’’. The Steel and Ter, which originated from the Parental, were developed for increased incidence of testicular teratoma [5, 6]. Using 129 mice, many kinds of ES cells with high frequency of germline transmission have been established. Consequently, a large number of gene-targeted mice have been created using the 129-derived ES cells, making 129 one of the most popular mouse strains in genetic research. However, 129 mice are also known to have poor reproductive performance with small litters compared to those of other mice such as C57BL/6 [7–9], making them difficult to breed. To date, the cause of these small litters is un
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