It is well known that mature oocytes can reprogram somatic nuclei in the process of nuclear transfer (NT) 1 and remodel male gametes in normal fertilization. An intriguing question is whether blastomeres from cleavage-stage embryos retain reprogramming activities. Recently, Egli et al.2 reported that one blastomere of the two-cell-stage embryo can successfully reprogram a donor nucleus, resulting in the formation of an embryonic stem (ES) cell line or a chimeric mouse. Therefore, it is interesting to investigate whether two-cell blastomeres can remodel male gametes as mature oocytes do. Using mature sperm and round spermatid as donors, we found that two-cell blastomeres can partially remodel mature sperm, but can completely remodel round spermatids to produce blastocysts carrying male chromosomes in all the cells, from which normal functional ES cells can be generated. Our results demonstrate that remodeling ability persists in the blastomeres of two-cell embryos.

To determine whether the two-cell blastomere can remodel male gametes to generate normal diploid embryos, we produced haploid two-cell blastomeres as recipients. Diploid parthenogenetic embryos can be achieved conventionally by activation of MII oocytes in the presence of cytochalasin B (CB), which prevents extrusion of the second polar body. To derive the haploid two-cell-stage embryos, CB was removed from the activation medium. In the control experiments, a total of 109 oocytes were activated for 6 h in CB-free activation medium. Most of the oocytes (104/109) extruded the second polar body and formed one pronucleus. Although all of them divided into the two-cell stage, only 14% (15/104) developed into blastocysts, consistent with a previous report 3. The other five oocytes, which did not extrude the second polar body and formed two pronuclei, developed into blastocysts. Next, we analyzed the cell-cycle stage of haploid two-cell embryos by culturing them in medium containing 5′-bromodeoxyuridine (Brdu). We observed that two-cell embryos usually formed at 18 h and entered the S phase at 19 h after parthenogenetic activation.

Log in or create a free account to read this content

Gain free access to this article, as well as selected content from this journal and more on nature.com

Access through your institution

or

Sign in or create an account
Continue with Google
Continue with ORCiD