Parthenogenetic embryonic stem cells have been obtained in mouse and in primates. However, it would be desirable to have an alternative experimental model that could be used to investigate the therapeutic potential of these cells. For this purpose, we generated parthenogenetic pig blastocysts from in vitro-matured oocytes activated by sequential exposure to 10 μM ionomycin for 5 min and 2 mM 6-DMAP for 3 h. Inner cell masses were isolated by immunosurgery and plated on mitotically inactivated STO fibroblast feeder layers in 4-well dishes. Cells were incubated in 5% CO2 at 37°C in low glucose DMEM/F10 medium supplemented with 1000 IU/mL of mouse recombinant LIF, 10% Knockout serum replacer (Gibco, Italy), and 5% FBS. Within 3 days, circular colonies with distinct margins of small round cells were observed on both substrates. When a colony enlarged enough to cover half or more of the well surface, cells were trypsinized in clumps never reaching single-cell suspension and passaged to a newly prepared well. The expression of a gene panel was examined by RT-PCR on a portion of the cells at each passage. Oct-4 and nanog were used as markers of pluripotency. Interferon-τ, α-Amilase, Bone Morphogenetic Protein-4, and Neurofilament were used as markers of trophectoderm, endoderm, mesoderm, and ectoderm differentiation respectively. After 4 passages, three colonies expressed Oct-4 and nanog and were negative for all four differentiation markers. Two colonies at the 5th and 7th passages maintained nanog but not Oct-4 expression, while remaining negative to all of the other genes. To induce the formation of embryoid bodies (EBs), cells were cultured in 50-μL droplets of medium without LIF. Initiation of differentiation of EBs was confirmed through both morphological examination and molecular analysis; mesodermal, ectodermal, and endodermal markers were all expressed by Day 9 of culture and Oct-4 and nanog expression was completely down-regulated. Interestingly, when EBs were returned to adherent culture conditions patches of differentiated cells tended to form, spontaneously differentiating into mesodermal, endodermal, or neuroectodermal cell monolayers. The present data suggest that it is possible to establish putative embryonic stem cells from pig parthenotes. Further studies are in progress to determine their ability to stably maintain the undifferentiated state.
This work was supported by MIUR COFIN 20022074357 and Fondazione CARIPLO.
Derivation of sheep embryonic stem cells under optimized conditions
