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Mouse embryonic stem cells (mESCs) have played a key role in the newly emerging fields of stem cell research. The traditional derivation and culture of mESCs have been based on the use of mouse embryonic fibroblasts (MEFs) treated with exogenous leukemia inhibitory factor (LIF). However, the rapid senescence of MEFs, coupled with the high cost of LIF, has significantly hampered the widespread use of mESCs in stem cell research. Thus, we present a novel exogenous LIF-free culture system for general mESCs applications, comprising fibroblast-like cells derived from the rabbit spleen (RSFs). We demonstrated that mESCs cultured on RSFs (mESCs-RSFs) maintained all mESC features after prolonged LIF-free culture, including alkaline phosphatase, cell surface markers (SSEA-1), molecular markers (OCT-4, NANOG, TERT, REX-1), karyotype, and pluripotency. The high expression level of both LIF and WNT3A in the RSFs may account for their ability to maintain mESCs without exogenous LIF. Moreover, this exogenous LIF-free culture system was verified to be of microbiological quality through analysis with electron transmission microscopy.

Embryonic stem cells (ESCs) often display high rates of apoptosis and spontaneous differentiation in routine culture, thus bring the proliferation of these cells highly inefficient. Moreover, little is known about the factors that are indispensable for sustaining self-renewal. To surmount these issues, we established transgenic mES cell lines expressing the enhanced green fluorescent protein (EGFP) under the control of the Rex1 promoter which is a key regulator of pluripotency in ES cells. In addition, we provided a simplified and improved protocol to derive transgenic mESCs from single cell. Finally, we showed that embryoid body (EB) development was faster than adherent differentiation in terms of differentiation ratio by real-time tracking of the EGFP expression. Therefore, these cell lines can be tracked and selected both in vitro and in vivo and should be invaluable for studying the factors that are indispensable for maintaining pluripotency.