Putative embryonic stem cell lines from pig embryos.

Embryonic stem cells (ES cells) were first established in the mouse, and they represent a population of pluripotent, undifferentiated cells derived from early embryos that is capable of proliferating without any limitation in an undifferentiated state. These cells retain the ability to differentiate in vitro or in vivo into derivates of all three germ layers, and when injected into blastocysts, they can participate in the formation of all tissues, including gonads (germ-line chimeras). It is possible to transfect them with a gene of interest, and the resulting transgenic cell lines can also be used for production of chimeras. Unfortunately, mammalian germ-line chimeras that can carry an inserted gene into their progeny have only been produced in the mouse. Logically, before application of stem cell therapies into a human medicine, it is necessary to verify the efficiency and safety of these methods with an acceptable animal model. The pig is currently used as a very convenient animal for pre-clinical applications, and therefore establishment of porcine ES cell lines is highly needed; unfortunately, no convincing ES cell lines have been produced in this species (and other domestic animals) to date. In this article, we discuss the recent advances in this field, especially oriented on possible reasons and obstacles why derivation of porcine ES cell lines is still unsuccessful.