Background
Embryonic stem (ES) cells have been derived from the inner cell masses (ICM) of blastocysts in many species. They are capable of unlimited, undifferentiated proliferation on feeder cell layers and remain karyotypically normal and phenotypically stable. In addition, ES cells have the ability to differentiate into a wide variety of cell types in vitro and in vivo. Human ESCs are normally maintained on MEFs in conjunction with media containing a serum replacement. Human ESCs can also be cultured feeder-free on Matrigel®, laminin, or fibronectin in MEF-conditioned medium. Unfortunately, MEFs can be only passaged approximately five times before undergoing senescence. This requires the fresh derivation of MEFs in large quantities from mice and the preparation of a frozen stock which can be thawed, growth inactivated, and used as feeder cells when needed. This process is not only cumbersome but also requires a significant devotion of time and reagents, most often resulting in the generation of a heterogeneous population of stromal cells that varies in its capacity to support stem cell expansion.
JK1 is an murine immortalized SMA+ CD34+ testicular stromal cell line. It supports long-term proliferation of numerous types of stem cells including pluripotent ESCs, germ line-derived stem cells such as spermatogonial progenitor cells (SPCs) and multipotent adult spermatogonial-derived stem cells (MASCs), and primordial germ cell (PGC)-derived embryonic germ cells (EGCs). The JK1 feeder line has maintained its capacity for promoting stem cell self-renewal even after serial passaging during the course of more than one year (Ref. 1).