Human embryonic stem cells (hESC) differentiate spontaneously in culture and develop a complex microenvironment comprising of autologously derived niche that in turn supports their pluripotency. The basic hypothesis that we deal with is that hESCs undergoing differentiation, sequentially generate trophectoderm and endoderm lineages and thereafter influence further events through the production of growth factors. These factors control the fate of hESCs either by promoting or retarding the recruitment of new cells in the differentiation program. This scenario therefore represents an analog of the in vivo situation in which extra-embryonic tissues influence the behavior of the inner cell mass (ICM). The premise of the paper is the Rho kinase inhibitor Y27632 that can spatiotemporally alter this balance between pluripotency and differentiation. To evaluate the composition and inclination of lineage specification during spontaneous differentiation, we have studied the hESC colonies and their surrounding niche as interdependent entities. We show that the population of fibroblastic niche that surrounds hESC colonies co-expresses trophectoderm and niche cell markers including SSEA1, hCG, progesterone, HAND1, pSmad1 and FGFR1 as early as day 4. A sudden increase in the expression of GATA4 and AFP secretion indicated putative endoderm formation on day 6 in both control and Y27632 treated cultures. On day 6, 20 μM of Y27632 supplementation significantly reduced the trophectoderm-like niche population without affecting endoderm formation, enhanced the average size and number of hESC colonies, decreased IGF1 secretion thereby improving the pluripotency. Overall our findings support the afore mentioned hypothesis and demonstrate that closely packed epithelial trophectoderm-like cells bordering the hESC colonies present an initial and imminent localized niche which is spatiotemporally regulated. Such advances in understanding the behavior and modulation of hESC and its surrounding niche would facilitate better differentiation protocols for applications in regenerative medicine and drug screening.
Keywords: Human embryonic stem cells, Y27632, trophectoderm-like niche, primitive endoderm, IGF2
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