Symposia Abstracts

Do hierarchies within a stem cell lineage consist of a discrete series of stages with progressively narrowing developmental potential, or a continuum with reversible transitions along the way towards the fully differentiated state? Human embryonic stem cell (hESC) cultures are not homogenous but contain a mixed population of cells that we hypothesise represent stages in a differentiation hierarchy. Transcriptional analysis of immunologically defined subpopulations in hESC cultures indicates that spontaneous differentiation occurs continuously, with a steady decline in expression of markers of pluripotentiality across the cell population. Signaling between the stem cells and their early differentiated progeny likely influences cell fate, to drive hESC grown under standard conditions towards neurogenesis. Analysing hESC subpopulations and dissecting the molecular basis of paracrine interactions in the cultures will enable better understanding and control of stem cell renewal and commitment. We previously reported an efficient method that induces selective neural differentiation from mouse embryonic stem (ES) cells by using coculture with PA6 stromal cells (Kawasaki et al, Neuron, 2000; PNAS, 2002). This method, named SDIA, can be used to generate a wide variety of neural tissues including midbrain dopaminergic neurons and motor neurons. However, SDIA-treated ES cells rarely differentiate into telencephalic cells. In the present report, we demonstrate directed differentiation of telencephalic precursors from ES cells by using optimized serum-free suspension culture (SFEB culture). Treatment with Wnt and Nodal antagonists (Dkk1 and LeftyA) during the first five days of SFEB culture causes nearly selective neural differentiation in ES cells (w90%). In the presence of Dkk1 (or Dkk1C LeftyA), SFEB induces efficient generation (w35%) of the cells expressing the telencephalic marker Bf1. Wnt3a treatment during the late culture period increases the pallial telencephalic population (Pax6 C cells yield up to 75% of Bf1 C cells) while Shh significantly promotes basal telencephalic differentiation (Nkx2.1 C , Islet1/2 C) at the cost of pallial telencephalic differentiation. Thus, floating aggregates of ES cells in default of caudalizing signals generate naı¨ve telencephalic precursors that acquire subregional identities by responding to extracellular patterning signals. We also discuss the possibility of generating other CBS tissues by modifying the SFEB conditions. Stem cells are required to support the indeterminate growth style of plants. Meristems are a plant's stem cell niches that foster stem cell survival and the production of descendants destined for differentiation. In shoot meristems, stem cell fate is decided at the populational level by an exchange of signals between the …