The name says it all. When stem cells differentiate, their fate may be determined AUTONOMOUSLY (ie, no external cues are required) or CONDITIONALLY (where their fate is determined by external factors)
Autonomous specification results from cell-intrinsic properties , which arise from a cleavage of a cell with asymmetric cytoplasmic determinants or morphogenetic determinants. Thus, the fate of the cell depends on factors segregated into the cytoplasm during cleavage Autonomous specification happens right from the beginning, when egg cytoplasm is partitioned asymmetrically. It is interesting to note that this contribution is entirely maternal , since the sperm donates only a negligible amount of cytoplasm.
Conditional specification happens by cell-extrinsic cues such as cell-cell contact and temporal-spatial signaling (wnt, notch, fgf, bmp, hedgehog, etc). This depends a lot on the relative position of the cell in the embryo.
Cell specificaion in C.elegans is a wonderful model to illustrate these concepts. With the first division of the zygote, there are two cells, the P1 cell and the AB cell. The P1 cell is able make all of its fated cells while the AB cell can only make a portion of the cells it was fated to produce. Thus, the first division gives the autonomous specification of the two cells, but the AB cells require a conditional mechanism to produce all of its fated cells.
Ectoderm, and more specifically, neuroepithelium is widely regarded as the default program in vertebrate development . That is, in the absence of any other differentiating signal, a stem cell will proceed towards neuroepithelial fate. Countless studies have demonstrated this phenomenon in cell and animal models , as reviewed in THIS ARTICLE