Immunohistochemical analysis of sectioned EBs treated with cytokines and BMP-4. Representative analysis of individual EBs isolated and sectioned (see “Materials and methods”) for analysis of (A) CD45 and alkaline phosphatase (AP) activity at specific time points indicated during hESC differentiation. Scale bars measure 200 μm. (B) High resolution microscopy of indicated regions of day 15 EBs analyzed for CD45 and AP activity. Scale bars in main panels measure 100 μm; in insets, 20 μm.