Ultrastructure of Three Mouse Embryonic Stem Cell Lines: A Comparative Study

Mouse embryonic stem cells can differentiate in vitro and form embryoid bodies containing structures and cells that differ among all three germ layers. The fine structure of mouse ES cells and that of their respectively differentiated EBs were analyzed by transmission electron microscopy. We observed that mES cells remained undifferentiated and expressed undifferentiated markers such Oct4, Nanog, Sox2 and SSEA1 both protein and gene expression. Differentiated EBs expressed different marker genes and proteins form the three germ layer including KDR, GATA4, Vimentin and β III tubulin. The ultrastructure of undifferentiated mES cells revealed the presence of microvilli on the cell surfaces, large and deep infolded nuclei, low cytoplasm: nuclear ratios, frequent occurrence of lipid droplets, non-prominent Golgi apparatti and smooth endoplasmic reticuli. We also observed on mES cells prominent juvenile mitochondria and free ribosomes-rich cytoplasm; however the modified polysomes were cell line-dependent. Ultrastructure of EB showed different cell arrangements which indicate the different stages of EB development and differentiation. The morphologies of Balb/C and 129 W9.5 EBs were very similar at day 4, while C57BL/6 EBs were distinct at this time, however later in development they all looked alike. This finding suggested that differentiation of EBs from different cell lines occurs in the same pattern but not at the same rate, thus showing a slower proliferation rate ofC57BL/6 EBs than Balb/C and 129 W9.5 EBs. Conversely, we believe some cells from the Balb/C and 129 W9.5 ES lines had already started to differentiate when grown as colonies. The ultrastructure results of these mES cells revealed some differentiating features such as a dilated profile of a rough endoplasmic reticulum. We also measured low expression levels of undifferentiated markers on the outer cells of the Balb/C and 129W9.5 mES colonies, which therefore suggested a greater and faster differentiation potentiality as EBs.