Figure 1. Ultrastructure of Undifferentiated and Differentiated Human Embryonic Stem Cells.

(A): A representative undifferentiated human embryonic stem cell with a large euchromatin-rich nucleus and scant cytoplasm. The nucleus contains a prominent nucleolus and strands and clumps of heterochromatin distributed throughout. Rare mitochrondria with simple structure and few cristae are present. (B): Undifferentiated cytoplasm is simple and primarily comprised of glycogen, free ribosomes and lipid droplets. Organelles are rare and generally less mature in structure. (C): A cell after 8 days of differentiation shows a significant increase in cytoplasm area. The nucleus is slightly condensed and contains more heterochromatin that is distributed in a diffuse granular pattern and a dense rim inside the nuclear envelope. (D): A typical differentiation day 8 cell contains far more complex cytoplasm than the undifferentiated cell, with increases in protein synthetic machinery including Golgi apparatus and rough endoplasmic reticulum. (E): A cell after 17 days of differentiation contains a nucleus with finely dispersed heterochromatin, a less prominent nucleolus and granular distribution with a dense rim. The cytoplasm comprises significantly more area than the undifferentiated cell's. (F): A group of cardiomyocytes after 17 days of differentiation contains highly specialized organelles, including contractile apparatus and desmosomes.

Abbreviations: Ds = Desmosome. Gly = Glycogen. MF = myofilaments. Nu = Nucleus. GA = Golgi Apparatus. RER = Rough Endoplasmic Reticulum.