Extended Data Figure 6. Electron microscopy of an 8 month old human brain organoid.

a. Whole organoid before vibratome sectioning shows regional structures. Red line shows approximate location of the vibratome slice. b. Example 40 nm section. c. Regular finger-like membrane stack could indicate development of the outer segment of a retinal rod cell. d. Stacked endoplasmic reticulum. e. Example synapse with crystalline-looking arrangement of vesicles. f. Putative muscle cell characterized by a large cell body with a diameter of about 30 μm highlighted in an organoid section stained with eosin-haematoxylin. Scale bar, 50 μm. g. Example of a muscle cell showing a distribution of the mitochondria near the cell surface, typical for muscle cells. Scale bar, 10 μm. h. Large cell body with muscle fibers highlighted in blue. Enlarged red square region shows muscle fibers with sarcomeres. Scale bar, 10 μm (low magnification) and 1 μm (enlargement). i. Left, one of the dark cell bodies appearing as dark spots on the organoid surface. Right, cell reaching to the surface of the organoid. Scale bar, 10 μm. j. Several cell bodies with different morphologies and cytoplasm content highlighted in color. Scale bar, 10 μm.