This transmission electron micrograph of a section through a late orthochromatic erythroblast (normoblast) from human bone marrow shows shallow depressions along the membrane surface, indicated by arrows. The thickened membrane at these sites suggests the formation of coated vesicles during micropinocytosis in these erythropoietic cells. The nucleus contains much compact heterochromatin as transcription ceases and is followed by it's ejection.
Figure 53 from Chapter 2 (Specializations of the Free Surface) of 'The Cell, 2nd Ed.' by Don W. Fawcett M.D. A PDF copy of the accompanying chapter is available on the ASCB's BioEDUCATE website.