Transmission electron micrograph of a bone marrow orthochromatic erythroblast from guinea pig showing terminal differentiation in erythrocyte formation. Accompanying the cessation of most transcription is the conversion of most nuclear chromatin to dense heterochromatin. Heterochromatin is excluded from regions of nuclear envelope with nuclear pores. This micrograph shows nuclear condensation at an advanced stage in. Figure 116 from Chapter 4 (Nucleus) of 'The Cell, 2nd Ed.' by Don W. Fawcett M.D. A PDF copy of the corresponding chapter is available on the ASCB's BioEDUCATE website.