Figures 220 (upper) and 221 (lower) from Chapter 7 (Mitochondria) of 'The Cell, 2nd Ed.' by Don W. Fawcett M.D. Mitochondrial density, and the number of cristae per mitochondrion are related to energy demands of each particular cell type and tissue. In the upper figure, epithelial cells from mouse are moderately active, and thus this mitochondrion has an average number of lamellar cristae. In the lower figure, rapidly contracting skeletal muscle from cricothyroid muscle of the bat has high energy requirements. Correspondingly, this mitochondrion contains many long and closely packed cristae. A PDF copy of the accompanying chapter is available on the ASCB’s BioEDUCATE website.