Fig. 2. Mitochondria-rich (MR) cells in the lamprey gill epithelium. (A) Intercalated MR cell between two pavement cells in the gill epithelium of a downstream migrant (young adult) of Geotria australis. The apical surface is enlarged by slender microplicae and the apical cytoplasm contains large numbers of membranous tubules and vesicles. (B) Higher magnification of the membrane of the microplicae, showing a coat of studs on the cytoplasmic side (arrows). (C) Ammocoete MR cells in the gill epithelium of a Petromyzon marinus larva, which, in contrast to the intercalated MR cells, lack cytoplasmic vesicles and tubules and exhibit only a moderate enlargement of their apical surface. Scale bars, 2 µm (A), 0.2 µm (B) and 2 µm (C).