Fig. 3. (A) Freeze fracture of an intercalated mitochondria-rich (MR) cell of a Petromyzon marinus larva. Rod-shaped particles are present in the apical membrane (top of the micrograph) and in the membranes of cytoplasmic tubules and vesicles (arrows), while the basolateral membrane (bottom of micrograph) contains only globular particles. (B) Higher magnification of the apical part of the intercalated MR cell shown in A. The apical and cytoplasmic membranes contain a few globular particles in addition to rod-shaped particles. (C) Scanning electron micrograph of an intercalated MR cell surrounded by ammocoete MR cells in the gill epithelium of a larval Geotria australis. Scale bars, 0.5 µm (A), 0.25 µm (B) and 2 µm (C).