Assemblies of linear arrays of particles in the apical plasma membrane of mitochondria-rich cells in the gill epithelium of the atlantic hagfish (Myxine glutinosa)

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Abstract
In the gill epithelium of the Atlantic hagfish Myxine glutinosa a mitochondria‐rich cell type is described which ultrastructurally resembles the chloride cells in other fish species. Freeze‐fracture replicas reveal different supramolecular structures in the luminal plasma membrane and in the intracellular amplification (tubular system) of the basolateral plasma membrane of these mitochondria‐rich cells. In the luminal plasma membrane ordered assemblies of particles and fibril‐like elements are regularly found. On the P face, the assemblies are composed of up to 20 linear arrays of particles which are preferentially located in the microvillar membrane on which they show a helicoidal orientation. The arrays are formed by globular (diameter 8–9 nm) and rod‐shaped (length 16–20 nm) particles, which occasionally are so tightly end‐to‐end attached that they generate fibril‐like structures. The distances between adjacent arrays within an assembly measure 10–15 nm. On the E face complementary patterns of linear grooves are present. These assemblies have not previously been demonstrated in the freeze‐fractured plasma membrane of branchial chloride cells. In the membrane of the tubular system, emerging from the basolateral plasma membrane of the mitochondria‐rich cells, infrequently repeating rows of 7–8 nm particles are present in addition to randomly distributed globular particles (diameter 7–8 nm). Complementary patterns of grooves are present on the E face. These intramembranous structures resemble the repetitive elements which have been previously described in the tubular system of branchial chloride cells in euryhaline teleosts and the subunits of which are considered to be the sites of ion pumps.