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Carbonic anhydrase (CAH) activity was biochemically measured and histochemically localized (at both the light and electron microscope levels) in isolated opercular membranes from teleost fish, Fundulus heteroclitus, adapted to freshwater (FW), seawater (SW), and double-strength seawater (2 x SW). The normal morphology of this membrane showed that its epithelial portion consisted of five cell types: (1) chloride cells, which have been previously implicated as responsible for the active chloride transport across the epithelium; (2) mucous cells; (3) pavement cells, which formed the major portion of the free epithelial surface; (4) supportive cells, which had an abundance of intermediate (10 nm)-type filaments suggesting a structural role for these cells; and (5) vesicular cells, which were characterized by various types of membrane-bound vesicles, including lysosomes, and numerous free ribosomes. Vesicular cells may be stem cells and/or endocrine cells. Hansson's histochemical method for CAH revealed cobalt sulfide reaction product confined to the following structures in fish from each environment: (1) chloride cells: throughout the cytoplasm and some nuclear staining; (2) mucous cells: throughout the cytoplasm, some nuclear staining, and some in mucous granules; (3) vesicular cells: confined to lysosomes, some of the vesicles, and nucleoli; (4) a small portion of the intracellular space between adjacent vesicular cells and supportive cells; and (5) supportive cells: in nucleoli and occasionally in larger membrane-bound lysosomelike structures. Acetazolamide (10(-5) M) and potassium cyanate (KCNO) (10(-1) M) in Hansson's incubation medium completely inhibited the formation of reaction product. Biochemical determination of CAH activity on vascularly perfused, isolated opercular membranes showed no statistically significant difference in enzyme activity between environmental groups. The following units of activity/mg opercular membrane protein were measured: FW: 0.63 +/- 0.02; SW: 0.43 +/- 0.08; 2 x SW: 0.64 +/- 0.09.

Ultrastructure of neurosecretory cells of the preoptic nucleus in immature sterlet (Acipenser ruthenus L.) was studies for the first time. The dorsal zone of the preoptic nucleus houses light, dark, and pycnomorphic neurosecretory cells, which contain varying numbers of elementary secretory granules measuring 1995 plus or minus 32 A in diameter. Light cells containing numerous secretory granules with a diameter of 1707 plus or minus 27 A are predominant in the middle zone. Pycnomorphic cells are absent in this zone. The ventral zone contains only poorly differentiated cells with solitary elementary granules measuring 1100-1300 A in diameter. The neurosecretory cells also differ with respect to the number and structural characteristics of their organoids. Thus, neurosecretory cells in the preoptic nucleus are found in different morphofunctional states, a circumstance that reflects the sequence of their development in ontogenesis and different phases of the secretory cycle.

Cells of the gas gland of the perch Perca fluviatilis L., stimulated to increased generation of gas by the repeated emptying of the swim-bladder, were examined in the electron microscope. Intense activity of the nuclear envelope was demonstrated. Simple vesicles originating from the external nuclear membrane and the so-called multivesicular bodies derived from the outpocketings of both membranes of the nuclear envelope were observed. The multivesicular bodies were filled with numerous fine vesiculae arising from the active proliferation of their internal membrane. The authors offer two alternative mechanisms of formation of fine vesiculae inside the multivesicular bodies and the mechanism of the tearing away of these bodies from the nuclear envelope.