Ernst S A, Dodson W C, Karnaky K J
J Cell Biol. 1980 Nov;87(2 Pt 1):488-97. doi: 10.1083/jcb.87.2.488.
The structural features of the chloride-secreting opercular epithelium of seawater-adapted killifish (Fundulus heteroclitus) were examined by thin-section and freeze-fracture electron microscopy, with particular emphasis on the morphological appearance of occluding junctions. This epithelium is a flat sheet consisting predominantly of groups of mitochondriarich chloride cells with their apices associated to form apical crypts. These multicellular groups are interspersed in an otherwise continuous pavement cell epithelial lining. The epithelium may be mounted in Ussing-type chambers, which allow ready access to mucosal and serosal solutions and measurement of electrocal properties. The mean short-circuit current, potential difference (mucosal-side negative), and DC resistance for 19 opercular epithelia were, respectively, 120.0 +/- 18.2 microA/cm2, 12.3 +/- 1.7 mV, and 132.5 +/- 26.4 omega cm2. Short-circuit current, a direct measure of Cl- transport, was inhibited by ouabain (5 micron) when introduced on the serosal side, but not when applied to the mucosal side alone. Autoradiographic analysis of [3H]-ouabain-binding sites demonstrated that Na+,K+-ATPase was localized exclusively to basolateral membranes of chloride cells; pavement cells were unlabeled. Occluding junctions between adjacent chloride cells were remarkably shallow (20-25 nm), consisting of two parallel and juxtaposed junctional strands. Junctional interactions between pavement cells or between pavement cells and chloride cells were considerably more elaborate, extending 0.3-0.5 micron in depth and consisting of five or more interlocking junctional strands. Chloride cells at the lateral margins of crypts make simple junctional contacts with neighboring chloride cells and extensive junctions with contiguous pavement cells. Accordingly, in this heterogeneous epithelium, only junctions between Na+,K+-ATPase-rich chloride cells are shallow. Apical crypts may serve, therefore, as focal areas of high cation conductivity across the junctional route. This view is consistent with the electrical data showing that transmural resistance across the opercular eptihelium is low, and with recent studies demonstrating that transepithelial Na+ fluxes are passive. The simplicity of these junctions parallels that described recently for secretory cells of avian salt gland (Riddle and Ernst, 1979, J. Membr. Biol., 45:21-35) and elasmobranch rectal gland (Ernst et al., 1979, J. Cell Biol., 83:(2, Pt. 2):83 a[Abstr.]) and lends morphological support to the concept that paracellular ion permeation plays a central role in ouabain-sensitive transepithelial NaCl secretion.
通过超薄切片和冷冻蚀刻电子显微镜检查了适应海水的鳉鱼（Fundulus heteroclitus）鳃盖氯化物分泌上皮的结构特征，特别强调了紧密连接的形态外观。该上皮是一个扁平片层，主要由富含线粒体的氯化物细胞群组成，这些细胞的顶端相连形成顶端隐窝。这些多细胞群散布在其他方面连续的扁平上皮细胞衬里中。该上皮可以安装在Ussing型小室中，便于获取黏膜和浆膜溶液并测量电生理特性。19个鳃盖上皮的平均短路电流、电位差（黏膜侧为负）和直流电阻分别为120.0±18.2微安/平方厘米、12.3±1.7毫伏和132.5±26.4欧姆厘米²。短路电流是Cl⁻转运的直接指标，当从浆膜侧引入哇巴因（5微米）时会受到抑制，但仅应用于黏膜侧时则不会。对[³H] - 哇巴因结合位点的放射自显影分析表明，Na⁺,K⁺ - ATP酶仅定位于氯化物细胞的基底外侧膜；扁平上皮细胞未被标记。相邻氯化物细胞之间的紧密连接非常浅（20 - 25纳米），由两条平行且并列的连接链组成。扁平上皮细胞之间或扁平上皮细胞与氯化物细胞之间的连接相互作用要复杂得多，深度延伸0.3 - 0.5微米，由五条或更多相互连锁的连接链组成。隐窝边缘的氯化物细胞与相邻氯化物细胞形成简单的连接接触，并与相邻的扁平上皮细胞形成广泛的连接。因此，在这种异质上皮中，只有富含Na⁺,K⁺ - ATP酶的氯化物细胞之间的连接是浅的。因此，顶端隐窝可能作为跨连接途径的高阳离子传导的焦点区域。这一观点与表明跨鳃盖上皮的跨膜电阻较低的电生理数据一致，并且与最近证明跨上皮Na⁺通量是被动的研究一致。这些连接的简单性与最近描述的鸟类盐腺分泌细胞（Riddle和Ernst，1979年，《膜生物学杂志》，45:21 - 35）和板鳃类直肠腺（Ernst等人，1979年，《细胞生物学杂志》，83:(2, Pt. 2):83 a[摘要]）的情况相似，并为细胞旁离子渗透在哇巴因敏感的跨上皮NaCl分泌中起核心作用的概念提供了形态学支持。
