Ericson L E, Nilsson M
Department of Anatomy, University of Göteborg, Sweden.
Toxicol Lett. 1992 Dec;64-65 Spec No:365-73. doi: 10.1016/0378-4274(92)90209-3.
The thyroid epithelium is morphologically and functionally polarized, with an apical surface facing the follicular lumen containing colloid and a basolateral surface facing the interstitium. Iodination and thyroid hormone synthesis occur in the colloid at the apical plasma membrane. The introduction by Mauchamp et al. of primary cultures of porcine thyroid cells grown as a polarized, confluent monolayer on a filter in a bicameral chamber system has now made it possible to study in more detail the barrier function and vectorial ion transport in the thyroid epithelium. The follicular cells form a very tight monolayer (transepithelial resistance > 6000 ohm cm2) and establish a transepithelial potential difference (apical medium negative) of about 20 mV. These parameters are rapidly influenced by TSH, mainly by an action on apical sodium channels, and by EGF. The integrity of the barrier is, as in other epithelia, dependent on extracellular calcium. A calcium-dependent cell adhesion molecule, uvomorulin, is expressed at the lateral plasma membrane surface. EGF induces cell proliferation as well as migration of some of the epithelial cells to a position below the monolayer, which however maintains its polarity and barrier function. In contrast, during TPA-induced proliferation the barrier function is disrupted. Iodide is vectorially transported in basoapical direction while the epithelial layer is virtually impermeable for iodide transfer in the opposite direction. Iodide is concentrated in the cell by the basolateral "iodide-pump" and its efflux across the apical plasma membrane is rapidly and selectively increased by TSH via cAMP. EGF inhibits vectorial basoapical iodide transport mainly by reducing the iodide permeability of the apical plasma membrane. Together, these recent observations indicate that the ion content of the follicular lumen is strictly controlled by the thyroid epithelium.
甲状腺上皮在形态和功能上呈极性分布，其顶端表面朝向含有胶体的滤泡腔，基底外侧表面朝向间质。碘化作用和甲状腺激素合成发生在顶端质膜处的胶体中。Mauchamp等人在双室系统的滤器上培养猪甲状腺细胞，使其形成极性汇合单层，这使得更详细地研究甲状腺上皮的屏障功能和向量离子转运成为可能。滤泡细胞形成非常紧密的单层（跨上皮电阻>6000欧姆·平方厘米），并建立约20 mV的跨上皮电位差（顶端介质为负）。这些参数会受到促甲状腺激素（TSH）的快速影响，主要是通过对顶端钠通道的作用，以及表皮生长因子（EGF）的影响。与其他上皮一样，屏障的完整性依赖于细胞外钙。一种钙依赖性细胞粘附分子——桥粒芯蛋白，在外侧质膜表面表达。EGF诱导细胞增殖以及一些上皮细胞迁移到单层下方的位置，但单层仍保持其极性和屏障功能。相比之下，在佛波酯（TPA）诱导的增殖过程中，屏障功能会被破坏。碘离子沿基底顶端方向进行向量转运，而上皮层实际上对碘离子的反向转运是不可渗透的。碘离子通过基底外侧的“碘泵”在细胞内浓缩，促甲状腺激素通过环磷酸腺苷（cAMP）迅速且选择性地增加其通过顶端质膜的外流。EGF主要通过降低顶端质膜对碘离子的通透性来抑制碘离子的基底顶端方向的向量转运。总之，这些最新观察结果表明，滤泡腔内的离子含量受到甲状腺上皮的严格控制。
