Davies J A, Garrod D R
Department of Anatomy, University of Edinburgh Medical School, UK.
Bioessays. 1997 Aug;19(8):699-704. doi: 10.1002/bies.950190810.
Epithelia can be defined morphologically as tissues that line surfaces, and ultrastructurally with reference to their cells' apico-basal polarity and possession of specific cell-cell junctions. Defining the epithelial phenotype at a molecular level is more problematic--while it is easy to name proteins (e.g. keratins) expressed by a 'typical' epithelium, no known molecules are expressed by every epithelium but by no other tissues. Cells can differentiate to and from the epithelial state as part of normal development, as a response to disease or when manipulated in culture. Many factors (matrix components, adhesion molecules, growth factors, transcription factors) have been identified that can trigger these transitions of phenotype in specific cases, but to date no general master regulators of the epithelial state have been found. The epithelial state may therefore be controlled by multiple regulatory genes so that there is no single molecule responsible for all of the diverse types of epithelium that exist in higher animals.
上皮组织在形态学上可定义为覆盖表面的组织，在超微结构上则可依据其细胞的顶-基极性以及特定细胞间连接的存在来定义。在分子水平上定义上皮表型则更具问题——虽然很容易列举出由“典型”上皮组织表达的蛋白质（如角蛋白），但不存在已知分子是所有上皮组织都表达而其他组织都不表达的。作为正常发育的一部分、对疾病的反应或在培养中进行操作时，细胞可以分化为上皮状态或从上皮状态分化出来。已经确定了许多因素（基质成分、黏附分子、生长因子、转录因子），在特定情况下这些因素可以触发表型的这些转变，但迄今为止尚未发现上皮状态的通用主调节因子。因此，上皮状态可能由多个调控基因控制，以至于不存在单个分子能负责高等动物中存在的所有不同类型的上皮组织。
