epithele

例句：

1. 1. The epithelial cells are responsible for protecting the underlying tissues.
   上皮细胞负责保护下面的组织。
2. 2.The epithelium is the first line of defense against foreign invaders.
   上皮是抵御外来入侵者的第一道防线。
3. 3.The epithelium is a thin layer of cells that lines the body's surface.
   上皮是一层覆盖身体表面的细胞层。