epithelial layer

例句：

1. 1.Epithelial cells are responsible for the protection of the skin from external factors.
   上皮细胞负责保护皮肤免受外界因素侵害。
2. 2. The epithelial layer is a thin layer of cells that lines the surface of organs and tissues.
   上皮层是一层覆盖器官和组织表面的薄细胞层。
3. 3. Epithelial cells can also undergo differentiation into specialized cell types.
   上皮细胞还可以分化成专门的细胞类型。