It's this tube-like structure, which is made of many, many epithelial cells all directly adjacent to each other, is what separates the outside world from our internal bodies.

这些管状结构,也是由许许多多上皮细胞,互相连接而成的,它使体内和体外,区分开来

耶鲁公开课 - 生物医学工程探索课程节选