Question

what is thin epithelium

Answer 1

The thin, membranous tissue that lines most of the internal and external surfaces of an animal's body

Answer 2

Simple epithelium is one cell thick; that is, every cell is in direct contact with the underlying basement membrane. In general, it is found where absorption and filtration occur. The thinness of the epithelial barrier facilitates these processes .
for more reading https://en.wikipedia.org/wiki/Epithelium#Simple_epithelium