Definition of guano. : a fertilizer containing the accumulated excrement of seabirds or bats; broadly : excrement especially of seabirds or bats.
This is a cross section through the small intestine. The tall, vertical cells seen at the top of the tissue are columnar cells (one is shown - red arrow), which make up simple columnar epithelium (the cells at the bottom of the image are connective tissue, which will be discussed later). Since columnar cells are quite thick, they do not readily allow passive diffusion.
Epithelial cells are specialized cells which line many different kinds of tissue. Columnar epithelial cells are named because they are somewhat rectangular in shape and resemble a column in their orientation. Columnar epithelial cells are often specialized so that they can have a variety of different functions in the tissues which they line.