The striking difference in gross characters of these two tumours is reflected also in their histology. In a villous papilloma the mucus-secreting epithelium covers the outside of the supporting central core of connective tissue and blood-vessels and the secretion is poured out on the surface, whereas in an adenoma the secreting cells are arranged as in tubular
glands and the mucus is therefore discharged into a confined space.
Why should these two tumours, derived from a similar source and composed of identical elements, differ so strikingly both in appearance and intimate structure?
The most obvious reply is that the differences are due to the fact that a papilloma projects from the surface and has therefore ample room in which to expand whereas the development of an adenoma is to some extent restricted by the tissues in which it is embedded. This explanation presupposes that a papilloma is derived from surface epithelium whereas an adenoma is
derived from epithelium deeper down in the mucous membrane.