Composed of long chains of molecules called nucleotides. The specific sequence of a segment of these nucleotides codes for a specific cell function. This segment is a gene. Usually refers to a disorder in the maturation process of squamous epithelium. Dysplasias are diagnosed by microscopic examination of a biopsy specimen.
Skin is an external lining.
The lining of the intestinal tract is a glandular lining. Epithelial linings are often exposed to the environment. You can trace a path from the skin through the mouth to the esophagus, stomach, small intestine, large intestine and then out the anus and back to the skin. Another example would be from the skin through the vagina, cervix, uterus, fallopian tubes to the surface of the ovary and into the lining of the abdominal cavity. Or, skin, nipple, milk duct into a milk gland of the breast.