Category 1: mildly distorted glandular structures, branching villi and tubular crypt proliferation, mild nuclear and cellular atypism, and intact basement membrane; Category 2: moderately or severely distorted glandular structures with branching villi, severe nuclear and cellular atypism, increased mitotic figures, increased atypical mucous retention, and intact basement membrane; Category 3: evident infiltration of tumor cell nests with tumor stromal induction, increased mitotic figures, penetration of the basement membrane, and invasion of the lamina propria or muscularis mucosae.
Source: wiktionary