Table 1.

Colorectal low- and high-grade dysplasia characterisation.

	Low-grade dysplasia	High-grade dysplasia
Extension	–	Changes must involve more than two glands (except in tiny biopsies of polyps)
Low power magnification	Lack of architectural complexity suggests low-grade dysplasia throughout	Alterations have to be enough to be identified atlow power: complex architectural abnormalities, epithelium looks thick, blue, disorganised and “dirty”
Cytology/architecture	Does not combine cytological high-grade dysplasia with architectural high-grade features	Needs to combine high-grade cytological and high-grade architectural alterations
Architectural features*	Gland crowding, showing parallel disposition, with no complexity (no back-to-back or cribriforming); Global architecture may vary from tubular to villous	Complex glandular crowding and irregularity; Prominent budding; Cribriform appearance and back-to-back glands; Prominent intra-luminal papillary tufting
Cytological features**	Nucleus are enlarged and hyperchromatic, many times cigar-shaped; Nucleus maintain basal orientation (only up to the lower half of the height of the epithelium, although in some cases we can see glands with full-thickness nuclear stratification - this is not HGD if the archite- ture is bland); There is no loss of cell polarity or pleomorphism; No atypical mitosis; Maintained cytological maturation (mucin)	Noticeably enlarged nuclei, often with a dispersed chromatin pattern and evident nucleoli; Loss of cell polarity or nuclear stratification to the extent that the nuclei are distributed within all 1/3 of the height of the epithelium; Atypical mitoses; Prominent apoptosis/necrosis, giving the lesion a “dirty” appearance; Lack of cytological maturation (loss of mucin)

*Architectural features: gland morphology and placement; **Cytological features: cell level characteristics.