AgNOR

AgNOR refers to silver-stained nucleolar organizer regions. Nucleolar organizer regions are chromosomal segments that contain ribosomal RNA gene clusters and are involved in ribosome biogenesis. In histology and cytology, AgNOR staining uses a silver nitrate–based technique to visualize argyrophilic proteins associated with these regions. Under light microscopy, AgNORs appear as black or brown dots within nuclei, typically concentrated in or near the nucleolus.

The number, size, and distribution of AgNORs in a cell reflect ribosomal RNA transcriptional activity and, by extension, cellular proliferative status. Cells undergoing active proliferation tend to show more numerous and larger AgNOR dots than quiescent cells. Because of this relationship, AgNOR assessment has been explored as a supplementary marker of proliferative activity in various tissues and neoplasms, and has been used in both histopathology and cytology to help distinguish malignant or highly proliferative lesions from benign or reactive ones. It has been studied in cancers such as breast, prostate, bladder, and oral/oropharyngeal carcinomas, often in conjunction with established proliferation markers like Ki-67.

Limitations of AgNOR analysis include lack of universal standardization in staining protocols and counting methods, potential interobserver variability, and dependence on tissue handling and fixation. These factors can affect comparability of results across laboratories. Consequently, AgNOR is best used as an adjunct to morphological assessment and other biomarkers rather than as a standalone diagnostic or prognostic test.