Characterisation of Kürsteiner canals of parathyroid: imparting relevance to a one-and-a-quarter-century-old concept.

Aims: Kürsteiner canals (KC) were described at least 125 years ago as pharyngeal pouch embryological remnants of parathyroid and thymic development. While considered precursors for a subset of parathyroid cysts and salivary heterotopias (SH), they remain enigmatic. We now define a comprehensive phenotype of KC remnants and investigate their role in a spectrum of parathyroid lesions.

Methods and results: `Sixty-two cystic and 22 non-cystic parathyroid lesions (73 patients) were retrieved from our institutional archive (2011-23) and evaluated for the presence of KC and prevalence of KC phenotype in parathyroid hormone (PTH)-positive and PTH-negative cysts. KC phenotype was defined as: cysts and tubules with surrounding sclerosis; bland, unilayered lining with frequent nuclear indentation of lumina; vesicular chromatin relative to chief cells; attenuated eosinophilic to 'hyper-cleared' cytoplasm; and staining pattern PTH-negative, SOX-10-positive, CK7-positive, GATA-3-positive and PAX-9 dim, a subset with oestrogen/progesterone receptor (ER/PR) positivity. Thirty PTH-negative cysts were identified in the neck/mediastinum; 14 of this group also showed SH. Thirty-two PTH-positive cysts included: 11 cystic parathyroid adenomas, 17 hyperplastic parathyroids, and four carcinomas. KC showed two distinct subtypes and were often found near PTH-negative cysts. PTH-negative cysts were associated with inferior parathyroids, SOX-10 positivity, fibrosclerosis, vesicular nuclei indenting cyst lumina and hyper-cleared or attenuated eosinophilic cytoplasm.

Conclusions: KC are common in parathyroids and show a distinct histological and immunohistochemical profile, with an inferior predilection favouring branchial cleft III distribution. Diagnostically, the high prevalence of this phenotype in PTH-negative cysts and salivary heterotopia supports derivation of non-functioning cysts from KC. Conversely, PTH-positive cysts are more compatible with cystic change within hyperfunctioning glands.