Expression of occludin in human rectal carcinoid tumours as a possible marker for glandular differentiation.

Aims: To examine whether or not the tight junction-associated transmembrane protein occludin is expressed in rosette or gland-like structures in human rectal carcinoid tumours. The tight junction is crucial for the formation and maintenance of organized tubular structures in glandular epithelia. Previous studies have reported the presence of glandular structures in carcinoid tumours, though they are not believed to arise from glandular epithelium. Methods and results: The expression profiles of occludin in 40 carcinoid tumours were examined immunohistochemically, using an anti-occludin monoclonal antibody. In eight (20%) samples of typical carcinoid tumours, a small number of rosette-like tubular structures outlined by occludin were detected. Conclusions: Tight junction-associated molecules, including occludin, are thought to be one of the most characteristic structural markers of polarized glandular structures. The results of the present study provide supportive evidence that carcinoid tumour cells are capable of glandular differentiation.