Barrett's oesophagus. Endoscope view of the lower oesophagus (gullet) of a patient showing Barrett's oesophagus. The entrance to the stomach (black hole) is seen at centre left. This is a condition in which the cells lining the lower oesophagus change into cells similar to those which line the stomach, a type of change known as metaplasia. This is usually due to prolonged acid reflux, where stomach acid enters the oesophagus. Here, stomach lining cells are red and the normal oesophageal lining is pink. This condition often develops into a malignant tumour, and requires frequent monitoring.