Toughest layer in esophagus is

The wall of oesophagus has the usual four layers viz., mucosa, submucosa, muscularis externa and an external adventitia. The oesophagus does not have a serous covering except over a sho length near its lower end. The Mucosa - The mucous membrane of the oesophagus shows several longitudinal folds that disappear when the tube is distended. The mucosa is lined by stratified squamous epithelium, which is normally not keratinised. Occasional melanocytes and endocrine cells are present. A columnar epithelium, similar to that lining the cardiac end of the stomach, may extend for some distance into the abdominal pa of the oesophagus. Finger-like processes (or papillae) of the connective tissue of the lamina propria project into the epithelial layer (just like dermal papillae). This helps to prevent separation of epithelium from underlying connective tissue. At the upper and lower ends of the oesophagus some tubuloalveolar mucous glands are present in the lamina propria. The muscularis mucosae is absent or poorly developed in the upper pa of the oesophagus. It is distinct in the lower pa of the oesophagus, and is thickest near the eosophagogastric junction. It consists chiefly of longitudinal muscular fibres, but a few circular fibres are also present. The Submucosa - The only special feature of the submucosa is the presence of compound tubuloalveolar mucous glands. Small aggregations of lymphoid tissue may be present in the submucosa, specially near the lower end. Some plasma cells and macrophages are also present. The Muscularis Externa- The muscle layer consists of the usual circular and longitudinal layers. However, it is unusual in that the muscle fibres are paly striated and paly smooth. In the upper one-third (or so) of the oesophagus the muscle fibres are entirely of the striated variety, while in the lower onethird all the fibres are of the smooth variety. Both types of fibres are present in the middle onethird of the oesophagus. The Adventitia - The muscle layer of the oesophagus is surrounded by dense fibrous tissue that forms an adventitial coat for the oesophagus. The lowest pa of the oesophagus is intra-abdominal and has a covering of peritoneum. REF : Inderbir Singh's Textbook of Human Histology, seventh edition, pg.no., 264,265.