The oesophagus is a 25 cm long, 2 cm diameter muscular tube which joins the pharynx to the cardiac orifice of the stomach. The stratified squamous epithelium lining the buccal cavity continues through the pharynx and down the oesophagus. The lowest 2 cm or so of the oesophagus which lies within the abdominal cavity is normally lined with gastric mucosa and covered by peritoneum. The stratified squamous epithelium provides a tough impermeable lining resisting the abrasive nature of food boluses, whilst the gastric mucosal lining resists damage by gastric acid. The lumen of the oesophagus is highly folded in the relaxed state. The pH of the normal oesophageal lumen is usually between 6 and 7.
The oesophagus has four coats, a fibrous external layer, a muscular layer, a submucous or areolar layer and an internal or mucous layer (Figure 4.1).
a) The fibrous coat consists of elastic fibres embedded in a layer of areolar tissue.
b) The muscular layer is composed of circular muscle surrounded by longitudinal muscle. Smooth muscle is found in the lower third of the oesophagus, striated muscle in the upper part and both types are found in the middle section.
c) The areolar or submucous coat contains the larger blood vessels, nerves and mucous glands. It loosely connects the mucous and muscular coats.
d) The mucosal layer consists of a layer of stratified squamous epithelium, one of connective tissue, and a layer of longitudinal muscle fibres, the muscularis mucosae. It forms longitudinal folds in its resting state which disappear when distended.
Two types of secretory glands are found in the oesophagus. The majority of oesophageal glands are simple glands located in the lamina propria, but in the lower 5 cm of the oesophagus the glands are compound and identical to the cardiac glands of the stomach (Chapter 5) which secrete mucus rather than acid.
The oesophageal glands are distributed throughout the length of the oesophagus and are located in the submucosa. These are small racemose glands of the mucous type and each open into the lumen by a long duct which pierces the muscularis mucosae. There are probably no more than 300 in total, of which the majority lie in the proximal half of the oesophagus. The relatively few glands present make the oesophagus a moist rather than a wet environment with the majority of fluid coming from swallowed saliva (approximately 1 litre per day). The principal reason for secretion is to lubricate food and protect the lower part of the oesophagus from gastric acid damage.
Submucous coat Circular muscle
Submucous coat Circular muscle
\ Longitudinal muscle
The oesophagus possesses both sympathetic and parasympathetic innervation. Extrinsic innervation consists of a supply from the vagus nerves and sympathetic fibres derived from the cervical and thoracic ganglia. The intrinsic supply is derived from the Auerbach and Meissner plexuses. Neurites are found in the circular muscle of the oesophagus. They also run over the surfaces of interstitial cells which are not present in the longitudinal layer. These cells are called interstitial cells of Cajal and they possess a round or oval nucleus and a long, flat broad process which extends between the muscle fibres. The precise function of these cells has not yet been discovered, but it is thought that they coordinate muscle contraction.
The distal or lower oesophageal sphincter, also called the cardia, represents the transition between the oesophagus and the stomach. As no definite anatomical sphincter exists, there are several definitions of the lower oesophageal sphincter:
a) the junction of squamous and columnar epithelium, b) the point at which the oesophagus enters the stomach, c) the junction between the oesophageal inner muscle layer and the inner layer of oblique muscle of the stomach (Figure 4.2).
These features all occur in the human stomach within 1 cm of each other. The definition by manometry is a high pressure zone, 2 to 6 cm in length, with an intraluminal pressure of 15 to 40 mm Hg above intragastric pressure. This "sphincter" prevents gastrooesophageal reflux, i.e. acidic gastric contents from reaching stratified epithelia of the oesophagus, where it can cause inflammation and irritation.
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