Chapter 310 Embryology, Anatomy, and Function of the Esophagus
The esophagus is a hollow muscular tube, separated from the pharynx above and the stomach below by 2 tonically closed sphincters. Its primary function is to convey ingested material from the mouth to the stomach. Largely lacking digestive glands and enzymes, and exposed only briefly to nutrients, it has no active role in digestion.
Embryology
The esophagus develops from the postpharyngeal foregut and can be distinguished from the stomach in the 4 wk old embryo. At the same time, the trachea begins to bud just anterior to the developing esophagus; the resulting laryngotracheal groove extends and becomes the lung. Disturbance of this stage can result in congenital anomalies such as tracheoesophageal fistula. The length of the esophagus is 8-10 cm at birth and doubles in the 1st 2-3 yr of life, reaching ∼25 cm in the adult. The abdominal portion of the esophagus is as large as the stomach in an 8 wk old fetus but gradually shortens to a few millimeters at birth, attaining a final length of ∼3 cm by a few years of age. This intra-abdominal location of both the distal esophagus and the lower esophageal sphincter (LES) is an important antireflux mechanism, because increases in intra-abdominal pressure are also transmitted to the sphincter, augmenting its defense. Swallowing can be seen in utero as early as 16-20 wk of gestation, helping to circulate the amniotic fluid; polyhydramnios is a hallmark of lack of normal swallowing or of esophageal or upper gastrointestinal tract obstruction. Sucking and swallowing are not fully coordinated before 34 wk of gestation, a contributing factor for feeding difficulties in premature infants.
Anatomy
The luminal aspect of the esophagus is covered by thick, protective, nonkeratinized stratified squamous epithelium, which abruptly changes to simple columnar epithelium at the stomach’s upper margin at the gastroesophageal junction (GEJ). This squamous epithelium is relatively resistant to damage by gastric secretions (in contrast to the ciliated columnar epithelium of the respiratory tract), but chronic irritation by gastric contents can result in morphometric changes (thickening of the basal cell layer and lengthening of papillary ingrowth into the epithelium) and subsequent metaplasia of the cells lining the lower esophagus from squamous to columnar. Deeper layers of the esophageal wall are composed successively of lamina propria, muscularis mucosae, submucosa, and the 2 layers of muscularis propria (circular surrounded by longitudinal). The 2 delimiting sphincters of the esophagus, the upper esophageal sphincter (UES) at the cricopharyngeus muscle and the LES at the GEJ, constrict the esophageal lumen at its proximal and distal boundaries. The muscularis propria of the upper third of the esophagus is predominantly striated, and that of the lower 2/3 is smooth muscle. Clinical conditions involving striated muscle (cricopharyngeal dysfunction, cerebral palsy) affect the upper esophagus, whereas those involving smooth muscle (achalasia, reflux esophagitis) affect the lower esophagus. The muscular LES and the mucosal “Z-line” of the GEJ may be discrepant up to several centimeters.
Function
The esophagus can be divided into 3 areas: the UES, the esophageal body, and the LES. At rest, the tonic LES pressure is normally ∼20 mm Hg; values <10 mm Hg are usually considered abnormal, although it seems that competence against retrograde flow of gastric material is maintained if the LES pressure is >5 mm Hg. The LES pressure rises during intragastric pressure amplifications, whether caused by gastric contractions, abdominal wall muscle contractions (“straining”), or external pressure applied to the abdominal wall. It also rises in response to cholinergic stimuli, gastrin, gastric alkalization, and certain drugs (bethanechol, metoclopramide, cisapride). The UES pressure is more variable and often higher than that of the LES; it decreases almost to zero during deep sleep and it increases markedly during stress and straining. The UES and LES relax briefly to allow material to pass through during swallowing, belching, reflux, and vomiting. They can contract in response to subthreshold levels of reflux (esophagoglottal closure reflex).
Swallowing is initiated by elevation of the tongue, propelling the bolus into the pharynx. The larynx elevates and moves anteriorly, pulling open the relaxing UES, while the opposed aryepiglottic folds close. The epiglottis drops back to cover the larynx and direct the bolus over the larynx and into the UES. The soft palate occludes the nasopharynx. The primary peristalsis thus initiated is a contraction originating in the oropharynx that clears the esophagus aborally (Fig. 310-1
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