Esophagus extends between the lower edges of upper esophageal sphincter (UES) to the upper edge of lower esophageal sphincter (LES), and both of these edges are best defined functionally. They are not discernible anatomically, even on the autopsy specimen. Esophageal length varies between 20 and 25 cm and it is not related to an individual's height [50]. Esophagus traverses in the posterior mediastinum of the chest and comes in close anatomical relationship with the aorta, trachea, heart, and vertebral column. Esophagus sits to the right of the aorta in its upper extent and anterior to it in the lower extent, with the left atrium in close relationship in the anterior aspect. Even though relatively vertical in it courses, there are two gentle curvatures that are visible on the barium esophagogram. Similar to the rest of gastrointestinal tract, esophagus is made up of several layers, i.e., mucosa, muscularis mucosa, submucosa, and muscularis propria. Muscularis mucosa is thin, two- to three-cells thick, and oriented in the longitudinal axis. On the other hand, in the muscularis propria, also called as muscularis externa, the dominant muscle layer is organized into inner circular and outer longitudinal muscle layers, each of which is several muscle cells thick. Both circular and longitudinal muscle layers, based on the ultrasound images, are approximately 0.75 mm thick under baseline resting conditions in the live humans [51,52]. Circular muscle is continuous with the cricopharyngeus muscle and the inferior pharyngeal constrictor at the cranial end and with the muscles of the LES at the audal end. On the other hand, the longitudinal muscle layer originates from the dorsal, superior, and lateral margins of cricoid cartilage as two fascicles, cricoesophageal tendons, leaving a triangular gap in its most cranial and posterior aspect (Laimer's triangle) (Figure 2). As the fibers proceed caudally, they surround circular muscle completely and at the inferior end are inserted into the circular muscles of LES. Longitudinal muscle layer is thicker than the circular muscle in its proximal part, but the two layers are equal in their distal extent. Histological studies show that the muscle fibers are arranged spirally in the proximal part. Magnetic resonance diffusion tensor imaging, a relatively novel technique to decipher the orientation of muscle fibers, found helical arrangement in the proximal part of the bovine esophagus, but distally, fibers were arranged in the longitudinal and circular axis [53] (Figure 8). Inter-muscular septum that contains myenteric plexus resides in between the two muscle layers. Esophagus is unique, unlike any other organ in the body, it is made up of partly skeletal and partly smooth muscles. Upper part is entirely skeletal (2–4 cm), the middle, a mixture of skeletal and smooth muscle (Figure 9), and the lower part, 11 cm or so in length is entirely smooth. Upper esophageal sphincter is composed of all skeletal muscles and lower esophageal sphincter of all smooth muscles. Based on the studies in mice embryo, esophagus is comprised of entirely smooth muscles at the beginning that slowly transdifferentiate into the skeletal muscles during later embryological age until few days after birth [54]. However, this issue has been debated by other investigators [55]. Transdifferentiation of fully differentiated phenotype cells is not commonly seen in the other body tissue types and has been of significant interest to the developmental biologists.
FIGURE 8
DSI tractography demonstrating the three-dimensional myoarchitecture of the esophagus. DSI with tractography was employed to image the mesoscopic fiber tract structure of the intact excised bovine esophagus. Fibers were color-coded according to the helix (more...)
FIGURE 9
Smooth and striated muscle composition in human esophagus. The outer and inner layers of the tunica muscularis from each tissue block were investigated and demonstrated separately. Dotted rectangles indicate areas, in which the outer layer was 2- to 4-folds (more...)
Neuromuscular anatomy of the lower esophageal sphincter has fascinated many because contrary to expected, no consistent thickening of the muscles at the gastroesophageal junction is found on autopsy specimens. However, in vivo intraluminal ultrasound imaging in the live humans clearly show a region of thick circular and longitudinal muscle layers in the LES region [56]. The muscle thickness increases and decreases with increase and decrease in the LES pressure, which suggests that the absence of muscle tone in the autopsy specimen account for the lack of muscle thickness. Liebermann–Meffert found an oblique gastroesophageal ring (GER) at the junction between the left side of the esophagus and the greater curvature of stomach, which was the site of greatest muscular thickness. It tapered toward cephalic and caudal direction for a length of approximately 31 mm (Figure 10) [57]. The muscle bundles split 10 mm above the GER and for a length of 25 mm to form short transverse muscle clasps on the right (lesser curvature of the stomach) and oblique fibers on the left (greater curvature of stomach). Oblique fibers form a collar around the left lower end of the esophagus that extends caudally and toward the lesser curvature. How do clasp and sling fibers result in a circumferential squeeze is not clear. As discussed later, clasp and sling fibers show marked differences in their physiological properties and have been referred to as two distinct lower esophageal sphincters [58]. Ultrastructural studies show that the LES muscle, unlike esophageal muscle, shows inward invagin*tions related to its state of tonic contraction [59]. The nerve varicosities in the LES are no different than that of the esophagus.
FIGURE 10
Anatomy of lower esophageal sphincter. Note that the muscle fibers of the LES are not circular, rather they are organized as clasp and sling fibers (from Liebermann-Meffert et al., Gastroenterology 1979;76:31–8).
The crural diaphragm, which forms the hiatus for entry of esophagus from the chest into the abdomen is formed in the right crus of the diaphragm; its inner or medial fibers are oriented in the circumferential direction, and lateral fibers are directed in an oblique craniocaudal fashion [60]. Embryologically, crural diaphragm develops in the dorsal mesentery of the esophagus while the costal diaphragm develops from myoblasts originating in the lateral body wall [61]. Also referred as the pinch co*ck action of diaphragm, crural diaphragm provides a strong sphincter mechanism at the lower end of the esophagus that has been appropriately called as the “external lower esophageal sphincter [63].” The LES and crural diaphragm are anchored to each other by the phrenoesophageal ligament, a condensation of the loose areolar tissue. It may form two leaves that extends from the under surface of diaphragm and attaches to the esophagus, approximately at the upper border of the LES. Because of the firm anchoring of LES and crural diaphragm by the phrenoesophageal ligament, the two structures move together with inspiration and expiration but can separate during longitudinal esophageal muscle contraction related to peristalsis [63] and transient LES relaxation [64].
