Interspecies Differences in Electromechanical and Histological Characteristics of Human and Swine Esophagus1

Abstract

The esophagus (muscular tube that conveys food from the oral cavity to the stomach through the process of peristalsis) is a vital organ that plays a critical role in the digestive system. Temporary or permanent injury of the esophagus can have severe clinical implications. Any form of physical injury or loss of function will result in lower quality of life. Esophageal injury can happen in many ways, for example, esophageal cancer, gastroesophageal reflux disease, Barrett's esophagus, achalasia, tylosis, during interventional procedures (treatment of cardiac arrhythmias), and use of tobacco and alcohol to name a few. Though chemotherapy remains the first line of treatment, in severe cases, esophagectomy can be inevitable. There have been numerous efforts in researching xenografts and developing artificial esophagus that possess all the necessary qualities of native esophagus including peristalsis [1]; however, this still remains an active area of investigation. Hence, in order to address these unmet clinical needs, it is essential to better understand both the anatomical and physiological properties of the native esophagus that can not only be used to develop artificial biomaterials but also be used to test and qualify biomaterials for form and function.These studies were approved by the University of Minnesota Institutional Animal Care and Use Committee. Fresh esophagus biopsies were obtained (immediately after surgical resection) from healthy castrated male Yorkshire-cross swine (mean weight of approximately 70 kg). Fresh human tissue biopsies of esophagus were obtained (within 6 hr after surgical resection) through research collaboration with LifeSource (St. Paul, MN).Physiological responses were obtained via tissue-bath studies. The details regarding muscle bundle preparation, the tissue-bath system, and stimulation protocols have been described previously [2]. Briefly, muscle bundles bathed in Krebs solution were maintained at 37 °C and electric field stimulations were applied to elicit muscle contractions.The biopsied lengths of swine (n = 2) and human (n = 1) esophagi were divided into five sections as shown in Fig. 1. Five slides for each section were prepared with H&E stain (hematoxylin and eosin) and Masson's trichrome stain. The slides were viewed using the Olympus BX90 microscope. The muscularis externa was identified under 10 × magnification, and then analyzed under higher magnification to better distinguish muscle types. Smooth muscle cells were identified by one centrally placed nucleus in a spindle shaped cell. Skeletal muscle cells were identified by multiple peripherally placed nuclei in a tubular-shaped cell with characteristic striations. The analysis was continued in a circumferential pattern in order to ensure visualization of the entire muscularis externa. Images were digitally saved using the attached Olympus DP71 camera and DP programmer.Contrasting differences were observed in the electromechanical responses of human and swine esophagus muscle bundles. The contraction durations of swine esophagus muscle bundles were approximately 0.7 s as compared to 7 s for the human esophagus muscle bundles (Fig. 2). Depending on which section of the human esophagus (superior, middle, or inferior) muscle bundles were stimulated, different responses were observed, representative examples of which are shown in Fig. 3. We hypothesize that the initial peak results from the fast-twitch muscle fibers followed by prolonged contractions of the slow-twitch muscle fibers. These responses change phases as more inferior muscle bundles are simulated. The wavelike spontaneous contractions are speculated to be ones associated with peristalsis. In contrast, swine esophagus muscle bundles had fairly consistent electromechanical response (along its whole length), i.e., irrespective of which section of the esophagus muscle bundles were stimulated. Although a small prolonged contraction was observed following the peak wave in inferior sections, it was not comparable to those observed from the human tissue samples.In order to assess relative variability in the spatial distributions of skeletal and smooth muscle fibers along the length of these esophagi, standard histological slides were prepared using H&E and Masson's trichrome stain. Contrasting differences were observed between the human and swine tissue types. Swine esophagus slides exhibited mainly skeletal muscle fibers in both the longitudinal and circular muscularis layers throughout the specimen. The superior end of human esophagus consisted of mainly skeletal muscle fibers in both the outer longitudinal and inner circular muscularis layers (Fig. 4). The middle and inferior sections consisted of mainly smooth muscle fibers in both layers (Figs. 5 and 6). In the human esophagus, we expected to observe a transition zone, consisting of mixed muscle fibers (i.e., skeletal and smooth combined), but did not observe such likely because the transition zone was relatively small and was missed during specimen preparation.In this investigation, we uniquely assessed the electromechanical and histological properties of both human and swine esophagus. We observed contrasting species differences, which were attributed to the unique spatial distribution of the skeletal, smooth, and mixed muscle fibers, and also due to the differences in motor innervation of neural supply along the entire length of the swine and human esophagi [3,4] which have been reported previously. It is clear that a thorough understanding of interspecies differences, physiological, and anatomical properties will be essential for qualifying the potential for xenografts and/or artificial biomaterials for future medical use. Along with this, numerous other considerations will have to be addressed, such as, organ rejection, geometrical dimensions for transplant, and mechanical and neural control. Additionally, these data may provide new insights as to symptomatic changes that occur when esophageal tissues become damaged.