Regeneration Capacity of the Upper Esophageal Sphincter in Adult and Aging Rats

Abstract

Swallowing difficulty is a common problem in the elderly. Dysfunction of the upper esophagus sphincter (UES) is one of the common causes. The UES regulates the passage of food between the pharynx and the esophagus and prevents esophageal air insufflation and esophagopharyngeal reflux. Previous studies have indicated an aging-associated physical muscular bar at the upper opening of the esophagus. This study aimed to investigate the regeneration capacity of the UES and its aging effect. Specifically, this study used immunohistochemistry and western blot methods to examine the activity of the adult myogenic stem cells (MSCs) of the UES in normal adult and aging rats.

A total of 25 male Sprague-Dawley rats were used in this study. They were grouped as the young adult (n=16; 4 months old; average bodyweight of 501 ± 10.8 g; mean ± SEM) and aging rats (n=9; 24 months old; average bodyweight of 827 ± 28.44 g; mean ± SEM). The laryngopharynx, UES and cervical esophagus were sampled from each rat. Since the muscular regeneration capacity is determined by activity of the MSCs, in this study, the evaluation of the MSCs in adult and aging rats were performed at cellular and protein levels using immunohistochemistry and western blot methods, respectively. We used Pax7 and key muscle regulatory factors (MRFs) - MyoD, Myf5 and Myogenin as markers for the MSCs.

Our results demonstrate that, quantitatively, the rat UES contained abundant Pax7+ve MSCs but a very small number of MRF+ve cells. The number of Pax7+ve cells decreased significantly during the aging process whereas the number of Myogenin+ve cells remained unchanged in both adult and aging rats. MyoD and Myf5 solely expressed in the adult and aging rats, respectively. Their numbers of MyoD+ve cells in the normal adult and Myf5+ve cells in the aging were similar and at a very low level.

Our results suggest that as Pax7 is expressed in both quiescent and activated MSCs but the quiescent MSCs do not usually express any MRFs, the majority of the MSCs in the UES of both adult and aging rats are the quiescent MSCs. Although the MSCs in the UES are activated in both normal adult and aging rats, the activation rate is very low and constant during the adult period and aging process. As Myogenin expression normally indicates the terminal differentiation of the MSC, the number of fusion-competent MSCs (Myogenin+ve cells) and the fusion rate of the activated MSCs are small and constant in the normal and aging UES.

We concluded that the rat UES is capable of regeneration as it contains the adult MSCs which express at least one of Pax7, MyoD, Myf5 and Myogenin. Such regeneration capacity exists throughout adult and aging periods but with different regulation mechanisms as MyoD and Myf5 solely expressed in the adult and aging rats, respectively.