Comparative Histoanatomy of the Epiglottis and Preepiglottic Space of the Lemur Larynx

Abstract

An adequate understanding of the evolution of the larynx presumably clarify the physiology and functional histoanatomy of the present-day human organ. This study investigated the comparative histoanatomy of the epiglottis and preepiglottic space of the lemur larynx (a non-human primate without vocal tract). Two normal adult ring-tailed lemur larynges obtained from natural death were investigated. The whole organ serial section technique was employed. The histologic structures of the lemur epiglottis and preepiglottic space differed considerably from that of human adults. The lemur epiglottis was proportionally thick and vertical. It was connected to the thyroid cartilage anteroinferiorly without an intervening thyroepiglottic ligament and to the hyoid bone anteriorly with intervening large bundles of collagen fibers, indicating that the lemur epiglottis does not play the role of retroflection. The lemur larynges did not have a preepiglottic space. These findings also reflect the fact that lemurs do not have a descended larynx and do not acquire the pharyngeal space of the vocal tract. The lemur epiglottis was composed of fibrocartilage, adipose tissue and a small amount of elastic cartilage covered with mucosa (lamina propria and pseudostratified ciliated columnar epithelium). The histologic findings also indicate that the lemur epiglottis is not flexible and does not play the role of retroflection. The results of this study are consistent with the hypothesis that, in the process of evolution, the histologic structures of epiglottis and preepiglottic space likely change to make the larynx descend and lengthen the pharyngeal space of the vocal tract for speech production in humans. Moreover, the distribution of the human preepiglottic space likely allows the epiglottis to more effectively play the role of retroflection during swallowing in order to prevent aspiration.