Fibrous structure of connective tissue in the vocal fold of the Japanese monkey (Macaca fuscata)

Fibrous structures in the vocal fold were studied in 8 adult Japanese monkeys. Their vocal folds were fixed with formalin and longitudinal and cross-sections were prepared. Some of the samples were treated with 10% NaOH to digest cellular components and elastic fibers, and some of them were treated with 90% formic acid to digest cellular components and collagen fibers. Each sample was then fixed with OsO4, dehydrated, dried at the critical point, ion-coated, and studied under a scanning electron microscope. The lamina propria mucosae in Japanese monkeys was thinner than that in humans and consisted of a superficial layer rich in connective tissue and a deep layer poor in this tissue. Both collagen fibers and elastic fibers mostly ran straight, and the fiber distribution and morphology slightly differed according to the depth of the layer. Their density was higher in upper layers. In the muscle layer, connective tissue surrounding muscle fibers was scarce. The fibrous structure of the monkey vocal fold is simpler than that of human vocal fold, and these findings reflect the short and monotonous phonation of monkeys.