[Age-related development of the arrangement of connective tissue fibers in the lamina propria of the human vocal folds--scanning electron microscopic examination with digestion method].

The lamina propria of the human vocal fold consists of a superficial, intermediate, and deep layer. This stratified structure is thought to facilitate phonation. Each layer has different physical properties based on different alignment and distribution of collagen and elastic fibers. In the present study, developmental changes in vocal fold structure were studied in human fetuses, infants, and children, with special reference to the pattern of distribution of collagen and elastic fibers. Vocal fold specimens were obtained at autopsy from 5 fetuses, 7 neonates, 3 infants, 3 children at the age of 1 year, 3 children at 3 years, 3 children at 5 years, 3 children at 12 years, and 5 subjects at ages ranging from 15 to 22 years. Prior to the examination of collagen fibers, elastic fibers and cells were dissolved with 10% sodium hydroxide treatment. Prior to the examination of elastic fibers, collagen fibers and cells were dissolved by treatment with 90% formic acid. The specimens were then dehydrated, dried, ion-coated with platinum, and examined with a scanning electron microscope. In fetuses and infants, thin, coiled fibers were found distributed densely in the anterior, posterior, and deep parts of the lamina propria, while irregular thick fibers were sparsely seen in the superficial layer of the vocal fold. In children aged 1 to 3 years, the dense fibers in the deep part decreased, and the longitudinal fibers in the superficial layer increased. In children at 5 years of age, longitudinal collagen and elastic fibers were noted in all of the layers of the vocal fold. The distribution of fibers was uniform irrespective of the depth. At 12 years of age, thin, coiled fibers were noted in the superficial layer, while thin, irregular fibers were found in the deep layer. At 17 years, differentiation of the superficial and deep layers was more evident. In male subjects after adolescence the curvature of curly collagen fibers decreased, and the diameter of fibers increased. The present findings suggest that the development of the vocal fold in childhood occurs in two steps. In the first step, dense fibers in the anterior, posterior, and deep parts of the lamina propria found in fetuses and infants shift to the anterior and posterior ends of the vocal fold, between which longitudinal fibers appear. During this step only simple phonation is possible. In the second step, differentiation of the superficial and deep layers occurs, and the stratified structure of the vocal fold appears in the teens. This step is probably related to the complicated modality of phonation in this age group. In males, the development of the vocal fold is completed after changes in collagen fibers during the mutation.