Mucociliary activities in fetal rabbits.

There have been many morphological investigations on the generation of respiratory epithelium. However, the mucociliary activity of fetal respiratory epithelium has never yet been discussed. In the present work, ciliary activity and mucociliary function in the nose and the trachea of fetal rabbits were studied and, moreover, their respiratory epithelial cells were observed by scanning and transmission electron microscopy. Ciliary activity as noted on the 26th day of fetal life (day 26) for the first time in both of the nasal and the tracheal region and this activity was already equivalent to that in adult rabbits. Mucociliary transport function in either region was first noted on day 27. A quantitative as well as a qualitative immaturity of the respiratory epithelium was recognized on the last day of observation (day 29). The quantitative immaturity is characterized by 1) the ratio of ciliated to non-ciliated cells being lower than in the adult epithelium, 2) each ciliated cell possessing about three-fifths as many cilia as those of a full-grown cell, and 3) some cilia being smaller than full-grown ones and the qualitative immaturity by the directional disorder of the basal foot. No differences were observed between a cilium on day 25 or earlier, another on day 26 or later, and a full-grown cilium. It is suggested that cilia of the respiratory epithelium are morphologically prepared for motion and are activated on day 26 by changes in their surroundings, and that the poorer mucociliary transport in fetuses despite an almost normal ciliary beating is due in fairly large part to their qualitative immaturity (directional disorder of the basal foot).