Histological Studies on Innervation of Lung of Human Embryo

Abstract

The ground plexus around the bronchus and bronchial branches, the secondary plexus in the inner side of the bronchial cartilages and the plexus under the bronchial epithelium are in remarkable formation in 4th month embryo. These plexuses become poorer in development as the bronchial branches approach their ends, and in the small uncartilaged bronchial branches, there are found only weakly developed plexus around their walls. Some nerve elements are observed penetrating into the circumference of the bronchuli and alveolar ducts.
The distribution of the nerve cells in the lung of human embryo is much the same as in adult. They aggregate along or in the nerve bundles of ground plexus, to form capsulated or non-capsulated ganglia, but sometimes sporadic existence of 1 or 3 cells is also observable.
The nerve cells in the lung of 4th month embryo are large cells containing ober-sized cell nucleus and protoplasm staining red under my silver staining and including extremely minute fibril net. In general, the form is as yet round and infantile, but some have already grown a few processes and appear stellar in shape. By the 6th month, these cells show some increase in size, the fibril net-work becomes more marked and the number of cells with processes also rises somewhat. In the 8th month, the above developments become more accentuated, the processes in particular showing powerful development. However, the development of nerve processes is as yet far weaker than in adult, the distinction between the cells of Types I and II of Dogiel being impossible at this stage.
The termination of vegetative fibres, in the lung of human embryo as well as of adult, form Stöhr's terminalreticulum, which is very conspicuous in the 4th month already.
Also in the lung of human embryo, the existence of a considerable number of sensory fibres and their endings, apparently originating in the vagus nerve, is observable. However, the endings are as yet of simple nature, no such complex endings as found in adult lung being represented. Consequently, such complex sensory endings as those connected with blood pressure falling reflex (Sunder-Plassmann) have never been found.
Such infantility in terminal formation is of course the most marked in 4th month embryo. The sensory endings are represented by unbranched or simple branched terminations in the muscle layer or the subepithelial tissue of the bronchial branches. Intraepithelial fibres are found, but they also show unbranched. or very simple branched formation and are very small in number, as compared with those in adult lung.
The distribution of the aorta nerve of sensory nature concerned with blood pressure falling reflex is much wider in the embryo than in the adult, extending partially to the root of a. pulmonalis (Yabuki¹⁵⁾), and as I have found, to its distal prolongation. The sensory endings seen here are much simpler in scale than those in the root of a. pulmonalis but the same in general structure as the latter, being represented by simple branched endings conssisting of 2 or 3 branches showing peculiar change in size running along the muscle fibres in the media. These are found in good development in 4th month embryo, but fall into degeneration in the later months.
No existence of sensory terminations may be found around the pulmonary vein.