Histology and Innervation of Lung in Bat

Abstract

The bronchial branches of bats are very short, but the bronchioli are rather long and pass over into alveolar ducts and sacs. Pieces of cartilage are found only at the incipient part of the bronchial branches, and these very often show signs of ossification. The mucous membrane in such a cartilaged part do not show any fold formation.The bronchial branches lose rather rapidly in size, but their fine structure undergo little essential change throughout, even down to the smallest branches. They are covered by a 1-3 rowed ciliated epithlium containing no goblet cells and a row of well-developed circular muscle bundles surrounds the thin propria, so that the mucous membrane shows a conspicuous formation of longitudinal folds. This muscle layer is lined by an adventitia and frequently contains lymphocytic gatherings, which, besides, are also often found in the propria. Bronchial glands are found nowhere in the bronchial branches.Since the bronchioli are devoid of alveoli, it is impossible to make distinction between bronchioli terminales and bronchioli respiratorii. The epithelium here is a single-row ciliated one, surrounded by a row of very thinly arranged smooth muscle fibres, and no folds are formed in the mucous membrane. The alveolar ducts and sacs are nothing but alveoli arranged in consecution. These are composed of nucleated and unnucleated flat cells and blood capillaries are found lining the outside of them. The distinction between interlobular septa and interstitial connective tissue is very hard in bats. The pleura visceralis consists of an endothelial cell layer and a thin connective tissue layer.The v. pulmonalis has a media consisting of heart muscle tissue showing great fluctuation of thickness and reaching down to the periphery. Accordingly, the vein shows extremely mutable cross-sections.Most of the nerve bundles forming the peribronchial plexus of bat run along the outside of the adventitia toward the periphery. Frequent gatherings of nerve cells are seen along these bundles, but even the largest of such ganglia do not comprise more than 20 cells. The nerve cells do not lose their essential characteristic of multipolarity, but their nerve processes are extremely ill-developed, so that many of the cells present the appearance of being apolar. Thus, the development of sympathetic nerve cells is much inferior to that in dog (SAITO).The nerve bundles of the peribronchial plexus are formed of unmyelinated vegetative fibres and medullated sensory fibres, but in bat, it is noteworthy that the latter are markedly in large number, unlike the case in man or dog. It is needless to say that the vegetative fibres here also end always in STOHR's terminal reticula. Vegetative fibres are also found running along the outside of the v. pulmonalis. These run into the heart muscle tissue of its media, as in the heart, to form conspicuous terminal reticula there.The sensory fibres found in the plexus penetrate from the adventitia through the muscle layer into the propria, and sometimes further into the epithelium, to form their terminations. A small number of terminations are also formed en route in the muscular layer too. Beside unbranched and simple branched terminations, we find not rarely sensory terminations Type I concerned with blood pressure fall reflex in the muscle layer.The number of sensory fibres running into the propria is small in the bronchioli and their terminal mode is limited to the simplest unbranched endings, but in the bronchial branches, especially, in the large and the intermediate branches, the number is larger and complex branched terminations are also found here and there beside the unbranched and the simple branched terminations. In such a complex termination, the terminal branches gradually lose in size and usually end in sharp points in the propria, but some of the branches frequently end in intraepithelial fibres.