Some observations on the obliteration of Ductus arteriosus Botalli using the electron microscope

Summary

The main findings reported in the paper are as follows.

• 1. The cells, including endothelial and smooth-muscle cells, came to associate closely with each other, and elastic (membranes) layers intervening among them were interrupted by the cytoplasmic projections of endothelial and muscle cells before birth. In the next stage (just before birth), disorder of cellular arrangement occurred remarkably. Some of the endothelial and smooth-muscle cells protruded into the vascular lumen. Later the central part of the obliterated D. a. was occupied by modified smooth-muscle cells, and most of them became necrotic. However, some of the smooth-muscle cells did not degenerate and seemed to change into fibroblast-like cells in the peripheral regions. During these processes, mitosis of intimal cells was found in a few cases.

• 2. During the obliteration, mucoid substance appeared in the cytoplasm of the smooth-muscle cells instead of myofilaments, and fusion of smooth-muscle cells sometimes occurred in the inner and middle parts of the muscle cell layers. Just after birth, inclusion bodies identified as lysosomes were found randomly distributed in the cytoplasm of the smooth-muscle cells.

• 3. Nerve endings, fibroblasts and mast cells were scarcely seen in the ductus arteriosus at the early stage of the closure.

• 4. In embryos treated with cortisone and vitamin A by the authors' method, unlike the untreated controls, mucoid substance did not appear in the smooth-muscle cells, and the myofibrils remained vigorous, till a later stage. Further, two sets of myofibrils were observed in a single cell. Generally speaking, endothelium and smooth-muscle cells in treated embryos were thin compared with those of controls. However, the obliteration of ductus arteriosus was rather accelerated in treated embryos.