Ultrastructural and experimental evidence of myocardial cell differentiation into connective tissue cells in embryonic chick heart

Abstract

The embryonic cardiac cells originate from a very homogeneous population of myocytes which, by differentiation, will form the atria, the ventricles, the conus and the truncus arteriosus. In the early stages of chick embryo development ($\text{3}\text{1}\text{2}\text{days}$), the wall of the truncus is formed by a layer of myocardial cells, and at about the 7th day it is formed by layers of fibroblasts, collagen, elastin and smooth muscle. The purpose of this investigation was to elucidate the ultrastructural events which occur during the development of the truncus, and to investigate if 5-bromodeoxyuridine (BrdU) could modify its differentiation.

Our sequential ultrastructural observations of the myocardial layer of the truncus showed that the myocardial cells progressively acquire a fibroblastic phenotype. This was shown by the continuous diminution in the content of myofibrils and the increase in collagen, elastin and an amorphous granular material in the intercellular space. On the other hand, 5-bromodeoxyuridine prevented the “transformation” of the myocardium into fibroblast cells, and the truncus remained as a beating structure. From our results, we conclude that the myocardial cells of the truncus are able to differentiate into cells of a fibroblastic type.