Adult and embryonic laboratory rats have been used as a mammalian model organism in biomedical research, descriptive and experimental cardiac embryology, and experimental teratology. There have been, however, considerable variations and discrepancies concerning the developmental staging of the rat embryo in the reported literature, which have resulted in several controversies and inconsistencies. Therefore, we carried out a careful anatomical and histological study of rat cardiac morphogenesis from the premorphogenetic period to the mature heart in a newborn pup. A correlation between the chronology and morphological features of the heart and embryo or newborn was made. We provide a simple and comprehensive guide relating the developmental timing and fate of the embryonic components of the heart and their morphological changes in the rat based on in vivo labeling studies in the chick. We also compare the timing of heart development in rats, humans, and mice.
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