The origin and histology of the cardiac mucosa remains controversial. The classical concept that the cardiac mucosa is of gastric origin has been challenged by those who advocate that the cardiac mucosa results from a metaplastic esophageal process. Some regard cardiac mucosa as consisting solely of pure mucous glands, whereas others accept the presence of isolated parietal cells within the mucous gland (mixed glands). In this study, we have clarified the presence and site of origin of the cardiac mucosa and its histological composition. To do so we studied the microscopic characteristics of the gastric side of the squamous-columnar junction (SCJ) of 77 autopsied fetuses of different gestational ages (prenatal group) and of infants, young children, and adolescents (postnatal group). We evaluated the presence or absence of a transitional zone, defined as the area between the squamous esophageal and oxyntic mucosa, the glandular composition of the transitional zone (i.e., pure mucous and mixed glands), and the presence or absence of inflammation. Our study revealed that a transitional zone with the microscopic characteristics of cardiac mucosa was universally present at the SCJ. The microscopic characteristics of this zone varied with age. Both pure mucous and mixed glands were observed. We conclude that the cardiac mucosa is partially if not entirely the result of normal embryonic gastric development. Both mucous and mixed glands constitute normal components of the cardiac mucosa.