Prichard's structures or minute endocardial deformities with lacunas of capillary size lined by plump endothelial cells located in the fossa ovalis are an age-related alteration of unknown origin. In this report the histogenesis, the proliferative potential and the incidence in the aged of these structures are investigated. We have undertaken a prospective histological study of the fossa ovalis in a series of 111 consecutive hearts of patients aged >/=70 years obtained at autopsy. Included in this study was immunohistochemical staining for vimentin, CD31, CD34, thrombomodulin, c-kit (CD117), Ki67 (MIB1) and vascular endothelial growth factor receptor 2 in six cases showing these structures. Prichard's structures were observed in 50 hearts (45%), and were more frequent in males. We confirmed that these structures are age-related phenomena. Subendothelial structures were more common than intracavitary structures. Most individuals had Prichard's structures located on the right side; however, the structures were more numerous on the left side of the fossa ovalis. The immunohistochemical study revealed that Prichard's structures consisted of adult, fully differentiated, postmitotic-type endothelial cells. We suggest that Prichard's structures are formed by infolding of the endothelial lining of the endocardium of the fossa ovalis as an irritational response to altered blood flow, eddies or turbulence. A parallel can be established between Prichard's structures and senile angiomas as both structures increase with age; they are not genuine neoplasms; and they are overgrowths made up of endothelial cells with terminal differentiation. We propose that Prichard's structures are the cardiac equivalent of cutaneous senile angioma.