This study was undertaken in an attempt to determini whether there is a distinct histological difference between the human normally closing ductus arteriosus anda pe. rsistent ductus arteriosus. The microscopical investigation was done in 42 specimens ofhuman ductus arteriosusfrom subjects ranging in agefrom 12 hours afterpremature delivery to 32years. The ducts derivedfrom hearts withvarious congenital malformations, in some of which patency of the ductus was essentialfor survival, as well asfrom hearts without other congenital malformations. Because no histological differences werefound between the isolatedpatent ductus arteriosus and those associated with other congenital heart anomalies, the specimers were classified according to age. Stages of a normal anatomical closing process were not encountered in patients over 4 months ofage with a patent duct. Younger material showed either a histologically normalstage of anatomical closure or an abnormality ofthe ductus wall. This abnormal histology is mainly characterised by an aberrant distribution of elastic material, the most conspicuous feature being a thick, wavy, unfragmenteI subendothelial elastic lamina. In respect of the question as to whether the observed histological abnormality forms part ofa primary anomaly of the ductus arteriosus or is secondary to the prolonged patency, it appears that most of the evidence provides support for the view that a primary anatomical defect of the ductus wall is responsible for persistence qf the ductus arteriosus.

The human ductus arteriosus has been the subject of many studies, butmost of these investigations concerned normal anatomical closure (eg Jager and Wollenman, 1942; Danesino et al., 1955; Bakker, 1962; HofEmann, 1964; Desligneres and Larroche, 1970) or physiological closure (eg Eldridge and Hultgren, 1955; Adams and Lind, 1957; Rudolph et al., 1961). Many studies have also been based on animal material (eg Sciacca and Condorelli, 1960; Hoefsmit, 1967; Hornblad, 1969; Broccoli and Carinci, 1973). The mono-graph by Cassels (1973) entitled The Ductus Arteriosus provides an abundance of information on the human ductus arteriosus, covering a great many fields of interest and giving a good review of the literature. There is, however, sparse material pub-lished on the histology of persistent ductus arterio-sus. Some comments are given by Bakker (1962), Desligneres and Larroche (1970), and Cassels et al.(1975). With respect to the pathogenesis of the persistent type, Cassels et al.(1975) think it reasoniable to conclude that in some groups of heart mal.-formations the haemodynamic oxygen tension plays a significant role in the patency of the duct,