**2.2 Embryology**

Duodenal atresia results from an impaired development of the intestinal tube in early fetal life. The most common theory of duodenal atresia development was first described by Tandler et al. in 1900 and is based on failure of the solid core of duodenum to recanalize, also called the developmental arrest ("epithelial plug") theory [24–26]. Duodenal development is initiated at the beginning of the 4th gestational week from the distal part of the foregut and the proximal portion of the midgut. Between 5 and 6 weeks of intrauterine life, the lumen of the duodenum temporarily obliterates as a result of intensive proliferation of epithelial cells. Vacuolization, or degeneration of epithelial cells, occurs during the 11th week of gestation and leads to recanalization of the duodenal lumen [27]. Failure of this sequence in fetal development leads to duodenal atresia.

The vascular theory described in the middle of the twentieth century gained less popularity; however, it has been utilized for explanation of the types of duodenal atresia. According to this theory, a vascular accident leads to intrauterine bowel necrosis [28, 29].

Congenital duodenal obstruction can be caused by internal or external factors. Internal etiologies include intestinal atresia, stenosis, duodenal web, and intraluminal duodenal ("windsock") diverticulum. The latter is a fenestrated membrane in the second portion of the duodenum [30] prolapsing into the lumen of the distal duodenum. These internal forms of CDO are explained by the developmental arrest theory. External duodenal obstruction can result from annular pancreas, malrotation, or preduodenal portal vein. Most authors believe that the annular pancreas alone cannot cause CDO, but only accompanies atresia or stenosis of the intestinal lumen. Annular pancreas is an associated anomaly in patients with CDO [31]. However, the cause of CDO is failure of embryological development, not external compression of the duodenal lumen with the excessive pancreatic tissue [6]. This is well supported by the data obtained during the study of the embryologic development of the digestive tract.

CDO can be classified into pre- and postampullar. The latter is more common and is detected in 85% of affected newborns. In cases of complete obstruction, a dilated stomach and proximal duodenum are visualized, along with pyloric hypertrophy. The digestive tract distal to the obstruction is in a dormant state, excluding rare forms of type I atresia. In such cases, the membrane causing duodenal obstruction prolapses into the distal segment of the duodenum and its significant distention, obfuscating the true location of the obstruction. This anomaly is similar to a duodenal "windsock" diverticulum [32].
