PARAPAGUS CONJOINED TWIN HOLSTEIN CALF

Background: Conjoined twins with a doubling of all cranial structures, having two vertebral columns along their entire body length yet with a single pelvis, have not been described in the literature. The cause of conjoined twinning is incompletely understood; however, two main th eories (fission and fusion) have been proposed and disputed. Methods: A dicephalus, tetrabrachius (parapagus) conjoined twin Holstein heifer calf was studied. Results: Two normal heads were present on two necks . The twins were fused in the thoracic region. There were four forelim bs and an abnormal orientation of ribs. Two separate vertebral columns along the length of the animal ended with two tails. There was a sing le pelvis, and only two hindlimbs were present. The musculature of the medial forelimbs was complete but abnormally positioned. Some medial structures, caudal to the thorax, failed to develop in these twins. Th ere were two hearts, each one supplying one half of the body. The cran ial vasculature was doubled and normal. Caudally there were two aortas , each supplying the respective half of the twin. The right caudal ven a cava drained all caudal parts of the body while the left caudal vena cava drained only the liver. There were two sets of lungs. Each twin had a separate esophagus that entered a separate stomach. The right ab omasum (fourth stomach chamber) was herniated through the diaphragm in to the thoracic cavity. The two duodenums from each stomach fused dist al to the pyloric sphincters. Caudal to this point of fusion, all stru ctures of the digestive and urogenital systems were single. The calf h ad a single anus and vulva. Conclusions: The anatomical findings in th is twin suggest a fission event followed by fusion of parallel embryon ic axes. (C) 1998 Wiley-Liss, Inc.