Two-headed, two-necked conjoined twin calf with partial duplication of thoracoabdominal structures: role of blastocyst hatching.

Gross anatomical analysis of a derodidymic monosomic stillborn male calf from an embryo transfer recipient was carried out. Two normal heads were present on two necks which were fused at the shoulders. Although the ribs were abnormal in shape and number, there was one trunk and four legs. The vertebrae were double and partially fused from the thoracic region to the sacrum, which was singular and normal. In the thoracic region there was a single vertebral canal that contained two incompletely fused spinal cords which shared common meninges. In the lumbar region there was a spina bifida. The medial neck musculature was variously fused. Two esophagi entered one enlarged rumen and the liver and gallbladder had extra lobes. Two sets of lungs occupied three pleural cavities and also part of the pericardial cavity. Two hearts were joined by a common cavernous venous sinous. The vasculature cranial to the heart reflected the doubling. The vasculature caudally was singular. The calf was uniscrotal and ipsilaterally cryptorchid. Our results are consistent with the hypothesis that atypical "hatching," that is, emergence of the blastocyst from the zona pellucida, may cause anomalous twinning.