Anatomy and embryology in cephalothoracopagus twins.

Anatomical features are described in a case of cephalothoracopagus female twins with laterally fused heads. There was a single foregut, shared equally by the two individuals. The tracheae, lungs, hearts, livers, and gallbladders were also shared by the two individuals. They were located in the anterior and posterior regions of the joined bodies. These peculiarities occur in other cases of cephalothoracopagus twinning reported in the literature, and may have been caused by spatial problems in the developing twin embryos. In particular, each heart is considered to be a composite organ, formed by tissue from both individuals. The occipital region of the skull was double, but the remainder of the cranium was single. The oral cavity was divided in two by a midline mandibular body thought to represent the fused medial portions of two mandibles. Each compartment so formed contained a tongue. This midline body is considered responsible for a cleft lip and palate. Finally, we propose that anencephaly in this and other cases of laterally fused heads is associated with mechanical difficulty in closing the rostral neuropore.