ABSTRACT
The ddN strain of mice were kept in a chamber with reduced atmospheric pressure (225 mmHg) for 5 hours on their eighth (group A), ninth (group B), tenth (group C), eleventh (group D) or twelfth (group E) day of pregnancy and examined both for external malformations and for skeletal changes in the foetuses. For skeletal examinations they were cleared by the Dawson method.
In group A, besides two cases of atlanto-occipital fusions, malformations of upper cervical vertebrae were seen. In group B, cervical vertebrae were chiefly involved in malformations, but the thoraco-lumbar ones were also disturbed. In group C, besides a few cervical involvements, lower thoracic and lumbar malformations were the most striking. In group D, malformations shifted to the sacro-caudal level, while in group E the malformations ranged from the lumbar to the caudal level, though they were low in frequency.
These changes were fusion, adhesion, bifurcation, absence, reduction in size, asymmetry of the primordial posterior rami, disordered segmentation, etc. The primordium of the vertebral body was also involved. At the thoracic level ribs were also involved in deformities when their corresponding vertebrae were affected. In the lowest part of the thoracic level, the absence or vestigial of the rib was often seen in group C.
In an overall picture, a clear-cut head-to-tail gradient shift of malformations occurs as the developmental stage of oxygen deficiency advances. This sequence is in good agreement with that shown by Ingalls et al. and by Degenhardt.
Atlanto-occipital fusions and abnormalities in the upper cervical region recall the conditions seen in iniencephalia, Klippel-Feil syndrome and platybasia in human beings. Some of the malformations in the vertebrae at other levels resembled those in vertebral rachischisis in man.
