Mice homozygous for either of two mutations, chondrodysplasia (cho) or cartilage matrix deficiency (cmd), have short-limbed chondrodystrophy. This phenotype includes retrognathia, relative macroglossia, and cleft palate. It has been postulated that the cleft palate in these mice is the result of tongue obstruction during palatogenesis. Agnathia associated with microglossia is an independent spontaneously occurring defect in the strains bearing these mutations. The coincidental occurrence of agnathia-microglossia with chondrodystrophy lends itself to the study of the mechanism of cleft palate formation. We examined approximate midsagittal histological sections of normal and chondrodystrophic newborn mice, both with and without agnathia. Mandibular measurements and examinations of palate closure and tongue structure were made from photographic prints. Typical chondrodystrophic mutants with cleft palates had a mean mandibular length that was 66% of normal and a tongue that appeared large relative to the shortened mandible. Chondrodystrophic mutants with agnathia and microglossia had a mean mandibular length that was further reduced to 30% of normal, yet had a closed palate. We also observed two nonagnathic chondrodystrophic mutants that had slightly decreased mandibular lengths, microglossia, and closed palates. These observations suggest that tongue obstruction during palatogenesis is the pathogenetic mechanism of cleft palate in chondrodystrophic mice. A similar tongue obstruction hypothesis has been proposed as the mechanism of cleft palate formation in the human Pierre Robin sequence, which consists of retrognathia, glossoptosis, and cleft palate. This mechanistic hypothesis has been challenged, but our findings support the tongue obstruction hypothesis in the Robin cleft.

Day-18 fetal mice with chondrodysplasia (CHO) have been shown to have pulmonary hypoplasia and thoracic volume reduction, and it has been hypothesized that the former is a secondary effect of the latter. To establish a pathogenetic relationship between thoracic volume reduction and pulmonary hypoplasia, it must first be demonstrated that the decrease in thoracic volume precedes or coincides with the morphologic effect on the lungs. At 24-h intervals through gestation days 14 to 18, the thoracic volume and lung histopathology for normal and chondrodystrophic fetuses were estimated by image analysis of frontal histologic sections. Relative to normal littermates, the average thoracic volumes of day-16, day-17, and day-18 mutants were significantly decreased by 18%, 34%, and 49%, respectively. In the mutants' lungs the histology was normal through day 16, but on days 17 and 18 the frequency of the more distended primary saccules and the amount of potential airspace were decreased relative to normal. These results support the hypothesis that the relative decrease in thoracic volume of the mutant precedes the departure from normal for primary saccule development. The reduction effect on thoracic volume therefore is pathogenetically related to the hypoplastic lungs of fetal mice with lethal chondrodystrophy.

In a previous study we estimated fetal mouse thoracic volume by use of paraffin casts. While this procedure provided useful information, it did not allow histologic examination of thoracic viscera. In the present study the thoracic volumes of day 14-18 fetal mice were determined through serial histological sections. The thoracic cavity was traced from the sections and the area of each tracing was determined by computer image analysis. These areas were summed and then multiplied by the thickness of each section to derive the thoracic volume. This procedure thus permitted both volumetric determinations and histological inspection of the thoracic viscera. In addition, two randomized sampling methods designed to increase the utility of such volumetric estimates were compared for reliability. The method best suited for this study was a random stratified sampling method because it reproduced estimates with minimal standard deviation.