Pathological observations on hereditary achondroplasia in the rabbit have been described. At autopsy, the chief features of interest are: reduced size with disproportionately shortened extremities and large head, cutaneous and subcutaneous edema of variable degree and distribution, small shortened bones with a cartilaginous appearance and texture, immature teeth, and cleft palate in one-fourth the cases; blood-stained fluid in the thoracic and abdominal cavities; a comparatively small heart pointing to the right of the midline, a very large firm thymus, a large pale soft spleen, a large swollen liver with red mottling, and a stomach distended with thin greenish mucus but no milk.

The mean relative weights of all organs in terms of the net body weight were larger than those of normal new-born litter mates. The mean actual weights of the kidneys, the brain, and especially the spleen and the thymus were also larger than their respective normal values, those of the heart, liver, and adrenals were slightly smaller, while that of the pituitary was the same.

Histologically, all endochondral cartilages show marked abnormalities of differentiation with pronounced deficiency of ossification. Calcification of membranous bones is likewise deficient. The histological abnormalities of the long bones are very similar to, if not identical with, those characteristic of human fetal chondrodystrophy, the creeper fowl condition, the "bull-dog" calf, and achondroplasia of the dog.

No histological evidence was found in any organ which would suggest a basis for a responsible causal agent of the abnormality. Minor to marked vascular dilatation and congestion and edema is a variable feature but is fairly widely distributed. The changes in the thyroid indicate an active gland. The cellular pattern of the pituitary is characterized by some increase in basophilic cells. The lymphoid elements of the spleen are more or less depleted. The hemopoietic tissue in the spleen and liver is reduced in amount. In blood obtained from the heart, there is a reduction in the numbers of red and white cells and platelets and in the hemoglobin content as well; immature cells and particularly normoblasts are comparatively numerous.

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