Description:
<jats:title>Abstract</jats:title><jats:p>Pregnant rats were treated daily with intramuscular injections of 5 or 10 mg cortisone acetate dissolved in 0.5 ml normal saline, or orally with 20,000 IU vitamin D<jats:sub>2</jats:sub> dissolved in 0.5 ml olive oil, or both. Controls received olive oil and normal saline, or vitamin D<jats:sub>2</jats:sub>. Treatment was from day 8 or 10 of gestation until the animals were killed on day 18, 20, or 22. The higher dose of cortisone shortened the diaphyses and epiphyses of long bones. Concomitant administration of vitamin D<jats:sub>2</jats:sub> caused further bone shortening and hypomineralization. The main microscopic alterations in the fetal bones of cortisone‐treated rats were changes in cartilaginous intercellular substance, narrowing of the hypertrophic and calcifying cartilage cell layer, and inhibition of endochondral ossification. Bone trabeculae were very wide and irregular. The bone matrix had abnormal staining properties. Bone marrow spaces were obliterated and contained a very large number of fibroblasts, preosteoblasts, and osteoclasts. The number of osteoclasts was further increased by treatment with vitamin D<jats:sub>2</jats:sub>. It is presumed that cortisone passed through the placenta and affected fetal bones, causing the microscopic changes. Concomitant administration of vitamin D<jats:sub>2</jats:sub> potentiated the effects of cortisone on bone, and also produced certain specific changes.</jats:p>