Segmental aplasia of a uterine horn in two RccHan:WIST rats

Aplasia of the uterine horn caused by developmental defects has been reported in several species but has not been reported in RccHan:WIST rats. We encountered spontaneous segmental aplasia of a right uterine horn in two RccHan:WIST rats and detailed its pathological characteristics. The right uterine horn of both rats had similar gross and histological appearances. At necropsy, there was segmental loss of the tissues corresponding to normal right uterine horn, which consisted of a fibrous band connected to the uterine cervix. A cystic structure with clear and colorless fluid was observed in the cranial segment of the right uterine horn close to the right oviduct. The cystic structure was thought to be a partially developed tissue to the right uterine horn. The cystic structure seemed to be derived from the right uterine horn. Histologically, a single layer of cuboidal epithelium lined the luminal surface of the cystic structure, the endometrium was thin, and no uterine glands were observed. The fibrous band was composed of alpha-SMA positive smooth muscle cells, connective tissue, and blood vessels, but cytokeratin AE1/AE3 positive epithelium and uterine endometrium were absent. Based on these gross findings and histological features, segmental aplasia of a uterine horn was diagnosed. To our knowledge, these cases of segmental aplasia of a uterine horn are the first ones described in RccHan:WIST rats.