α-Smooth Muscle Actin-positive Stromal Cells Reactive to Estrogens Surround Endometrial Glands in Rats but not Mice

In human endometrium, α-smooth muscle actin (α-SMA)-positive stromal cells (SMA-SCs) surround endometrial glands, and the α-SMA expression is regulated by estrogen. The biological significance of these cells remains to be elucidated, and no information is available with regard to their animal counterparts. The present study, therefore, investigated SMA-SCs in the uteri of female Donryu rats and CD-1 mice. SMA-SCs with morphological similarities to those in the human were detected around the endometrial glands in normal cycling rats, but not in mice. Furthermore, the rat SMA-SCs disappeared after ovariectomy but returned with estrogen replacement in a duration- dependent manner, suggesting the regulatory role of estrogens similar to the human situation. Thus, SMA-SCs are present in rats, but not in mice, with characteristics close to their human counterparts. Their biological significance now needs to be elucidated by comparative studies. (J Toxicol Pathol 2005; 18: 47-52)