Calbindin-D9k expression in the pregnant cow uterus and placenta.

Calbindin-D9k (CaBP9k) is a vitamin D-dependent, calcium binding protein first identified in the cytoplasm of the intestinal epithelial cell. Using biotin-streptavidin immunohistochemistry, CaBP9k was localized to the maternal caruncular epithelium, fetal chorionic epithelium, and trophoblastic binucleated cells of the bovine placenta. Within the maternal epithelium the intensity of staining increases from second trimester pregnancies to term pregnancies, indicating a higher intracellular concentration of CaBP9k in the epithelium at term. Luminal and glandular epithelium of the non-caruncular endometrium also stained positively for CaBP9k in all stages of pregnancy observed. No CaBP9k was identified within the stroma or myometrium of the pregnant cow uterus. The increased level of CaBP9k in the caruncular epithelium during the last trimester is hypothesized to be in response to the rising demand for calcium to aid in the mineralization of the fetal skeleton. CaBP9k may play a role in enhancing calcium transport across the placenta in cattle.