Aquaporin Water Channels In The Bovine Ovary

The follicles of the bovine ovary are large, rapidly growing fluid filled structures. Follicular fluid originates in the ovarian capillary network but nothing is known about how it is transferred over the endothelium, through the ovarian stroma and across the follicle wall into the antrum. We hypothesize that specific aquaporin (AQP) water channels control fluid flow in different regions of the ovary. Using custom‐designed rabbit polyclonal antisera, we have made a preliminary immunohistochemical identification of aquaporin molecules in abattoir‐derived ovarian tissue sections. AQP1 was located in the somatic cells of small, pre‐antral follicles but not in early or large antral follicles. It was also present in the endothelium of stroma and theca interna in most parts of the ovary. AQP1 also stained erythrocytes, and these could be seen penetrating the antrum following the peri‐ovulatory breakdown of the basement membrane. AQP3 and AQP 4 showed clear staining in the oocytes of antral follicles, and AQP4 also exhibited diffuse expression in the stroma. AQP2 was undetected in the ovary, despite strong and specific expression in cortical and medullary collecting ducts of kidneys from the same animals. These results indicate both site‐specific and stage‐specific expression of aquaporins in the ovary. In particular, they suggest that AQP1 and AQP4 influence endothelial and stromal transport and that the transition from pre‐antral to antral development involves a loss of AQP1 expression in the granulosa.