Immunolocalization of retinol-binding protein, cellular retinoic acid-binding protein I and retinoid X receptor beta in the porcine reproductive tract during the oestrous cycle

Retinoid-binding proteins and nuclear receptors are expressed in the reproductive tissues of different species and their expression is hormonally regulated. In the present study, we demonstrated immunocytochemically the temporal and spatial localization of retinol-binding protein (RBP), cellular retinoic acid-binding protein I (CRABPI) and retinoid X receptor beta (RXRbeta) in porcine ovary, oviduct and uterus during the oestrous cycle. RBP and CRABPI were localized in the cytoplasm, whereas RXRbeta occurred in the nucleus. RBP was not detected in either the ovary or the oviduct at any stage of the oestrous cycle. CRABPI was present in luteal cells of the ovary only during dioestrus and in glandular and ciliated cells of the oviduct during oestrus. In the ovary, RXRbeta was always present in granulosa cells and germinal epithelium, with highest levels observed during oestrus. In the uterus, RXRbeta was present throughout the cycle in both the endometrium and the myometrium. However, changes in RXRbeta were observed in the endometrium, with highest levels observed during dioestrus. RBP and CRABPI could be observed in the endometrium only during dioestrus. The results show that the occurrence of retinoid-binding proteins and nuclear receptors in individual tissues of the reproductive tract are strongly dependent on the stage of the oestrous cycle. In the oviduct, the expression of CRABPI seems to be dependent on oestrogen, whereas in the uterus the expression of RBP and CRABPI is influenced by progesterone. The association of expression in different sections of the reproductive tissues investigated shows that the presence of specific proteins involved in retinoid metabolism was dependent on events associated with ovulation, the migration of the oocyte through the oviduct and the possible implantation of the blastocyst into the uterus.