Histology and expressions/localizations of estrogen and progesterone receptors in the reproductive tract during the natural ovarian cycle of female domestic cats ovulating spontaneously.

Reproductive tract structural and functional properties are key to the success of fertilization, early embryo development, implantation, and healthy pregnancy. Because of the lack of in-depth knowledge in cat species, the objectives of the study were to comprehensively characterize (1) structural properties and (2) expressions as well as localizations of nuclear estrogen and progesterone receptors in the oviduct and the uterus during different phases of the natural ovarian cycle of spontaneously ovulating cats. After routine ovario-hysterectomies, cat reproductive tracts were grouped in 5 categories based on the ovarian morphology (inactive, early follicular, late follicular, early luteal, or late luteal phases). In each category, serial sections of the oviduct and the uterus were analyzed for histomorphometry, cell proliferation, gene expression of estrogen (ESR1) and progesterone (PGR) nuclear receptors, and immunolocalizations of estrogen receptor alpha (ERα) and PGR. Oviductal ampulla reached >2-mm diameter during the late follicular phase because of the significant increase of its mucosal and muscular layer thickness. Uterine horns reached >5-mm diameter during the luteal phase for the same reasons. Maximum height of epithelial cells was 12 μm in the oviduct at the late follicular phase or in the uterine horns at the early luteal phase (in addition to the high number of glandular cells in the uterus). Cell proliferation was moderately active in the oviduct at all phases, indicating cell hypertrophy being mainly at the origin of the size changes. In the uterus, cell proliferation was more active in the glandular cells at the late follicular and early luteal phase. Expression of ESR1 was high in inactive, early, and late follicular phases and then dropped at the early and late luteal phases in the oviductal ampulla only, with no preferential localization of ERα. Expression of ESR1 did not vary in the uterine horns between the phases, with ERα preferentially located in the glandular cells at the early and late luteal phases. Expression of PGR was unchanged in the oviduct across the phases with PGR located in all cells. Expression of PGR was at the highest during the early and late follicular as well as the early luteal phase in the uterine horns with preferential localization of PGR in the glandular cells during the luteal phase. Those combined findings generated in adult, healthy cats represent comprehensive references that can be used to compare with individuals having fertility issues or after hormonally-stimulated ovarian cycles.