Establishment and characterization of a coculture system of equine endometrial epithelial and stromal cells

To investigate the equine endometrium as close to the in vivo situation as possible, we established a coculture system for epithelial and stromal cells (ECs/SCs). ECs and SCs were isolated from nine endometrial tissue specimens. ECs obtained as glandular formations were cultivated on one side of the semipermeable membrane of a Millicell^® insert. After 2 days, SCs (2 × 10⁴ cells/membrane) were seeded onto the other side of the same membrane. During cocultivation, the low serum containing culture medium (Theuß et al., 2010) was supplemented with different concentrations and combinations of 17β-estradiol (2.0-3.0 pg/ml medium) and progesterone (0.5-15.0 ng/ml medium). Once the cocultures formed continuous cell layers as determined by phase-contrast microscopy, the membranes were fixed and processed for light microscopical examination. Cytokeratin 19, steroid hormone receptors and the uterine proteins uteroglobin and calbindin_D9k were detected using immunocytochemistry to determine the degree of culture purity and functional cellular differentiation. The culture purity of the EC layer averaged ≥95%. Uteroglobin and calbindin_D9k were consistently expressed in ECs, while hormone receptors were predominantly absent in both cell populations. An explicit cytomorphological epithelial differentiation with formation of round-oval to polygonal cell forms was encountered in ≤50% of all ECs and independent of supplemented steroids. Based on the findings altogether, and despite the partly absent congruence to the in situ prerequisites, we established a standardized and reproducible coculture system, which offers a basic approach for studies of physiologic and pathophysiologic issues in the mare.