Development of endometriosis-like lesions after transplantation of human endometrial fragments onto the chick embryo chorioallantoic membrane.

The chick embryo chorioallantoic membrane (CAM) bioassay was used to investigate the early pathogenesis of endometriosis. Endometrial fragments were explanted onto the CAM. The grafts including the surrounding CAM were excised at 24, 48 or 72 h after explantation, fixed and embedded in paraffin. Immunohistochemical analysis was used to distinguish endometrial cells. To identify cells of human origin, in-situ hybridization was performed using a probe specific for human chromosome 1. After 24 h, direct contact between endometrial stromal as well as epithelial cells and the mesenchymal layer of the CAM was observed. Invasion of both stromal cells and intact endometrial glands into the mesenchymal layer was observed after 48 h. At 72 h, endometriosis-like lesions were observed in the mesenchymal layer. Positive staining with antibodies to vimentin and pan-cytokeratin was observed in the invading cells as well as in the lesions. In the lesions these positively stained cells showed in-situ hybridization signals for human chromosome 1, confirming their human origin. In conclusion, after 3 days of incubation, endometriosis-like lesions consisting of human endometrial glands and stromal cells were found in the mesenchymal layer of the CAM. These lesions apparently resulted from the invasion of intact human epithelial structures and stromal cells.