Implantation : morphological and biochemical characterization of the receptive human endometrium

Abstract

Our knowledge of the human implantation process, and the role of the endometrium. during this process, is very scarce. The endometrium is only receptive for implantation during a limited period, the so-called implantation window, after which it becomes refractory to the blastocyst. The ovarian steroid hormones regulate the cyclic changes in the endometrium.

The epithelial cells form protrusions, the so-called pinopodes, on their apical side at the time of implantation. The function of the pinopodes has not been elucidated, but these structures might prevent the cilia from sweeping off an approaching blastocyst. It is possible that blastocysts adhere to adhesion molecules on the pinopodes.

An endometrial biopsy was obtained from normal healthy women during the luteal phase of their menstrual cycle. The endometrial biopsies were divided and one piece was used for examination in scanning electron microscopy, while the other piece was used for immunohistochemistry and/or tissue culture.

In the first study, a newly developed method was used to accomplish implantation in vivo by exposing the biopsy strips in culture to a blastocyst. In the second study it was established that the use of a culture insert in presence of Matrigel, and an oxygen concentration of 5%, is optimal for culturing endometrial strips. The epithelial cells were seen to cover the injured surface, suggesting a healing process.

The third study showed that pinopodes develop when the progesterone levels were increasing and that the pinopodes regress when the progesterone levels were decreasing. The pinopodes were found mainly on days 6 to 9 after the LH surge. The progesterone receptor B level decreased in epithelial cells concomitantly with the appearance of pinopodes. This study suggests that progesterone is directly or indirectly involved in the development of pinopodes.

The concomitant presence of Glutaredoxin (Grx) and pinopodes was investigated in the fourth study. The Grx staining was intense in the epithelial cells when the pinopodes were present and the most intense staining was found in pinopodes on the apical side of the luminal epithelial cells. The function of Grx is not known, but it is likely that it protects the endometrium from oxidative stress during the implantation process. The strong staining of Grx implies that it is possible to use Grx as a biochemical marker of pinopode appearance and thus the implantation window.

Heparin binding epidermal growth factor-like growth factor (HB-EG17) exists both as a membrane-bound growth factor and as a free growth factor. In the fifth study, it was shown that the immunostaining of HB-EG17 was strongest when the pinopodes were present and it was also shown, by using confocal microscopy, that HB-EG17 is present on the surface of the pinopodes.

In conclusion, a correct dating of the endometrium is important for implantation studies. Progesterone seems to be the main regulator, directly or indirectly, of changes in the receptive endometrium and thus the development of pinopodes and the up-regulation of HB-EGF and Grx. It is possible to achieve implantation in vitro. The healing process during the culture mimics the healing after blastocyst invasion. HB-EGF, known to be involved in healing processes, is likely to promote the healing process in the endometrium.