Semin Reprod Med 2000; 18(3): 229-236
DOI: 10.1055/s-2000-12561
Copyright © 2000 by Thieme Medical Publishers, Inc., 333 Seventh Avenue, New York, NY 10001, USA. Tel.: +1(212) 584-4662

Endometrial Receptivity: Changes in Cell-Surface Morphology

George Nikas
  • Department of Obstetrics & Gynecology, Karolinska Institute, Stockholm, Sweden
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Publication History

Publication Date:
31 December 2000 (online)

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ABSTRACT

Ovulation and fertilization trigger embryonic development and endometrial differentiation by corpus luteum progesterone production. These two synchronous processes couple about 1 week later, when the blastocyst begins to implant in the now receptive endometrium (implantation window). Receptivity is a state of endometrial differentiation marked by a change in epithelial morphology: the hairy-like cell microvilli fuse to a single flower-like membrane projection called the ``pinopode.'' Scanning electron microscopy of sequential endometrial biopsies shows that pinopodes form briefly (1-2 days), and their numbers correlate with implantation. On average, the formation of pinopodes is earlier in stimulated (days 19-20) and later in artificial (days 21-22) compared with natural cycles (days 20-21). There is, however, a wide (up to 5 days) variation between women in the cycle days on which pinopodes form. These results suggest the existence of a narrow and discrete implantation window in humans. Detection of pinopodes is a potential clinical marker to assess endometrial receptivity.