Estrogen is a critical determinant that specifies the duration of the window of uterine receptivity for implantation

Abstract

Many underlying causes of human infertility have been overcome by using in vitro fertilization (IVF) and embryo transfer (ET) techniques. Nevertheless, implantation rates in IVF programs remain low despite the transfer of apparently healthy embryos. This suggests that there are problems with the differentiation of the uterus to the receptive state in response to the ovarian hormones estrogen and progesterone. The molecular basis of this receptive state when the uterine environment is conducive to blastocyst acceptance and implantation remains poorly understood. Normally, the "window" of uterine receptivity lasts for a limited time. Using ETs and the progesterone-treated delayed-implantation model in mice, we demonstrate here that levels of estrogen within a very narrow range determine the duration of the window of uterine receptivity. Although estrogen at different physiological concentrations can initiate implantation, we find that the window of uterine receptivity remains open for an extended period at lower estrogen levels but rapidly closes at higher levels. The uterine refractoriness that follows the receptive state at high estrogen levels is accompanied by aberrant uterine expression of implantation-related genes. These results suggest that careful regulation of estrogen levels is one of the important factors for improvement of female fertility in IVFET programs.

Figure 1

Schematic outline of experimental designs.

Figure 2

Uterine gene expression in receptive and refractory uteri. (a) Lif, Ptgs2, and Hegfl expression is aberrant at the site of a blastocyst in a refractory uterus induced by 25 ng of E₂. Recipient mice were ovariectomized on day 4 of pseudopregnancy and injected daily with 2 mg of P₄ to induce the condition of delayed implantation. On day 7, the recipients received the first injection of E₂ at 3 or 25 ng. On day 8, blastocysts were transferred into these recipients immediately followed by a second injection of 3 ng of E_2. Uterine sections containing blastocysts were processed for in situ hybridization 24 h later. (Magnification, ×100.) Arrows indicate the location of blastocysts. (b) Uterine expression of Areg and Ptgs1 becomes aberrant at a higher E₂ level. Pseudopregnant mice ovariectomized on day 4 were injected daily with P₄ to induce the condition of delayed implantation. On day 7, they received an injection of 3 or 25 ng of E₂. Uteri were processed for in situ hybridization 24 h later. (Magnification, ×40.) Note that although Hoxa-10 expression is similar at 3 or 25 ng of E₂, the expression of amphiregulin and COX-1 is aberrant at 25 ng of E₂. le, luminal epithelium; ge, glandular epithelium; s, stroma; myo, myometrium. (c) Uterine Lif expression is different at higher and lower doses of E₂. As stated above, pseudopregnant mice ovariectomized on day 4 were treated with P₄ daily to induce the condition of delayed implantation. On day 7, they received 3 or 25 ng of E₂ or received the first injection of E₂ at 3 or 25 ng followed by a second injection of 3 ng of E₂ 24 h later. Uteri were processed for in situ hybridization at different times. Results of Lif expression at 24 h are shown. (Magnification, ×100.) E₂ at 3 ng as one or two injections failed to detect glandular Lif expression, whereas 25 ng of E₂ as a single injection induced this expression. The expression was undetectable when the first E₂ injection was at 25 ng followed by a second injection at 3 ng. Representative sections of day 4 (D4) pregnant uterus showing glandular Lif expression and of P₄-treated uteri showing the absence of Lif expression were used as positive and negative controls, respectively.

Figure 3

A scheme depicting modulation of the window of receptivity in the P₄-primed uterus in response to changing estrogen levels. This scheme shows that estrogen at a low threshold level extends the window of uterine receptivity for implantation, but higher levels rapidly close this window, transforming the uterus into a refractory state.