Ultrastructure of human uterine epithelium at the time of implantation after postovulatory administration of norethindrone

Derivation of hormone-responsive human endometrial organoids and stromal cells from cryopreserved biopsies

The human endometrium is a dynamic tissue that undergoes cyclic changes in response to sex steroid hormones to provide a receptive status for embryo implantation. Disruptions in this behavior may lead to implantation failure and infertility; therefore, it is essential to develop an appropriate in vitro model to study endometrial changes in response to sex hormones. In this regard, the first choice would be human endometrial cells isolated from biopsies that could be used as monolayer cell sheets or to generate endometrial organoids. However, the need for fresh samples and short-time viability of harvested endometrial biopsy limits these approaches. In order to overcome these limitations, we sought to develop an efficient, simple, robust and reproducible method to cryopreserve human endometrial biopsies that could be stored and/or shipped frozen and later thawed to generate endometrial organoids and endometrial stromal cells (EnSCs). These cryopreserved biopsies could be thawed and used to generate simple endometrial organoids or organoids for co-culture with matched stromal cells that are functionally responsive to sex hormones as similar as the organoids generated from fresh biopsy. An optimal endometrial tissue cryopreservation method would allow the possibility for endometrial tissue biobanking to enable future organoid generation from both healthy tissues and pathological conditions, and open new venues for generate endometrial assembloids, consisting of epithelial organoids and primary stromal cells.

The hormonal control of implantation

The hormonal control of implantation in mammalian species with and without embryonic diapause is described. In a majority of species displaying the obligate form of diapause the corpora lutea appear to exhibit a low level of steroidogenic activity throughout diapause, full luteal activity being resumed just before the initiation of implantation. Fluctuations in the plasma levels of oestrogen and progesterone during diapause may serve to prime the uterus for implantation. In species exhibiting the facultative form of diapause, such as the rat and mouse, both progesterone and nidatory oestrogen are required for the induction of implantation. In species not displaying embryonic diapause implantation will take place in the presence of progesterone alone. In the light of these considerations the selection of animal models for drug-screening purposes and possible new approaches to contraception are discussed.

Endometrial receptivity: Clinical assessment in relation to fertility, infertility, and antifertility

Fertility in humans and other mammalian species depends absolutely on synchronous events that render the developing blastocyst and the receiving uterus competent for implantation. Endometrial receptivity is defined as the period during which the endometrial epithelium acquires functional, but transient, ovarian steroid-dependent status supportive to blastocyst acceptance and implantation. Once inside the uterus, the blastocyst is surrounded by an intact luminal epithelium, which is considered to act as barrier to its attachment, except for this short period of high endometrial receptivity to blastocyst signal(s). Its transport and permeability properties, in conjunction with cellular action of the endometrium and the embryo, have been suggested to influence creation and maintenance of informational and nutritional status of uterine luminal milieu. This period, also termed as the 'window of implantation,' is limited to days 20-24 of menstrual cycle in humans. However, establishment of endometrial receptivity is still a biological mystery that remains unsolved despite marked advances in our understanding of endometrial physiology following extensive research associated with its development and function. This review deals with various structural, biochemical, and molecular events in the endometrium coordinated within the implantation window that constitute essential elements in the repertoire that signifies endometrial receptivity and is aimed to achieve a better understanding of its relationship to fertility, infertility, and for the development of targeted antifertility agents for human use and welfare.

In vitro models of human blastocyst implantation

This paper reviews different in vitro models used for the study of blastocyst implantation in animals and the human. Furthermore, results from human blastocyst-endometrial interactions in vitro, investigated by scanning electron microscopy (SEM), light microscopy (LM) and transmission electron microscopy (TEM), are presented. SEM demonstrates the preference of human blastocysts to adhere to pinopode-presenting areas on endometrial cell cultures. LM and TEM show that the first morphological sign of cell contact, defined as junction formation, is present at the apical-to-lateral border of endometrial epithelial cells, whereas trophoblast attachment to apical endometrial epithelial plasma membranes was not observed. More advanced stages illustrate that the human blastocyst penetrates the epithelial lining by the intrusive penetration mechanism.

Presence of uterine pinopodes at the embryo-endometrial interface during human implantation in vitro

In order to study changes occurring on the surfaces of human endometrial epithelial cells in the presence of an implanted blastocyst, we used scanning electron microscopy for investigation of five endometrial biopsies and three human implantation sites obtained in vitro. All specimens showed areas with endometrial pinopodes, separated by cells displaying microvilli or cilia at the apical surface. Pinopode formation was more pronounced in endometrial biopsies than in cell cultures. All blastocysts adhered to pinopode presenting cells. Endometrial surface changes were not seen around the blastocysts. The results of this study demonstrate that cultured endometrial epithelial cells are capable of pinopode formation. Furthermore, endometrial epithelial pinopodes, generally considered as a marker of endometrial receptivity, seem to be directly involved in the adhesion of the blastocyst to the endometrial surface.

Scanning electron microscopy of human preimplantation endometrium in normal and clomiphene/human chorionic gonadotropin-stimulated cycles

Embryo transfer after in vitro fertilization of oocytes obtained from unstimulated or stimulated women is usually carried out earlier than when an embryo developed in vivo reaches the womb. The possible asynchrony between the developmental stage of the embryo and that of the endometrium at transfer might reduce the prospects for implantation; so also might any secondary effect of stimulation. Preimplantation endometrium dated by plasma hormone analyses of 17 beta-estradiol and progesterone in five unstimulated and eight stimulated women was surveyed. Three features were examined, namely, the frequency, distribution, and appearance of ciliated cells, nonciliated cells, and apical protrusion. Wide regional variations were observed, but no continuous or consistent changes or differences were found from ovulation to the time for implantation in unstimulated and stimulated cycles. Therefore, from the morphologic point of view, the surface might be ready to accept a transferred embryo at any time within a range of days after ovulation.

Action of midcycle contraceptive (R 2323) on the human endometrium

Over 2,148 cycles of midcycle oral administration of R 2323 (50 mg. per day on Days 15,16, and 17), the authors recorded a drug-failure pregnancy rate of 5 per cent and an unusually regular cycle length of 28 +/- 2 days. During this trial, endometrial biopsies obtained in the luteal phase were examined by light and electron microscopy and compared to pretreatment biopsies. Light microscopy indicated a weakly secretory endometrium suggestive of some, albeit low, progesterone impregnation. Ultrastructural examination revealed deleterious changes in the development of the nucleolar channel system and giant mitochondria and a delay in the migration of glycogen granules. This low progesterone impregnation could be explained either by a direct effect of R 2323 on cell ultrastructure or by interference with progesterone availability. It would appear that R 2323 acts as a temporary substitute for progesterone at the receptor level but that it does not induce all the biological manifestations of this hormone, in particular, the endometrial changes required for implantation.