Leptin and its receptor are involved in endocrine and paracrine regulation of metabolism, obesity and reproduction. Here, we describe the detection of the functional long isoform receptor of leptin in human endometrium. The leptin receptor protein was shown to be expressed in glandular and luminal epithelium and is periodically regulated throughout the menstrual cycle, demonstrating main expression in follicular and mid-luteal phase. In contrast, leptin receptor mRNA is detectable by reverse transcription–polymerase chain reaction (RT–PCR) as a constitutive component. Since RT–PCR analyses showed that leptin is not expressed in this tissue, the present study suggests that the human endometrium is a novel target for leptin. Therefore, we investigated 11 subfertile patients who underwent two biopsies in one menstrual cycle. The patients presented with a repetitive endometrial maturation defect, but showed adequate serum hormone concentrations and normal steroid hormone receptor expression and down-regulation in the endometrium. These patients were, however, deficient for expression of the functional leptin receptor. These analyses provide evidence that the lack of the leptin receptor in an ovulatory cycle may contribute to subfertility by a yet undefined `endometrial factor'.

Apoptosis in the human endometrium up to now has been detected during the mid to late luteal phase and therefore connected to the onset of the menstrual shedding. However, there is increasing evidence that regulated apoptosis may be important during decidualization and implantation. To investigate a possible role for apoptosis in the human endometrium and its regulation, we correlated the immunolocalization of the apoptosis regulatory protein bcl-2 and the proliferation marker Ki67 to the in-situ nuclear DNA fragmentation - a key feature of apoptosis - detected by using the terminal deoxynucleotidyl transferase-mediated dUTP nick-end-labelling (TUNEL) method during the menstrual cycle. Whereas proliferation and bcl-2-expression were predominantly detected in the glandular compartment during the proliferative phase, only single apoptotic cells could be shown during this period. During the transformation of the endometrium (days 15-19) proliferation and bcl-2 expression decreased markedly and there was no sign of apoptosis. At the beginning of the implantation window (days 19-20) we could detect the first signs of apoptosis in the glandular epithelia in the basalis, which extended to the functionalis during the luteal phase. Proliferation and bcl-2 expression are limited to the stromal compartment comprising the large granular lymphocytes - during this time, and extend in parallel with apoptosis from the basal to the functional layers. Apoptosis therefore may be related to the loss of the protective effect of bcl-2 and may have significance for the establishment of an endometrium adequately prepared for successful implantation.

The human endometrium is characterized by exceptional plasticity, as evidenced by rapid growth and differentiation during the menstrual cycle and fast tissue remodeling during early pregnancy. Past work has rarely addressed the role of cellular mechanics in these processes. It is becoming increasingly clear that sensing and responding to mechanical forces are as significant for cell behavior as biochemical signaling. Here, we provide an overview of experimental evidence and concepts that illustrate how mechanical forces influence endometrial cell behavior during the hormone-driven menstrual cycle and prepare the endometrium for embryo implantation. Given the fundamental species differences during implantation, we restrict the review to the human situation. Novel technologies and devices such as 3D multifrequency magnetic resonance elastography, atomic force microscopy, organ-on-a-chip microfluidic systems, stem-cell-derived organoid formation, and complex 3D co-culture systems have propelled the understanding how endometrial receptivity and blastocyst implantation are regulated in the human uterus. Accumulating evidence has shown that junctional adhesion, cytoskeletal rearrangement, and extracellular matrix stiffness affect the local force balance that regulates endometrial differentiation and blastocyst invasion. A focus of this review is on the hormonal regulation of endometrial epithelial cell mechanics. We discuss potential implications for embryo implantation.

Introduction This study investigated subfertile patients with abnormally thin endometrium after infertility treatment. As they had adequate serum concentrations of hormones, an endometrial factor for subfertility was suspected. Methods To elucidate the cause of subfertility, endometrial biopsies were taken in each patient in the late proliferative and mid-secretory phases of one menstrual cycle. Endometrial biopsies from women with normal menstrual cycles and confirmed fertility who were undergoing hysterectomy for benign uterine disease were used as positive controls. The tissue samples were investigated for steroid hormone receptor expression and for the proliferation marker Ki-67. Immunohistochemistry was performed with antibodies against the marker molecules for endometrial receptivity – β₃ integrin, VEGF, LIF, and CD56 (large granular lymphocytes, LGLs). Results The steroid hormone receptors for estrogen (E2) and progesterone (P) were expressed normally (at the first biopsy) and were down-regulated (at the second biopsy) within the cycle. Strikingly, all of the marker molecules investigated showed negative or weak and inadequate expression in the mid-secretory phase. Numbers of LGLs remained as low as in the proliferative phase. In contrast, fertile patients were found to express these marker molecules distinctly in the mid-secretory phase. Conclusions It may be hypothesized that a severe deficiency of these angiogenesis-related marker molecules leads to defective development of the endometrium, which remains thin. Deficient angiogenetic development may thus provide an explanation for the endometrial factor that causes infertility. Further investigations will need to focus on identifying the regulating factors that act between steroid receptor activation and the expression of these marker molecules. Einleitung Diese Studie untersuchte subfertile Patientinnen mit einem abnorm dünnen Endometrium nach einer Infertilitätsbehandlung. Da ausreichende Hormonkonzentrationen im Blut dieser Frauen gemessen wurden, lag die Vermutung nahe, dass ein endometrialer Faktor für die verminderte Fertilität eine Rolle spielen könnte. Methoden Um möglichen Ursachen für die verminderte Fertilität auf den Grund zu gehen, wurden bei jeder Patientin Biopsien des Endometriums in der späten Proliferationsphase sowie in der mittleren Sekretionsphase eines Menstruationszyklus durchgeführt. Endometriale Biopsien von fertilen Frauen mit regulärem Menstruationszyklus, die sich einer Hysterektomie wegen gutartiger Gebärmutterveränderungen unterziehen mussten, dienten als Positivkontrollen. Die Gewebeproben wurden auf die Expression von Steroidhormonrezeptoren sowie mit dem Proliferationsmarker Ki-67untersucht. Mithilfe von Antikörpern gegen Markermoleküle der endometrialen Rezeptivität: β3-Integrin, VEGF, LIF und CD56 (große granuläre Lymphozyten, LGLs), wurden immunhistochemische Untersuchungen durchgeführt. Ergebnisse Die Steroidhormonrezeptoren für Östrogen (E2) und Progesteron (P) waren bei der ersten in einem Zyklus durchgeführten Biopsie normal exprimiert und in der zweiten Biopsie im selben Zyklus herunterreguliert. Auffallend war, dass die Expression aller untersuchten Marker in der mittleren Sekretionsphase negativ oder schwach bzw. ungenügend war. Die Anzahl der LGLs blieb so niedrig wie in der Proliferationsphase. Dagegen wurden diese Markermoleküle bei fertilen Patientinnen in der mittleren Sekretionsphase deutlich exprimiert. Schlussfolgerungen Es wurde die Hypothese aufgestellt, dass die starke Defizienz dieser Markermoleküle, die in Beziehung zur Angiogenese stehen, zu einer ungenügenden Ausbildung des Endometriums führt, das dann dünn bleibt. Eine unzureichende angiogenetische Entwicklung mag daher der Grund für eine endometriale Insuffizienz sein, die zur Infertilität führt. Weitergehende Studien sind nötig, um die Faktoren zu identifizieren, die zwischen der Steroidrezeptor-Aktivierung und der Expression dieser Markermoleküle regulierend wirken.