Mammalian reproduction is more inefficient than expected and embryo/conceptus implantation into the maternal endometrium is considered to be a rate-limiting process. Although extensive physiological and structural diversity exists among mammalian species, the basic molecular mechanisms underlying successful implantation are conserved. The extensive use of genetically engineered mouse models has provided considerable information on uterine receptivity for embryo implantation. The molecular mechanisms and cellular processes identified thus far require further validation in other mammalian species. In this review, representative ovarian steroid hormone-induced signaling pathways controlling uterine adaptation are presented based on the results of rodent studies. Selected examples of functional conservation in mammals, such as humans and cattle, are briefly described. To date, molecular therapeutic trials for fertility improvement have not been conducted. Considerable efforts are required to provide further understanding of these molecular mechanisms. Such understanding will contribute to the development of reliable clinical diagnostics and therapeutics for implantation failure, leading to reproductive success in a wide variety of mammals in the future.

Understanding of central nervous system mechanisms underlying age-related infertility remains limited. Fibril α-synuclein, distinct from its monomeric form, is implicated in age-related diseases. Notably, fibril α-synuclein spreads among neurons, similar to prions, from damaged old neurons in cortex and hippocampus to healthy neurons. However, less is known whether α-synuclein propagates into oxytocin neurons, which play crucial roles in reproduction. We compared α-synuclein expression in the oxytocin neurons in suprachiasmatic nucleus (SCN), supraoptic nucleus (SON), paraventricular hypothalamic nucleus (PVN), and posterior pituitary (PP) gland of healthy heifers and aged cows to determine its role in age-related infertility. We analyzed mRNA and protein expression, along with Congo red histochemistry and fluorescent immunohistochemistry for oxytocin and α-synuclein, followed by confocal microscopy with Congo red staining. Both mRNA and protein expressions of α-synuclein were confirmed in the bovine cortex, hippocampus, SCN, SON, PVN, and PP tissues. Significant differences in α-synuclein mRNA expressions were observed in the cortex and hippocampus between young heifers and old cows. Western blots showed five bands of α-synuclein, probably reflecting monomers, dimers, and oligomers, in the cortex, hippocampus, SCN, SON, PVN, and PP tissues, and there were significant differences in some bands between the young heifers and old cows. Bright-field and polarized light microscopy did not detect obvious amyloid deposition in the aged hypothalami; however, higher-sensitive confocal microscopy unveiled strong positive signals for Congo red and α-synuclein in oxytocin neurons in the aged hypothalami. α-synuclein was expressed in oxytocin neurons, and some differences were observed between young and old hypothalami.

Embryonic transfer of bovine blastocysts produced by in vitro fertilization is widely utilized-despite a compromised conception rate. It has been suggested that a set of four evaluation criteria for judging the quality of embryos, based on the timing of early cleavages and proper morphologies of embryos, can effectively predict pregnancy success. These blastocysts are hereafter referred to as four-criteria-compliant blastocysts. The same criteria should be used to modify the culture media to improve embryo quality. For example, culture media is often supplemented with nonessential amino acids (NEAA) at a uniform concentration despite the major variation in their concentration in the oviductal fluid. In the present study, the effects of the embryo culture medium, namely CR1, supplemented with all seven MEM NEAA or six of them, excluding one at a time, were examined. All media, except for the medium that did not contain proline and serine, tended to improve the efficiency of producing four-criteria-compliant blastocysts, and excluding alanine was particularly effective. The absence of alanine resulted in the rapid occurrence of the first cleavage and pronuclear formation of fertilized oocytes in the alanine-free medium compared to that in the medium containing alanine. These results suggested that alanine hinders certain events involved in the progression of early embryogenesis, which is necessary to achieve the four criteria that provide a benchmark for pregnancy. Therefore, a significantly higher percentage of embryos satisfied the recommended criteria and developed into four-criteria-compliant blastocysts when developed in alanine-free medium than in alanine-containing medium.

For the intracytoplasmic sperm injection (ICSI) procedure in pigs, an electrical pulse (EP) has been used as an effective method for oocyte stimulation, but unlike sperm, EP is unable to induce Ca²⁺ oscillations. In this study, we investigated the effects of generating artificial Ca²⁺ oscillations with phospholipase Cζ (PLCζ) mRNA, a candidate sperm factor, on fertilization, embryonic development, and gene expression after ICSI. Firstly, the concentration of PLCζ mRNA of a fixed volume (1.0 pl) that would induce a pattern of Ca²⁺ rise similar to that of in vitro fertilized (IVF) sperm was examined and determined to be 300 ng/μl. Secondly, the effects of oocyte stimulation methods on fertilization and embryonic development were investigated. ICSI-oocytes were activated by EP (EP group) or by PLCζ mRNA (PLCζ group). Furthermore, IVF-oocytes (IVF group) and ICSI-oocytes with and without an injection of buffer (buffer and untreated groups, respectively) were used as controls. It was found that the rates of normal fertilization in the PLCζ and EP groups were significantly higher than those in the buffer and untreated groups. The blastocyst formation rates did not differ among the groups. The embryo quality in the EP group was inferior to those in the PLCζ and IVF groups. Additionally, the expression level of a proapoptosis-related gene (Caspase-3) in the EP group was significantly higher than those in the PLCζ and IVF groups. Our data suggest that oocyte activation by PLCζ mRNA has the effect of improving embryo quality.

Understanding how stress hormones induce apoptosis in oviductal epithelial cells (OECs) and mural granulosa cells (MGCs) can reveal the mechanisms by which female stress impairs embryonic development and oocyte competence. A recent study showed that tissue plasminogen activator (tPA) ameliorates corticosterone-induced apoptosis in MGCs and OECs by acting on its receptors low-density lipoprotein receptor-related protein 1 (LRP1) and Annexin A2 (ANXA2), respectively. However, whether tPA is involved in corticotropin-releasing hormone (CRH)-induced apoptosis and whether it uses the same or different receptors to inhibit apoptosis induced by different hormones in the same cell type remains unknown. This study showed that CRH triggered apoptosis in both OECs and MGCs and significantly downregulated tPA expression. Moreover, tPA inhibits CRH-induced apoptosis by acting on ANXA2 in both OECs and MGCs. While ANXA2 inhibits apoptosis via phosphatidylinositol 3-kinase/protein kinase B (PI3K/Akt) signaling, LRP1 reduces apoptosis via mitogen-activated protein kinase (MAPK) signaling. Thus, tPA used the same receptor to inhibit CRH-induced apoptosis in both OECs and MGCs, however used different receptors to inhibit corticosterone-induced apoptosis in MGCs and OECs. These data helps understand the mechanism by which female stress impairs embryo/oocyte competence and proapoptotic factors trigger apoptosis in different cell types.

The present study examined whether male resveratrol intake affected mitochondrial DNA copy number (mt-cn) and telomere length (TL) in blastocysts fathered by young and aged male mice. C57BL/6N male mice supplied with water or water containing 0.1 mM resveratrol were used for embryo production at 14–23 and 48–58 weeks of age. Two-cell-stage embryos were collected from the oviducts of superovulated female mice (8–15 weeks old) and cultured for 3 days until the blastocyst stage. Mt-cn and TL levels were measured by real-time polymerase chain reaction. Resveratrol intake did not affect body weight or water consumption. Resveratrol intake increased the expression levels of SIRT1 in the liver, the antioxidative ability of serum, and extended TL in the heart, whereas there was no significant difference in mt-cn in the heart or TL in sperm. The rate of blastocyst development was significantly lower in aged male mice than in younger mice, and resveratrol intake increased the total number of blastocysts derived from both young and aged males. Resveratrol intake did not affect mt-cn or TL in blastomeres of blastocyst-stage embryos derived from young mice, but significantly increased both mt-cn and TL in blastomeres of blastocysts derived from aged fathers. In conclusion, resveratrol intake increased mt-cn and TL levels in blastocysts derived from aged male mice.

Intracytoplasmic sperm injection (ICSI) is clinically used to treat obstructive/nonobstructive azoospermia. This study compared the efficacy of ICSI with cauda epididymal and testicular sperm in Wistar (WI) and Brown-Norway (BN) rats. The transfer of ICSI oocytes with cryopreserved epididymal and testicular WI sperm resulted in offspring production of 26.2% and 3.7%–4.7%, respectively (P < 0.05). Treatments for artificial oocyte activation (AOA) and acrosome removal improved pronuclear formation in BN-ICSI oocytes; however, only AOA treatment was effective in producing offspring (3.7%–6.5%). In the case of ICSI with testicular sperm (TESE-ICSI), one offspring (0.6%) was derived from the BN-TESE-ICSI oocytes. The application of AOA or a hypo-osmotic sperm suspension did not improve the production of TESE-ICSI offspring. Thus, outbred WI rat offspring can be produced by using ICSI and less efficiently by using TESE-ICSI. Challenges in producing offspring by using ICSI/TESE-ICSI in inbred BN strain require further investigation.

This study aimed to develop a method to evaluate the quality of bovine in vitro fertilized (IVF) embryos based on gene expression profiling via whole-transcriptome amplification. The expression of 11 developmentally important genes in individual bovine in vivo-derived (IVD) and IVF embryos were examined. Gene expression profiling was conducted by classifying the expression level of each gene in individual embryos as low, medium, or high. The IVF group had a higher (P < 0.01) proportion of embryos with low expression of SOX2, NANOG, and FGF4. In addition, a correlation analysis between the expression levels of each gene in individual embryos demonstrated that the relationship between gene expression differed with respect to IVD and IVF embryos. Our results suggest that the expression profiling of developmentally important genes using IVD embryos as normal controls could be a useful indicator for evaluating the quality of bovine IVF embryos.