Imaging filopodia dynamics in the mouse blastocyst

Transforming the concept of connectivity: unveiling tunneling nanotube biology and their roles in brain development and neurodegeneration

Tunneling nanotubes (TNTs) are thin tubular membrane protrusions that connect distant cells, generating a complex cellular network. Over the past few decades, research on TNTs has provided important insights into their biology, including structural composition, formation mechanisms, modulators, and functionality. It has been discovered that TNTs allow cytoplasmic continuity between connected cells, facilitating fast intercellular communication via both passive and active exchange of materials. These features are pivotal in the nervous system, where rapid processing of inputs is physiologically required. TNTs have been implicated in the progression of neurodegenerative diseases and cancer in various in vitro models, and TNT-like structures have also been observed in the developing brain and in vivo. This highlights their significant role in pathophysiological processes. In this comprehensive review we aim to provide an extensive overview of TNTs, starting from key structural features and mechanisms of formation and describing the main experimental techniques used to detect these structures both in vitro and in vivo. We focus primarily on the nervous system, where the discovery of TNTs could prompt a reconsideration of the brain functioning as individual units (the neuronal theory of Cajal) versus neurons being physically connected, as Golgi believed. We illustrate the involvement of TNTs in brain development and neurodegenerative states and highlight the limitations and future research needs in this field.

The Istanbul Consensus update: a revised ESHRE/ALPHA consensus on oocyte and embryo static and dynamic morphological assessment† ‡

This European Society of Human Reproduction and Embryology (ESHRE)/Alpha Scientists in Reproductive Medicine (ALPHA) consensus document provides several novel recommendations to assess oocyte and embryo morphology and rank embryos for transfer. A previous ALPHA/ESHRE consensus on oocyte and embryo morphological assessment was published in 2011. After more than a decade, and the integration of time-lapse technology into embryo culture and assessment, a thorough review and update was needed. A working group consisting of ALPHA members and ESHRE Special interest group of Embryology members formulated recommendations on oocyte and embryo assessment. The working group included 17 internationally recognized experts with extensive experience in clinical embryology. Seven members represented ALPHA and eight members represented ESHRE, along with two methodological experts from the ESHRE central office. Based on a systematic literature search and discussion of existing evidence, the recommendations of the Istanbul Consensus (2011) were reassessed and, where appropriate, updated based on consensus within the working group. A stakeholder review was organized after the updated draft was finalized. The final version was approved by the working group, the ALPHA Executive Committee and the ESHRE Executive Committee. This updated consensus paper provides 20 recommendations focused on the timeline of preimplantation developmental events and morphological criteria for oocyte, zygote and embryo assessment. Based on the duration of embryo culture, recommendations are given on the frequency and timing of assessments to ensure consistency and effectiveness. Several criteria relevant to oocyte and embryo morphology have not been well studied, leading to either a recommendation against their use for grading or for their use in ranking rather than grading. Future updates may require further revision of these recommendations. This document provides embryologists with advice on best practices when assessing oocyte and embryo quality based on the most recent evidence.

The Istanbul consensus update: a revised ESHRE/ALPHA consensus on oocyte and embryo static and dynamic morphological assessment†,‡

STUDY QUESTION

What are the current recommended criteria for morphological assessment of oocytes, zygotes, and embryos?

SUMMARY ANSWER

The present ESHRE/Alpha Scientists in Reproductive Medicine consensus document provides several novel recommendations to assess oocyte and embryo morphology and rank embryos for transfer.

WHAT IS KNOWN ALREADY

A previous Alpha Scientists in Reproductive Medicine/ESHRE consensus on oocyte and embryo morphological assessment was published in 2011. After more than a decade, and the integration of time-lapse technology into embryo culture and assessment, a thorough review and update was needed.

STUDY DESIGN, SIZE, DURATION

A working group consisting of Alpha Scientists in Reproductive Medicine executive committee members and ESHRE Special interest group of Embryology members formulated recommendations on oocyte and embryo assessment.

PARTICIPANTS/MATERIALS, SETTING, METHODS

The working group included 17 internationally recognized experts with extensive experience in clinical embryology. Seven members represented Alpha Scientists in Reproductive Medicine and eight members represented ESHRE, along with to two methodological experts from the ESHRE central office. Based on a systematic literature search and discussion of existing evidence, the recommendations of the Istanbul Consensus (2011) were reassessed and, where appropriate, updated based on consensus within the working group. A stakeholder review was organized after the updated draft was finalized. The final version was approved by the working group, the Alpha executive committee and the ESHRE Executive Committee.

MAIN RESULTS AND THE ROLE OF CHANCE

This updated consensus paper provides 20 recommendations focused on the timeline of preimplantation developmental events and morphological criteria for oocyte, zygote, and embryo assessment. Based on duration of embryo culture, recommendations are given on the frequency and timing of assessments to ensure consistency and effectiveness.

LIMITATIONS, REASONS FOR CAUTION

Several criteria relevant to oocyte and embryo morphology have not been well studied, leading to either a recommendation against their use for grading or for their use in ranking rather than grading. Future updates may require further revision of these recommendations.

WIDER IMPLICATIONS OF THE FINDINGS

This document provides embryologists with advice on best practices when assessing oocyte and embryo quality based on the most recent evidence.

STUDY FUNDING/COMPETING INTEREST(S)

The consensus meeting and writing of the paper were supported by funds from ESHRE and Alpha Scientists in Reproductive Medicine. The working group members did not receive any payment. G.C. declared payments or honoraria for lectures from Gedeon Richter and Cooper Surgical. A.C. declared text book royalties (Mastering Clinical Embryology, published 2024), consulting fees from Cooper Surgical, Gedeon Richter and TMRW Life Sciences, honoraria for lectures from Merck, Ferring, and Gedeon Richter, and participation in the HFEA Scientific Advances Committee; she also disclosed being treasurer and vice-president of Alpha Scientists in Reproductive Medicine, a shareholder in Care Fertility Limited and Fertile Mind Limited, and having stock options in TMRW Life Sciences and U-Ploid Biotechnology Ltd. L.R. declared consulting fees from Organon, payments or honoraria for lectures from Merck, Organon, IBSA, Finox, Geden Richter, Origio, Organon, Ferring, Fundation IVI; she also disclosed being a member of the Advisory Scientific Board of IVIRMA (Paid) and a member of the Advisory Scientific Board of Nterilizer (unpaid). I.S. declared payments or honoraria for lectures from Vitrolife and Cooper Surgical, and stock options from Alife Health. M.A. declared payments or honoraria for lectures from Vitrolife and support for attending meetings from Vitrolife and Cooper Surgical (both unrelated to this manuscript). The other authors have no conflicts of interest to declare.

DISCLAIMER

This Good Practice Recommendations (GPRs) document represents the consensus views of the members of this working group based on the scientific evidence available at the time of the meeting. GPRs should be used for information and educational purposes. They should not be interpreted as setting a standard of care or be deemed inclusive of all proper methods of care or be exclusive of other methods of care reasonably directed to obtaining the same results. They do not replace the need for application of clinical judgement to each individual presentation, or variations based on locality and facility type.

Preimplantation trophectoderm: A 'quick-fix' protector for embryo survival?

The trophectoderm (TE) epithelium forms the outer layer of the mammalian blastocyst and generates the blastocoel through vectorial transport. Its differentiation during cleavage, studied mainly in mouse, is integrated with blastocyst morphogenesis with key roles for cell polarisation, asymmetric cell divisions, cell signalling, regulatory transcription factors and cellular inheritance. The TE provides a physical and cellular protection to the emerging lineages of the embryo essential for the integrity of blastocyst development. Here, two examples of TE differentiation are considered in some detail where this immediate protective function for embryo survival is assessed: (i) cellular processes from TE at the polar-mural junctional zone in the early blastocyst that later form filopodia traversing the blastocoel, and (ii) the endocytic system which matures and polarises during differentiation. Understanding the broad role for TE in regulating early morphogenesis and environmental protection of the embryo, including these two examples, have clinical as well as biological relevance.

Artificial removal of the zona pellucida at the pronuclear stage: an exploratory study to improve embryo fragmentation

Objective

To investigate whether artificial removal of zona pellucida (ZP) at the pronuclear stage improves good-quality embryos and blastocyst development in patients with difficulty conceiving because of severe fragmentation in early-cleavage stage.

Design

Exploratory investigation.

Setting

Reproductive center.

Patients

Thirty-four patients scheduled for assisted reproductive technology (ART) treatment in our clinic between February 2020 and January 2021 were selected to undergo ZP removal according to their ART-related medical history. In total, 173 two pronuclei zygotes were obtained, with 101 allocated to the ZP-free group and 72 to the ZP-intact group. All patients had zygotes allocated in both ZP-free and ZP-intact groups.

Interventions

Oocytes that were confirmed to be normally fertilized were placed in sucrose-containing HEPES medium for approximately 5 minutes and cultured under one of the following two conditions: ZP-free, in which the ZP was completely removed from the ooplasm by laser irradiation and a medium-blowing method with a biopsy pipette; and ZP-intact. Subsequently, embryos were either freshly transferred on day 2 (ZP-intact group only, according to patients' wishes), or day 5/6, or they were cryopreserved on day 5/6/7 for future embryo transfer cycles for both groups.

Main Outcome Measures

Rates of good-quality embryos, blastocyst development, morphologically good-quality blastocyst development, and cryopreservation.

Results

In ZP-free embryos, the inter-blastomere adhesion was not disturbed and the fragmentation was significantly decreased that resulted into significant improvement in all measured parameters compared with the ZP-intact group.

Conclusions

Artificial ZP removal at the pronuclear stage may prevent excessive fragmentation leading to good-quality blastocysts. Moreover, patients with recurrent ART failure may achieve successful pregnancies.

Cytoplasmic strings in human blastocysts: hypotheses of their role and implications for embryo selection

STUDY QUESTION

What are the implications of the presence cytoplasmic strings (Cyt-S) and their quantity and dynamics for the pre-implantation development of human blastocysts?

SUMMARY ANSWER

Cyt-S are common in human embryos and are associated with faster blastocyst development, larger expansion, and better morphological quality.

WHAT IS KNOWN ALREADY

Cyt-S are dynamic cellular projections connecting inner cell mass and trophectoderm (TE) cells, that can be observed during blastocyst expansion. Their prevalence in human embryos has been estimated to be between 44% and 93%. Data relevant to their clinical implications and role in development are lacking, limited, or controversial.

STUDY DESIGN, SIZE, DURATION

Retrospective study conducted at a single IVF center between May 2013 and November 2014 and involving 124 pre-implantation genetic testing for aneuploidy cycles in a time-lapse incubator with ≥1 blastocyst biopsied and vitrified (N = 370 embryos assessed). These cycles resulted in 87 vitrified-warmed single-euploid blastocyst transfers.

PARTICIPANTS/MATERIALS, SETTING, METHODS

ICSI, continuous blastocyst culture (Days 5-7), TE biopsy of fully expanded blastocysts without Day 3 zona pellucida drilling, qPCR to assess uniform full-chromosome aneuploidies, and vitrification were all performed. Only vitrified-warmed euploid single-embryo-transfers were conducted. Blastocyst morphological quality was defined according to Gardner's criteria. The AI-based software CHLOE™ (Fairtility) automatically registered timings from time of starting blastulation (tSB) to biopsy (t-biopsy, i.e. blastocyst full-expansion) as hours-post-insemination (hpi), embryo area (including zona pellucida in µm2), and spontaneous blastocyst collapses. One senior embryologist manually annotated Cyt-S presence, quantity, timings, and type (thick cell-to-cell connections and/or threads). All significant associations were confirmed through regression analyses. All couples', cycles', and embryos' main features were also tested for associations with Cyt-S presence, quantity, and dynamics.

MAIN RESULTS AND THE ROLE OF CHANCE

About 94.3% of the patients (N = 117/124) had ≥1 embryo with Cyt-S. Out of a total of 370 blastocysts, 55 degenerated between blastulation and full-expansion (N = 55/370, 14.9%). The degeneration rate among embryos with ≥1 Cyt-S was 10.8% (N = 33/304), significantly lower than that of embryos without Cyt-S (33.3%, N = 22/66, P < 0.01). Of the remaining 315 viable blastocysts analyzed, 86% (N = 271/315; P < 0.01) had ≥1 Cyt-S, on average 3.5 ± 2.1 per embryo ranging 1-13. The first Cyt-S per viable embryo appeared at 115.3 ± 12.5 hpi (85.7-157.7), corresponding to 10.5 ± 5.8 h (0.5-31) after tSB. Overall, we analyzed 937 Cyt-S showing a mean duration of 3.8 ± 2.7 h (0.3-20.9). Cyt-S were mostly threads (N = 508/937, 54.2%) or thick cell-to-cell connections becoming threads (N = 382/937, 40.8%) than thick bridges (N = 47/937, 5.0%). The presence and quantity of Cyt-S were significantly associated with developmentally faster (on average 6-12 h faster) and more expanded (on average 2700 µm2-larger blastocyst's area at t-biopsy) embryos. Also, the presence and duration of Cyt-S were associated with better morphology. Lastly, while euploidy rates were comparable between blastocysts with and without Cyt-S, all euploid blastocysts transferred from the latter group failed to implant (N = 10).

LIMITATIONS, REASONS FOR CAUTION

Cyt-S presence and dynamics were assessed manually on seven focal planes from video frames recorded every 15 min. The patients included were mostly of advanced maternal age. Only associations could be reported, but no causations/consequences. Lastly, larger datasets are required to better assess Cyt-S associations with clinical outcomes.

WIDER IMPLICATIONS OF THE FINDINGS

Cyt-S are common during human blastocyst expansion, suggesting their physiological implication in this process. Their presence, quantity and dynamics mirror embryo viability, and morphological quality, yet their role is still unknown. Future basic science studies are encouraged to finally describe Cyt-S molecular nature and biophysical properties, and Artificial Intelligence tools should aid these studies by incorporating Cyt-S assessment.

STUDY FUNDING/COMPETING INTEREST(S)

None.

TRIAL REGISTRATION NUMBER

N/A.

Developmental perturbation in human embryos: Clinical and biological significance learned from time-lapse images

Background

Time-lapse technology (TLT) has gained widespread adoption worldwide. In addition to facilitating the undisturbed culture of embryos, TLT offers the unique capability of continuously monitoring embryos to detect spatiotemporal changes. Although these observed phenomena play a role in optimal embryo selection/deselection, the clinical advantages of introducing TLT remain unclear. However, manual annotation of embryo perturbation could facilitate a comprehensive assessment of developmental competence. This process requires a thorough understanding of embryo observation and the biological significance associated with developmental dogma and variation. This review elucidates the typical behavior and variation of each phenomenon, exploring their clinical significance and research perspectives.

Methods

The MEDLINE database was searched using PubMed for peer-reviewed English-language original articles concerning human embryo development.

Main findings

TLT allows the observation of consecutive changes in embryo morphology, serving as potential biomarkers for embryo assessment. In assisted reproductive technology laboratories, several phenomena have not revealed their mechanism, posing difficulties such as fertilization deficiency and morula arrest.

Conclusion

A profound understanding of the biological mechanisms and significance of each phenomenon is crucial. Further collaborative efforts between the clinical and molecular fields following translational studies are required to advance embryonic outcomes and assessment.

Development and validation of a nomogram for predicting ongoing pregnancy in single vitrified-warmed blastocyst embryo transfer cycles

Introduction

The global adoption of the "freeze-all strategy" has led to a continuous increase in utilization of single vitrified-warmed blastocyst embryo transfer (SVBT) owing to its clinical effectiveness. Accurate prediction of clinical pregnancy is crucial from a patient-centered perspective. However, this remains challenging, with inherent limitations due to the absence of precise and user-friendly prediction tools. Thus, this study primarily aimed to develop and assess a nomogram based on quantitative clinical data to optimize the efficacy of personalized prognosis assessment.

Materials and methods

We conducted a retrospective cohort analysis of ongoing pregnancy data from 658 patients with infertility who underwent SVBT at our center between October 17, 2017, and December 18, 2021. Patients were randomly assigned to the training (n=461) or validation (n=197) cohort for nomogram development and testing, respectively. A nomogram was constructed using the results of the multivariable logistic regression (MLR), which included clinical covariates that were assessed for their association with ongoing pregnancy.

Results

The MLR identified eight significant variables that independently predicted ongoing pregnancy outcomes in the study population. These predictors encompassed maternal physiology, including maternal age at oocyte retrieval and serum anti-Müllerian hormone levels; uterine factors, such as adenomyosis; and various embryo assessment parameters, including the number of fertilized embryos, blastocyst morphology, blastulation day, blastocyst re-expansion speed, and presence of embryo string. The area under the receiver operating characteristic curve in our prediction model was 0.675 (95% confidence interval [CI], 0.622-0.729) and 0.656 (95% CI, 0.573-0.739) in the training and validation cohorts, respectively, indicating good discrimination performance in both cohorts.

Conclusions

Our individualized nomogram is a practical and user-friendly tool that can provide accurate and useful SVBT information for patients and clinicians. By offering this model to patients, clinical stakeholders can alleviate uncertainty and confusion about fertility treatment options and enhance patients' confidence in making informed decisions.

The salt-and-pepper pattern in mouse blastocysts is compatible with signaling beyond the nearest neighbors

Embryos develop in a concerted sequence of spatiotemporal arrangements of cells. In the preimplantation mouse embryo, the distribution of the cells in the inner cell mass evolves from a salt-and-pepper pattern to spatial segregation of two distinct cell types. The exact properties of the salt-and-pepper pattern have not been analyzed so far. We investigate the spatiotemporal distribution of NANOG- and GATA6-expressing cells in the ICM of the mouse blastocysts with quantitative three-dimensional single-cell-based neighborhood analyses. A combination of spatial statistics and agent-based modeling reveals that the cell fate distribution follows a local clustering pattern. Using ordinary differential equations modeling, we show that this pattern can be established by a distance-based signaling mechanism enabling cells to integrate information from the whole inner cell mass into their cell fate decision. Our work highlights the importance of longer-range signaling to ensure coordinated decisions in groups of cells to successfully build embryos.

Human embryo live imaging reveals nuclear DNA shedding during blastocyst expansion and biopsy

Proper preimplantation development is essential to assemble a blastocyst capable of implantation. Live imaging has uncovered major events driving early development in mouse embryos; yet, studies in humans have been limited by restrictions on genetic manipulation and lack of imaging approaches. We have overcome this barrier by combining fluorescent dyes with live imaging to reveal the dynamics of chromosome segregation, compaction, polarization, blastocyst formation, and hatching in the human embryo. We also show that blastocyst expansion mechanically constrains trophectoderm cells, causing nuclear budding and DNA shedding into the cytoplasm. Furthermore, cells with lower perinuclear keratin levels are more prone to undergo DNA loss. Moreover, applying trophectoderm biopsy, a mechanical procedure performed clinically for genetic testing, increases DNA shedding. Thus, our work reveals distinct processes underlying human development compared with mouse and suggests that aneuploidies in human embryos may not only originate from chromosome segregation errors during mitosis but also from nuclear DNA shedding.

The importance of cytoplasmic strings during early human embryonic development

Objectives: During human in vitro fertilisation (IVF) treatments, embryologists attempt to select the most viable embryos for embryo transfer (ET). Previously, embryos were evaluated based on light microscopic morphological parameters. However, this is currently accomplished by morphokinetic analysis of time-lapse recordings. This technique provides us the opportunity to observe cytoplasmic strings at the blastocyst stage. The aim of this work was to examine the relationship between the presence of cytoplasmic strings (CS) and the embryo viability in human in vitro fertilised embryos. Study design: Herein, we present an evaluation of the morphokinetic data on the development of embryos obtained during IVF treatments performed at the Division of Assisted Reproduction between December 2020 and March 2021. The dynamics of embryo development, embryo morphology, and morphokinetic scores generated by a time-lapse system were compared between the presence of cytoplasmic strings (CS+) and their absence (CS-) at the blastocyst stage. Results: The development of 208 embryos from 78 patients was examined. Moreover, 81.2% of the embryos had CS in the blastocyst stage; 77% of CS existed in embryos created by conventional IVF, while 86% of CS existed in embryos fertilised by intracytoplasmic sperm injection (ICSI) (p = 0.08). A greater number of CS+ embryos developed into a higher quality blastocyst (52.1% vs. 20.5%, p = 0.02). The morphokinetic score values characterising the development of embryos, such as Known Implantation Data Score (KIDScore) and Intelligent Data Analysis (iDAScore), were higher in CS+ groups (KID: 6.1 ± 2.1 vs. 4.7 ± 2.07; iDA: 8.0 ± 1.9 vs. 6.8 ± 2.3, p < 0.01). The dynamics of the early embryo development were similar between the two groups; however, CS+ embryos reached the blastocyst stage significantly earlier (tB: 103.9 h vs. tB: 107.6 h; p = 0.001). Conclusion: Based on our results, the number of embryos with cytoplasmic strings was higher than that without cytoplasmic strings, and its presence is not related to the fertilisation method. These embryos reached the blastocyst stage earlier, and their morphokinetic (KIDScore and iDAScore) parameters were better. All these results suggest that the presence of CS indicates higher embryo viability. The examination of this feature may help us make decisions about the embryos with higher implantation potential.

Bovine embryos release extracellular vesicles with differential miRNA signature during the compaction and blastulation stages

Pre-implantation embryos release extracellular vesicles (EVs) to extracellular environment. In this work it is hypothesized that the EVs miRNA cargo will vary during pre-implantation development due to the constant changes in gene expression that take place through this period. The concentration, size and miRNA cargo of EVs secreted by competent bovine embryos during the period from compaction to blastulation (Day 3-7) were analyzed. For this analysis tow developmental windows were defined: W2 from 8-cells (D3) to morula (D5) and W3 from morula (D5) to blastocyst (D7). For W2, in vitro produced embryos were individually cultured in EVs-depleted medium from D3 to D5; culture media were collected and assigned to Group W2. Morulae were kept in culture up to blastocyst stage to determine the developmental competence. For W3, D5 morulae were collected and cultured individually in EVs-depleted medium up to blastocyst stage; culture media were assigned to Group W3, and blastocysts were kept in culture up to day 11 to define their competence. The mean size of EVs was similar between groups, however, EVs concentration was lower in W2. A total of 140 miRNAs were identified. From them, 79 were differentially expressed between the groups, 28 upregulated and 51 downregulated. miRNAs differentially detected between both developmental windows participate in the regulation of signaling pathways which crucial for embryonic development. It was concluded that the secretion of EVs is regulated by the developmental progress of the embryo during the pre-implantation period.

Detailed analysis of cytoplasmic strings in human blastocysts: new insights

The aim of this study was to determine if there was an association between the presence of cytoplasmic strings (CS) and their characteristics, with blastocyst quality, development and clinical outcome in human blastocysts. This two-centre cohort study was performed between July 2017 and September 2018 and involved a total of 1152 blastocysts from 225 patients undergoing in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI). All embryos were cultured in Embryoscope+ and were assessed for CS using time-lapse images. A single assessor examined all blastocysts and reviewed videos using the EmbyroViewer® Software. Blastocyst quality was assessed on day 5 of embryo development. The number of CS, location and duration of their activity was recorded on days 5/6. A positive association between the presence of CS in human blastocysts with blastocyst quality was identified. Blastocysts with a higher number of CS present, were of higher quality and were in the more advanced stages of development. Top quality blastocysts had CS activity present for longer, as well as having a higher number of vesicles present travelling along the CS. Blastocysts that had CS present, had a significantly higher live birth rate. This study has confirmed that a higher number of CS and vesicles in human blastocysts is associated with top quality blastocysts and is not a negative predictor of development. They had a higher number of CS present that appeared earlier in development and, although ceased activity sooner, had a longer duration of activity. Blastocysts with CS had a significant increase in live birth rate.

Extracellular vesicles throughout development: A potential roadmap for emerging glioblastoma therapies

Extracellular vesicles (EVs) are membrane-delimited vesicular bodies carrying different molecules, classified according to their size, density, cargo, and origin. Research on this topic has been actively growing through the years, as EVs are associated with critical pathological processes such as neurodegenerative diseases and cancer. Despite that, studies exploring the physiological functions of EVs are sparse, with particular emphasis on their role in organismal development, initial cell differentiation, and morphogenesis. In this review, we explore the topic of EVs from a developmental perspective, discussing their role in the earliest cell-fate decisions and neural tissue morphogenesis. We focus on the function of EVs through development to highlight possible conserved or novel processes that can impact disease progression. Specifically, we take advantage of what was learned about their role in development so far to discuss EVs impact on glioblastoma, a particular brain tumor of stem-cell origin and poor prognosis, and how their function can be hijacked to improve current therapies.

Noninvasive autologous mitochondria transport improves the quality and developmental potential of oocytes from aged mice

OBJECTIVE

To establish an optimized autologous mitochondria transport technique for oocyte-aging rescue, which minimizes both the patient's pains and the damage to oocytes.

DESIGN

Experimental laboratory study.

SETTING

Laboratory.

ANIMAL(S)

Institute of Cancer Research mice.

INTERVENTION(S)

The murine umbilical cord mesenchymal stem cells were isolated from the female pup and cryopreserved. After the female aged, its germinal vesicle (GV) oocytes were collected and treated to weaken the zona pellucida. Its autologous umbilical cord mesenchymal stem cells were induced into granulosa cells (iGCs). The zona-weakened GV oocytes were aggregated with iGCs into iGC-oocyte complexes. Then, these complexes were cultured in growth-differentiation factor 9-containing media for 3 days. Next, they were subjected to in vitro maturation and fertilization. Presumptive zygotes were cultured for 24 hours, and the cleaved 2-cell embryos were selected for embryo transfer.

MAIN OUTCOME MEASURE(S)

The oocyte quality was determined by examining mitochondrial ultrastructure using transmission electron microscopy, the adenosine triphosphate content using a luminometer, and intracellular reactive oxygen species levels by confocal microscopy. The spindle organization in mature oocytes was examined by confocal microscopy. The developmental potential of oocytes was evaluated by monitoring the in vitro embryo development and the birth rate after embryo transfer.

RESULT(S)

Mitochondria migrated from iGCs into the GV oocyte via transzonal filopodia. The maturation rate, quality, and developmental potential of these oocytes were substantially increased. Furthermore, the birth rate after embryo transfer has been improved.

CONCLUSION(S)

This approach used noninvasive procedures to collect mitochondria donor cells and optimized mitochondria transfer manipulations; thus, it may have potential in ameliorating oocyte-aging-related subfertility.

Extracellular Vesicles and Membrane Protrusions in Developmental Signaling

During embryonic development, cells communicate with each other to determine cell fate, guide migration, and shape morphogenesis. While the relevant secreted factors and their downstream target genes have been characterized extensively, how these signals travel between embryonic cells is still emerging. Evidence is accumulating that extracellular vesicles (EVs), which are well defined in cell culture and cancer, offer a crucial means of communication in embryos. Moreover, the release and/or reception of EVs is often facilitated by fine cellular protrusions, which have a history of study in development. However, due in part to the complexities of identifying fragile nanometer-scale extracellular structures within the three-dimensional embryonic environment, the nomenclature of developmental EVs and protrusions can be ambiguous, confounding progress. In this review, we provide a robust guide to categorizing these structures in order to enable comparisons between developmental systems and stages. Then, we discuss existing evidence supporting a role for EVs and fine cellular protrusions throughout development.

Cytoplasmic strings between ICM and mTE are a positive predictor of clinical pregnancy and live birth outcomes: A time-lapse study

Background

Elective single blastocyst transfer (eSBT) is considered to reduce the incidence of multiple pregnancy compared to double embryo transfer. Blastocyst selection is the key to achieving pregnancy. In the past, morphological assessment was the main criterion used to select blastocyst. Some important morphological parameters are considered to be clinically valuable, such as cytoplasmic strings traversing from the inner cell mass (ICM) and mural trophectoderm (mTE).

Methods

In this study, 1,267 elective frozen-thawed eSBT cycles cultured in a time-lapse culture system from January 2018 to May 2019 were included. Blastocysts were grouped into “present” and “absent” according to the appearance of cytoplasmic strings between ICM and mTE cells. The “present” group was further categorized according to the quantity of cytoplasmic strings between the ICM and mTE cells.

Results

A time-lapse analysis indicated that cytoplasmic strings between ICM and mTE were more visible among good quality blastocysts. Furthermore, blastocysts with cytoplasmic strings showed higher clinical pregnancy and live birth rates (P = 0.011 and 0.003), while no significant differences were observed in abortion rate and birth weight (P = 0.466 and 0.556).

Conclusions

In conclusion, although the results of previous studies about cytoplasmic strings have been controversial, the present time-lapse analysis provides evidence for the first time that cytoplasmic strings between ICM and mTE cells are a positive predictor of clinical pregnancy and live birth outcomes in elective frozen-thawed single blastocyst transfer cycles.

Cytoneme-like protrusion formation induced by LAR is promoted by receptor dimerization

Actin-based protrusions called cytonemes are reported to function in cell communication by supporting events such as morphogen gradient establishment and pattern formation. Despite the crucial roles of cytonemes in cell signaling, the molecular mechanism for cytoneme establishment remains elusive. In this study, we showed that the leukocyte common antigen-related (LAR) receptor protein tyrosine phosphatase plays an important role in cytoneme-like protrusion formation. Overexpression of LAR in HEK293T cells induced the formation of actin-based protrusions, some of which exceeded 200 µm in length and displayed a complex morphology with branches. Upon focusing on the regulation of LAR dimerization or clustering and the resulting regulatory effects on LAR phosphatase activity, we found that longer and more branched protrusions were formed when LAR dimerization was artificially induced and when heparan sulfate was applied. Interestingly, although the truncated form of LAR lacking phosphatase-related domains promoted protrusion formation, the phosphatase-inactive forms did not show clear changes, suggesting that LAR dimerization triggers the formation of cytoneme-like protrusions in a phosphatase-independent manner. Our results thus emphasize the importance of LAR and its dimerization in cell signaling.

This article has an associated First Person interview with the first author of the paper.

Summary: We showed that the formation of cytoneme-like protrusions, which function in cell signaling, is induced by LAR and clarified that it is LAR dimerization which promotes protrusion formation.

Rhes protein transits from neuron to neuron and facilitates mutant huntingtin spreading in the brain

Rhes (RASD2) is a thyroid hormone-induced gene that regulates striatal motor activity and promotes neurodegeneration in Huntington disease (HD) and tauopathy. Rhes moves and transports the HD protein, polyglutamine-expanded huntingtin (mHTT), via tunneling nanotube (TNT)-like membranous protrusions between cultured neurons. However, similar intercellular Rhes transportation in the intact brain was unknown. Here, we report that Rhes induces TNT-like protrusions in the striatal medium spiny neurons (MSNs) and transported between dopamine-1 receptor (D1R)-MSNs and D2R-MSNs of intact striatum and organotypic brain slices. Notably, mHTT is robustly transported within the striatum and from the striatum to the cortical areas in the brain, and Rhes deletion diminishes such transport. Moreover, Rhes moves to the cortical regions following restricted expression in the MSNs of the striatum. Thus, Rhes is a first striatum-enriched protein demonstrated to move and transport mHTT between neurons and brain regions, providing new insights into interneuronal protein transport in the brain.

The Ism between Endothelial Cilia and Endothelial Nanotubules Is an Evolving Concept in the Genesis of the BBB

The blood–brain barrier (BBB) is fundamental in maintaining central nervous system (CNS) homeostasis by regulating the chemical environment of the underlying brain parenchyma. Brain endothelial cells (BECs) constitute the anatomical and functional basis of the BBB. Communication between adjacent BECs is critical for establishing BBB integrity, and knowledge of its nanoscopic landscape will contribute to our understanding of how juxtaposed zones of tight-junction protein interactions between BECs are aligned. The review discusses and critiques types of nanostructures contributing to the process of BBB genesis. We further critically evaluate earlier findings in light of novel high-resolution electron microscopy descriptions of nanoscopic tubules. One such phenotypic structure is BEC cytoplasmic projections, which, early in the literature, is postulated as brain capillary endothelial cilia, and is evaluated and compared to the recently discovered nanotubules (NTs) formed in the paracellular spaces between BECs during barrier-genesis. The review attempts to elucidate a myriad of unique topographical ultrastructures that have been reported to be associated with the development of the BBB, viz., structures ranging from cilia to BEC tunneling nanotubules (TUNTs) and BEC tethering nanotubules (TENTs).

An investigation of mechanisms underlying mouse blastocyst hatching: a ribonucleic acid sequencing study

OBJECTIVE

To investigate the regulatory mechanisms and signaling molecules underlying hatching in mouse embryos.

DESIGN

Experimental laboratory study using a mouse embryo model.

SETTING

University-based basic scientific research laboratory.

ANIMALS

A total of 40 B6C3F1 × B6D2F1 mouse embryos were used in this study.

INTERVENTION(S)

Frozen/thawed mouse embryos, at the 8-cell stage, were cultured in vitro for 2 days. The resulting hatching and prehatching blastocysts were then used for complementary deoxyribonucleic acid (cDNA) library preparation and ribonucleic acid (RNA) sequencing analysis (n = 8 for each group). Differentially expressed genes were then used for downstream functional analysis. In addition, a list of genes related to developmental progression in humans was used to identify genes that were potentially related to the hatching of human embryos.

MAIN OUTCOME MEASURE(S)

Differentially expressed genes, enriched Gene Ontology terms and canonical pathways, clustered gene networks, activated upstream regulators, and common genes between a gene list of hatching-related genes in mice and a gene list associated with developmental progression in humans.

RESULT(S)

A total 275 differentially expressed genes were identified between hatching and prehatching blastocysts: 230 up-regulated and 45 down-regulated genes. Functional enrichment analysis suggested that blastocyst hatching in vitro is an adenosine triphosphate (ATP)-dependent process that involves protein biosynthesis and organization of the cytoskeleton. Furthermore, by regulating cell motility, the RhoA signaling pathway (including Arpc2, Cfl1, Gsn, Pfn1, Tpi1, Grb2, Tmsb10, Enah, and Rnd3 genes) may be a crucial signaling pathway during hatching. We also identified a cluster of genes (Krt8, Krt7, Cldn4, and Aqp3) that exerted functional roles in cell-cell junctions and water homeostasis during hatching. Moreover, some growth factors (angiotensinogen and fibroblast growth factor 2) and endocrine factors (estrogen receptor and prolactin) were predicted to be involved in the regulation of embryo hatching. In addition, we identified 81 potential genes that are potentially involved in the hatching process in human embryos.

CONCLUSION(S)

Our analysis identified potential genes and molecular regulatory pathways involved in the blastocyst hatching process in mice; we also identified genes that may potentially regulate hatching in human embryos. Our findings enhance our knowledge of embryo development and provide useful information for further exploring the mechanisms underlying embryo hatching.

Specification and role of extraembryonic endoderm lineages in the periimplantation mouse embryo

During mammalian embryo development, the correct formation of the first extraembryonic endoderm lineages is fundamental for successful development. In the periimplantation blastocyst, the primitive endoderm (PrE) is formed, which gives rise to the parietal endoderm (PE) and visceral endoderm (VE) during further developmental stages. These PrE-derived lineages show significant differences in both their formation and roles. Whereas differentiation of the PE as a migratory lineage has been suggested to represent the first epithelial-to-mesenchymal transition (EMT) in development, organisation of the epithelial VE is of utmost importance for the correct axis definition and patterning of the embryo. Despite sharing a common origin, the striking differences between the VE and PE are indicative of their distinct roles in early development. However, there is a significant disparity in the current knowledge of each lineage, which reflects the need for a deeper understanding of their respective specification processes. In this review, we will discuss the origin and maturation of the PrE, PE, and VE during the periimplantation period using the mouse model as an example. Additionally, we consider the latest findings regarding the role of the PrE-derived lineages and early embryo morphogenesis, as obtained from the most recent in vitro models.

The mini player with diverse functions: extracellular vesicles in cell biology, disease, and therapeutics

Extracellular vesicles (EVs) are tiny biological nanovesicles ranging from approximately 30-1000 nm in diameter that are released into the extracellular matrix of most cell types and in biofluids. The classification of EVs includes exosomes, microvesicles, and apoptotic bodies, dependent on various factors such as size, markers, and biogenesis pathways. The transition of EV relevance from that of being assumed as a trash bag to be a key player in critical physiological and pathological conditions has been revolutionary in many ways. EVs have been recently revealed to play a crucial role in stem cell biology and cancer progression via intercellular communication, contributing to organ development and the progression of cancer. This review focuses on the significant research progress made so far in the role of the crosstalk between EVs and stem cells and their niche, and cellular communication among different germ layers in developmental biology. In addition, it discusses the role of EVs in cancer progression and their application as therapeutic agents or drug delivery vehicles. All such discoveries have been facilitated by tremendous technological advancements in EV-associated research, especially the microfluidics systems. Their pros and cons in the context of characterization of EVs are also extensively discussed in this review. This review also deliberates the role of EVs in normal cell processes and disease conditions, and their application as a diagnostic and therapeutic tool. Finally, we propose future perspectives for EV-related research in stem cell and cancer biology.

The presence of cytoplasmic strings in human blastocysts is associated with the probability of clinical pregnancy with fetal heart

PURPOSE

Is the presence of cytoplasmic strings (CS) in human blastocysts associated with the probability of clinical pregnancy with fetal heart (CPFH) after transfer.

METHODS

This case-control study involved 300 single blastocyst transfers. 150 of these resulted in a CPFH (cases) while 150 did not (controls). All embryos were cultured in Embryoscope+ and AI software (IVY) was used to select the blastocyst with the highest score from the cohort for transfer. An embryologist, blind to the transfer outcome, recorded the CS number, location, and duration of their activity.

RESULTS

There was a significant difference in the number of blastocysts that contained CS, with 97.3% of women's blastocysts resulting in +CPFH containing the CS compared to 88.7% of blastocysts in women who did not have a pregnancy (p = 0.007, OR; 4.67, CI 95% 1.5-14.2). CS appeared 2.4 h earlier in embryo development in the +CPFH group compared to their negative counterparts (p = 0.007). There was a significant difference in the average number of CS/blastocyst with a higher number being present in those that achieved a clinical pregnancy (mean: 6.2, SD 2.9) compared to those that did not (mean: 4.6, SD 3.0) (p ≤ 0.0001). There was a significant increase in the number of vesicles seen traveling along the CS with more seen in the blastocysts resulting in a +CPFH (mean: 4.3 SD 2.1) compared to those in the -CPFH group (mean: 3.1, SD 2.1).

CONCLUSION

This study has shown that the presence of cytoplasmic strings in human blastocysts is associated with the probability of clinical pregnancy with fetal heart.

The unknown human trophectoderm: implication for biopsy at the blastocyst stage

Trophectoderm biopsy is increasingly performed for pre-implantation genetic testing of aneuploidies and considered a safe procedure on short-term clinical outcome, without strong assessment of long-term consequences. Poor biological information on human trophectoderm is available due to ethical restrictions. Therefore, most studies have been conducted in vitro (choriocarcinoma cell lines, embryonic and pluripotent stem cells) and on murine models that nevertheless poorly reflect the human counterpart. Polarization, compaction, and blastomere differentiation (e.g., the basis to ascertain trophectoderm origin) are poorly known in humans. In addition, the trophectoderm function is poorly known from a biological point of view, although a panoply of questionable and controversial microarray studies suggest that important genes overexpressed in trophectoderm are involved in pluripotency, metabolism, cell cycle, endocrine function, and implantation. The intercellular communication system between the trophectoderm cells and the inner cell mass, modulated by cell junctions and filopodia in the murine model, is obscure in humans. For the purpose of this paper, data mainly on primary cells from human and murine embryos has been reviewed. This review suggests that the trophectoderm origin and functions have been insufficiently ascertained in humans so far. Therefore, trophectoderm biopsy should be considered an experimental procedure to be undertaken only under approved rigorous experimental protocols in academic contexts.

Rhes Tunnels: A Radical New Way of Communication in the Brain's Striatum?

Ras homolog enriched in the striatum (Rhes) is a striatal enriched protein that promotes the formation of thin membranous tubes resembling tunneling nanotubes (TNT)-"Rhes tunnels"-that connect neighboring cell and transport cargoes: vesicles and proteins between the neuronal cells. Here the literature on TNT-like structures is reviewed, and the implications of Rhes-mediated TNT, the mechanisms of its formation, and its potential in novel cell-to-cell communication in regulating striatal biology and disease are emphasized. Thought-provoking ideas regarding how Rhes-mediated TNT, if it exists, in vivo, would radically change the way neurons communicate in the brain are discussed.

Loss of Cx43 in Murine Sertoli Cells Leads to Altered Prepubertal Sertoli Cell Maturation and Impairment of the Mitosis-Meiosis Switch

Male factor infertility is a problem in today’s society but many underlying causes are still unknown. The generation of a conditional Sertoli cell (SC)-specific connexin 43 (Cx43) knockout mouse line (SCCx43KO) has provided a translational model. Expression of the gap junction protein Cx43 between adjacent SCs as well as between SCs and germ cells (GCs) is known to be essential for the initiation and maintenance of spermatogenesis in different species and men. Adult SCCx43KO males show altered spermatogenesis and are infertile. Thus, the present study aims to identify molecular mechanisms leading to testicular alterations in prepubertal SCCx43KO mice. Transcriptome analysis of 8-, 10- and 12-day-old mice was performed by next-generation sequencing (NGS). Additionally, candidate genes were examined by qRT-PCR and immunohistochemistry. NGS revealed many significantly differentially expressed genes in the SCCx43KO mice. For example, GC-specific genes were mostly downregulated and found to be involved in meiosis and spermatogonial differentiation (e.g., Dmrtb1, Sohlh1). In contrast, SC-specific genes implicated in SC maturation and proliferation were mostly upregulated (e.g., Amh, Fshr). In conclusion, Cx43 in SCs appears to be required for normal progression of the first wave of spermatogenesis, especially for the mitosis-meiosis switch, and also for the regulation of prepubertal SC maturation.

Integration of luminal pressure and signalling in tissue self-organization

Many developmental processes involve the emergence of intercellular fluid-filled lumina. This process of luminogenesis results in a build up of hydrostatic pressure and signalling molecules in the lumen. However, the potential roles of lumina in cellular functions, tissue morphogenesis and patterning have yet to be fully explored. In this Review, we discuss recent findings that describe how pressurized fluid expansion can provide both mechanical and biochemical cues to influence cell proliferation, migration and differentiation. We also review emerging techniques that allow for precise quantification of fluid pressure in vivo and in situ. Finally, we discuss the intricate interplay between luminogenesis, tissue mechanics and signalling, which provide a new dimension for understanding the principles governing tissue self-organization in embryonic development.

Blastulation of a zygote to a hatched blastocyst without any clear cell division: an observational finding in a time-lapse system after in vitro fertilization

PURPOSE

To describe an interesting not previously described morphokinetic finding.

METHODS

Retrospective case report of a couple undergoing controlled ovarian stimulation (COS) followed by in vitro fertilization and blastocyst transfer.

RESULTS

We identified a unique finding of blastulation of a fertilized human zygote after conventional in vitro fertilization. The fertilized zygote did not show any clear cytokinesis until approximately 107 h post insemination, when it started dividing into a blastocyst. By 113 h post insemination, inner cell mass and trophectoderm cells could be clearly distinguished and the blastocyst was completely hatched by 136 h post insemination.

CONCLUSION

Time-lapse systems offer more detailed observations of embryonic development. Here, we report an atypical development of an embryo that was not described previously. We hope to become an insightful discussion among peers and incentive the publication of such findings in the future.

Fine intercellular connections in development: TNTs, cytonemes, or intercellular bridges?

Intercellular communication is a fundamental property of multicellular organisms, necessary for their adequate responses to changing environment. Tunneling nanotubes (TNTs) represent a novel means of intercellular communication being a long cell-to-cell conduit. TNTs are actively formed under a broad range of stresses and are also proposed to exist under physiological conditions. Development is a physiological condition of particular interest, as it requires fine coordination. Here we discuss whether protrusions shown to exist during embryonic development of different species could be TNTs or if they represent other types of cell structure, like cytonemes or intercellular bridges, that are suggested to play an important role in development.

Time-lapse imaging of cytoplasmic strings at the blastocyst stage suggests their association with spontaneous blastocoel collapse

RESEARCH QUESTION

To study the origin and temporal behaviour of cytoplasmic strings spanning the blastocoel (main objective) and their influence on treatment outcome (secondary objective).

DESIGN

This retrospective analysis of prospectively collected data was set up in a university medical centre. Patients who either underwent fresh (n = 95) or vitrified-warmed (n = 55) single blastocyst transfer were included. Time-lapse sequences of in-vitro developed blastocysts were screened for the presence of cytoplasmic strings. Pregnancies in string-positive and string-negative transfers were followed up to live birth.

RESULTS

A total of 387 blastocysts were obtained in the fresh cycles of 100 patients, corresponding to a blastocyst formation rate of 62.4%. Cytoplasmic strings were first detected around full stage (108.5 ± 6.4 h) in 170 blastocysts (43.9%). The number of strings varied (range: 1-7) and the duration of visibility was 5.2 ± 3.5 h. The occurrence of cytoplasmic strings was significantly associated with the presence of blastocoelic collapses (P < 0.001) but not with any of the annotated morphokinetic parameters. Live birth and neonatal outcome were the same for both string-positive and string-negative pregnancies. Moreover, collapses did not affect treatment outcome.

CONCLUSION

Time-lapse analysis of cytoplasmic strings at the blastocyst stage revealed that this morphological feature was not a negative predictor as previously reported. Although physiologically normal, at least some of the cytoplasmic strings are an artefact, possibly associated with blastocoelic collapses.

Rhes travels from cell to cell and transports Huntington disease protein via TNT-like protrusion

Tunneling nanotubes (TNT) are membranous tubes that connect two cells, but their functional roles and mechanism of biogenesis remain obscure. Sharma and Subramaniam demonstrate that Rhes, a brain-enriched protein, increases biogenesis of TNT-like cellular protrusions or “Rhes tunnels” through which Rhes travels from cell to cell and transports Huntington disease (HD) protein.

Tunneling nanotubes (TNT) are thin, membranous, tunnel-like cell-to-cell connections, but the mechanisms underlying their biogenesis or functional role remains obscure. Here, we report, Rhes, a brain-enriched GTPase/SUMO E3-like protein, induces the biogenesis of TNT-like cellular protrusions, “Rhes tunnels,” through which Rhes moves from cell to cell and transports Huntington disease (HD) protein, the poly-Q expanded mutant Huntingtin (mHTT). The formation of TNT-like Rhes tunnels requires the Rhes’s serine 33, C-terminal CAAX, and a SUMO E3-like domain. Electron microscopy analysis revealed that TNT-like Rhes tunnels appear continuous, cell–cell connections, and <200 nm in diameter. Live-cell imaging shows that Rhes tunnels establish contact with the neighboring cell and deliver Rhes-positive cargoes, which travel across the plasma membrane of the neighboring cell before entering it. The Rhes tunnels carry Rab5a/Lyso 20-positive vesicles and transport mHTT, but not normal HTT, mTOR, or wtTau proteins. SUMOylation-defective mHTT, Rhes C263S (cannot SUMOylate mHTT), or CRISPR/Cas9-mediated depletion of three isoforms of SUMO diminishes Rhes-mediated mHTT transport. Thus, Rhes promotes the biogenesis of TNT-like cellular protrusions and facilitates the cell–cell transport of mHTT involving SUMO-mediated mechanisms.

The cytoneme connection: direct long-distance signal transfer during development

During development, specialized cells produce signals that distribute among receiving cells to induce a variety of cellular behaviors and organize tissues. Recent studies have highlighted cytonemes, a type of specialized signaling filopodia that carry ligands and/or receptor complexes, as having a role in signal dispersion. In this Primer, we discuss how the dynamic regulation of cytonemes facilitates signal transfer in complex environments. We assess recent evidence for the mechanisms for cytoneme formation, function and regulation, and postulate that contact between cytoneme membranes promotes signal transfer as a new type of synapse (morphogenetic synapsis). Finally, we reflect on the fundamental unanswered questions related to understanding cytoneme biology.

Extracellular Vesicles: Decoding a New Language for Cellular Communication in Early Embryonic Development

The blastocyst inner cell mass (ICM) that gives rise to a whole embryo in vivo can be derived and cultured in vitro as embryonic stem cells (ESCs), which retain full developmental potential. ICM cells receive, from diverse sources, complex molecular and spatiotemporal signals that orchestrate the finely-tuned processes associated with embryogenesis. Those instructions come, continuously, from themselves and from surrounding cells, such as those present in the trophectoderm and primitive endoderm (PrE). A key component of the ICM niche are the extracellular vesicles (EVs), produced by distinct cell types, that carry and transfer key molecules that regulate target cells and modulate cell renewal or cell fate. A growing number of studies have demonstrated the extracellular circulation of morphogens, a group of classical regulators of embryo development, are carried by EVs. miRNAs are also an important cargo of the EVs that have been implicated in tissue morphogenesis and have gained special attention due to their ability to regulate protein expression through post-transcriptional modulation, thereby influencing cell phenotype. This review explores the emerging evidence supporting the role of EVs as an additional mode of intercellular communication in early embryonic and ESCs differentiation.

Cellular mechanisms responsible for cell-to-cell spreading of prions

Prions are infectious agents that cause fatal neurodegenerative diseases. Current evidence indicates that they are essentially composed of an abnormally folded protein (PrPSc). These abnormal aggregated PrPSc species multiply in infected cells by recruiting and converting the host PrPC protein into new PrPSc. How prions move from cell to cell and progressively spread across the infected tissue is of crucial importance and may provide experimental opportunity to delay the progression of the disease. In infected cells, different mechanisms have been identified, including release of infectious extracellular vesicles and intercellular transfer of PrPSc-containing organelles through tunneling nanotubes. These findings should allow manipulation of the intracellular trafficking events targeting PrPSc in these particular subcellular compartments to experimentally address the relative contribution of these mechanisms to in vivo prion pathogenesis. In addition, such information may prompt further experimental strategies to decipher the causal roles of protein misfolding and aggregation in other human neurodegenerative diseases.

The Origin of Animal Multicellularity and Cell Differentiation

Over 600 million years ago, animals evolved from a unicellular or colonial organism whose cell(s) captured bacteria with a collar complex, a flagellum surrounded by a microvillar collar. Using principles from evolutionary cell biology, we reason that the transition to multicellularity required modification of pre-existing mechanisms for extracellular matrix synthesis and cytokinesis. We discuss two hypotheses for the origin of animal cell types: division of labor from ancient plurifunctional cells and conversion of temporally alternating phenotypes into spatially juxtaposed cell types. Mechanistic studies in diverse animals and their relatives promise to deepen our understanding of animal origins and cell biology.

Role of connexin 43 in different forms of intercellular communication - gap junctions, extracellular vesicles and tunnelling nanotubes

Communication is important to ensure the correct and efficient flow of information, which is required to sustain active social networks. A fine-tuned communication between cells is vital to maintain the homeostasis and function of multicellular or unicellular organisms in a community environment. Although there are different levels of complexity, intercellular communication, in prokaryotes to mammalians, can occur through secreted molecules (either soluble or encapsulated in vesicles), tubular structures connecting close cells or intercellular channels that link the cytoplasm of adjacent cells. In mammals, these different types of communication serve different purposes, may involve distinct factors and are mediated by extracellular vesicles, tunnelling nanotubes or gap junctions. Recent studies have shown that connexin 43 (Cx43, also known as GJA1), a transmembrane protein initially described as a gap junction protein, participates in all these forms of communication; this emphasizes the concept of adopting strategies to maximize the potential of available resources by reutilizing the same factor in different scenarios. In this Review, we provide an overview of the most recent advances regarding the role of Cx43 in intercellular communication mediated by extracellular vesicles, tunnelling nanotubes and gap junctions.

Signaling filopodia in vertebrate embryonic development

Next to classical diffusion-based models, filopodia-like cellular protrusions have been proposed to mediate long range signaling events and morphogen gradient formation during communication between distant cells. An increasing wealth of data indicates that in spite of variable characteristics of signaling filopodia in different biological contexts, they represent a paradigm of intercellular crosstalk which is presently being unraveled in a growing literature. Here, we summarize recent advances in investigating the morphology, cellular basis and function of signaling filopodia, with focus on their role during embryonic development in vertebrates.

The first two cell-fate decisions of preimplantation mouse embryo development are not functionally independent

During mouse preimplantation embryo development, three distinct cell lineages are formed, represented by the differentiating trophectoderm (TE), primitive endoderm (PrE) and the pluripotent epiblast (EPI). Classically, lineage derivation has been presented as a two-step process whereby outer TE cells are first segregated from inner-cell mass (ICM), followed by ICM refinement into either the PrE or EPI. As ICM founders can be produced following the fourth or fifth cleavage divisions, their potential to equally contribute to EPI and PrE is contested. Thus, modelling the early sequestration of ICM founders from TE-differentiation after the fourth cleavage division, we examined ICM lineage contribution of varying sized cell clones unable to initiate TE-differentiation. Such TE-inhibited ICM cells do not equally contribute to EPI and PrE and are significantly biased to form EPI. This bias is not caused by enhanced expression of the EPI marker Nanog, nor correlated with reduced apical polarity but associated with reduced expression of PrE-related gene transcripts (Dab2 and Lrp2) and down-regulation of plasma membrane associated Fgfr2. Our results favour a unifying model were the three cell lineages are guided in an integrated, yet flexible, fate decision centred on relative exposure of founder cells to TE-differentiative cues.

Specialized filopodia: at the 'tip' of morphogen transport and vertebrate tissue patterning

For over a century, biologists have strived to unravel the mechanisms that establish how cells are informed of their position in the embryo and differentiate to give rise to complex organs and structures. However, the historical idea that one predominant mode of ligand transport, largely accounted for by free diffusion, can explain how all signaling molecules, known as morphogens, control tissue patterning has greatly hindered our ability to fully appreciate the complexities driving the delivery and reception of signaling molecules at a distance. In reality, a cell's shape, morphology, and location change continuously as development progresses. Thus, cellular context poses distinct challenges for morphogen transport in each unique cellular environment. Emerging studies reveal that some cells overcome such obstacles in an unexpected manner: via long, cellular projections, or specialized filopodia, that link distant cells and traffic signaling components. Here, we will review recent findings describing specialized filopodia and discuss the potential mechanisms and implications for filopodia-based long-range cell signaling and communication, particularly within the developing vertebrate embryo.

Structure-function relationships of ErbB RTKs in the plasma membrane of living cells

We review the states of the ErbB family of receptor tyrosine kinases (RTKs), primarily the EGF receptor (EGFR, ErbB1, HER1) and the orphan receptor ErbB2 as they exist in living mammalian cells, focusing on four main aspects: (1) aggregation state and distribution in the plasma membrane; (2) conformational features of the receptors situated in the plasma membrane, compared to the crystallographic structures of the isolated extracellular domains; (3) coupling of receptor disposition on filopodia with the transduction of signaling ligand gradients; and (4) ligand-independent receptor activation by application of a magnetic field.