An unusual case of a giant fetal facial tumour and review of the literature.

We present the case of a pregnant 32-year-old woman who presented with a giant fetal facial tumour at 22 weeks. The mass, initially 4 × 3.5 × 3 cm in size, was largely cystic with a small solid component. It subsequently increased to 9 × 9 × 10 cm. Significant compression effects on the fetal orbit, temple and infratemporal fossa, with potential compression of the optic nerve, were noted on ultrasound and MRI. The cyst required drainage twice in the pregnancy: firstly to reduce the compression effects and secondly to facilitate caesarean delivery. Postnatally, the baby had significant compression and displacement of the craniofacial skeleton from the mass effect. Postnatal histology revealed a diagnosis of a teratoma. This case highlights the complexities and challenges surrounding the diagnosis and management of a giant fetal facial tumour.

Prenatal findings and associated survival rates in fetal ventriculomegaly: A prospective observational study.

OBJECTIVES: Fetal ventriculomegaly is associated with varying degrees of genetic and structural abnormalities. The objective was to present the experience of fetal ventriculomegaly in a large European center in relation to: 1. grade of ventriculomegaly; 2. additional chromosomal/structural abnormalities; and 3. perinatal survival rates. METHODS: This was a prospective observational study of patients referred with fetal ventriculomegaly from January 2011 to July 2020. Data were obtained from the hospital database and analyzed to determine the rate of isolated ventriculomegaly, associated structural abnormalities, chromosomal/genetic abnormalities, and survival rates. Data were stratified into three groups; mild (Vp = 10-12 mm), moderate (Vp = 13-15 mm) and severe (Vp > 15 mm) ventriculomegaly. RESULTS: There were 213 fetuses included for analysis. Of these 42.7% had mild ventriculomegaly, 44.6% severe and 12.7% had moderate ventriculomegaly. Initial ultrasound assessment reported isolated ventriculomegaly in 45.5% fetuses, with additional structural abnormalities in 54.5%. The rate of chromosomal/genetic abnormalities was high,16.4%. After all investigations, the true rate of isolated VM was 36.1%. The overall survival was 85.6%. Survival was higher for those with isolated VM across all groups (P < 0.05). CONCLUSION: Ventriculomegaly is a complex condition and patients should be counselled that even with apparently isolated VM, there remains the possibility of additional genetic and/or structural problems being diagnosed in up to 10% of fetuses.

Parsing a plethora of pollen: the role of pollen size and shape in the evolution of Boraginaceae.

Pollen, the microgametophyte of seed plants, has an important role in plant reproduction and, therefore, evolution. Pollen is variable in, for example, size, shape, aperture number; these features are particularly diverse in some plant taxa and can be diagnostic. In one family, Boraginaceae, the range of pollen diversity suggests the potential utility of this family as a model for integrative studies of pollen development, evolution and molecular biology. In the present study, a comprehensive survey of the diversity and evolution of pollen from 538 species belonging to 72 genera was made using data from the literature and additional scanning electron microscopy examination. Shifts in diversification rates and the evolution of various quantitative characters were detected, and the results revealed remarkable differences in size, shape and number of apertures. The pollen of one subfamily, Boraginoideae, is larger than that in Cynoglossoideae. The diversity of pollen shapes and aperture numbers in one tribe, Lithospermeae, is greater than that in the other tribes. Ancestral pollen for the family was resolved as small, prolate grains that bear three apertures and are iso-aperturate. Of all the tribes, the greatest number of changes in pollen size and aperture number were observed in Lithospermeae and Boragineae, and the number of apertures was found to be stable throughout all tribes of Cynoglossoideae. In addition, the present study showed that diversification of Boraginaceae cannot be assigned to a single factor, such as pollen size, and the increased rate of diversification for species-rich groups (e.g. Cynoglossum) is not correlated with pollen size or shape evolution. The palynological data and patterns of character evolution presented in the study provide better resolution of the roles of geographical and ecological factors in the diversity and evolution of pollen grains of Boraginaceae, and provide suggestions for future palynological research across the family.

The rate of decline in fetal hemoglobin following intrauterine blood transfusion in the management of red cell alloimmunization.

OBJECTIVE: Hemolytic disease of the fetus and newborn is characterized by fetal anemia, secondary to maternal alloantibody-mediated fetal erythrocyte destruction. Despite our reliance on intrauterine blood transfusion (IUT) to maintain severely affected pregnancies, it remains difficult to predict the fetal response to an infusion of donor blood. Our objective was to determine the daily rate of decline in fetal hemoglobin following one, two, and three transfusions. We also evaluated the relationship between the fetal hemoglobin level and the corresponding doppler measurement of the fetal middle cerebral artery peak systolic velocity (MCA-PSV). STUDY DESIGN: A prospective observational study of all singleton pregnancies treated with intrauterine transfusion for fetal anemia secondary to maternal alloimmunization at the National Maternity Hospital, a tertiary referral centre, was conducted over a 10-year period (2011-2020). Demographic and clinical data was obtained from the electronic patient records. Ethical approval was granted by the Ethics and Research Committee of the National Maternity Hospital. RESULTS: A total of 90 intrauterine blood transfusions were performed in 41 fetuses affected by maternal alloimmunization, of which 70% (n = 29), 34% (n = 14) and 15% (n = 6) required a 2nd, 3rd, and 4th transfusion, respectively. The mean rate of decline in fetal hemoglobin following the first transfusion was 0.4 g/dl/day (range, 0.12-0.64 g/dl/day). The mean rate of decline was lower after repeat transfusions at 0.27 g/dl/day (range, 0.16-0.45 g/dl/day). The sensitivity of MCA-PSV threshold of 1.5 Multiples of the Median (MoM) to detect moderate-severe anaemia declined with rank of IUT, from 82% after one previous transfusion, to 75% after two or more previous transfusions. No fetal mortality was seen in our series. CONCLUSION: Knowledge of the expected rate of decline in fetal hemoglobin following an IUT aids in the determination of appropriate timing of subsequent transfusions in a fetus affected by red cell alloimmunization. We observed a reducing rate of daily decline in hemoglobin in fetuses requiring successive transfusions. Our findings suggest a reduced accuracy of the MCA-PSV threshold of 1.5 MoM in determining the optimal timing of 2nd, 3rd, and 4th transfusions.

Severe fetal ventriculomegaly: Fetal morbidity and mortality, caesarean delivery rates and obstetrical challenges in a large prospective cohort.

INTRODUCTION: Severe fetal ventriculomegaly (VM) is defined as an enlargement of the atria of the lateral cerebral ventricles (Vp) of greater than 15 mm. While it is well established that it confers significant risk of morbidity and mortality to the neonate, there is limited information pertaining to the caesarean delivery rates and the obstetric management of these complex cases. The aim of this study was twofold: firstly, to determine survival rates in fetuses with severe VM, and secondly to determine the caesarean delivery rates in continuing pregnancies. We explore the obstetric challenges associated with these difficult cases. METHODS: This was a prospective observational study of patients with antenatal severe VM, attending the Department of Fetal Medicine, National Maternity Hospital, Dublin, Ireland, from 1st January 2011 to 31st July 2020. Data were obtained from the hospital database and those with severe VM (Vp > 15 mm) were identified. The rates of chromosomal abnormalities, the survival rates and the caesarean delivery (CD) rates for the overall group were then determined. The data were then further sub-divided into two groups: 1. Vp < 20 mm and 2. Vp > 20 mm, and the results compared. Statistical analysis was performed using the Chi-Square test. RESULTS: A total of N = 95 pregnancies with severe VM were included for analysis, of which additional structural abnormalities on ultrasound were apparent in 67/95 (70.5%) and 28/95 (29.5%) had isolated severe VM. Chromosomal abnormalities were diagnosed in 15/95 (15.8%) of cases, with (2/28) 7.1% in the isolated SVM group versus (13/67) 19.4% in the non-isolated SVM group. The overall survival rate (excluding TOP) was 53/74 (71.6%), with 20/23 (86.9%) in the isolated SVM group. The overall CD rate was 47/72 (65.3%), which was significantly higher than the CD for the hospital during the same time period of 25.4% (P < 0.01). The data were subdivided into Vp < 20 and Vp > 20 and those with a Vp > 20 had higher rates of additional intracranial findings on ultrasound (Vp < 20 13/41 (31.7%) versus Vp > 20 32/54 (59.3%) (P < 0.05)) and macrocrania (Vp < 20 14/41 (34.1%) versus Vp > 20 35/54 (64.8%) (P < 0.05)). No significant difference was observed in the overall survival or CD rates between the two groups. CONCLUSION: In conclusion this study reports significant fetal morbidity and mortality with severe VM with high CD rates observed in this cohort. Significant challenges exist in relation to the obstetric management and counseling of parents regarding an often uncertain neonatal prognosis. In continuing pregnancies with significant macrocrania delivery plans should be individualized to improve neonatal outcomes where possible and minimize harm to the mother.

Caesarean section rates in primigravid women categorised by age and BMI.

The rising caesarean section (CS) rate is a complex issue, particularly in an increasingly heterogenous nulliparous population. The study aim was to stratify the CS rate in nulliparous women by age and BMI to determine if any difference existed. This was a retrospective review of CS procedures of nulliparous women in two centres in Ireland (2014 through 2017). Data were obtained for 17,177 women from the hospital databases and CS procedures determined for each age and BMI category. Significant differences were observed when CS rates were stratified in this manner. The CS rates for women <20 years/BMI < 18.5 was 8.8 versus 57.6% for women 35 - 39 years/BMI 30 - 34 and 76 - 100% for all women >45 years (p<.005). The development of customised charts subdivided by age and BMI may be a useful counselling tool and assist in the comparison of rates between units.Impact statementWhat is already known on this subject? It is well known that along with rising CS rates globally, there have also been significant changes in maternal demographics-with increasing maternal age at first birth and increasing maternal BMI. It is well established that both of these factors affect the rate of CS in a population.What do the results of this study add? This study sought to stratify the CS rate in nulliparous women by age and BMI to determine if any difference existed. The results of the study showed an increasing CS rate for increasing age and BMI categories that was statistically significant.What are the implications of these findings for clinical practice and/or further research? Additional research using larger population data sets could allow the development of customised charts for nulliparous women subdivided by age and BMI which could act as a useful counselling tool in clinical practice, as well as assist in the comparison of CS rates between units.

Conservative Management of Pelvic Organ Prolapse: Indian Contribution.

Pelvic organ prolapse (POP) is a very common condition which can greatly impact a woman's quality of life. Treatment options are individualized and typically involve a combination of physiotherapy, pessary insertion and surgical treatments. It is well known that nulliparous prolapse in India constitutes 1.5-2% of genital prolapse, while the incidence is even higher (5-8%) for young women who have delivered one or two children, thus making it one of the highest rates in the world. This has necessitated the development of numerous conservative surgical treatment options for POP, which allows women to retain their sexual and reproductive function and therefore allows for subsequent pregnancies. With the controversy surrounding the use of mesh, a variety of surgical treatment options should be considered. Such alternative treatments include the use of surgical sling procedures, which have been used widely in Indian practice for the treatment of POP for over 60 years. This review outlines some of the well-established conservative treatment options for POP. It also highlights the unique contribution of Indian Obstetricians in the development of these conservative surgical treatment options, from prominent Indian Gynecologists including Dr VN Shirodkar, Dr BN Purandare, VN Purandare, RP Soonawala, Brigadier SD Khanna and Dr RM Nadkarni.

Examining the role and relevance of the critical analysis and comparison of cesarean section rates in a changing world.

Cesarean section (CS) is one of the most commonly performed surgical operations in the world and has resulted in improved maternal and neonatal morbidity and mortality rates internationally. However, concerns have been raised regarding the ever increasing CS rates to what has been described as 'epidemic' proportions. Global CS rates have increased from 6.7% in 1990 to 19.1% in 2014. However, there is a vast variation in the CS rates between countries with CS rates of 44.3% reported across Latin America & the Caribbean and CS rates as low as 4.1% in central and West Africa. There is much controversy regarding the optimal figure for CS in a population. The optimal CS rates for a population have been recommend in various studies, ranging from 10% to 19%, above which no reported improvement in maternal and neonatal mortality rates is observed. This review examines the evolution of the changing indications for CS and increasing CS rates in a world where family sizes are reducing and maternal age at first pregnancy is increasing. Efforts must be made to agree on an appropriate classification system whereby CS rates can be compared accurately between units and countries as a useful tool to audit and monitor our practice. Obstetricians should consider the indications for each CS performed, be conscious of the CS rate in our own countries and institutions and most importantly, be cognizant of how the CS rate impacts the maternal and perinatal morbidity and mortality rates and adjust our practice accordingly, to minimize harm.

Spontaneous Human Myometrial Contractility in the Third Trimester of Pregnancy in Relation to Past Mode of Delivery.

OBJECTIVE: It is well established that women with a previous vaginal delivery have higher success rates in relation to vaginal birth after cesarean than those without. The aim of this study was to examine the effect of past mode of delivery on contractile parameters of human myometrium in vitro. STUDY DESIGN: Myometrial strips were excised from 64 women at cesarean delivery (CD) and recordings of spontaneous contractile activity analyzed and compared across three clinical groups: (1) women with no previous delivery (Group 1); (2) women with CD only (Group 2); and (3) women with a history of vaginal delivery and CD (Group 3). RESULTS: Myometrial samples from women in Group 3, women who had a previous vaginal delivery, had a significantly greater maximum amplitude of contractions (p < 0.05), a greater force (mean contractile force) of contractions (p < 0.01), and a faster rate of rise (p < 0.01) and relaxation of contractions (p < 0.05) than those in Groups 1 and 2. CONCLUSION: Many of the functional parameters of human uterine contractions are altered, or enhanced, in the women who have had a previous vaginal delivery, when compared with those without. This may partly explain the clinical differences observed in labor.

Clinical update on COVID-19 in pregnancy: A review article.

The data pertaining to the COVID-19 pandemic has been rapidly evolving since the first confirmed case in December 2019. This review article presents a comprehensive analysis of the current data in relation to COVID-19 and its effect on pregnant women, including symptoms, disease severity and the risk of vertical transmission. We also review the recommended management of pregnant women with suspected or confirmed COVID-19 and the various pharmacological agents that are being investigated and may have a role in the treatment of this disease. At present, it does not appear that pregnant women are at increased risk of severe infection than the general population, although there are vulnerable groups within both the pregnant and nonpregnant populations, and clinicians should be cognizant of these high-risk groups and manage them accordingly. Approximately 85% of women will experience mild disease, 10% more severe disease and 5% critical disease. The most common reported symptoms are fever, cough, shortness of breath and diarrhea. Neither vaginal delivery nor cesarean section confers additional risks, and there is minimal risk of vertical transmission to the neonate from either mode of delivery. We acknowledge that the true effect of the virus on both maternal and fetal morbidity and mortality will only be evident over time. We also discuss the impact of social isolation can have on the mental health and well-being of both patients and colleagues, and as clinicians, we must be mindful of this and offer support as necessary.

Morphological Analysis of Size and Shape (MASS): An integrative software program for morphometric analyses of leaves.

PREMISE: Morphometric analysis is a common approach for comparing and categorizing botanical samples; however, completing a suite of analyses using existing tools may require a multi-stage, multi-program process. To facilitate streamlined analysis within a single program, Morphological Analysis of Size and Shape (MASS) for leaves was developed. Its utility is demonstrated using exemplar leaf samples from Acer saccharum, Malus domestica, and Lithospermum. METHODS: Exemplar samples were obtained from across a single tree (Acer saccharum), three trees in the same species (Malus domestica), and online, digitized herbarium specimens (Lithospermum). MASS was used to complete simple geometric measurements of samples, such as length and area, as well as geometric morphological analyses including elliptical Fourier and Procrustes analyses. Principal component analysis (PCA) of data was also completed within the same program. RESULTS: MASS is capable of making desired measurements and analyzing traditional morphometric data as well as landmark and outline data. DISCUSSION: Using MASS, differences were observed among leaves of the three studied taxa, but only in Malus domestica were differences statistically significant or correlated with other morphological features. In the future, MASS could be applied for analysis of other two-dimensional organs and structures. MASS is available for download at https://github.com/gillianlynnryan/MASS.

Effects of maternal parity on response of human myometrium to oxytocin and ergometrine in vitro.

OBJECTIVE: The aim of this study was to compare the effects of oxytocin and ergometrine on the intrinsic contractile parameters of human uterine smooth muscle at term between primiparous and multiparous women. STUDY DESIGN: Myometrial biopsies were obtained from women undergoing planned caesarean section at term. The biopsies were dissected into eight uniform strips and mounted in tissue baths for isometric recording. The strips were challenged with increasing concentrations of oxytocin and ergometrine. Parameters of contractile activity, including mean contractile force (MCF) and maximum amplitude of contractions (MAMP) were recorded and analysed. Results were compared between primiparous (Group 1) and multiparous (Group 2) women. RESULTS: Myometrial biopsies were obtained from n = 11 donors (88 tissue strips), of which n = 5 were Group 1 and n = 6 were Group 2. In relation to oxytocin, the MAMP value observed was significantly greater in Group 2 than in Group 1 (151 ±â€¯18mN vs 67 ±â€¯14mN, P < 0.01). Regarding ergometrine, the MCF response was greater in Group 2 samples (24 ±â€¯10 mN) than that in Group 1 (18 ±â€¯2mN) (P < 0.05). CONCLUSION: Our findings highlight that women in a first pregnancy have a decreased response to both oxytocin and ergometrine in an in vitro setting when compared with women in a subsequent pregnancy, and this may have clinical implications regarding the management of postpartum haemorrhage in this cohort.

Questionnaire survey on women's views after a first caesarean delivery in two tertiary centres in Ireland and their preference for involvement in a future randomised trial on mode of birth.

OBJECTIVE: To assess the views of women after a first caesarean section (CS) on their birth experience, preference for future mode of birth and willingness to participate in a randomised controlled trial on mode of birth in a future pregnancy. DESIGN: Questionnaire survey. SETTING: Two tertiary maternity centres Ireland, Galway University Hospital, Galwayand the National Maternity Hospital, Dublin. PARTICIPANTS: Women with one previous CS. METHODS: Eligible women consented to participate, and postal surveys were forwarded. Results were collected and analysed. Results were compared between women who had elective operations and women who had emergency operations. PRIMARY OUTCOME MEASURES: The satisfaction levels of women after a first caesarean, their preference for mode of birth in a future pregnancy and their willingness to participate in a randomised trial on mode of birth. RESULTS: There were 347 completed surveys of 633 women who consented to participate (54.8%), of whom 285 and 62 had emergency and elective caesarean deliveries, respectively. In general, satisfaction ratings with the delivery were greater than 90%, with similar levels of satisfaction with the care received from doctors and midwives. Women who an emergency procedure expressed lower satisfaction levels with the information about the caesarean and the debriefing received afterwards than women who had a planned operation (p<0.05). For future mode of birth, 39.5% expressed a preference for vaginal birth after caesarean (VBAC) in a subsequent pregnancy, and 80% said they would consider involvement in a randomised trial in a future pregnancy. CONCLUSION: Debriefing and counselling women after a CS is an important part of pregnancy care and can significantly impact on a woman's overall birth experience. A significant proportion of this cohort considered VBAC as a future birth option. These data indicate that a randomised trial on mode of birth after caesarean would be viewed positively by women in our population.

Ovulation induction: comparing success rates between anovulatory and ovulatory cycles using different treatment protocols.

The management of patients with sub-fertility, particularly unexplained sub-fertility, is a sensitive and complex matter. This was a prospective observational study conducted from October 2016 to March 2017 in Galway, Ireland, the aim of which was to identify the clinical pregnancy rates (CPR) in women undergoing ovulation induction (OI) with timed sexual intercourse (TSI) or intrauterine insemination (IUI) and to compare them across two groups: (1) Anovulatory women and (2) ovulatory women with unexplained subfertility. Patients undergoing OI were recruited consecutively and OI regimens were prescribed as per local clinical protocol. The main observation was a higher CPR in the anovulatory group (18%) compared with the ovulatory group (CPR = 10%) (p < 0.05). No difference was observed in the CPR between the TSI and IUI groups. There are many studies to support the use of OI in the treatment of women with anovulatory subfertility, though the use of OI in ovulatory women is a more controversial issue. The treatment options offered to these patients need to be individualized to each couple and should consider their length of infertility, age, and financial means. Due to the lower cost and the less invasive nature of OI-treatment we conclude that a short treatment course could be offered as an acceptable alternative prior to IVF.

Maternal parity and functional contractility of human myometrium in vitro in the third trimester of pregnancy.

OBJECTIVE: It is well established that the duration of the first and second stages of labor are shorter in parous women than in their nulliparous counterparts, a phenomenon not well understood. The aim was to examine the effect of maternal parity on contractile parameters of human myometrium. DESIGN: Myometrial strips were excised from n = 74 women at cesarean delivery and recordings of contractile activity analyzed and compared across three clinical groups: 1. No previous delivery (P0); 2. One previous delivery (P1); 3. Greater than one previous delivery (P>1). RESULTS: There was a trend towards greater mean contractile force in the P>1 group than the P1 and P0 groups (P = 0.412). Frequency of contractions was less in the P1 group than in the P0 and P>1 groups(P = 0.027). No differences were observed in relation to all other parameters. CONCLUSION: Excluding frequency, no intrinsic differences were observed in the functional parameters of human uterine contractions in relation to parity.

Vaginal birth after caesarean section: Current status and where to from here?

Vaginal birth after caesarean (VBAC) delivery remains a controversial topic, and one for which there is a lack of robust data to guide clinicians and parturients regarding their best option for mode of delivery in a subsequent pregnancy. In many developed countries the trend observed in recent years is that of progressively reduced VBAC rates, and hence increased use of elective repeat caesarean section (ERCS). This factor has contributed, more than any other, to the disproportionately high caesarean section (CS) rates in many countries. With current CS rates varying between 30 and 50% in the developed world, a previous CS is the cited primary indication in approximately 30%. To compound matters, there are huge variations in the reported VBAC rates between different countries, regions and even institutions. This review has focused on the recent trends in VBAC attempt, success and overall rates internationally, with inclusion of figures for a period of 25 years from a single Irish institution. An analysis of the reported factors that influence VBAC success, or failure, is presented. The complex task of estimating risk, both perinatal and maternal, for women who pursue VBAC or ERCS, is included in this review. Finally, the current evidence base for clinical practice pertaining to VBAC is outlined, with inclusion of commentary regarding the future for this difficult area of obstetric practice.

Reduced Graphene Oxide on Nickel Foam for Supercapacitor Electrodes.

The focus of this paper is the investigation of reduced graphene oxide (GO)/nickel foam (RGON) samples for use as supercapacitor electrodes. Nickel foam samples were soaked in a GO suspension and dried before being subjected to two different methods to remove oxygen. Atmospheric pressure annealed (APA) samples were treated with a varying number (10-18) of nitrogen plasma jet scans, where sample temperatures did not exceed 280 °C. Furnace annealed (FA) samples were processed in an atmosphere of hydrogen and argon, at temperatures ranging from 600 °C to 900 °C. Environmental Scanning Electron Microscope (ESEM) data indicated that the carbon to oxygen (C:O) ratio for APA samples was minimized at an intermediate number of plasma scans. Fourier Transform Infrared Spectroscopic (FTIR) and Raman spectroscopic data supported this finding. ESEM analysis from FA samples showed that with increasing temperatures of annealing, GO is transformed to reduced graphene oxide (RGO), with C:O ratios exceeding 35:1. X-ray Photoelectron Spectroscopy (XPS) and X-ray diffraction (XRD) data indicated the formation of RGO with an increasing annealing temperature until 800 °C, when oxygen reincorporation in the surface atomic layers becomes an issue. Supercapacitors, constructed using the FA samples, demonstrated performances that correlated with surface atomic layer optimization of the C:O ratio.

Cell protrusion and retraction driven by fluctuations in actin polymerization: A two-dimensional model.

Animal cells that spread onto a surface often rely on actin-rich lamellipodial extensions to execute protrusion. Many cell types recently adhered on a two-dimensional substrate exhibit protrusion and retraction of their lamellipodia, even though the cell is not translating. Travelling waves of protrusion have also been observed, similar to those observed in crawling cells. These regular patterns of protrusion and retraction allow quantitative analysis for comparison to mathematical models. The periodic fluctuations in leading edge position of XTC cells have been linked to excitable actin dynamics using a one-dimensional model of actin dynamics, as a function of arc-length along the cell. In this work we extend this earlier model of actin dynamics into two dimensions (along the arc-length and radial directions of the cell) and include a model membrane that protrudes and retracts in response to the changing number of free barbed ends of actin filaments near the membrane. We show that if the polymerization rate at the barbed ends changes in response to changes in their local concentration at the leading edge and/or the opposing force from the cell membrane, the model can reproduce the patterns of membrane protrusion and retraction seen in experiment. We investigate both Brownian ratchet and switch-like force-velocity relationships between the membrane load forces and actin polymerization rate. The switch-like polymerization dynamics recover the observed patterns of protrusion and retraction as well as the fluctuations in F-actin concentration profiles. The model generates predictions for the behavior of cells after local membrane tension perturbations.