Introdução: Aneurismas e dissecções da aorta constituem as principais doenças da aorta, as quais podem ser submetidas a princípios e técnicas de tratamento cirúrgico em comum. A conduta clínica e cirúrgica continua sendo um desafio nos procedimentos eletivos, bem como em casos de emergência. Informações sobre variáveis prognósticas associadas independentemente com óbito hospitalar e no longo prazo, são escassas, havendo necessidade da identificação destes fatores para a avaliação apropriada sobre o risco cirúrgico desta população. Objetivos: Primário: identificar variáveis prognósticas associadas independentes ao óbito hospitalar em pacientes submetidos a procedimento cirúrgico para correção de doenças da aorta. Secundários: identificar variáveis prognósticas associadas independentes ao óbito tardio e ao desfecho clínico composto (óbito, sangramento, disfunção ventricular e complicações neurológicas). Casuística e Métodos: Delineamento transversal com componente longitudinal, com coleta de dados retrospectiva e prospectiva. Pacientes consecutivos, portadores de aneurisma de aorta ascendente ou dissecção crônica de aorta tipo A de Stanford, foram incluídos por meio de revisão de prontuários. Foram incluídos 257 pacientes, cujos critérios de inclusão envolviam aqueles que foram operados por dissecção crônica de aorta tipo A de Stanford e aneurisma de aorta ascendente. Foram excluídos pacientes com dissecção aguda de aorta, de qualquer tipo, e pacientes que tiveram aneurisma de aorta em outro segmento da aorta que não fosse a aorta ascendente. Os desfechos clínicos avaliados foram óbito, sangramento clinicamente relevante, complicações neurológicas e disfunção ventricular, fase hospitalar e óbito no longo prazo. As variáveis prognósticas avaliadas incluíram: demografia, fatores pré-operatórios, fatores intra-operatórios e complicações pós-operatórias. O seguimento médio foi de 970 dias. O tamanho de amostra foi definido por conveniência aliado a publicações prévias sobre o tópico. Análise univariada foi realizada para selecionar variáveis para serem inseridas no modelo multivariado para identificação das variáveis prognósticas independentemente associados aos desfechos clinicamente relevantes. Resultados: As seguintes variáveis prognósticas apresentaram associação independente como o risco aumentado de óbito na fase hospitalar (RC; IC95%; P valor): etnia negra (6.8; 1.54-30.2; 0,04), doença cerebrovascular (10.5; 1.12-98.7; 0,04), hemopericárdio (35.1; 3.73-330.2; 0,002), cirurgia de Cabrol (9.9; 1.47-66.36; 0,019), cirurgia de revascularização miocárdica (4.4; 1.31-15.06; 0,017), revisão de hemostasia (5.72 ;1.29-25.29; 0,021) e circulação extra-corpórea [min] (1.016; 1.007-1.026; 0,001). A presença de dor torácia associou-se independentemente com o risco reduzido de óbito hospitalar (0.27; 0.08-0.94; 0,04). As seguintes variáveis apresentaram associação independente com o risco aumentado do desfecho clínico composto na fase hospitalar: uso de antifibrinolítico (3.2; 1.65-6.27; 0,0006), complicação renal (7.4; 1.52-36.0; 0,013), complicação pulmonar (3.7; 1.5-8.8; 0,004), EuroScore (1.23; 1.08-1,41; 0,003) e tempo de CEC [min] (1.01; 1.00-1.02; 0,027). As seguintes variáveis apresentaram associação independente com o risco aumentado de óbito no longo prazo: doença arterial obstrutiva periférica (7.5; 1.47-37.85; 0,015), acidente vascular cerebral prévio (7.0; 1.46-33.90; 0,015), uso de estatina na alta hospitalar (4.9; 1.17-21.24; 0,029) e sangramento aumentado nas primeiras 24 horas (1.0017; 1.0003-1.0032; 0,021). Conclusão: Etnia negra, doença cerebrovascular, hemopericárcio, cirurgia de Cabrol, revascularização miocárdica cirúrgica associada, revisão de hemostasia e tempo de CEC associaram-se independentemente com risco aumentado de óbito hospitalar. A presença de dor torácica associou-se independentemente com o risco reduzido de óbito hospitalar. Doença arterial obstrutiva periférica prévia, acidente vascular cerebral prévio, uso de estatina na alta hospitalar e sangramento aumentado nas primeiras 24 horas associaram-se independentemente com risco aumentado de óbito no prazo longo. Uso de antifibrinolítico, complicação renal, complicação pulmonar, EuroScore e tempo de CEC associaram-se independentemente com o risco aumentado de desfecho clínico composto hospitalar (óbito, sangramento, disfunção ventricular e complicações neurológicas). / Introduction: Both aortic aneurisms and dissections constitute the main aortic diseases, sharing common principles and surgical procedure approaches. Medical and surgical management are seen as a medical challenge concerning elective procedures as well as in emergency cases. Data on prognostic variables independently associated with both hospital and long term death are scarce, leading to a need for appropriate identification of those factors for proper surgical risk evaluation of this population. Objectives: Primary: to identify prognostic variables independently associated with hospital death in patients who underwent surgical procedures for aortic disease correction. Secondary: to identify prognostic variables independently associated with long term death and with composite clinical endpoint (death, bleeding, ventricular dysfunction and neurological complications). Methods: Cross-sectional design plus a longitudinal component, with a retrospective and prospective data collection. Consecutive patients, diagnosed with ascendant aortic aneurism or type A of Stanford aortic chronic dissection were included by means of hospital chart revision and data extraction. A total of 257 patients were recruited and eligibility criteria included those who underwent surgical procedures due to ascendant aortic aneurism or type A of Stanford aortic chronic dissection. Patients with acute aortic dissection and with aortic aneurism in a different segment location other than ascendant aorta were excluded. Clinical endpoints evaluated were death, clinically relevant bleeding, ventricular dysfunction and neurological complications, during the hospital phase and long-term death. Prognostic variables evaluated included: demography, pre-operative factors, intra-operative factors and post-operative complications. Mean follow up was of 970 days. Sample size estimation was defined by a convenience sample along with previous publications. Univariate analysis was conducted to select key variables to be inserted in the multivariate model and to identify the prognostic variables independently associated with clinically relevant endpoints. Results: The following prognostic variables have been identified as independently associated with increased risk of hospital death (OR; 95%IC; P value): black ethnicity (6.8; 1.54-30.2; 0,04), cerebrovascular disease (10.5; 1.12-98.7; 0,04), hemopericardium (35.1; 3.73-330.2; 0,002), Cabrol operation (9.9; 1.47-66.36; 0,019), associated coronary artery bypass graft (4.4; 1.31-15.06; 0,017), reoperation for bleeding (5.72; 1.29-25.29; 0,021) and cardiopulmonary bypass time (CPB) [min] (1.016; 1.007-1.026; 0,001). Presence of chest pain was independently associated with reduced risk of hospital death (0.27; 0.08-0.94; 0,04). The following variables were independently associated with increased risk of composite clinical endpoint during hospital phase: antifibrinolitic use (3.2; 1.65-6.27; 0,0006), renal failure (7.4; 1.52-36.0; 0,013), respiratory failure (3.7; 1.5-8.8; 0,004), EuroScore (1.23; 1.08-1,41; 0,003) and cardiopulmonary bypass time (CPB) [min] (1.01; 1.00-1.02; 0,027). The following variables were independently associated with increased risk of long term death: peripheral obstructive arterial disease (7.5;1.47-37.85;0,015), previous stroke (7.0;1.46-33.90;0,015), at discharge statin use (4.9;1.17-21.24;0,029) and first 24-hour increased bleeding (1.0017;1.0003-1.0032;0,021). Conclusion: Black ethnicity, cerebrovascular disease, hemopericadium, Cabrol operation, associated coronary artery bypass graft, reoperation for bleeding, and cardiopulmonary bypass time were associated with increased risk of hospital death. Presence of chest pain was associated with reduced risk of hospital death. Peripheral obstructive arterial disease, previous stroke, at discharge statin use and first 24-hour increased bleeding were associated with increase risk of long-term death. Use of antifibrinolitic, renal failure, respiratory failure, EuroScore and cardiopulmonary bypass time were associated with increased risk of hospital composite clinical endpoint (death, bleeding, ventricular dysfunction and neurological complications).

INTRODUÇÃO: O diâmetro transverso máximo (DTM) de um aneurisma da aorta abdominal (AAA), a medida mais comumente utilizada para determinar quando uma intervenção cirúrgica está indicada nos portadores desta afecção, tem limitações e não reflete o exato risco de rotura. Parâmetros biomecânicos e morfológicos detectados pelo ultrassom (US) com propriedades tridimensionais (3D), podem ajudar a melhor entender o comportamento dos AAA e a determinar o melhor momento para o tratamento cirúrgico deste grupo de pacientes. OBJETIVOS: Verificar a capacidade e a viabilidade do ultrassom bidimensional (US-2D) acoplado com algoritmos de speckletracking em avaliar as propriedades biomecânicas da aorta em pacientes com e sem AAA. Mensurar o volume parcial dos AAA através do ultrassom tridimensional (US- 3D) e compará-lo com o volume estimado pela tomografia computadorizada (TC). MÉTODOS: Este foi um estudo prospectivo. Trinta e um pacientes com aortas normais (grupo 1), 46 pacientes com AAA de diâmetro máximo entre 3,0 - 5,5 cm (grupo 2) e 31 pacientes com diâmetro dos AAA >- 5,5 cm (grupo 3) foram submetidos ao exame de US 2D/3D da aorta infrarrenal, e as imagens foram pós-processadas antes de serem analisadas. No diâmetro máximo, o strain (deformação) circunferencial global e a rotação global máxima acessados pelo algoritmo de speckle-tracking 2D foram comparados entre os três grupos. O strain regional da parede posterior foi também comparado com o da parede anterior em todos os grupos. Os dados de volumetria obtidos usando o US-3D de 40 pacientes foram comparados com os dados de volumetria obtidos por uma TC contemporânea. RESULTADOS: A mediana do strain circunferencial global foi 2,0% (interquartile range (IR): 1,0 - 3,0), 1,0% (IR: 1,0 - 2,0) e 1,0% (IR: 1,0 - 1,75) nos grupos 1, 2 e 3 respectivamente (p < 0,001). A mediana da rotação global máxima diminuiu progressivamente dos grupos 1 ao 3 (1,38º (IR: 0,77 - 2,13), 0,80º (IR: 0,57 - 1,0) e 0,50º (IR: 0,31 - 0,75) (p < 0,001)). Na análise regiãoespecífica, o pico de strain na parede posterior foi significativamente maior que na parede anterior apenas no grupo 3 (p = 0,003). Os volumes dos AAA estimados pelo US-3D tiveram boa correlação com a TC (R-square = 0,76). CONCLUSÕES: O US é capaz de detectar parâmetros biomecânicos distintos entre aortas normais, aneurismas pequenos e aneurismas grandes. A propriedade 3D do US é capaz de determinar o volume dos AAA e apresenta boa correlação com o método padrão ouro (TC). Estudos prospectivos e com seguimento longo são necessários para aprofundar a compreensão não invasiva do comportamento biomecânico e morfológico dos AAA e correlacionar esses parâmetros com o risco de rotura / INTRODUCTION: The maximum transverse diameter of an abdominal aortic aneurysm (AAA), the most common measurement utilized to determine whether surgical intervention is indicated, has limitations and does not reflect the exact risk of rupture. Biomechanical and morphological parameters detected by ultrasound (US), including three-dimensional (3D) properties, can help to better understand the behavior of AAA and to determine the optimal approach to treatment in this group of patients. OBJECTIVES: To demonstrate the feasibility and the ability of the US with speckletracking algorithms to evaluate biomechanical parameters of the aorta in patients with and without abdominal aortic aneurysm (AAA). To measure the partial aneurysm volume by 3D-US and to compare it with the aneurysm volume measured by computed tomography (CT). METHODS: It was a prospective study. Thirty-one patients with normal aortas (group 1), 46 patients with AAA measuring 3.0 - 5.5 cm (group 2) and 31 patients with AAA >- 5.5 cm (group 3) underwent a 2D/3D US examination of the infrarenal aorta, and the images were post-processed prior to being analyzed. In the maximum diameter, the global circumferential strain and the global maximum rotation assessed by 2D speckle-tracking algorithms were compared among the three groups. The regional strain on posterior wall was also compared to that in the anterior wall for all groups. The volumetry data obtained using 3D-US from forty AAA patients was compared with the volumetry data obtained by a contemporary CT. RESULTS: The median global circumferential strain was 2.0% (interquartile range (IR): 1.0 - 3.0), 1.0% (IR: 1.0 - 2.0) and 1.0% (IR: 1.0 - 1.75) in groups 1, 2 and 3, respectively (p < 0.001). The median global maximum rotation decreased progressively from group 1 to 3 (1.38º (IR: 0.77 - 2.13), 0.80º (IR: 0.57 - 1.0) and 0.50º (IR: 0.31 - 0.75) (p < 0.001)). In the region-specific analysis, the strain in the posterior wall was significantly higher than anterior wall only in group 3 (p = 0.003). AAA volume estimations by 3D-US correlated well with CT (R-square = 0.76). CONCLUSIONS: The US can detect distinct biomechanical parameters between normal aorta, small aneurysms and big aneurysms. The 3D property of US is able to determine AAA volume and correlates well with the gold standard technique (CT). Prospective studies and with long-term follow-up are necessary in order to deepen the non invasive understanding of AAA biomechanical and morphological behavior and to correlate those parameters with rupture risk

Background: Arterial and venous thromboses associated with the coronavirus disease 2019 (COVID-19) have been well described. These events are caused by a hypercoagulable state due to endotheliopathy and infection-driven coagulopathy. There has been an ever-increasing number of documented cases of aortic thrombosis (AoT) in COVID-19 patients. We conducted a systematic review of current scientific literature to identify and consolidate evidence of AoT in COVID-19 patients. Methods: A systematic review of literature was conducted between March 15, 2020, and May 1, 2021, on PubMed and Cochrane databases. Additionally, a case from our facility was included. Results: A total of 38 studies (12 case series and 26 case reports) and a case from our facility describing AoT in 56 COVID-19 patients were included. Patients were aged 64.8 ± 10.5 years, were predominantly male (75%), and had several comorbidities. AoT was symptomatic in 82,14% of patients; however, when D dimers were reported, they were significantly elevated even in otherwise asymptomatic patients. Most patients had no previous history of aortic disease. Thrombosis was described in all parts of the aorta, with several cases reporting multiple locations. The median reported time until development of AoT was 10 days. Peripheral thrombosis occurred in 73.21% of cases, most commonly causing lower limb ischemia. Mortality rate was 30.4%. Conclusions: AoT can occur with no clinical symptoms or as a primary symptom in otherwise asymptomatic COVID-19 patients. D dimers are a highly sensitive diagnostic tool. Diagnosis of this condition prior to development of complications could be instrumental in saving many lives.

Analysis of phase-contrast MR images yields cardiac flow information which can be manipulated to produce accurate segmentations of the aorta. New phase contrast segmentation algorithms are proposed that use mean-based calculations and least mean squared curve fitting techniques. A GPU is used to accelerate these algorithms and it is shown that it is possible to achieve up to a 2760x speedup relative to the CPU computation times. Level sets are applied to a magnitude image, where initial conditions are given by the previous segmentation algorithms. A qualitative comparison of results shows that the algorithm parallelized on the GPU appears to produce the most accurate segmentation. After segmentation, particle trace simulations are run to visualize flow patterns in the aorta. A procedure for the definition of analysis planes is proposed from which virtual particles can be emitted/collected within the vessel, which is useful for future quantification of various flow parameters. / October 2014

Due to the complex anatomy of the heart, heart valves and aorta, blood flow in the aorta is known to be complex and can exhibit a swirling, or helical, flow pattern. The purpose of this thesis is to implement methods to quantify and visualize both the speed of helicity, referred to as the helicity density, and the direction of helicity, which is measured by the localized normalized helicity. Furthermore, the relationship between helicity and geometrical aorta parameters were studied in young and old healthy volunteers. Helicity and geometrical parameters were quantified for 22 healthy volunteers (12 old, 10 young) that were examined using 4D Flow MRI. The relation between helicity and the geometry of the aorta was explored, and the results showed that the tortuosity and the diameter of the aorta are related to the helicity, but the jet angle and flow displacement do not appear to play an important role. This suggests that in healthy volunteers the helical flow is primarily affected by the geometry of the aorta, although further trials should be performed to fully characterize the effects of aortic geometry. The results also show that the helicity changes with age between the two age groups and some of the geometrical parameters also has a significant difference between the age groups.

Die retrospektive Arbeit untersucht die in der Zeit von Januar 1993 bis Oktober 2002 an der Uniklinik Würzburg implantierten 294 aortodistalen Gefäßprothesen. Dabei erfolgte sowohl eine Auswertung der perioperativen Komplikationen als auch der Langzeitergebnisse. Mit Hilfe von Chi-Quadrat Tests und Kaplan-Meier Berechnungen konnte ein direkter Einfluss unterschiedlicher Faktoren auf die Ergebnisse nachgewiesen werden. In der Diskussion wurden die Ergebnisse internationalen Publikationen gegenübergestellt und mit alternativen Therapiemethoden verglichen. Zusammenfassend lässt sich die aortobifemorale Y-Prothese bei Patienten mit zufriedenstellendem Allgemeinzustand und einer proximal manifestierten pAVK angesichts eines durch andere Verfahren nicht erreichten Risiko-Nutzen Verhältnisses nach wie vor als Goldstandard empfehlen. / This retrospective work examines all 294 aortodistal grafts which were implantated by the surgical section of the university hospital Würzburg from the time of january 1993 till october 2002. Perioperative complications were as well in the centre of interest as longterm results. Chi-Quadrat-Tests and Kaplan-Meier-Statistics showed direct influence of several factors on the results. In the discussion the results were opposed to international publications and compared to alternative treatments. In conclusion the aortobifemoral graft in patients with satisfying general conditions and a proximal manifested pAOD due to the, by other methods never achieved, risk-benefit-ratio can still be adviced as the goldstandard.

ETB-Rezeptoren nehmen innerhalb der endothelialen Regulationsprozesse eine zentrale Rolle ein. In der vorliegenden Arbeit wurde der Frage nachgegangen, welchen Einfluss eine Salzbelastung auf den Blutdruck und die vaskuläre Funktion von ETB-Rezeptor-Knockout-Mäusen hat. In diesem Zusammenhang wurden männliche ETB-Rezeptor-Knockout-Mäuse parallel mit Wildtyp-Kontroll-Mäusen 15 Tage lang mit Standard- bzw. salzreichem Futter gehalten. Der systolische Blutdruck wurde ebenfalls dokumentiert. Nach 15 Tagen wurde den narkotisierten Tieren die Aorta descendens entnommen. An isolierten Aortenringen wurden in der Organkammer die Endothel-abhängige und -unabhängige vaskuläre Funktion untersucht. Die ETB-Rezeptor defizienten Mäuse bleiben – unter einer Haltung mit Standardfutter – normotensiv. Eine Hypertonie entwickeln die Tiere erst bei Verabreichung von salzreichem Futter. Die Endothel-abhängige Gefäßfunktion ist jedoch nicht nur bei den hypertensiven Tieren verändert, sondern bei allen ETB-Rezeptor defizienten Mäusen – unabhängig von Salzgehalt der Nahrung und Blutdruck. / The ETB receptor is involved in endothelial function. In the present study, we analysed whether salt alters endothelial function in rescued ETB receptor-deficient mice. Adult ETB-deficient mice were kept in parallel with wild-type control animals for 15 days on standard or salt-enriched chow, respectively. Systolic blood pressure was measured also and endothelium-dependent and endothelium-independent vascular function was assessed in isolated aortic rings. Systolic blood pressure increased on salt-enriched chow in ETB receptor-deficient mice, but neither in wild-type mice on high-salt diet nor in ETB receptor-deficient mice on standard chow.

The goal of this thesis was to investigate the interplay between sodium, glycosaminoglycans, vascular stiffness, and hypertension. In contrast to the traditional view of salt-dependent hypertension, recent studies have found that sodium accumulation can occur without commensurate fluid retention. Researchers hypothesise that this sodium is stored non-osmotically via association with negatively charged glycosaminoglycans (GAGs) in the extracellular matrix. The interaction of sodium and GAGs, the influence of sodium on GAG production, and the ability of GAGs to affect vascular stiffness are of key interest. This thesis first investigates the link between hypertension, vascular stiffness, and GAGs in ex vivo human aortae. Aortae from hypertensive donors were found to be stiffer than normotensive controls even after controlling for both pressure and age, a novel finding in humans. In these aortae, hypertension was associated with GAG remodelling, but not with changes in total GAG content. Next, an interventional rat study is presented to examine the effects of dietary salt on vascular stiffness and GAGs, and to distinguish between salt-dependent and blood pressure-dependent effects. In vivo vascular stiffness was found to be salt-dependent but pressure-independent, with ex vivo stiffness unaffected by salt. Ex vivo stiffness was also independent of aortic GAG content, similar to the human aortae described previously. GAG content in the skin was both salt-dependent and pressure-dependent. Finally, this thesis closes with an interventional study in humans. This study was designed to examine the effects of diuretic-induced salt loss on sodium storage, GAGs, and haemodynamics. An eight-day diuretic course corresponded to a ~10% reduction in skin sodium content, without associated water loss or cardiovascular changes. GAG mRNA expression was decreased in the skin, suggesting reduced GAG content. Pilot work from this study supports the use of 23Na MRI as a non-invasive measurement of skin sodium, but only for pre- vs post-treatment comparisons rather than absolute quantification. In conclusion, this thesis demonstrates that both salt and blood pressure influence GAG accumulation and distribution, but that GAGs do not directly affect vascular stiffness. However, GAGs do play a direct role in osmotically inactive sodium storage, which may modulate development of hypertension.

BACKGROUND: It is well established that thoracic aortic aneurysms (TAA) and abdominal aortic aneurysms (AAA) have different risk factors, clinical features, and genetic influences. Differences between and amongst subtypes of TAAs have received less attention. Despite observations of divergent clinical outcomes between ascending thoracic aortic aneurysms (ATAAs) and descending thoracic aortic aneurysms (DTAAs), etiologic factors determining the anatomic distribution of these aneurysms are not well understood. METHODS: From 3,247 patients registered in an institutional Thoracic Aortic Center Database from July 1992 through August 2013, we identified 921 patients with full aortic dimensional imaging by CT or MRI scan with TAA > 3.5 cm and without evidence of aortic dissection (AoD). Patients were analyzed in three groups: isolated ATAA (n=677), isolated DTAA (n=97), and combined ATAA and DTAA (n=146). RESULTS: Patients with a DTAA, alone or with coexistent ATAA, had significantly more hypertension (80.6% vs. 61.8%, p<.001) and a higher burden of atherosclerotic disease ( 86.7% vs. 7.5%, p<.001) ) and were more likely to be female (59.3% vs. 29.5%, P<.001). Conversely, patients with isolated ATAA were significantly younger (average age 59.5 vs. 71, p<.001), and contained almost every case of overt genetically-triggered TAA. Patients with isolated DTAA were demographically indistinguishable from patients with combined ATAA and DTAA. In follow up, patients with isolated DTAA, or with ATAA and DTAA, experienced significantly more aortic events (aortic dissection/rupture) and had higher mortality than patients with isolated ATAA. CONCLUSIONS: Based on patient characteristics and outcomes, subtypes of TAA emerge. DTAA with or without associated ATAA or AAA appears to be a disease more highly associated with atherosclerosis, hypertension, and advanced age. In contrast, isolated ATAA appears to be a clinically distinct entity with a higher burden of genetically triggered disease. These data have important implications for familial screening recommendations for TAA.

The production of fully functional haematopoietic stem cells (HSCs) for clinical transplantation is a highly sought after goal in the field of regenerative medicine. Given their capacity for extensive self-renewal and differentiation into any cell type, human pluripotent stem cells (hPSCs) provide a potentially limitless source of haematopoietic cells in vitro for clinical application. However, to date, fully functional HSCs have not been produced from hPSCs without the overexpression of transcription factors. In this study I first investigated the production of HSCs and haematopoietic progenitor cells (HPCs) in an established clinical-grade haematopoietic differentiation protocol. I demonstrated the efficient and reproducible production of HPCs but showed that the strategy did not produce fully functional HSCs that could repopulate the haematopoietic system of immune-deficient mice. Modification of the protocol by manipulation of the hedgehog signalling pathway and co-aggregation with OP9 stromal cells did not provide any significant enhancement of HPC production. To gain the required knowledge with which to improve our current protocol, I therefore switched my focus towards studying the development of HSCs in the early human embryo. It has been shown that HSCs first emerge from the ventral wall of the dorsal aorta in the aorta-gonad-mesonephros (AGM) region of the human embryo but the precise location and the mechanisms underpinning this process remain unknown. In this study, I established a culture system to map the spatio-temporal distribution of HSCs and to investigate the presence of HSC precursors. I showed that embryonic HSCs emerge predominantly around and above the vitelline artery entry point in the dorsal aorta and can be maintained in our explant culture system. I then performed RNA-sequencing of cells derived from AGM sub-regions, and this identified molecular signatures which could potentially underlie the ventral polarity of HSC emergence in the AGM. To elucidate the role of the stromal compartment in this unique haematopoietic niche, I derived stromal cell lines from the human AGM region and showed they were capable of supporting haematopoiesis in vitro. This work has provided some important insights into the mechanisms regulating HSC development in the human AGM region and identified interesting candidate molecules for future testing in differentiation protocols. This knowledge brings us a step closer to the successful in vitro production of HSCs for clinical use.