Principles of Heart Valve Engineering

By Arash Kheradvar

Principles of Heart Valve Engineering is the first comprehensive resource for heart valve engineering that covers a wide range of topics, including biology, epidemiology, imaging and cardiovascular medicine. It focuses on valves, therapies, and how to develop safer and more durable artificial valves. The book is suitable for an interdisciplinary audience, with contributions from bioengineers and cardiologists that includes coverage of valvular and potential future developments. This book provides an opportunity for bioengineers to study all topics relating to heart valve engineering in a single book as written by subject matter experts.

• Covers the depth and breadth of this interdisciplinary area of research
• Encompasses a wide range of topics, from basic science, to the translational applications of heart valve engineering
• Contains contributions from leading experts in the field that are heavily illustrated

• Language: English
• Publisher: Academic Press
• Release date: Aug 28, 2019
• ISBN: 9780128146620

Arash Kheradvar

Arash Kheradvar, M.D., Ph.D., FAHA is a Professor of Biomedical Engineering, Mechanical and Aerospace Engineering, and Medicine (Cardiology) at the University of California, Irvine. Dr. Kheradvar received M.D. from Tehran University of Medical Sciences in 2000 and Ph.D. in Bioengineering in 2006 from Caltech. His research interests are focused on heart valve engineering, cardiac fluid dynamics, and new cardiac imaging technologies. He is the author of two books and over 50 journal articles and the lead inventor of 45 issued and pending patents in cardiovascular area, mainly on heart valve technologies and imaging modalities. He is an elected Fellow to the American Heart Association by two councils of Cardiovascular Radiology and Intervention, and Cardiovascular Surgery and Anesthesia. Dr. Kheradvar founded KLAB, the Kheradvar research group, in 2007. The lab aims to study to the cardiovascular fundamental problems and devise translational solutions to ultimately help patients with cardiovascular disease by developing better diagnostic tools and more efficient therapeutic devices. KLAB is focused on research areas related to Heart Valve Engineering, Cardiovascular Imaging, and Cardiac Mechanics.

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Principles of Heart Valve Engineering

Edited by

Arash Kheradvar

University of California, Irvine, CA, United States

Table of Contents

Cover image

Title page

Copyright

Dedication

Contributors

Preface

Chapter 1. Clinical anatomy and embryology of heart valves

1.1. Atrioventricular valves

1.2. Semilunar valves

1.3. Epigenetic factors in heart valve formation

Chapter 2. Heart valves' mechanobiology

2.1. Introduction

2.2. Valvular interstitial cells

2.3. Cell signaling and microenvironment

2.4. Role of extracellular matrix in heart valve biomechanics

2.5. Extracellular matrix remodeling in heart valve disease

2.6. Mechanobiology considerations for tissue engineering atrioventricular and semilunar valves

2.7. Future directions

Chapter 3. Epidemiology of heart valve disease

3.1. Introduction

3.2. Epidemiology of heart valve disease in developed regions

3.3. Epidemiology of heart valve disease in developing regions

3.4. Epidemiology of congenital heart valve disease

Chapter 4. Surgical heart valves

4.1. Introduction: history of surgical heart valves

4.2. Mechanical valves

4.3. Bioprosthetic valves

4.4. Prosthetic heart valve selection and development

4.5. Unmet clinical needs and future areas of development

Chapter 5. Transcatheter heart valves

5.1. History of transcatheter heart valves

5.2. Transcatheter aortic valves

5.3. Transcatheter mitral valve repair and replacement

5.4. Pediatric transcatheter heart valves

Chapter 6. Tissue-engineered heart valves

6.1. Introduction

6.2. The living heart valve—taking inspiration from nature

6.3. Heart valve tissue engineering paradigms

6.4. The cellular players in heart valve tissue engineering

6.5. Scaffolds for heart valve tissue engineering

6.6. Bioreactors

6.7. Computational modeling

6.8. Minimally invasive delivery of tissue-engineered heart valves

6.9. Perspective on current challenges for heart valve tissue engineering

Chapter 7. Computer modeling and simulation of heart valve function and intervention

7.1. Introduction

7.2. Governing equations

7.3. Structural modeling

7.4. Fluid–structure interaction

7.5. Conclusions and future outlook

Chapter 8. In vitro experimental methods for assessment of prosthetic heart valves

8.1. Hydrodynamic evaluation

8.2. Particle image velocimetry

8.3. Accelerated wear testing

8.4. Structural assessment

8.5. Structural component fatigue assessment

8.6. Corrosion assessment

8.7. Summary

Chapter 9. Transvalvular flow

9.1. Fluid dynamics of transmitral flow

9.2. Fluid dynamics of the aortic valve

9.3. Fluid dynamics of the valves of the right heart

Chapter 10. Heart valve leaflet preparation

10.1. Alternative fixation chemistries

10.2. Anticalcification strategies

10.3. No fixation

10.4. Alpha-gal removal

10.5. Different types of tissues

10.6. Physical treatments

10.7. Testing the efficacy of a tissue and its chemical treatments

10.8. Stentless valves

10.9. Surgeon factors

10.10. Unmet needs and opportunities

Chapter 11. Heart valve calcification

11.1. Native valves

11.2. Bioprosthetic valves

11.3. Structure and pathology of aortic valves

Chapter 12. Immunological considerations for heart valve replacements

12.1. Introduction

12.2. Heart valve transplants

12.3. Mechanical heart valves

12.4. Tissue valves

12.5. Transcatheter valves

12.6. Conclusions and future directions

Chapter 13. Polymeric heart valves

13.1. Introduction

13.2. History of polymeric valves

13.3. Design considerations and challenges

13.4. Investigational valves

13.5. Summary and conclusions

Chapter 14. Regulatory considerations

14.1. The sins of the father

14.2. The need for documented procedures

14.3. Risk versus reward

14.4. Risk management

14.5. Objective performance criteria

14.6. Making sausages

14.7. Failure of preclinical models

14.8. A case study

14.9. Closing

Appendix. Bernoulli’s equation, significance, and limitations

Index

Copyright

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Notices

Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary.

Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility.

To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein.

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ISBN: 978-0-12-814661-3

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Dedication

I would like to dedicate this book to my wife Ladan for her constant love and support, to my children Aryana and Ario for being the reason I would never give up, and to my beloved parents for their never-ending devotion and care.

Contributors

Hamza Atcha

Department of Biomedical Engineering, University of California Irvine, Irvine, CA, United States

The Edwards Lifesciences Center for Advanced Cardiovascular Technology, University of California Irvine, CA, United States

Ali N. Azadani,     Department of Mechanical & Materials Engineering, Ritchie School of Engineering and Computer Science, University of Denver, Denver, CO, United States

Stefanie V. Biechler,     Director of Marketing Collagen Solutions PLC Minneapolis, MN, United States

Carlijn V.C. Bouten

Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, the Netherlands

Institute for Complex Molecular Systems (ICMS), Eindhoven University of Technology, Eindhoven, the Netherlands

Lakshmi Prasad Dasi,     Department of Biomedical Engineering, The Ohio State University, Columbus, OH, United States

Linda L. Demer,     Departments of Medicine, Physiology, & Bioengineering, University of California, Los Angeles, Los Angeles, CA, United States

Craig J. Goergen,     Weldon School of Biomedical Engineering, Purdue University, West Lafayette, IN, United States

Richard L. Goodwin,     Biomedical Sciences, University of South Carolina School of Medicine, Greenville, SC, United States

K. Jane Grande-Allen,     Department of Bioengineering, Rice University, Houston, TX, United States

Boyce E. Griffith,     Department of Mathematics, Carolina Center for Interdisciplinary Applied Mathematics, Computational Medicine Program, and McAllister Heart Institute, University of North Carolina, Chapel Hill, NC, United States

Elliott M. Groves,     Division of Cardiology, College of Medicine, University of Illinois at Chicago, Chicago, IL, United States

Megan Heitkemper,     Department of Biomedical Engineering, The Ohio State University, Columbus, OH, United States

Svenja Hinderer,     Natural and Medical Sciences Institute (NMI), University of Tübingen, Reutlingen, Germany

Geoffrey D. Huntley,     Department of Cardiovascular Medicine, Mayo Clinic, Rochester, MN, United States

Harkamaljot S. Kandail,     Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, the Netherlands

Arash Kheradvar

Department of Biomedical Engineering, University of California Irvine, Irvine, CA, United States

Department of Medicine, Division of Cardiology, University of California Irvine, CA, United States

The Edwards Lifesciences Center for Advanced Cardiovascular Technology, University of California Irvine, CA, United States

Wendy F. Liu

Department of Biomedical Engineering, University of California Irvine, Irvine, CA, United States

Department of Chemical Engineering and Materials Science, University of California Irvine, CA, United States

The Edwards Lifesciences Center for Advanced Cardiovascular Technology, University of California Irvine, CA, United States

Wenbin Mao,     Tissue Mechanics Laboratory, The Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, GA, United States

Petra Mela

Department of Biohybrid & Medical Textiles (BioTex), AME – Institute of Applied Medical Engineering, Helmholtz Institute, RWTH Aachen University, Aachen, Germany

Medical Materials and Implants, Department of Mechanical Engineering, Technical University of Münich, Münich, Germany

Madeline Monroe,     Department of Bioengineering, Rice University, Houston, TX, United States

Daisuke Morisawa

Department of Biomedical Engineering, University of California Irvine, Irvine, CA, United States

The Edwards Lifesciences Center for Advanced Cardiovascular Technology, University of California Irvine, CA, United States

Vuyisile T. Nkomo,     Department of Cardiovascular Medicine, Mayo Clinic, Rochester, MN, United States

Niema M. Pahlevan,     Department of Aerospace & Mechanical Engineering, University of Southern California, CA, United States

Evan H. Phillips,     Weldon School of Biomedical Engineering, Purdue University, West Lafayette, IN, United States

Mohammad Sarraf,     Division of Cardiovascular Disease, School of Medicine, University of Alabama at Birmingham, Birmingham, AL, United States

Anthal I.P.M. Smits

Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, the Netherlands

Institute for Complex Molecular Systems (ICMS), Eindhoven University of Technology, Eindhoven, the Netherlands

Wei Sun,     Tissue Mechanics Laboratory, The Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, GA, United States

Jeremy J. Thaden,     Department of Cardiovascular Medicine, Mayo Clinic, Rochester, MN, United States

Yin Tintut,     Departments of Medicine (Cardiology), Physiology & Orthopaedic Surgery, University of California, Los Angeles, CA, United States

Ivan Vesely

Class III Medical Device Consulting, Gaithersburg, MD, United States

CroiValve Limited, Dublin, Ireland

Amadeus Zhu,     Department of Bioengineering, Rice University, Houston, TX, United States

Preface

Heart valves are living tissue structures that ensure adequate blood flow passes from one heart chamber to the next without the possibility of backflow. Native heart valves are among the body's most enduring tissues, with the ability to grow during the pediatric years. However, these tissue structures cannot regenerate or repair themselves spontaneously. Although heart valve disease is etiologically diverse—it can be acquired or congenital—phenotypically, it results in either valve stenosis or regurgitation. However, since the valves cannot repair themselves, medical interventions are always required to remedy these diseases. No drug currently exists to cure heart valve disease, and all interventions are based on surgical or transcatheter repair or replacement of the diseased valve. These issues have inspired researchers to seek effective and long-lasting means of mitigating damaged heart valves.

The very first successful heart valve repair is reported to have been performed by Dr. Walton Lillehei in 1957, followed by the first successful artificial heart valve implantation, by Dr. Albert Starr, in 1960s. Since then, heart valve engineering has been behind all major advances in treating patients with heart valve disease. Successful translation of a technology for heart valve replacement or repair from a research lab to a patient's heart takes many steps, from bench testing and acute animal studies to chronic animal studies and major clinical trials. These steps require many years of hard work by a multidisciplinary team, not to mention enormous amounts of funding.

To engineer a heart valve, many technical issues must be considered. Issues such as creating tissue structures that are able to resist deterioration and having designs that avoid thrombus formation, to name just two among many examples, challenge us to develop heart valve technologies that last longer while working seamlessly. Therefore, experts from diverse backgrounds—such as, but not limited to, mechanobiology, cardiology, physiology, tissue culture, mathematical modeling, fluid dynamics, and polymer science—often form core teams to develop new heart valve–related technologies. A new heart valve technology should be tested according to regulatory authorities' safety and efficacy guidelines before it can be used in humans. These verification, validation, and preclinical and clinical feasibility studies depend on close collaborative efforts among a group of expert engineers, regulatory bodies, and physicians from academia and industry, a group whom this book aims to address.

A few years ago, I led the publication of a series of four review articles on emerging trends in heart valve engineering in the Annals of Biomedical Engineering. During that effort, I realized that the field lacked a comprehensive textbook addressing this ever-expanding area of research and development. Inspired by that realization, and with the intention of disseminating the science and knowledge of heart valve engineering, I asked experts in different areas of heart valve research and development to assist me in this crucial effort. The present work is the first of its kind to comprehensively bring together a variety of techniques and disciplines from the current state of the art in heart valve engineering within a single book that can be used by students, scholars, engineers, and physicians. An elite group of internationally known experts on different aspects of heart valves contributed to it. Completing all the book chapters entailed over 2   years of efforts working on topics ranging from mechanobiology, engineering, epidemiology, and imaging to heart valve–focused therapies, among others. We hope this volume will generate new conversations among educators and scholars and spur continual improvements in these technologies.

I am indebted to the many outstanding faculty members from a range of disciplines and to leading heart valve experts, who donated their time and effort to produce carefully crafted chapters on topics at the cutting edge of work that is critical to research and development in heart valve engineering. I remain grateful to the whole Elsevier team, especially to Joshua Means, Sheela B. Josy, and Mohanambal Natarajan, for their support, patience, and guidance during all the stages of book production. Finally, I would like to thank all my past mentors, present and past trainees, and collaborators, whose guidance, hard work, and friendship have inspired me to pursue research in heart valve engineering, a field that continues to astonish and reward me. I hope our efforts in this book will likewise be beneficial for educators and scholars around the world who are interested in heart valve research and development.

Arash Kheradvar,     University of California, Irvine, CA, United States

Chapter 1

Clinical anatomy and embryology of heart valves

Richard L. Goodwin ¹ , and Stefanie V. Biechler ²       ¹ Biomedical Sciences, University of South Carolina School of Medicine, Greenville, SC, United States      ² Director of Marketing Collagen Solutions PLC Minneapolis, MN, United States

Abstract

Unidirectional blood flow is essential and the primary function of heart valves. Malformation and dysfunction of these complex structures result in potentially fatal pathologies. In this chapter, the formation, anatomy, and histology of the four cardiac valves will be described. These four cardiac valves can be further classified as two atrioventricular (AV) valves and two semilunar valves; however, each valve is unique. The two classes of valves differ in how the valve leaflets are supported when they are undergoing mechanical loading. To prevent regurgitation, the AV valves use a tension apparatus, which is composed of fibrocartilage-containing chordae tendineae (heart strings) and extensions of ventricular myocardium known as papillary muscles. On the other hand, the semilunar or ventriculoarterial valve leaflets are self-supporting, each having three leaflets that collapse onto thickened edges as they snap shut. Despite the differences in the adult cardiac valves, they appear to develop in similar ways. Both the AV and semilunar valve primordia appear early in heart development as acellular swellings between the primitive myocardium and the endocardium. These swellings or cushions are filled with proteoglycans and glycosaminoglycans making them jelly-like in consistency. During development, genetic and mechanical factors shape and remodel the soft cardiac cushions into tough, complex, fibrous tissues that are capable of withstanding the increasing demand as adult physiology is achieved. Increasing evidence supports a fundamental role for mechanical forces in the formation and homeostasis of valve tissues. However, much remains unknown about the specific molecular mechanisms that transduce the various forces which the valves are subjected to during the cardiac cycle. Defining these mechanisms will be a key in the development of new replacement valve technologies and novel therapeutic approaches to treating malformations and dysfunctions of cardiac valves.

Keywords

AV cushion; Chordae tendineae; ECM; Morphology; Semilunar valves; VIC

1.1 Atrioventricular valves

1.1.1 Embryology

1.1.2 Morphology

1.1.3 Histology

1.2 Semilunar valves

1.2.1 Embryology

1.2.2 Morphology

1.2.3 Histology

1.3 Epigenetic factors in heart valve formation

References

1.1. Atrioventricular valves

1.1.1. Embryology

The heart is first formed as a simple tube from anterior lateral splanchnic mesoderm as the flat, trilaminar embryonic disc rolls into a cylinder. The growing prosencephalon and the closing gut tube endoderm bring the left and right lateral mesoderms together ventrally at the midline of the developing embryo [1]. At this stage or even a bit before, the primitive myocardium begins to spontaneously contract. The formation of the primitive heart tube is critical to further development of the embryo as it relies on effective hemodynamics to support the ontogenesis of other structures.

Though the cardiac valves play a central role in the maintenance of unidirectional blood flow for the entire cardiovascular system, other tissues have valves, including some veins and lymphatic vessels. It is important to note that a valve-like structure is formed, transiently, between the left and right atria known as the foramen ovale. This structure allows placentally derived oxygen- and nutrient-rich blood to pass from the right atrium to the left atrium, allowing it to be distributed systemically during fetal development. Following the first breath and perfusion of the pulmonary vascular, the blood pressure of the right side of the circulation drops below that of the systemic left side blood pressures, physiologically closing the foramen. Over time, the septum primum and the septum secundum fuse, leaving a thumbprint-shaped indentation on the atrial septum known as the fossa ovale. Failure of this foramen to close results in atrial septal defects of varying degree and severity.

The acelluar cushions are largely composed of the glycosaminoglycans hyaluronan and chondroitin sulfate, which yield a soft, jelly-like consistency, giving it the name, cardiac jelly (Fig. 1.1). Nonetheless these soft, pliable atrioventricular (AV) cushions do contribute to unidirectional blood flow in the early embryonic circulation. The myocardium of the AV junction produces the initial extracellular matrix (ECM) of the cushions. This provides the substrate that cells will use to migrate into the cushions and produce the tissues of the mature valves and supporting structures.

The majority of the cells populating the AV cushions are derived from endocardial cells of the inferior and superior AV cushions as well as significant contribution of epicardially derived cells that have undergone an epithelial-to-mesenchymal transformation (EMT) (Fig. 1.1C). During this process, cells detach from the simple epithelium that lines the interior and exterior of the heart and migrate into the matrix-filled cushions. The cells of this newly formed mesenchyme become VICs, which remodel and maintain the ECM into the complex, stratified valve leaflets [2]. The endocardial cells that cover the valves have been reported to retain their ability to undergo EMT throughout adult life [3]. Under pathological conditions, these endothelial cells transform and migrate into the mesenchyme of the valve leaflets and adversely contribute to valve disease. The roles that other cell types, such as macrophages and other immune cells, play in development and disease of valve tissues are beginning to gain increased attention by investigators, as they appear to be key regulators of homeostasis and pathology [4].

The inferior and superior AV cushions fuse, forming the septum intermedium, which physically separates the left (systemic) and right (pulmonary) sides of the circulatory system. As development continues, lateral AV cushions emerge on the left and right sides and fuse with the inferior and superior endocardial tissues, providing the cells that will go on to form the AV septum, AV valve leaflets, and supporting tissues. In lineage tracing studies, neural crest cells were detected in the AV septum and shown to have migrated from the top of the neural tube into the heart via pharyngeal arches 3, 4, and 6. The roles that specific cells play in the differentiation and their contributions to eventual adult cardiac structures are not clear despite intensive and ongoing efforts. It is critical that these studies be brought to their fruition, as defects in the AV valvuloseptal tissues are amongst the most lethal.

During normal development of the AV septum, the ostia of the atria anatomically align with the AV valves and the ventricular chambers. Subsequent fibroadipose development of the AV septum provides a foundation for the remodeling of the endocardial cushions into the valve leaflets. The AV septum and its fibrous cardiac skeleton also act as an electrical insulator that allows for the atrial, ventricular delay of the cardiac cycle. Housing the AV node of the cardiac conductance pathway, malformations of this region impact cardiac rhythm and function and are thus critically pathological.

Development of the valve leaflets and tension apparatus of AV valves is generally thought to be driven by a remodeling process in which cushion cells differentiate into ECM-producing VICs that create the stratified fibroelastic connective tissue of the valve leaflets and the fibrocartilage-like chordae tendineae. This remodeling occurs in humans during infancy and early childhood. The mechanisms that drive the differentiation of cells into VICs versus cells of the chordae tendineae are not clear [5]. Hemodynamically driven differentiation is an attractive, though, not well-tested mechanism. Malformations of these structures include prolapse, stenosis, and atresia.

The molecular mechanisms that create and maintain the tissues of the cardiac valves have a long history of investigation. Decades of research studies using a variety of model systems have delineated the molecular signaling pathways that are critical for the induction, differentiation, and maturation of cardiac valves [2,5]. These processes can be divided into four stages: endocardial cushion formation, endocardial transformation, growth and remodeling, and stratification (Fig. 1.2).

AV valve formation is initiated when the myocardium of the AV canal produces the cardiac jelly that fills the superior and inferior AV cushions. Along with the ECM proteins, these cells secrete morphogens that activate overlying endocardial cells to disconnect from neighboring endothelial cells and migrate into the ECM of the cushions. Myocardially derived BMP2 signals initiate transformation of the AV canal endocardial cells, while canonical Wnt and TGF-β signaling are critical for sustaining EMT [6]. Endocardially derived Notch and VEGF signaling are also required for EMT, and several other well-characterized signaling pathways that are summarized in Fig. 1.2.

In addition to the molecules above, transcription factors Twist1 and Tbx20 are critical for the proliferation and differentiation of newly formed mesenchymal cells. Interestingly, VEGF becomes a negative regulator of VIC proliferation at the post-EMT stage of valve development [6]. During this stage of valve development, the matricellular protein, periostin, becomes highly expressed in the developing cushions and is necessary for the differentiation of VICs into ECM-producing fibroblasts within valve cushions [2]. As its name denotes, periostin is also involved in bone development. In fact, valve development involves a number of molecules that have been implicated in the development of bone and cartilage. Another similarity between bone and cardiac valve development is the BMP-driven expression of Sox9 [6]. However, there is a tendon-like gene expression pattern in the differentiation of the chordae tendineae of AV valves, involving Fibroblast Growth Factor (FGF), scleraxis, and tenascin.

As the valve leaflets mature, they become more complex with specific combinations of ECM proteins deposited in different locations within the valve [2]. This results in the formation of three distinct layers within valves, which are discussed in detail below. Here, it is important to note that the bone-like expression pattern remains in the collagen-rich fibrosa layer, which is dependent on NFATc1, whereas a cartilage-like expression pattern has been found in the proteoglycan- and glycosaminoglycan-rich spongiosa layer. The third layer, which is on the flow facing side of the valve leaflet, has a smooth muscle-like ECM, which contains elastin and collagen [2]. However, the molecular regulation of this layer has yet to be clearly defined.

Figure 1.1 Overview of cardiac development.(A) The heart initiates as a tube composed of endothelial cells (ECs), cardiac jelly (CJ), and myocyte cells (MCs). The tube is initially linked to the foregut (FG) via the dorsal mesocardium (DM), but this connection later breaks away as looping occurs. (B) As the tube bends and twists, cushions filled with CJ form. Atrioventricular cushions (AVCs) form in the AV canal, and outflow cushions (OFCs) form at the heart outlet where they receive a cellular contribution from neural crest–derived cells (NCCs). The future right and left ventricles, RV and LV, and the future right and left atria, RA and LA, are defined. (C) (i) Signaling from the MCs to the ECs induce EMT. (ii) Activated ECs lose their cell–cell junctions and invade the CJ. (iii) The activated ECs begin to express mesenchymal cell (MesC) markers. (D) The future heart chambers are in their final location when the outflow tract (OFT), atria, and ventricles begin to septate. (E) Muscular protrusions grow from the heart wall to form the atrial septum (AS) and ventricular septum (VS). The AS protrusion has a cap, the dorsal mesenchymal protrusion (DMP), that connects with the AVCs. (F) After EMT, the AVCs elongate into leaflets that are lined with valvular endothelial cells (VECs) and valvular interstitial cells (VICs). (G) Blood flows through the developed heart in the following order: vena cava, RA, tricuspid valve (TV), RV, pulmonary valve (PV), pulmonary artery, lungs, pulmonary veins, LA, mitral valve (MV), LV, aortic valve (AoV), aorta, body (H) Atrioventricular (AV) valves are composed of three layers: the elastin-rich atrialis, the water-rich spongiosa, and the collagenous fibrosa. The leaflet tip is tethered to the heart wall via the chordae tendineae. (I) Semilunar (SL) valves have the same three layers, but the elastin-rich layer is referred to as the ventricularis. The leaflet cusps end in thick, fibrous tips known as the nodules of Arantius in the AoV or nodules of Morgagni in the PV. The trilaminar leaflets and associated support structures function to withstand flow-induced forces.

1.1.2. Morphology

As discussed above, the AV valves differ from the semilunar valves in that the AV valve leaflets have a tension apparatus that consists of the chordae tendineae and the papillary muscles. The chordae tendineae are string-like extensions off of the valve leaflets that connect the AV leaflets to the papillary muscles of the ventricles (Fig. 1.3). The papillary muscles are invested with conductive tissue that are closely connected to the branch bundles and thus are amongst the earliest regions to contract in the ventricles, tensing the leaflet and preparing the structure to withstand systole. Failure to do so results in regurgitation of blood back into the atrium, resulting in loss of cardiac output. Generally, the orifice of the left ostia is bigger than the right, though this can change as a result of malformation or pathology.

The mitral valve, or bicuspid valve, separates the left atrial and ventricular chambers and has two valve leaflets, the anterior (aortic) and posterior (mural). The chordae tendineae of these leaflets coalesce into two well-defined papillary muscles located near the apex of the left ventricular chamber. During systole, the two leaflets have one zone of apposition that seals off the AV ostia and prevents regurgitation back into the left atria. Clinically, this crescent-shaped zone is divided into the anterolateral commissure and the posteromedial commissure, which enables anatomical description of areas of regurgitation or prolapse [7].

The tricuspid valve separates the right atrial and ventricular chambers and has three valve leaflets: the anterior; posterior; and the mural. The chordae tendineae from these leaflets coalesce into three clusters of papillary muscles in the right ventricle. The papillary muscles of the right ventricle are less organized and more variable than those of the left papillary muscles [7]. The moderator band, an important cardiac conductance tissue, is incorporated within the septomarginal trabecula, which is a myocardial structure that connects the anterior papillary muscle of the right ventricle to the interventricular septum. Being trifoliate, there are three zones of apposition and three commissures in the tricuspid valve: the anteroseptal; the anteroposterior; and the posterior.

Figure 1.2 Signaling in cardiac development.Each phase of cardiac development is associated with biochemical signals that are still being fully elucidated. During cushion formation, myocyte cells (MCs) signal to the endothelial cells (ECs) and induce alignment and proliferation. At the same time, the MCs secrete hyaluronic acid (HA) and, in the outflow cushions (OFCs), versican to fill the cardiac jelly (CJ) space. The CJ maintains a gradient of growth factors (GFs), allowing different stages of development to be triggered at different times. Epithelial-to-mesenchymal transformation (EMT) begins with EC activation and MC secretion of fibronectin (FN). The activated ECs phenotypically change as they lose their cell–cell junctions and migrate into the CJ. Inside the CJ, these cells begin to express mesenchymal cell (MesC) markers. Flow and proteinases stop EMT and the cushions grow and remodel. The VICs in the CJ have regulated proliferation, and EC proliferation is thought to slow as extracellular matrix (ECM) is increasingly deposited in the CJ. MesCs differentiate into ECM-secreting cells characteristic of mature valves. A mature valve exhibits three distinct layers that are regulated with unique signaling mechanisms, and the atrioventricular (AV) valves have a tendon-like support apparatus that undergoes signaling similar to cartilage and tendons.

Figure 1.3 Clinical anatomy of left heart valves.The semilunar and atrioventricular valves have unique structures that provide support and anchor the valves to the wall. The three leaflets of the semilunar valves have commissures at the wall juncture (depicted for the aortic valve) and the atrioventricular valves are attached to the papillary muscle via a tension apparatus, chordae tendineae (depicted for the mitral valve). 

(Left) From Frank H. Netter, Atlas of Human Anatomy – 4th Edition, 2006; (Right) CNRI/Science Photo Library.

1.1.3. Histology

The tissues that make up valve leaflets of both AV and semilunar valves have a similar overall design. A common feature of all cardiac valve leaflets is that they are organized into three layers (Fig. 1.1, panels H and I). The first layer of cells under the endocardial epithelium on the flow side of the leaflet contains densely packed cells that are surrounded by an elastic connective tissue. This layer is named the atrialis in AV valves and the ventricularis in semilunar valves. Elastic fibers are radially oriented from the hinge of the leaflet to the coapting edge [7]. The composition and organization of the matrix allows for extension and recoil of this layer as the valve opens and closes. The middle layer of valve cells is called the spongiosa and contains sparsely distributed cells embedded in ground substance, which is largely composed of proteoglycans. This layer is thought to carry out a cushioning function for the valves. The layer on the back (nonflow) side of the valve leaflets is called the fibrosa. As its name implies, it is a dense connective tissue containing large bundles of insoluble, fibrous Type I collagen, giving it a comparatively stiff quality. These fibers are circumferentially oriented, providing tensile strength to the leaflet. Together, the three layers of the valve leaflets provide a balanced mix of stiffness, pliability, and recoil, giving it the mechanical properties necessary for healthy valves to be competent when closing and compliant when opening. Alterations in the composition and organization of these layers are associated with numerous valve pathologies including valve calcification and myxomatous valves.

Another difference between the mitral and tricuspid AV valves, in addition to the number of leaflets and the size of the annuli, is the thickness of the valve leaflets. The mitral valve leaflets are thicker than those of the tricuspid. However, this difference is not evident until after birth, indicating that the increasing hemodynamic load experienced on the systemic side of the circulation is driving this morphogenesis.

The annuli of the valves are composed of dense connective tissue and provide a firm foundation to anchor the hinges of the leaflets. Type I collagen is the main ECM protein of the valve annuli, forming the major components of the cardiac skeleton. With the exception of the pulmonary valve, cardiac valve annuli are embedded in the AV septum with the aorta being wedged between the tricuspid and mitral valves, making this a highly complex region of the heart.

1.2. Semilunar valves

1.2.1. Embryology

The cushions that go on to contribute to the semilunar valves appear just after the cushions of the AV canal. These conotruncal cushions form as oppositely opposed ridges that spiral down the truncus arteriosus, which is the single outflow vessel, or arterial pole, of the tubular heart. This single outflow tract is divided into the pulmonary and aortic arteries as the conotruncal cushions become populated with cells, grow, and fuse at midline, creating the septum intermedium, which physically separates left and right sides of the arterial pole of the heart and sets the stage for complete aorticopulmonary septation [2].

Septation of the truncus arteriosus occurs from the inside out, with the tunica intima of the two newly formed vessels forming first, followed by the generation of their tunica medias, and, finally, their adventitias. In this way, the single outflow vessel is divided into two completely formed great arteries. The completeness of this separation is particularly evident in the transverse pericardial sinus, which separates the infundibulum of the pulmonary trunk from the aorta. Failure of proper formation and progression of the aorticopulmonary septum can result in life-threatening lesions including persistent truncus arteriosus or subclinical lesions such a small ventricular septal defect. This is because the aorticopulmonary septum fuses with the muscular ventricular septum that arises between the left and right ventricular chambers, becoming the membranous component of the ventricular septum. Neural crest cells have been found to contribute to both the AV valves and the semilunar valves; however, their role in morphogenesis of the outflow tract is particularly critical. Failure of neural cells to populate and migrate with the aorticopulmonary septum results in persistent truncus arteriosus.

In combination with the remodeling of the conotruncal cushions of the embryonic outflow tract, the leaflets of the semilunar valves develop from another set of cushions in the outflow tract, the intercalated cushions, which form adjacent to the conotruncal cushions. Once again in a manner similar to the AV valves, cardiac jelly filled swellings appear between the myocardium and endocardium, become cellularized by endocardial EMT, and undergo ECM remodeling over time into the highly organized valve leaflets. Much remains unknown about mechanisms that regulate the number and positioning of semilunar valve leaflet anlagen that differentiate into the trifoliate adult semilunar valves. Defects in these structures result in bicuspid and stenotic semilunar valves.

1.2.2. Morphology

The two semilunar valves have similar structures with three pocket-like leaflets arranged such that they are competent without the tension apparatus that is found in the AV valves. The three leaflets of the semilunar valves have three commissures, which act as anchoring points to support the juncture to the wall at the base of the leaflets (Fig. 1.3). The semilunar leaflet geometry creates spaces behind the leaflets known as the sinuses of Valsalva. The U-shaped base of the semilunar valve leaflets creates triangular-shaped areas in the walls of the great arteries that are not occupied by either valve tissue or valve sinuses. Thickened nodes are present at the tip of each leaflet, known as the nodules of Arantius in the aortic valve and the nodules of Morgagni in the pulmonary valve. These nodules exhibit an enlarged spongiosa layer making them characteristically elastic structures that can act to support the extreme hemodynamic forces present at the point of valve closure or the valvular orifice (Fig. 1.11I). The pulmonary valve differs from the aorta in that it has a column of myocardium, the infundibulum, to support its root. However, the aortic root is embedded in the connective tissue of AV septum. The aortic valve leaflets are thicker than those of the pulmonary, but again, this difference appears to develop postpartum as a response to the increased load that is required for systemic circulation. Another difference between this class of cardiac valves is the presence of coronary artery ostia in two of the aortic valve sinuses.

The aortic valve is situated in the middle of the AV septum, wedged between the mitral and tricuspid annuli and wrapped by the pulmonary infundibulum. The left and right sinuses of the aorta have the openings of the left and right coronary arteries. Therefore, the sinus of the posterior leaflet is known as the noncoronary sinus. Interestingly, this leaflet is the only aortic leaflet that does not appear to have any contribution of neural crest cells; however, this may be due to the fact this leaflet is deeply embedded in between the mitral and tricuspid annuli and may be inaccessible to neural crest cells migrating down the aorticopulmonary septum during development.

As mentioned above, the aortic valve, unlike the pulmonary valve, is surrounded by fibrous tissues. A portion of the aortic valve is continuous with the fibrous aspects of the mitral valve, including its anterior leaflet. The left fibrous trigone of the aortic root is also continuous with the mitral valve. The right fibrous trigone is continuous with the membranous portion of the ventricular septum, which is derived from the aorticopulmonary septum. The aortic root is bulbous in appearance and contains the annulus and sinuses of the valve leaflets. At the distal attachments of the valve leaflets, the aorta becomes cylindrical and is called the sinotubular junction. This marks the end of the aortic valve and the beginning of the ascending portion of the aorta.

The pulmonary valve is positioned at a distinctly different angle than the other three cardiac valves. The elongated, funnel-shaped infundibulum of the right ventricular outlet wraps around the aortic root. This sleeve of myocardium acts to support the pulmonary valve root. The three leaflets of the pulmonary valve are the left, right, and anterior. Common malformations of this valve include atresia and stenosis, which can be associated with other cardiac lesions, as in the case of tetralogy of Fallot, and result in cyanosis.

1.2.3. Histology

As described above, the histological organization of the semilunar valve leaflets is similar to that of the AV valves. They share a common tissue architecture, having the three tissue layers of the fibrosa, spongiosa, and elastic layer, known as the ventricularis in the semilunar valves. The semilunar leaflets are significantly thinner than the AV leaflets. Another difference between the AV and semilunar leaflets is that the distal edges of the semilunar leaflets are thick and contain a bulbous structure in the middle of the free edge known as the nodule of Arantius. These modifications and their overall geometry allow for the semilunar valves to seal as they close during diastole, preventing regurgitation.

Recently a new anatomical structure has been described in the root of the aorta. This prelymphatic organ appears to be distinct from the adventitia and is thought to provide a shock absorber function. This structure contains a series of interconnected vessels that are produced by bundles of fibrous collagen and sporadically lined with CD34   +   cells that have not been well-characterized [8]. This initial report did not appear to investigate whether this structure is present in the pulmonary trunk.

1.3. Epigenetic factors in heart valve formation

As previously indicated, hemodynamics is thought to play an important and fundamental role in the morphogenesis of the cardiac valves. In particular, the shear stresses and pressures of the developing cardiovascular system appear to play a formative role in the remodeling of the EMC of the cardiac cushions as they morph into the fibrous tissues of the valves [9]. While substantial clinical and experimental literature support the no flow/no grow hypothesis, the specific molecular mechanisms that are used to transduce these mechanical signals into distinct cellular activities are not well-characterized. These studies are frequently confounded by the chicken/egg paradox. For instance, individuals with bicuspid aortic valves have a high risk of developing calcified leaflets that become incompetent. Is this pathological calcification caused by the abnormal geometry that results in an aberrant aortic flow field or by the same process that drove the altered anatomy? New 3D in vitro model systems in which mechanical forces can be carefully controlled are coming on line that will allow for the direct testing of mechanotransduction pathways in specific mechanical environments [10,11]. These studies will be important in designing new therapies for these deadly valve diseases.

Other investigations using surgically created hemodynamic abnormalities have been carried out on avian embryos and have revealed that at its earliest stages, valve development is regulated by blood flow by affecting EMT [12]. Not surprisingly, similar approaches have found that altered hemodynamics drives alterations in the expression and deposition of critical ECM proteins [13].

The mechanisms by which developing valve tissues sense and transduce mechanical signals have been aided by the discovery of primary cilia on VICs within the developing valve cushions [14]. Primary cilia have been implicated in valve development previously, but only in early endocardial cells [15]. Primary cilia have long been known as cellular structures that sense and mediate responses to mechanical forces. Thus, their discovery in cushion mesenchyme indicates that forces other than shear stress, such as deformation-inducing pressures, could be important regulators of valve development.

Gestational diabetes has recently been reported to be an epigenetic regulator of valve development. Fetal hyperglycemia results in the increase of reactive oxygen species, which has been reported to result in cardiac malformations. Specifically, malformations of the outflow tract were associated with hyperglycemia and decreased nitrous oxide signaling [16].

Much progress has been made in delineating the genetic pathways that are critical in the progression of valve development and disease. However, the roles that epigenetic factors play in these processes are in their infancy and require more intense study.

References

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Chapter 2

Heart valves' mechanobiology

Madeline Monroe, Amadeus Zhu, and K. Jane Grande-Allen     Department of Bioengineering, Rice University, Houston, TX, United States

Abstract

Heart valves are subjected to unrelenting mechanical forces as they perform their constant vital function of precisely opening and closing. Understanding how these mechanics fit in with the development of valve disease has become more closely researched—particularly via development of in vitro experimental setups that incorporate these mechanical cues. In addition, the mechanics of the inherent extracellular matrix environment has been implicated in differential development of valvular cells and has wide-reaching effects toward study of valvular disease progression and design of tissue-engineered valves. Here, we describe how internal and external mechanical features affect valves on a cellular level,