Developmental Biology and Musculoskeletal Tissue Engineering

Principles and Applications

Martin J. Stoddart, April M. Craft, Girish Pattappa, Oliver F.W. Gardner

• Publisher: Academic Press
• Year: 2018
• Print ISBN: 9780128114674
• eBook ISBN: 9780128115381
• Topic: Medicine
• Subtopic: Medical Theory, Practice & Reference

Developmental Biology and Musculoskeletal Tissue Engineering: Principles and Applications focuses on the regeneration of orthopedic tissue, drawing upon expertise from developmental biologists specializing in orthopedic tissues and tissue engineers who have used and applied developmental biology approaches. Musculoskeletal tissues have an inherently poor repair capacity, and thus biologically-based treatments that can recapitulate the native tissue properties are desirable. Cell- and tissue-based therapies are gaining ground, but basic principles still need to be addressed to ensure successful development of clinical treatments. Written as a source of information for practitioners and those with a nascent interest, it provides background information and state-of-the-art solutions and technologies.Recent developments in orthopedic tissue engineering have sought to recapitulate developmental processes for tissue repair and regeneration, and such developmental-biology based approaches are also likely to be extremely amenable for use with more primitive stem cells.- Brings the fields of tissue engineering and developmental biology together to explore the potential for regenerative medicine-based research to contribute to enhanced clinical outcomes- Initial chapters provide an outline of the development of the musculoskeletal system in general, and later chapters focus on specific tissues- Addresses the effect of mechanical forces on the musculoskeletal system during development and the relevance of these processes to tissue engineering- Discusses the role of genes in the development of musculoskeletal tissues and their potential use in tissue engineering- Describes how developmental biology is being used to influence and guide tissue engineering approaches for cartilage, bone, disc, and tendon repair

Chapter 1

Developmental Biology of Musculoskeletal Tissues for Tissue Engineers

Noriaki Ono¹, and Henry M. Kronenberg^{2 1}University of Michigan School of Dentistry, Ann Arbor, MI, United States ²Massachusetts General Hospital and Harvard Medical School, Boston, MA, United States

Abstract

Understanding the fundamental process of musculoskeletal development is essential for tissue engineers to develop a more effective approach for regeneration. Learning how stem cells and signals are used to orchestrate the process of musculoskeletal development is particularly important. In this chapter, we will provide a brief overview on the skeletal development and the recent progress on stem cells and cell lineages governing this process. The emerging concept is that multiple and distinct stem cell types are enlisted to support bone growth, maintenance, and repair. We will need to identify more detailed mechanisms of their cell fate specification and differentiation so that tissue engineers can robustly take advantage of the diversity of these skeletal stem cells to achieve more functional regeneration.

Keywords

Bone marrow; Bone marrow stromal cell; Chondrocyte; Endochondral bone formation; Growth plate; Intramembranous bone formation; Osteoblast; Perichondrium; Skeletal development; Skeletal stem cell

Introduction

The musculoskeletal system is a highly complex unit in which there are five major elements—bones, muscles, tendons that connect the former two elements, cartilages, and menisci—that function together to achieve locomotion. Although these elements are equally important, bones have been historically regarded as a central component of the musculoskeletal system. Bones have strong and rigid structures owing to mineralized matrix, yet they grow explosively in early life and maintain their strength throughout life. Contrary to their inert appearance, bones are strikingly multifunctional. The primary functions of bones are to protect vital organs and act as levers whereby muscle contraction leads to body movement. In addition, bone cells support hematopoiesis in the adjacent marrow space and secrete hormones that regulate carbohydrate and mineral ion metabolism, as well as fertility and brain function.

Because of their primary functions, bones, muscles, and tendons are by far the most commonly injured tissues in the body. These tissues possess amazing capabilities to repair various degrees of damage incurred on them, ranging from microscopic to substantial damages that disrupt tissue continuity. Tissue engineering has provided great promise in musculoskeletal tissue regeneration. The current approaches are intended to enhance innate regenerative capabilities by supplementing appropriate cells, signals, and scaffolds. Despite some success in many settings, existing approaches have certain limitations in their applicability to musculoskeletal regeneration. Generally speaking, tissue engineers can rebuild the lost component only when there are sufficient preexisting structures. In other words, they still cannot build skeletal components from nothing. Therefore, the extent that tissue engineers can regenerate now is therefore still at an infancy stage. There has been great progress in developing prostheses, but they do not have the same biological functions as living bones and critically lack many important aspects such as growth and regeneration. As many young and old patients suffer from substantial loss of important musculoskeletal tissues such as digits, limbs, face, skull, or dental structures, more efficient ways for functional regeneration are highly desirable. More specifically, tissue engineers will need to develop a more comprehensive approach to recapitulate the process of development. Thus, it is important to learn how stem cells and signals are used to orchestrate the development of musculoskeletal tissues. In this chapter, we will review the fundamental process of musculoskeletal development that can be instrumental for tissue engineering.

Developmental Origin of Musculoskeletal Tissues

Developmentally, most musculoskeletal tissues are derived from the mesoderm, except those of the craniofacial complex that are derived from the neural crest, or otherwise known as ectomesenchyme. The mesoderm is formed between the ectoderm and the endoderm as a result of gastrulation. Of its subdomains, the paraxial and lateral plate mesoderm are particularly relevant to the formation of musculoskeletal tissues. The paraxial mesoderm first organizes into somites that give rise to the myotome, sclerotome, and dermatome. The myotome gives rise to skeletal muscles, and the sclerotome gives rise to bones and tendons of the vertebrae (reviewed in Refs. [1,2]).

Limb development undergoes more complex and dynamic processes requiring heterotopic interactions between the ectoderm and the underlying mesoderm. The limb bud, a structure formed in early development, is established by proliferation of mesenchymal cells that originate from the lateral plate mesoderm and the myotome. These limb mesenchymal cells stimulate formation of a signaling center in the ectoderm, termed the apical ectodermal ridge (AER). The AER expresses fibroblast growth factors (FGFs) to communicate with its underlying mesenchyme to pattern the proximal to distal axis by antagonizing retinoic acid signaling present in the proximal limb. They simultaneously stimulate formation of the zone of polarizing activities within the mesenchyme, which expresses sonic hedgehog that patterns the anterior–posterior axis (reviewed in Refs. [3–6]). These signaling centers orchestrate proper development of the limb by recruiting mesenchymal cells from the lateral plate mesoderm to form bones and tendons, also from the myotome to form skeletal muscles.

Development of the craniofacial structure is even more complex than limb development, requiring mesenchymal cells with two distinct origins of the neural crest and the mesoderm (reviewed in Refs. [7,8]). In the face, pharyngeal arches, a series of bulges located laterally, develop through complex interactions among all the primary germ layers and the neural crest (reviewed in Ref. [9]). Of these contributing cell types, neural crest cells give rise to the connective and skeletal element of each arch. Although most of facial skeletal structures are derived purely from the neural crest cells, mesodermal and neural crest cells are intricately combined in the cranial structures, particularly in the calvaria and cranial base, illustrating the complexity of craniofacial development.

Formation of Mesenchymal Condensations

Bones assume very different shapes in different parts of the human body, but they are formed through only two common mechanisms: intramembranous and endochondral bone formation. Intramembranous bone formation is a straightforward process in which undifferentiated mesenchymal cells directly differentiate into osteoblasts that lay down the mineralized matrix. Intramembranous bones (or dermal bones) evolve earlier in the early fish and comprise part of the skull in mammals regardless of developmental origins of mesenchymal cells in the neural crest or the mesoderm. By contrast, endochondral bone formation is a complex process in which initial cartilaginous templates are later replaced by bones.

In both mechanisms, a primordial structure called mesenchymal condensations frames the future domain of bones. In this process, mesenchymal cells in a specific domain of the embryonic tissue temporarily stop proliferating, then align together to form cell clusters that exclude blood vessels. How self-organization of mesenchymal cells is induced remains unknown. It has been suggested that signaling pathways induced by transforming growth factor β, bone morphogenetic proteins (BMPs), and FGFs regulate formation of mesenchymal condensations. In addition, intrinsic and extrinsic cellular changes are considered to modify the responsiveness of condensing cells to various signals, including reorganization of the cytoskeleton and intercellular adhesions, extracellular matrix milieu, and hypoxic conditions [10]. Formation of mesenchymal condensations is thus a critical step to initiate subsequent steps of differentiation.

Endochondral Bone Formation

Most bones in mammals are formed through endochondral bone formation (Fig. 1.1). This process is highly sequential, thus represents one of the best examples of organogenesis requiring heterotypic cellular interactions [11]. In this process, mesenchymal cells in condensations further differentiate into two distinct but closely related cells types, chondrocytes and perichondrial cells. Chondrocytes develop in the vasculature-free central portion of condensations, whereas perichondrial cells develop in the highly vascularized outer layer of condensations. This process is initiated when condensing mesenchymal cells start to express the transcription factor Sox9, a master regulator of chondrogenesis [12,13]. Indeed, Sox9 is absolutely required for these mesenchymal cells to stay organized within condensation. Sox9 can directly bind to regulatory elements of major cartilage matrix genes, including those encoding collagens (such as type II, IX, and XI collagen) and proteoglycans (e.g., aggrecan). As a result, mesenchymal cells in condensations become programmed as chondrocytes or perichondrial cells, which surround the chondrocytes.

Figure 1.1 The process of endochondral bone formation.

(A) Bone anlage is formed by condensing mesenchymal cells. (B) Mesenchymal cells differentiate into chondrocytes, and their surrounding cells become perichondrial cells. (C) Chondrocytes continue to proliferate; those in the center stop proliferating and become hypetrophic chondrocytes. (D) First osteoblast precursors appear in the perichondrium adjacent to hypertrophic chondrocytes. (E) Osteoblast precursors invade into the cartilage template along with blood vessels.

Formation of the Fetal Growth Plate

Chondrocytes restart proliferation in a relatively uniform manner within the cartilaginous template. As the cartilage enlarges, chondrocytes stop proliferating in the center, drastically change their cell morphology, and become special chondrocytes termed hypertrophic chondrocytes. How this initial hypertrophy is triggered is largely unknown. In the limb, initially continuous mesenchymal condensations undergo the processes termed segmentation and cavitation to give rise to individual skeletal elements. The interzone, a localized dense region in condensations, at a later stage becomes the joint. Secreted proteins, Wnt9a and Gdf5, regulate early joint formation, and the transcription factor c-Jun plays impor...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• List of Contributors
• Preface
• Acknowledgments
• Chapter 1. Developmental Biology of Musculoskeletal Tissues for Tissue Engineers
• Chapter 2. The Mechanics of Skeletal Development
• Chapter 3. Development, Tissue Engineering, and Orthopedic Diseases
• Chapter 4. Limb Synovial Joint Development From the Hips Down: Implications for Articular Cartilage Repair and Regeneration
• Chapter 5. Stem Cell-Based Approaches for Cartilage Tissue Engineering: What Can We Learn From Developmental Biology
• Chapter 6. Endochondral Ossification: Recapitulating Bone Development for Bone Defect Repair
• Chapter 7. Challenges in Cell-Based Therapies for Intervertebral Disc Regeneration: Lessons Learned From Embryonic Development and Pathophysiology
• Chapter 8. Developmental Biology in Tendon Tissue Engineering
• Chapter 9. Biomimetic Tissue Engineering for Musculoskeletal Tissues
• Chapter 10. Clinical Translation of Cartilage Tissue Engineering, From Embryonic Development to a Promising Long-Term Solution
• Index