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## **Meet the editor**

Dr. Margarita Valdés Flores received M.D. at Autonomous University of Coahuila in 1989, title of Genetic specialist in 1995 and Ph.D in 2002, at the National Autonomous University of Mexico. From 1998, Dr. Valdés Flores is working at National Rehabilitation Institute as Genetics specialist and Researcher. She supervises the Masters and Doctorate in Medical Sciences Program and

in the Genetics Specialty of Medicine, at National Autonomous University of Mexico. Currently, Dr. Valdés Flores is member of the National System of Researchers, the Mexican Academy of Sciences and Mexican Academy of Surgery. Between her research lines, highlights the investigation of the genetic component of Mexican population in relation to osteoporosis. Most of her published articles are related with these investigations.

Contents

**Preface VII**

León-Suárez

**Osteoporosis 67**

Chapter 1 **Molecular Aspects of Bone Remodeling 1**

Chapter 2 **Genetic Diseases Related with Osteoporosis 29**

Rafael Velázquez-Cruz

Alma Y. Parra-Torres, Margarita Valdés-Flores, Lorena Orozco and

Margarita Valdés-Flores, Leonora Casas-Avila and Valeria Ponce de

Chapter 3 **Serum Leptin and Bone Turnover Markers in Postmenopausal**

Chapter 4 **Modification of Sex Hormones with RGD-Peptide: A Strategy of**

Ming Zhao, Yuji Wang, Jianhui Wu and Shiqi Peng

Chapter 5 **Oxidative Stress and Antioxidants in the Risk of Osteoporosis**

Chapter 6 **Pathogenesis, Clinical Diagnosis and Treatment, and Animal**

Chapter 7 **Osteoporosis and Nutrition — Nutrition, Anthropometry and**

Olga Cvijanović, Sandra Pavičić Žeželj, Silvija Lukanović, Nenad Bićanić, Robert Domitrović, Dragica Bobinac and Željka Crnčević

Yan Zhang and Yoseph Asmelash Gebru

**Bone Mineral Density in Women 179**

**— Role of the Antioxidants Lycopene and Polyphenols 117**

Mehreen Lateef, Mukhtiar Baig and Abid Azhar

**Improving HRT and Other Secondary**

**Osteoporosis Therapy 87**

**Models for Ckd-Mbd 163**

L.G. Rao and A.V. Rao

Orlić

### Contents

### **Preface XI**


Preface

In the last decades the world has faced several epidemic health problems; among them are osteoporosis, osteoarthritis, diabetes mellitus, obesity and cardiovascular diseases. All these diseases share characteristics as their multifactorial and polygenic origin and the extensive involvement of the environment in their etiology strongly associated with the actual lifestyle (bad dietary habits, sedentary life and eating disorders, among others). At present, the above mentioned diseases do not exclusively affect elderly people and are currently afflict‐ ing young adults and even children, obligating us to intensify our efforts to prevent them. The progressive loss of bone mineral density and bone microarchitecture deterioration in osteoporosis, promote the occurrence of fractures which can develop complications and dis‐ abilities and imply high costs to the healthcare services. There is no doubt that the efforts should focus on prevention and on strengthening of programs directed toward protecting and improving the bone health to induce bone mass gain in early stages of life and to limit the potential risks related with the onset of the disease. Actually a great number of investi‐ gations throughout the world continuously contribute with information about functioning of bone tissue and the physiopathology of osteoporosis which enable researchers to explore

**Dr. Margarita Valdés-Flores**

Mexico D.F.

Health Secretary (Secretaria de Salud),

Genetics Unit, National Institute of Rehabilitation,

Faculty of Medicine, Universidad Nacional Autónoma Mexico

new therapeutic possibilities in bone resorption disorders.


### Preface

Chapter 8 **Bone Mineral Density and High-Performance Aerobic Activity**

Luiz Eugênio Garcez Leme and Maria do Carmo Sitta

Chapter 9 **Influence of the Nutrition on Bone Health of Children and**

Chapter 10 **The Effectiveness of Progressive Load Training Associated to**

Satoshi Iwase, Naoki Nishimura and Tadaaki Mano

**the Proprioceptive Training for Prevention of Falls in Women**

Lucas Teixeira, Stella Peccin, Kelson Silva, Tiago Teixeira, Aline Mizusaki Imoto, Joelma Magalhães and Virgínia Trevisani

**in Older Adults Experience in Brazil 193**

**Adolescents 207** Emilio González-Jiménez

**VI** Contents

**with Osteoporosis 215**

**Osteoporosis 241**

Chapter 12 **Osteoporosis in Spaceflight 259**

Chapter 11 **Anabolic Agents as New Treatment Strategy in**

Tulay Okman-Kilic and Cengiz Sagiroglu

In the last decades the world has faced several epidemic health problems; among them are osteoporosis, osteoarthritis, diabetes mellitus, obesity and cardiovascular diseases. All these diseases share characteristics as their multifactorial and polygenic origin and the extensive involvement of the environment in their etiology strongly associated with the actual lifestyle (bad dietary habits, sedentary life and eating disorders, among others). At present, the above mentioned diseases do not exclusively affect elderly people and are currently afflict‐ ing young adults and even children, obligating us to intensify our efforts to prevent them.

The progressive loss of bone mineral density and bone microarchitecture deterioration in osteoporosis, promote the occurrence of fractures which can develop complications and dis‐ abilities and imply high costs to the healthcare services. There is no doubt that the efforts should focus on prevention and on strengthening of programs directed toward protecting and improving the bone health to induce bone mass gain in early stages of life and to limit the potential risks related with the onset of the disease. Actually a great number of investi‐ gations throughout the world continuously contribute with information about functioning of bone tissue and the physiopathology of osteoporosis which enable researchers to explore new therapeutic possibilities in bone resorption disorders.

**Dr. Margarita Valdés-Flores**

Genetics Unit, National Institute of Rehabilitation, Health Secretary (Secretaria de Salud), Faculty of Medicine, Universidad Nacional Autónoma Mexico Mexico D.F.

**Chapter 1**

**Molecular Aspects of Bone Remodeling**

Bone is a dynamic tissue in constant change; maintenance of bone mass throughout life relies on the bone remodeling process, which continually replaces old and damaged bone with new bone. This remodeling is necessary to maintain the structural integrity of the skeleton and allows the maintenance of bone volume, the repair of tissue damage and homeostasis of calcium and phosphorous metabolism. This process allows the renewal of 5% of cortical bone and trabecular 20% in a year, and although the cortical portion makes up most of the bone (75%), the metabolic activity is ten times greater in the trabecular since the relationship between surface and volume is greater in this, which is achieved by an annual renewal of 5-10% of bone volume and although this remodeling takes place throughout life, your balance is positive only during the first three decades. The skeleton is particularly dependent on mechanical informa‐ tion to guide the resident cell population towards adaptation, maintenance and repair; a wide range of cell types depend on mechanically induced signals to enable appropriate physiolog‐ ical responses. The bone remodeling has two main phases: a resorption phase, consisting of the removal of old bone by osteoclasts, and a later phase of formation of new bone by osteo‐ blasts that replaces the tissue previously resorbed. While osteoclasts are derived from hema‐ topoietic precursor cells and degrade the bone matrix, osteoblasts originate from mesenchymal stem cells, they deposit a collagenous bone matrix and orchestrate its mineralization. While the interaction of bone cells with their mechanical environment is complex, an understanding of mechanical regulation of bone signaling is crucial to understanding bone physiology, the etiology of bone diseases such as osteoporosis, and to the development of interventions to improve bone strength. The clinical importance of bone formation has stimulated a lot of research aimed at understanding its mechanism. Much knowledge has been gained in the recent years, especially in relation with the signaling pathways controlling osteoblast differ‐ entiation. The purpose of this chapter is to review current knowledge on biochemical and

> © 2013 Parra-Torres et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

> © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

and reproduction in any medium, provided the original work is properly cited.

Alma Y. Parra-Torres, Margarita Valdés-Flores, Lorena Orozco and Rafael Velázquez-Cruz

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/54905

**1. Introduction**

### **Chapter 1**

### **Molecular Aspects of Bone Remodeling**

Alma Y. Parra-Torres, Margarita Valdés-Flores, Lorena Orozco and Rafael Velázquez-Cruz

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/54905

### **1. Introduction**

Bone is a dynamic tissue in constant change; maintenance of bone mass throughout life relies on the bone remodeling process, which continually replaces old and damaged bone with new bone. This remodeling is necessary to maintain the structural integrity of the skeleton and allows the maintenance of bone volume, the repair of tissue damage and homeostasis of calcium and phosphorous metabolism. This process allows the renewal of 5% of cortical bone and trabecular 20% in a year, and although the cortical portion makes up most of the bone (75%), the metabolic activity is ten times greater in the trabecular since the relationship between surface and volume is greater in this, which is achieved by an annual renewal of 5-10% of bone volume and although this remodeling takes place throughout life, your balance is positive only during the first three decades. The skeleton is particularly dependent on mechanical informa‐ tion to guide the resident cell population towards adaptation, maintenance and repair; a wide range of cell types depend on mechanically induced signals to enable appropriate physiolog‐ ical responses. The bone remodeling has two main phases: a resorption phase, consisting of the removal of old bone by osteoclasts, and a later phase of formation of new bone by osteo‐ blasts that replaces the tissue previously resorbed. While osteoclasts are derived from hema‐ topoietic precursor cells and degrade the bone matrix, osteoblasts originate from mesenchymal stem cells, they deposit a collagenous bone matrix and orchestrate its mineralization. While the interaction of bone cells with their mechanical environment is complex, an understanding of mechanical regulation of bone signaling is crucial to understanding bone physiology, the etiology of bone diseases such as osteoporosis, and to the development of interventions to improve bone strength. The clinical importance of bone formation has stimulated a lot of research aimed at understanding its mechanism. Much knowledge has been gained in the recent years, especially in relation with the signaling pathways controlling osteoblast differ‐ entiation. The purpose of this chapter is to review current knowledge on biochemical and

© 2013 Parra-Torres et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

physiological mechanisms of remodeling bone, with particular attention to the role in the cell involved, the process, regulation signals into the control and pathophysiology of bone remodeling (diseases).

### **2. Cells involved in bone remodeling**

Two bone cell lineages have been identified: cells of the osteoblast lineage (osteoblasts, osteocytes and bone-lining cells) and bone resorbing cells (osteoclasts) that together with their precursor cells and associated cells (e.g. endothelial cells, nerve cells) are organized in specialized units called bone multi cellular units (BMU). The main function of the BMU is to mediate a bone ''rejuvenation" mechanism called ''bone remodeling". Bone remodeling maintains the integrity of the skeleton by removing old bone of high mineral density and high prevalence of fatigue micro-fractures through repetitive cycles of bone resorption and bone formation [1]. This is a direct and crucial interaction that has been well established in vivo. Once osteoblasts and osteoclasts are fully differentiated, there is a less direct relationship [2]. Despite the know close physiological interactions of the two main cellular systems in bone, there are effectively separate and distinct origins of osteoblast (hematopoietic cell origin) and stromal/osteoblast linages from the developing fetus onward in mammalian development; circulating osteogenic precursor cells are blood-borne cells that express a variety of osteoblastic markers and are able to form bone in vivo. Strong evidence suggests that cells are derived from bone marrow and are of hematopoietic origin [3].

### **2.1. Osteoblast**

Osteoblasts derive from mesenchymal precursor cells, which also originate, chondrocytes (cartilage), adipocytes (bone marrow stroma), fibroblasts (periosteum), and adventitial reticular cells (bone marrow stroma). Although the claim that bone marrow stromal cell can also give raise chondrocytes, myoblasts, adipocytes and tendon cells, depending on the transcription factors that regulate the pathway [4]. There are four stages that have been identified in osteoblast differentiation: the preosteoblast, osteoblast, osteocyte and bone-lining cell that histologically these cells stain positively for alkaline phosphatase, however, only mature osteoblasts have the ability to produce mineralized tissue [5], and can be identified by their cuboidal morphology and strong alkaline phosphatase positivity. The master gene that encodes for a protein involved in the osteogenic differentiation process from mesenchymal precursors is the nuclear transcriptional factor Runx-2 (Runt related transcription factor 2, cbfa-1) (Figure 1) [6].

into a cartilage model that resembles the shape of the bone [8]. They are also responsible for the mineralization, although the exact mechanism by which mineralization occurs remains unclear [9]. Mature osteoblasts have one of three fates: they undergo apoptosis, differentiate further into osteocytes or become quiescent lining cells. Approximately 50 to 70% of osteoblasts

**Figure 1.** Osteoblasts derive from Mesenchymal Stem Cells. Osteoclasts derive from by the fusion of mononuclear pro‐ genitors of the monocyte/macrophage family, and Osteocytes are non-proliferative differentiated cells of the osteo‐

Molecular Aspects of Bone Remodeling http://dx.doi.org/10.5772/54905 3

Osteocytes are non-proliferative, terminally differentiated cells of the osteoblast lineage, how osteoblasts transform into osteocytes is dependent on the mode of ossification (Figure 1). They reside both in the mineralized bone matrix and in newly formed osteoid, locked inside small lacuna spaces in the hard substance of bone are smaller than osteoblasts and have lost many of their cytoplasmic organelles [11-13]. They compose over 90–95% of all bone cells in the adult skeleton and are thought to respond to mechanical strain to send signals of resorption or formation, due to their distribution throughout the bone matrix and extensive interconnec‐ tivity, osteocytes are thought to be one if not the major bone cell type responsible for sensing mechanical strain and orchestrating signals of resorption and formation. Evidence suggests that the primary function of the osteocyte relates to the determination and maintenance of bone structure. Osteocytes are mechanosensors capable of transducing musculoskeletal derived mechanical input into biological output [14], the osteocyte appears to be capable of

undergo apoptosis [10].

**2.2. Osteocyte**

blast lineage.

The osteoblast resides along the bone surface at sites of active bone formation. They secrete type 1 collagen, the basic building block of bone; non-collagenous proteins including osteo‐ calcin and alkaline phosphatase, which is essential for mineral deposition [7]. The principal function of the osteoblast is bone formation and these occur via two distinct mechanisms: the intramembranous ossification (flat bones of the skull and most of the clavicle) and the endo‐ chondral ossification, which produces most bones, involves the transformation of mesenchyme

**Figure 1.** Osteoblasts derive from Mesenchymal Stem Cells. Osteoclasts derive from by the fusion of mononuclear pro‐ genitors of the monocyte/macrophage family, and Osteocytes are non-proliferative differentiated cells of the osteo‐ blast lineage.

into a cartilage model that resembles the shape of the bone [8]. They are also responsible for the mineralization, although the exact mechanism by which mineralization occurs remains unclear [9]. Mature osteoblasts have one of three fates: they undergo apoptosis, differentiate further into osteocytes or become quiescent lining cells. Approximately 50 to 70% of osteoblasts undergo apoptosis [10].

### **2.2. Osteocyte**

physiological mechanisms of remodeling bone, with particular attention to the role in the cell involved, the process, regulation signals into the control and pathophysiology of bone

Two bone cell lineages have been identified: cells of the osteoblast lineage (osteoblasts, osteocytes and bone-lining cells) and bone resorbing cells (osteoclasts) that together with their precursor cells and associated cells (e.g. endothelial cells, nerve cells) are organized in specialized units called bone multi cellular units (BMU). The main function of the BMU is to mediate a bone ''rejuvenation" mechanism called ''bone remodeling". Bone remodeling maintains the integrity of the skeleton by removing old bone of high mineral density and high prevalence of fatigue micro-fractures through repetitive cycles of bone resorption and bone formation [1]. This is a direct and crucial interaction that has been well established in vivo. Once osteoblasts and osteoclasts are fully differentiated, there is a less direct relationship [2]. Despite the know close physiological interactions of the two main cellular systems in bone, there are effectively separate and distinct origins of osteoblast (hematopoietic cell origin) and stromal/osteoblast linages from the developing fetus onward in mammalian development; circulating osteogenic precursor cells are blood-borne cells that express a variety of osteoblastic markers and are able to form bone in vivo. Strong evidence suggests that cells are derived from

Osteoblasts derive from mesenchymal precursor cells, which also originate, chondrocytes (cartilage), adipocytes (bone marrow stroma), fibroblasts (periosteum), and adventitial reticular cells (bone marrow stroma). Although the claim that bone marrow stromal cell can also give raise chondrocytes, myoblasts, adipocytes and tendon cells, depending on the transcription factors that regulate the pathway [4]. There are four stages that have been identified in osteoblast differentiation: the preosteoblast, osteoblast, osteocyte and bone-lining cell that histologically these cells stain positively for alkaline phosphatase, however, only mature osteoblasts have the ability to produce mineralized tissue [5], and can be identified by their cuboidal morphology and strong alkaline phosphatase positivity. The master gene that encodes for a protein involved in the osteogenic differentiation process from mesenchymal precursors is the nuclear transcriptional factor Runx-2 (Runt related transcription factor 2,

The osteoblast resides along the bone surface at sites of active bone formation. They secrete type 1 collagen, the basic building block of bone; non-collagenous proteins including osteo‐ calcin and alkaline phosphatase, which is essential for mineral deposition [7]. The principal function of the osteoblast is bone formation and these occur via two distinct mechanisms: the intramembranous ossification (flat bones of the skull and most of the clavicle) and the endo‐ chondral ossification, which produces most bones, involves the transformation of mesenchyme

remodeling (diseases).

2 Topics in Osteoporosis

**2.1. Osteoblast**

cbfa-1) (Figure 1) [6].

**2. Cells involved in bone remodeling**

bone marrow and are of hematopoietic origin [3].

Osteocytes are non-proliferative, terminally differentiated cells of the osteoblast lineage, how osteoblasts transform into osteocytes is dependent on the mode of ossification (Figure 1). They reside both in the mineralized bone matrix and in newly formed osteoid, locked inside small lacuna spaces in the hard substance of bone are smaller than osteoblasts and have lost many of their cytoplasmic organelles [11-13]. They compose over 90–95% of all bone cells in the adult skeleton and are thought to respond to mechanical strain to send signals of resorption or formation, due to their distribution throughout the bone matrix and extensive interconnec‐ tivity, osteocytes are thought to be one if not the major bone cell type responsible for sensing mechanical strain and orchestrating signals of resorption and formation. Evidence suggests that the primary function of the osteocyte relates to the determination and maintenance of bone structure. Osteocytes are mechanosensors capable of transducing musculoskeletal derived mechanical input into biological output [14], the osteocyte appears to be capable of relating the intensity of strain signals and the distribution of the strain throughout the whole bone into signals to regulate [15]. Microdamage in the bone matrix has been shown to initiate bone remodeling, the osteocytes located near these sites undergo apoptosis correlated with increased bone remodeling due to enhanced RANKL production and an increase in osteoclast formation [16], and the osteocytes may be the major source of RANKL during bone remodeling [17-19]. For some time it has been estimated that the average life of this cell would be 25 years. The percentage of dead osteocytes increases with age senescence, being from 1% to 75% rise in the eighth decade [20,21].

fibrils in the cytoplasm [28]. It has been proposed that bone lining cells play a role in bone remodeling by preventing the inappropriate interaction of osteoclast precursors with the bone surface. It is thought that the signals that initiate osteoclast formation may stimulate the bone lining cells to prepare for bone resorption, through the actions of collagenase which digests a thin layer of non-mineralized bone, revealing the mineralized matrix underneath [29,30]. The bone lining cells migrate to form a canopy over the remodeling area, particularly at sites adjacent to osteoclasts, creating a microenvironment (in phagocytosis of collagen at the bone surface) for the coupling required during bone remodeling. It has been proposed that the bone lining cells are responsible for the cell to cell interactions between RANKL and RANK receptor

Molecular Aspects of Bone Remodeling http://dx.doi.org/10.5772/54905 5

The normal bone remodeling is a process that couples bone resorption and bone formation, it occurs in discrete locations and involves a group of different kinds of cells and takes 2 to 5 years for an area on the bone surface to complete one bone remodeling cycle [32]. The bone tissue is morphologically and physiologically separated from the marrow by bone lining cell; the process of cancellous bone remodeling occurs on the surface of trabeculae at the boundary between bone and marrow. In normal bone length of the remodeling is about 200 days, with the majority of that time (approx. 150 days) devoted to bone formation [33]. The bone remod‐ eling takes place in the BMU and the skeleton contains millions which comprises the next: osteoclasts that resorbing the bone, the osteoblasts that replacing the bone, the osteocytes within the bone matrix, the bone lining cells that covering the bone surface and the capillary blood supply. All BMU are in different stages, and the life span of individual cells in a BMU is much shorter than that of a BMU [31,35,36]. Mechanical stress in the bone can be sensed by osteocytes that can signal giving to lining cells to form a new BMU at cortical or cancellous surfaces and estimates that the duration is 2-8 months [12]. The bone remodeling follows

*Activation Phase-* The first stage of bone remodeling involves detection of an initiating remod‐ eling signal, the activation is a continuing process that occurs at the cutting edge of the BMU, and this signal can take several forms as a direct mechanical strain on the bone that results in structural damage or hormone (e.g. estrogen or PTH) action on bone cells in response to more systemic changes in homeostasis [32]. Conceivably, osteocyte apoptosis and possible release of osteotropic growth factors and cytokines could be attractants for blood vessels, which would then subsequently initiate the formation of a resorptive of the bone remodeling compartment which are a prerequisite for osteogenesis, including bone development, fracture healing, and cortical bone remodeling that support recruitment of osteoblast progenitors to bone remod‐ eling sites, thus highlight a link between activation of bone remodeling on the cancellous bone surfaces and activation of neighbouring bone marrow events [12,34,36,37]. The mechanical environment to which bone cells are exposed is a dynamic milieu of biophysical stimuli that includes strain, stress, shear, pressure, fluid flow, streaming potentials and acceleration. While ultimately it may not be possible to separate specific effects of each of these factors, it is clear

coordination of distinct and sequential phases of this process, (Figure 2):

on osteoclast precursors [31].

**3. Bone remodeling: The process**

### **2.3. Osteoclast**

Osteoclasts, which are the only cells capable of resorbing bone, are multinucleated giant cells formed from by the fusion of mononuclear progenitors of the monocyte/macrophage family in a process termed osteoclastogenesis (Figure 1) [22], they are located on endosteal surfaces within the Haversian system and on the periosteal surface beneath the periosteum, in the bone has only two to three per μm3 [23]. Osteoclasts are terminally differentiated myeloid cells that are uniquely adapted to remove mineralized bone matrix. These cells have distinct morpho‐ logical and phenotypic characteristics that are routinely used to identify them, including multinuclearity and expression of tartrate-resistant acid phosphatase and the calcitonin receptor. Osteoclast differentiation is supported by cells of the osteoblast lineage that express membrane-bound receptor activator (RANK) of RANKL (NF-kB ligand) and macrophagecolony stimulating factor (M-CSF) [22]; this process is also regulated by a secreted decoy receptor of RANKL, osteoprotegerin (OPG), which functions as a paracrine inhibitor of osteoclast formation [24]. The balance between OPG and RANKL regulates bone resorption and formation and one imbalance of the RANKL/OPG system have been implicated in the pathogenesis of various primary and secondary bone malignancies [25]. In the motile state the osteoclast migrate from the bone marrow to their resorptive site and in the resorptive phase they exert their bone resorbing function, in each state the osteoclast display morphological differences [26], the motile osteoclasts are flattened, non-polarised cells and they are charac‐ terised by the presence of membrane protrusions (lamellipodia), and podosome. Upon reaching the resorptive site, osteoclasts become polarised through cytoskeletal reorganization, results in the formation of a ruffled border, sealing zone, functional secretory domain and basolateral membrane. The sealing zone is an osteoclast specific structure, which separates the acidic resorptive environment from the rest of the cell, forming an organelle free area [27].

### **2.4. Bone-lining cells**

The bone lining cells constitute a subpopulation of the osteoblast family. Bone lining cells were characterized by their long, slender, and flattened appearance; and their association with the bone surface at sites where a thin no mineralized collagen layer was present [28]. Although not being osteoblasts in the sense that they produce an osteoid layer, belong to the same lineage as osteoblasts for the following reasons: they are alkaline phosphatase positive, respond to PTH, and are associated with the bone surface. The bone lining cells contained a low level of labeled osteocalcin, and they have electron-dense vacuoles containing crossbanded collagen fibrils in the cytoplasm [28]. It has been proposed that bone lining cells play a role in bone remodeling by preventing the inappropriate interaction of osteoclast precursors with the bone surface. It is thought that the signals that initiate osteoclast formation may stimulate the bone lining cells to prepare for bone resorption, through the actions of collagenase which digests a thin layer of non-mineralized bone, revealing the mineralized matrix underneath [29,30]. The bone lining cells migrate to form a canopy over the remodeling area, particularly at sites adjacent to osteoclasts, creating a microenvironment (in phagocytosis of collagen at the bone surface) for the coupling required during bone remodeling. It has been proposed that the bone lining cells are responsible for the cell to cell interactions between RANKL and RANK receptor on osteoclast precursors [31].

### **3. Bone remodeling: The process**

relating the intensity of strain signals and the distribution of the strain throughout the whole bone into signals to regulate [15]. Microdamage in the bone matrix has been shown to initiate bone remodeling, the osteocytes located near these sites undergo apoptosis correlated with increased bone remodeling due to enhanced RANKL production and an increase in osteoclast formation [16], and the osteocytes may be the major source of RANKL during bone remodeling [17-19]. For some time it has been estimated that the average life of this cell would be 25 years. The percentage of dead osteocytes increases with age senescence, being from 1% to 75% rise

Osteoclasts, which are the only cells capable of resorbing bone, are multinucleated giant cells formed from by the fusion of mononuclear progenitors of the monocyte/macrophage family in a process termed osteoclastogenesis (Figure 1) [22], they are located on endosteal surfaces within the Haversian system and on the periosteal surface beneath the periosteum, in the bone

are uniquely adapted to remove mineralized bone matrix. These cells have distinct morpho‐ logical and phenotypic characteristics that are routinely used to identify them, including multinuclearity and expression of tartrate-resistant acid phosphatase and the calcitonin receptor. Osteoclast differentiation is supported by cells of the osteoblast lineage that express membrane-bound receptor activator (RANK) of RANKL (NF-kB ligand) and macrophagecolony stimulating factor (M-CSF) [22]; this process is also regulated by a secreted decoy receptor of RANKL, osteoprotegerin (OPG), which functions as a paracrine inhibitor of osteoclast formation [24]. The balance between OPG and RANKL regulates bone resorption and formation and one imbalance of the RANKL/OPG system have been implicated in the pathogenesis of various primary and secondary bone malignancies [25]. In the motile state the osteoclast migrate from the bone marrow to their resorptive site and in the resorptive phase they exert their bone resorbing function, in each state the osteoclast display morphological differences [26], the motile osteoclasts are flattened, non-polarised cells and they are charac‐ terised by the presence of membrane protrusions (lamellipodia), and podosome. Upon reaching the resorptive site, osteoclasts become polarised through cytoskeletal reorganization, results in the formation of a ruffled border, sealing zone, functional secretory domain and basolateral membrane. The sealing zone is an osteoclast specific structure, which separates the acidic resorptive environment from the rest of the cell, forming an organelle free area [27].

The bone lining cells constitute a subpopulation of the osteoblast family. Bone lining cells were characterized by their long, slender, and flattened appearance; and their association with the bone surface at sites where a thin no mineralized collagen layer was present [28]. Although not being osteoblasts in the sense that they produce an osteoid layer, belong to the same lineage as osteoblasts for the following reasons: they are alkaline phosphatase positive, respond to PTH, and are associated with the bone surface. The bone lining cells contained a low level of labeled osteocalcin, and they have electron-dense vacuoles containing crossbanded collagen

[23]. Osteoclasts are terminally differentiated myeloid cells that

in the eighth decade [20,21].

has only two to three per μm3

**2.4. Bone-lining cells**

**2.3. Osteoclast**

4 Topics in Osteoporosis

The normal bone remodeling is a process that couples bone resorption and bone formation, it occurs in discrete locations and involves a group of different kinds of cells and takes 2 to 5 years for an area on the bone surface to complete one bone remodeling cycle [32]. The bone tissue is morphologically and physiologically separated from the marrow by bone lining cell; the process of cancellous bone remodeling occurs on the surface of trabeculae at the boundary between bone and marrow. In normal bone length of the remodeling is about 200 days, with the majority of that time (approx. 150 days) devoted to bone formation [33]. The bone remod‐ eling takes place in the BMU and the skeleton contains millions which comprises the next: osteoclasts that resorbing the bone, the osteoblasts that replacing the bone, the osteocytes within the bone matrix, the bone lining cells that covering the bone surface and the capillary blood supply. All BMU are in different stages, and the life span of individual cells in a BMU is much shorter than that of a BMU [31,35,36]. Mechanical stress in the bone can be sensed by osteocytes that can signal giving to lining cells to form a new BMU at cortical or cancellous surfaces and estimates that the duration is 2-8 months [12]. The bone remodeling follows coordination of distinct and sequential phases of this process, (Figure 2):

*Activation Phase-* The first stage of bone remodeling involves detection of an initiating remod‐ eling signal, the activation is a continuing process that occurs at the cutting edge of the BMU, and this signal can take several forms as a direct mechanical strain on the bone that results in structural damage or hormone (e.g. estrogen or PTH) action on bone cells in response to more systemic changes in homeostasis [32]. Conceivably, osteocyte apoptosis and possible release of osteotropic growth factors and cytokines could be attractants for blood vessels, which would then subsequently initiate the formation of a resorptive of the bone remodeling compartment which are a prerequisite for osteogenesis, including bone development, fracture healing, and cortical bone remodeling that support recruitment of osteoblast progenitors to bone remod‐ eling sites, thus highlight a link between activation of bone remodeling on the cancellous bone surfaces and activation of neighbouring bone marrow events [12,34,36,37]. The mechanical environment to which bone cells are exposed is a dynamic milieu of biophysical stimuli that includes strain, stress, shear, pressure, fluid flow, streaming potentials and acceleration. While ultimately it may not be possible to separate specific effects of each of these factors, it is clear that several of these parameters independently have the ability to regulate cellular responses and influence remodeling events within bone. Furthermore, components of these specific factors (such as magnitude, frequency, and strain rate) also affect the cellular response [38].

orientation is along the main loading direction trabecular, by contrast, are eroded as grooves

Molecular Aspects of Bone Remodeling http://dx.doi.org/10.5772/54905 7

*Reversal Phase-* This phase lasts ~9 days, occurs after the maximum eroded depth has been achieved. In the reversal period the osteoclasts undergo apoptosis whilst osteoblasts are recruited and begin to differentiate [44], therefore the reversal phase is a transition from osteoclast to osteoblast activity [35]. After withdrawal of the osteoclast from the resorption pit, bone-lining cells enter the lacuna and clean its bottom from bone matrix leftovers. This cleaning proves to be a prerequisite for the subsequent deposition of a first layer of proteins (collage‐ nous) in the resorption pits and form a cement line (glycoprotein) that helps in attaching

*Formation Phase-* The bone formation by the osteoblasts lasts the longest, and is slower than bone resorption, involves new bone formation and mineralization. It was proposed that the coupling molecules were stored in the bone matrix and liberated during bone resorption. TGFβ appears to be a key signal for recruitment of mesenchymal stem cells to sites of bone resorption and osteoclasts produce the coupling factors [32,45], once mesenchymal stem cells or early osteoblast progenitors have returned to the resorption lacunae, they differentiate [28, 34,46] and the proliferating osteoblasts forming multilayers of cells. Several genes associated with formation of the extracellular matrix (Type I collagen, fibronectin, and TGF-β) are actively expressed and then gradually decline being maintained at a low basal level during subsequent stages of osteoblast differentiation. Collagen type I is the primary organic component of bone and accumulation contributes, in part, to the cessation of cell growth. When proliferation ceases, proteins associated with bone cell phenotype are detected, e.g. alkaline phosphatase enzyme, osteocalcin [7,47]. Bone matrix is built up of type I collagen (88%) and the remaining 10% is composed of a large number of non-collagenous proteins (e.g. osteocalcin, osteonectin, bone sialoprotein and various proteoglycans) and lipids and glycosaminoglycans represent 1– 2% [48]. For bone to assume its final form, hydroxylapatite is incorporated into this newly deposited osteoid [47,49]. The extracellular matrix undergoes a series of modifications in composition and organization that renders it competent for mineralization that begins ~15 days after osteoid has been formed, and non-collagenous proteins participate in the process of matrix maturation, mineralization and may regulate the functional activity of bone cells. With the onset of mineralization, several other bone expressed genes are induced to maximal levels (bone sialoprotein, osteopontin and osteocalcin) [32,47]. The composition of bone is approxi‐ mately 10% cells, 60% mineral crystals (crystalline hydroxyapatite), and 30% organic matrix [48]. When an equal quantity of resorbed bone has been replaced, the remodeling cycle

*Termination Phase-* The termination signals are largely unknown, and include the terminal differentiation of the osteoblast. The role of osteocytes is emerging [12,32]. The cells then gradually flatten as they slow production, and finally they become quiescent lining cells. Some of the osteoblast differentiate into osteocytes and remain in the matrix [12]. The osteocytes may secrete inhibitory factors that slow the rate of bone formation as the resorbed cavity is nearly filled. Bone remodeling is mediated by a balance of osteoblast and osteoclast cell activity, which together, maintain bone mass and mineral homeostasis. Both decreased bone formation and

along the bone surface with a depth of 60–70 μm (Figure 2), [36].

osteoblasts (Figure 2), [28,41].

concludes (Figure 2).

**Figure 2.** Schematic presentation of trabecular and cortical bone remodeling by BMU. In trabecular, the osteoclast create Howship´s lacunas that are refilled by osteoblast, and in the cortical bone, the osteoclast erode bone tissue and are followed by osteoblast that refill the gap with new bone.

The osteocyte, which is uniquely situated in cortical bone to sense mechanical strain and load generated factors (e.g., fluid flow, streaming and pressure) through a connected network of sister cells contributes to the perception of and response to loading and unloading [12], this canalicular network responds to unloading, or a decrease in mechanical signals, with upre‐ gulation of the proteins sclerostin and RANKL that control bone remodeling at multiple levels. The long osteocytic processes are able to pass information between cells separated by hard tissue [16,19]. Osteoblast linage cells and bone marrow stromal cells (BMSCs) are thought to be the major cell types that express RANKL in support of osteoclastogenesis [39,40]; however the actual major source of RANKL in vivo is the osteocyte [12].

*Resorption Phase-* In this phase, the formation and activity of osteoclasts is controlled by cells of the osteoblast lineage that recruit osteoclast precursors to the remodeling site with the expression of the master osteoclastogenesis cytokines, CSF-1, RANKL, and OPG, is also modulated in response to PTH [32,45,46]. Remodeling is initiated by osteoclastic resorp‐ tion, which erodes a resorption lacuna, they attach to the bone surface, sealing a resorb‐ ing compartment that they acidify by secreting H+ ions, facilitating dissolution of the bone mineral and thereby exposing the organic matrix to proteolytic enzymes that degrade it, during resorption the bone matrix and bone mineral is digested. Some fragments can be used as biochemical markers for overall bone resorption [43]. The depth of which varies between 60-40 μm in young and older individuals, and the resorption period has a median duration of 30–40 days [45]. In cortical bone, the BMUs proceed by osteonal tunnelling, during which osteoclasts excavate a canal that is refilled by osteoblasts, the so-formed Haversian systems are 100–200 μm wide and may become as long as 10 mm; their orientation is along the main loading direction trabecular, by contrast, are eroded as grooves along the bone surface with a depth of 60–70 μm (Figure 2), [36].

that several of these parameters independently have the ability to regulate cellular responses and influence remodeling events within bone. Furthermore, components of these specific factors (such as magnitude, frequency, and strain rate) also affect the cellular response [38].

**Figure 2.** Schematic presentation of trabecular and cortical bone remodeling by BMU. In trabecular, the osteoclast create Howship´s lacunas that are refilled by osteoblast, and in the cortical bone, the osteoclast erode bone tissue and

The osteocyte, which is uniquely situated in cortical bone to sense mechanical strain and load generated factors (e.g., fluid flow, streaming and pressure) through a connected network of sister cells contributes to the perception of and response to loading and unloading [12], this canalicular network responds to unloading, or a decrease in mechanical signals, with upre‐ gulation of the proteins sclerostin and RANKL that control bone remodeling at multiple levels. The long osteocytic processes are able to pass information between cells separated by hard tissue [16,19]. Osteoblast linage cells and bone marrow stromal cells (BMSCs) are thought to be the major cell types that express RANKL in support of osteoclastogenesis [39,40]; however

*Resorption Phase-* In this phase, the formation and activity of osteoclasts is controlled by cells of the osteoblast lineage that recruit osteoclast precursors to the remodeling site with the expression of the master osteoclastogenesis cytokines, CSF-1, RANKL, and OPG, is also modulated in response to PTH [32,45,46]. Remodeling is initiated by osteoclastic resorp‐ tion, which erodes a resorption lacuna, they attach to the bone surface, sealing a resorb‐ ing compartment that they acidify by secreting H+ ions, facilitating dissolution of the bone mineral and thereby exposing the organic matrix to proteolytic enzymes that degrade it, during resorption the bone matrix and bone mineral is digested. Some fragments can be used as biochemical markers for overall bone resorption [43]. The depth of which varies between 60-40 μm in young and older individuals, and the resorption period has a median duration of 30–40 days [45]. In cortical bone, the BMUs proceed by osteonal tunnelling, during which osteoclasts excavate a canal that is refilled by osteoblasts, the so-formed Haversian systems are 100–200 μm wide and may become as long as 10 mm; their

are followed by osteoblast that refill the gap with new bone.

6 Topics in Osteoporosis

the actual major source of RANKL in vivo is the osteocyte [12].

*Reversal Phase-* This phase lasts ~9 days, occurs after the maximum eroded depth has been achieved. In the reversal period the osteoclasts undergo apoptosis whilst osteoblasts are recruited and begin to differentiate [44], therefore the reversal phase is a transition from osteoclast to osteoblast activity [35]. After withdrawal of the osteoclast from the resorption pit, bone-lining cells enter the lacuna and clean its bottom from bone matrix leftovers. This cleaning proves to be a prerequisite for the subsequent deposition of a first layer of proteins (collage‐ nous) in the resorption pits and form a cement line (glycoprotein) that helps in attaching osteoblasts (Figure 2), [28,41].

*Formation Phase-* The bone formation by the osteoblasts lasts the longest, and is slower than bone resorption, involves new bone formation and mineralization. It was proposed that the coupling molecules were stored in the bone matrix and liberated during bone resorption. TGFβ appears to be a key signal for recruitment of mesenchymal stem cells to sites of bone resorption and osteoclasts produce the coupling factors [32,45], once mesenchymal stem cells or early osteoblast progenitors have returned to the resorption lacunae, they differentiate [28, 34,46] and the proliferating osteoblasts forming multilayers of cells. Several genes associated with formation of the extracellular matrix (Type I collagen, fibronectin, and TGF-β) are actively expressed and then gradually decline being maintained at a low basal level during subsequent stages of osteoblast differentiation. Collagen type I is the primary organic component of bone and accumulation contributes, in part, to the cessation of cell growth. When proliferation ceases, proteins associated with bone cell phenotype are detected, e.g. alkaline phosphatase enzyme, osteocalcin [7,47]. Bone matrix is built up of type I collagen (88%) and the remaining 10% is composed of a large number of non-collagenous proteins (e.g. osteocalcin, osteonectin, bone sialoprotein and various proteoglycans) and lipids and glycosaminoglycans represent 1– 2% [48]. For bone to assume its final form, hydroxylapatite is incorporated into this newly deposited osteoid [47,49]. The extracellular matrix undergoes a series of modifications in composition and organization that renders it competent for mineralization that begins ~15 days after osteoid has been formed, and non-collagenous proteins participate in the process of matrix maturation, mineralization and may regulate the functional activity of bone cells. With the onset of mineralization, several other bone expressed genes are induced to maximal levels (bone sialoprotein, osteopontin and osteocalcin) [32,47]. The composition of bone is approxi‐ mately 10% cells, 60% mineral crystals (crystalline hydroxyapatite), and 30% organic matrix [48]. When an equal quantity of resorbed bone has been replaced, the remodeling cycle concludes (Figure 2).

*Termination Phase-* The termination signals are largely unknown, and include the terminal differentiation of the osteoblast. The role of osteocytes is emerging [12,32]. The cells then gradually flatten as they slow production, and finally they become quiescent lining cells. Some of the osteoblast differentiate into osteocytes and remain in the matrix [12]. The osteocytes may secrete inhibitory factors that slow the rate of bone formation as the resorbed cavity is nearly filled. Bone remodeling is mediated by a balance of osteoblast and osteoclast cell activity, which together, maintain bone mass and mineral homeostasis. Both decreased bone formation and increased bone resorption may result in bone loss. Therefore, the stimulation of bone formation may be another important factor for the prevention and treatment of bone loss (Figure 2).

acting through both systemic and local insulin-like growth factor (IGF) production, can stimulate bone formation and resorption. Glucocorticoids are necessary for bone cell differ‐ entiation during development. Indirect effects of glucocorticoids on calcium absorption and sex hormone production may, however, increase bone resorption (Table 1). O the other hand, probably the most important systemic hormone in maintaining normal bone turnover is estrogen. Estrogen deficiency leads to an increase in bone remodeling in which resorption overcome formation and bone mass decreases (Table 1). The increase in bone remodeling and in bone resorption in the estrogen deficient state is associated with an increase in bone formation at the tissue level [51]. Therefore, sex steroid deficiency is associated with a defect in bone formation. Based on the available evidence, there are currently at least three key mechanisms by which estrogen deficiency may lead to a relative deficit in bone formation through direct effects on osteoblasts: increased apoptosis, increased oxidative stress, and an increase in NF-kB activity (Figure 3). In addition, estrogen inhibits the activation of bone

Molecular Aspects of Bone Remodeling http://dx.doi.org/10.5772/54905 9

The parathyroid hormone (PTH) increases bone formation in bone diseases. The anabolic effects of PTH on bone formation are mediated through the PTH/PTH-related peptide (PTHrP) receptor-dependent mechanisms that generate multiple G protein-dependent signals (Table 1). PTH mediated cyclic AMP/protein kinase phosphorylates the osteoblast transcription factor Runx2, which in turn upregulates the expression of osteoblast genes. Intermittent PTH also activates ERK1/2-mitogen-activated protein kinase (MAPK) Erk1/2 and phosphatidylinositol phosphate (PI3K) signaling, resulting in increased osteoblastogenesis and osteoblast survival (Figure 3) [53]. PTH induces the synthesis of IGF-I that works with PTH in osteoblasts to stimulate osteoblast proliferation and differentiation as well as indirectly regulates osteoclast activity [54,55]. Also, PTH was inferred to interact with various local signaling molecules, including insulin-like growth factors and Wnt antagonist sclerostin (SOST) [55-57]. It was recently shown that, in addition to reducing SOST, PTH reduces Dkk1 expression and thereby increases Wnt signaling, which contributes to the anabolic effect of PTH in bone [58]. This does not exclude the possibility that PTH receptor signaling may increase bone mass and bone remodeling by affecting Wnt signaling in other cell types. Recent data indicate that the activation of the PTH receptor in T lymphocytes plays a role in PTH-induced bone formation and bone mass by promoting the production of Wnt10b by these cells [59]. These observations and the finding that PTH signaling also acts by phosphorylating the Wnt coreceptor LRP6 and β-catenin indicate that direct and indirect crosstalks between PTH and Wnt signaling are

Genetic studies in human and animal models suggest that the canonical Wnt/β-catenin pathway (Table 2), together with BMP signaling and key transcription factor RUNX2(CBFA1/ AML3), has an key role in skeletal development, osteoblast differentiation and bone formation [60,61]. Wnt/β-catenin signaling plays a significant role in promoting mesenchymal commit‐

remodeling, and this effect is most likely mediated via the osteocyte [52].

**4.2. Parathyroid hormone (TH) and PTHrP signals**

important mechanisms regulating bone formation.

**4.3. Wnt and Wnt antagonists**

### **4. Regulation signals into the control of bone remodeling**

### **4.1. Systemic regulation of bone remodeling**

The process of bone remodeling is essential for adult bone homeostasis. This control involves a complex mechanism compound by numerous local and systemic factors, and their expression and release is controlled finely. The main factor that affects normal bone remodeling is the regulation of osteoblasts and osteoclasts. Local and systemic factors can affect bone remodeling by directly or indirectly targeting mature cells and their respective progenitor cells. The metabolic functions of the bone are mediated by two major calcium-regulating hormones, parathyroid hormone (PTH) and 1,25-dihydroxy vitamin D (Table 1) [50].


? = Effects are not Known

\* PTH and vitamin D decrease collagen synthesis in high doses.
