Skeletal System Structure PDF

**Skeletal System Structure** Introduction Humans have an [endoskeleton](javascript:void(0)) that is derived from the [embryonic mesoderm](javascript:void(0)). The skeletal system is composed of bones and the intimately associated tissues that cover bones or serve as bone linkages. The term *bon...

**Skeletal System Structure** Introduction Humans have an [endoskeleton](javascript:void(0)) that is derived from the [embryonic mesoderm](javascript:void(0)). The skeletal system is composed of bones and the intimately associated tissues that cover bones or serve as bone linkages. The term *bone* can refer to a macroscopic unit (ie, one piece of the skeleton) or a tissue type with certain characteristic features. The human skeletal system is depicted in Figure 18.1. **Figure 18.1** The human skeletal system. This lesson introduces the structure of the skeletal system. A human skeleton with bones Description automatically generated Chapter 18: Skeletal System 583 18.1.01 Bone Types Bones and bone tissue can be classified based on characteristic traits, such as anatomical location, bone shape, and bone structure. When classifying bones based on anatomical location, the skeleton is divided into axial and appendicular components. The bones of the **axial skeleton** (ie, skull, spine, ribs, and sternum) make up the long axis of the skeleton, and the bones of the **appendicular skeleton** (eg, [femur](javascript:void(0)), [clavicles](javascript:void(0))) are appended (ie, attached) to those of the axial skeleton (Figure 18.2). **Figure 18.2** Classifying bones by anatomical location. When classifying bones according to shape, they can be grouped into several types: **long** (eg, the ulna in the forearm), **short** (eg, the cylindrical bones of the fingers), **flat** (eg, skull bones), or **irregular** (eg, vertebrae in the spine). Examples of bone shapes are illustrated in Figure 18.3.  Chapter 18: Skeletal System 584 **Figure 18.3** Classifying bones by shape. Bone classification based on intrinsic structure (ie, hard surface bone versus less dense interior bone) is discussed in detail in Concept 18.1.02. 18.1.02 Bone Structure **Bone** is a type of [connective tissue](javascript:void(0)) and is composed of both living bone cells and nonliving materials secreted by bone cells into the extracellular space. The components of the bone extracellular matrix are discussed in detail in Concept 18.1.03. Although a multitude of bone shapes exist, all bones share some common features. For example, all bones have the same fundamental intrinsic structure: an outer layer of hard, dense bone called **compact** or **cortical bone** and an inner, softer core known as **spongy bone**. In addition, long bones share a gross structure that includes the following features (shown in Figure 18.4): **Epiphyses** are rounded ends that make up joint surfaces and are covered by articular cartilage. The **diaphysis** is a hollow central shaft enclosing a **medullary cavity** filled with bone marrow. The **metaphyses** are regions where the diaphysis and epiphyses meet. The **epiphyseal (growth) plate**, a cartilaginous structure that lies between each epiphysis and metaphysis, is present only during childhood and serves as the site of longitudinal growth. When growth ceases, the growth plate is replaced with mature bone, forming the **epiphyseal line**. A skeleton with different bones Description automatically generated with medium confidence Chapter 18: Skeletal System 585 **Figure 18.4** Structure of a long bone. Compact bone is organized into structural units called **osteons**, or **Haversian systems**. Osteons are made up of **lamellae** (concentric rings of bone matrix) that surround a central **Haversian canal**, a cylindrical channel that runs parallel to the long axis of bone and through which blood vessels and nerves traverse. Within the lamellar matrix are tiny spaces called lacunae, each containing a mature, [mitotically](javascript:void(0)) inactive **osteocyte**, the most abundant non-marrow bone cell type. **Volkmann\'s (transverse) canals**, which run perpendicular to the long axis of bone, allow the passage of blood vessels and nerves between different Haversian canals. Microscopic channels called **canaliculi** allow osteocyte waste exchange and nutrient delivery (Figure 18.5).  Chapter 18: Skeletal System 586 **Figure 18.5** Microscopic structure of bone. Spongy bone consists of a porous, interconnected network of irregular fine spikes of bone called **trabeculae** (spongy bone is sometimes called trabecular bone). Spongy bone is less dense than cortical bone, and in all bones of young children and certain bones of adults, this bone type contains marrow. Bone marrow is composed of various types of cells, including fat cells (adipocytes) and [precursors](javascript:void(0)) for red and white blood cells. Marrow that actively produces blood cells is called red marrow because the [hemoglobin](javascript:void(0)) of red blood cell precursors imparts a reddish color. Inactive marrow is called yellow marrow and consists primarily of adipocytes. The two types of bone marrow are summarized in Table 18.1. A diagram of a human body Description automatically generated Chapter 18: Skeletal System 587 **Table 18.1** Bone marrow characteristics. Exposed bone surfaces (ie, those not covered by cartilage, tendon attachments, or ligaments) have a thin surface membrane layer called the **periosteum**. The periosteum provides some protection to bone and contains blood vessels, nerves, and a population of cells that can contribute to the repair of fractured bone. In this type of reparative bone formation (called **intramembranous ossification**), stem cells within the periosteum differentiate into osteoblasts (described in Concept 18.1.04) and secrete bone matrix. The ends of some bones are covered with cartilage, and cartilage-producing cells (chondrocytes) within bones play an important role in bone growth during development. Cartilage and cartilage-mediated bone growth are discussed further in Concept 18.1.06.  Chapter 18: Skeletal System 588 **Concept Check 18.1** Match the bone structure terms below with their description in the table. Canaliculi Haversian canals Lacunae Lamellae Osteons (Haversian system) Volkmann\'s canals **Structure** **Description** Structural units of compact bone Cylindrical channels that contain nerves and blood vessels and run *perpendicular* to the long axis of bone Tiny, osteocyte-containing spaces within the matrix surrounding Haversian canals Cylindrical channels that contain nerves and blood vessels and run *parallel* to the long axis of bone Tiny channels through which nutrient and waste exchange between osteocytes and the circulation occurs Concentric rings of bone matrix surrounding Haversian canals [**Solution**](javascript:void(0)) 18.1.03 Bone Matrix The **bone matrix** is the extracellular material surrounding the cells in bone and is formed from bone cell secretions and other components from the blood. The matrix is composed of both inorganic materials, such as **calcium phosphate**, and organic materials, such as **collagen**. Collagen is secreted by bone cells, and calcium phosphate (a salt) forms by the precipitation of calcium (ie, Ca2+) and phosphate (ie, PO43-) from the bloodstream. Precipitation of calcium phosphate salts onto collagen fibers in the matrix eventually results in the formation of **hydroxyapatite** crystals, which are responsible for the hardness of bone. Enzymes secreted by osteoblasts (Concept 18.1.04) catalyze the breakdown of calcium- and phosphate-containing compounds secreted from osteoblast vesicles, facilitating precipitation of Ca2+ and PO43- released from these compounds into crystals of hydroxyapatite. Secreted collagen provides a site for hydroxyapatite crystals to bind. Figure 18.6 shows the bone matrix structure from macro- to nanoscale. A blue check mark in a square Description automatically generated Chapter 18: Skeletal System 589 **Figure 18.6** Macro- to nanoscale bone structure. The bone matrix consists of both **mineralized** (ie, hard) and **unmineralized** (ie, soft) parts. The unmineralized bone matrix is called the **osteoid**. Specialized bone cells (discussed in Concept 18.1.04) mediate the transfer of calcium phosphate between the blood, osteoid, and mineralized bone matrix. In certain circumstances, such as in individuals with the bone disease osteoporosis or in astronauts whose bones are no longer loaded by gravity, bone matrix mineralization can be reduced, as shown in Figure 18.7. If severe enough, such demineralization can reduce bone mass and strength to the point that bones become fragile and fracture more easily. **Figure 18.7** Structure of bone with normal or reduced mineralization.  Chapter 18: Skeletal System 590 18.1.04 Bone Cells A whole bone (ie, one of the units of the skeleton) can be composed of several tissue types, including cartilage, marrow, and bone (ie, bone *tissue*), as depicted in Figure 18.8. The structural rigidity of the skeleton belies the dynamic nature of bones; for example, each day millions of new osteocytes and bone marrow cells are formed, and each year up to 10% of the skeleton is replaced. **Figure 18.8** Bone and bone tissue. [Connective tissues](javascript:void(0)), such as bone tissue, contain specialized cells that secrete and maintain an extensive extracellular matrix as well as other types of cells (see Concept 5.3.07). Bone tissue, which is sometimes connected by [gap junctions](javascript:void(0)), is made of several types of cells surrounded by bone matrix: **Osteoprogenitor cells** (sometimes called **osteogenic cells**) are mitotically active stem cells in bone that initially differentiate into osteoblasts. **Osteoblasts** are bone-forming cells that coordinate the incorporation of calcium and phosphate ions into bone. Osteoblasts secrete proteins that create the osteoid, or unmineralized bone matrix, into the extracellular space. Osteoblasts eventually become fully surrounded by bone matrix and differentiate into osteocytes. **Osteocytes** are mature (ie, fully [differentiated](javascript:void(0))) and mitotically inactive bone cells that maintain bone structure. Located within lacunae in the Haversian system (where they were originally osteoblasts), osteocytes can release signals to other bone cells to regulate compact bone remodeling. **Osteoclasts** are large cells that secrete [proteolytic](javascript:void(0)) enzymes and acid that promote **resorption** (breakdown) of the organic and mineral components of bone.  Chapter 18: Skeletal System 591 Osteocytes, osteoclasts, and osteoblasts function to maintain the strength and integrity of the skeleton through **bone remodeling**. During this process, old bone is resorbed by osteoclasts and new bone is deposited by osteoblasts. Osteoblasts form successive concentric layers of bone (lamellae), and as the osteoid secreted by osteoblasts mineralizes, some osteoblasts become trapped within lacunae (spaces) in the lamellar matrix. Eventually, these trapped osteoblasts become either flattened bone-lining cells or osteocytes within the interior of bone. The process of bone remodeling is summarized in Figure 18.9. **Figure 18.9** Bone remodeling. In healthy individuals, the process of bone remodeling is tightly regulated to ensure that bone mass and density remain constant. To maintain this consistency, the rate of bone resorption by osteoclasts equals the rate of bone deposition by osteoblasts. As discussed in Concept 18.1.02, certain bones contain bone marrow, which is made of various types of cells, including precursors for red and white blood cells (ie, erythrocytes and leukocytes, respectively). Such **bone marrow cells** are hematopoietic (ie, blood cell-producing) and remain active in some bones throughout life. Bone marrow and its role in the immune system are discussed in more detail in Concept 20.1.02. 18.1.05 Joints **Joints** are specialized components of the vertebrate musculoskeletal system where two or more bones articulate (ie, interact). There are several types of joints, including immovable joints (eg, [skull sutures](javascript:void(0))) and freely movable joints (eg, [hinge, ball-and-socket](javascript:void(0))). Freely movable joints, also known as **synovial joints**, consist of several structures that interact with bone and skeletal muscle, as shown in Figure 18.10: **Articular (hyaline) cartilage** surrounds the ends of bones within a joint, providing a smooth surface that absorbs compressive forces and reduces friction between interacting bones. A **joint cavity**, located between articulating bones, is lined by the synovial membrane (synovium), which produces specialized fluid known as **synovial fluid**. Synovial fluid lubricates joint surfaces and protects articular cartilage from excessive friction and damage. A diagram of a cell structure Description automatically generated Chapter 18: Skeletal System 592 A **fibrous layer** of connective tissue extends across a joint from the periosteal membranes of the articulating bones. This fibrous layer lies superficial to the synovial membrane. **Fat pads** are adipose tissue structures that provide additional cushion between bones in some synovial joints (eg, knee and hip joints). Ligaments and tendons are dense connective tissue structures. Ligaments attach bones to other bones, and tendons generally attach bones to surrounding muscles. **Figure 18.10** A freely movable (synovial) joint. Ligaments and tendons are important connective tissue structures at joints (Figure 18.11), as they stabilize the positioning of bones and enable the application of muscle force across joints. In this latter role, [muscle contraction](javascript:void(0)) transmits force to the muscle origin (the more proximal or less mobile end of a muscle) and insertion (the more distal or movable end of a muscle) points on the bones forming the joint. **Tendons** are strong, fibrous bands of connective tissue that, in the context of joints, anchor muscle by attaching it to bony structures. Outside of joints, tendons can attach muscles to other structures (eg, the [linea alba](javascript:void(0)) or other tendinous structures in the abdominal musculature). By transmitting the force generated by muscle contraction, tendons play an essential role in locomotion (movement). **Ligaments** are strong bundles of connective fibers in joints that connect bones to other bones, thereby stabilizing a joint by holding the bony structures together. Ligaments also help stabilize internal organs and in rare cases can serve as muscle attachment sites (eg, part of the transverse abdominis muscle of the abdomen has its origin in a ligament near the hip/lower abdomen).  Chapter 18: Skeletal System 593 **Figure 18.11** Tendons connect muscles to bones, and ligaments connect bones to other bones. 18.1.06 Cartilage **Cartilage** is a connective tissue made up of chondrocyte cells. These cells secrete a specialized extracellular matrix called chondrin, which contains collagen fibers, proteoglycans, and water (Figure 18.12). The firm but flexible structure of cartilage is resistant to compression and stretching. Most of the cartilage in the body is found in locations that require cushioning (eg, spine) and on the articulating surfaces of bone ends (ie, the bone surfaces that meet in a joint). Because cartilage lacks nerves and its own blood supply, this type of tissue must receive nutrients and oxygen via diffusion from surrounding fluids or vascularized areas. **Figure 18.12** Cartilage location and structure.  Chapter 18: Skeletal System Cartilage is classified as hyaline, elastic, or fibrous. **Hyaline cartilage** is the most abundant [cartilage type](javascript:void(0)) in the body and is found in the ribs, nose, trachea, and larynx. The articular cartilage that surrounds the ends of bones within a joint is also hyaline cartilage. **Elastic cartilage** is enriched with elastic fibers and is found where flexibility is important (eg, the ear). **Fibrous cartilage** is structurally reinforced by a high content of collagen fibers, which contributes to its resistance to deformation (eg, when compressed between vertebrae). Cartilage also plays a role in certain mechanisms of bone development. The process of **endochondral ossification** uses hyaline cartilage as a template for bone deposition by osteoblasts (as opposed to intramembranous ossification, in which osteoblasts in the periosteum secrete new bone in the absence of a cartilage template). During fetal development, the hyaline cartilage that composes the fetal skeleton is calcified (ie, replaced by bone). In children and adolescents, the epiphyseal (growth) plate of long bones is formed from hyaline cartilage and serves as the site of bone lengthening. The cartilaginous epiphyseal plate is present only during childhood. When growth ceases, the growth plate is replaced with mature bone, forming the **epiphyseal line**, as shown in Figure 18.13 **Skeletal System Function** Introduction Bone plays multiple important roles in the body, and the storage and release of calcium and phosphate in bone help maintain whole-body calcium and phosphorus homeostasis. Specific hormones regulate bone growth and development as well as calcium and phosphorus release. This lesson details the functions of bone and how the skeletal system is regulated. 18.2.01 Bone Function Vertebrates such as humans have an internal skeleton (endoskeleton) that consists of a bony vertebral column linked to other bones and tissues. The primary functions of the skeletal system are summarized in Table 18.2. **Table 18.2** Functions of the skeletal system. **Structural support** Bones provide a framework of structures to which other bones and tissues can attach. **Physical protection** Certain bones shield internal organs from physical trauma (eg, the skull protects the brain, the rib cage protects the lungs and heart, the vertebrae protect the spinal cord). **Mobility** By providing sites of attachment and joints across which muscles can exert force, the skeleton enables body movement. **Mineral and energy storage** Bones store and mobilize minerals (primarily phosphate and calcium) that add considerable strength to the skeleton. Additionally, yellow marrow present in some bones serves as a reservoir of stored energy in the form of fat. **Blood cell production (hematopoiesis)** Although cells in a variety of tissues are capable of producing blood cells during development or in response to certain stressors, bone marrow is typically the major site of blood cell production after birth. 18.2.02 Endocrine Control of Skeletal System Bone structure and function are influenced throughout life by [hormones](javascript:void(0)). Hormones regulate bone mass primarily through their influences on bone growth, bone density, and handling of the body\'s calcium stores, 99% of which are in the form of calcium phosphate-containing crystals in bone. Endocrine regulation of the skeletal system occurs in the context of a complex [system of hormones](javascript:void(0)) that regulates numerous processes in the body (Table 18.3). In many cases, the hormones involved act upon other endocrine tissues as well as target cells in bone. For example, in addition to individual influences on bone, both **thyroid hormone** and **estradiol** can augment growth hormone secretion. Despite this complexity, a few general themes exist. Chapter 18: Skeletal System 596 For example, during childhood and adolescence, **growth hormone** and thyroid hormone increase bone mass by promoting the synthesis of new bone necessary for linear bone growth. Abnormally high levels of either hormone can cause increased bone growth, whereas low growth hormone or thyroid hormone levels are associated with suboptimal bone growth. Some of the effects of thyroid hormone are likely due to its promotion of growth hormone synthesis. **Vitamin D**, in addition to being a nutrient (ie, a [fat-soluble vitamin](javascript:void(0))) consumed in the diet, can be synthesized in the body and act as a [steroid hormone](javascript:void(0)) that regulates bone growth and development. Vitamin D undergoes successive modifications in the liver and kidneys that increase its activity. The most active form of vitamin D is called vitamin D3, or **calcitriol**. Calcitriol promotes absorption of calcium from ingested food and [reabsorption](javascript:void(0)) of calcium in the kidneys. Inadequate vitamin D levels during childhood can cause rickets, a disease in which bone structure is abnormal. **Table 18.3** Hormonal modulators of bone mass via influences on linear growth and development and bone density. **Hormone** **Effect on linear bone growth and development** **Effect on adult bone density** Growth hormone \+ \+ Thyroid hormone \+ \- Vitamin D \+ \+ Estradiol \+ \+ Testosterone \+ \+ Parathyroid hormone \- \- Calcitonin \+ \+ The **sex hormones** estradiol (a type of estrogen) and testosterone are also associated with linear bone growth. The impact of both sex hormones on growth is likely due in part to their stimulation of growth hormone secretion. Testosterone can be converted to estradiol, so some (but not all) bone responses to testosterone also occur indirectly through estradiol. Estradiol contributes to the adolescent growth spurt but, in parallel, also promotes the slow conversion of chondrocytes to a senescent (aging) phenotype (see Concept 10.5.02). When senescence eventually occurs, the growth plates close and linear bone growth ends. Although linear bone growth eventually ends, bone remodeling and the need to regulate blood calcium levels continues throughout life, and hormonal regulation of these processes remains vital. A major hormonal player in such regulation is [parathyroid hormone](javascript:void(0)), which indirectly stimulates bone resorption by stimulating osteoblasts to secrete factors promoting osteoclast activity. Parathyroid hormone also influences bone by stimulating loss of phosphate via urine excretion. Another hormone, [calcitonin](javascript:void(0)), is thought to prevent excessive blood calcium concentrations by limiting bone resorption. Growth hormone and sex hormones continue to promote increased bone density throughout life, as does vitamin D. In contrast, there is a general inverse relationship between thyroid hormone levels and bone mineral density in adulthood, and excessive thyroid hormone concentrations can overstimulate bone resorption relative to bone deposition, leading to pathologically reduced bone density.