{"id":589,"date":"2021-01-18T03:06:47","date_gmt":"2021-01-18T03:06:47","guid":{"rendered":"https:\/\/pressbooks.publishdot.com\/anatomyphysiology\/chapter\/10-3-bone-structure\/"},"modified":"2021-12-07T09:38:34","modified_gmt":"2021-12-07T09:38:34","slug":"10-3-bone-structure","status":"publish","type":"chapter","link":"https:\/\/pressbooks.publishdot.com\/anatomyphysiology\/chapter\/10-3-bone-structure\/","title":{"raw":"10.3 Bone Structure","rendered":"10.3 Bone Structure"},"content":{"raw":"
\n

Learning Objectives<\/strong><\/p>\n\n<\/header>\n

\n\nBy the end of this section, you will be able to:\n
    \n \t
  • Identify the anatomical features of a bone<\/li>\n \t
  • Define and list examples of bone markings<\/li>\n \t
  • Describe the histology of bone tissue<\/li>\n \t
  • Compare and contrast compact and spongy bone<\/li>\n \t
  • Identify the structures that compose compact and spongy bone<\/li>\n \t
  • Describe how bones are nourished and innervated<\/li>\n<\/ul>\n<\/div>\n<\/div>\nBone tissue (osseous tissue) differs from other tissues in the body. Bone is hard and many of its functions depend on that characteristic hardness. Later discussions in this chapter will show that bone is also dynamic in that its shape adjusts to accommodate stresses. This section will examine the gross anatomy of bone first and then move on to its histology.\n

    Gross Anatomy of Bone<\/h2>\nThe structure of a long bone allows for the best visualisation of all the parts of a bone (Figure 10.3.1). A long bone has two parts: the\u00a0diaphysis<\/strong>\u00a0and the\u00a0epiphysis<\/strong>. The diaphysis is the tubular shaft that runs between the proximal and distal ends of the bone. The hollow region in the diaphysis is called the\u00a0medullary cavity<\/strong>, which is filled with yellow marrow. The walls of the diaphysis are composed of dense and hard\u00a0compact bone<\/strong>.\n\n[caption id=\"attachment_588\" align=\"aligncenter\" width=\"610\"]\"Diagram Figure 10.3.1. Anatomy of a long bone.<\/strong> A typical long bone shows the gross anatomical characteristics of bone.[\/caption]\n\nThe wider section at each end of the bone is called the epiphysis<\/strong> (plural = epiphyses), which is filled with spongy bone<\/strong>. Red marrow fills the spaces in the spongy bone. Each epiphysis meets the diaphysis at the metaphysis, the narrow area that contains the\u00a0epiphyseal plate<\/strong>\u00a0(growth plate), a layer of hyaline (transparent) cartilage in a growing bone. When the bone stops growing in early adulthood (approximately 18\u201321 years), the cartilage is replaced by osseous tissue and the epiphyseal plate becomes an epiphyseal line.\n\nThe medullary cavity has a delicate membranous lining called the\u00a0endosteum<\/strong>\u00a0(end- = \u201cinside\u201d; oste- = \u201cbone\u201d), where bone growth, repair, and remodelling occur. The outer surface of the bone is covered with a fibrous membrane called the\u00a0periosteum<\/strong>\u00a0(peri\u2013 =\u00a0\u201caround\u201d or \u201csurrounding\u201d). The periosteum contains blood vessels, nerves, and lymphatic vessels that nourish compact bone. Tendons and ligaments also attach to bones at the periosteum. The periosteum covers the entire outer surface except where the epiphyses meet other bones to form joints (Figure 10.3.2). In this region, the epiphyses are covered with\u00a0articular cartilage<\/strong>, a thin layer of cartilage that reduces friction and acts as a shock absorber.\n\n[caption id=\"attachment_588\" align=\"aligncenter\" width=\"936\"]\"Periosteum Figure 10.3.2. Periosteum and endosteum.<\/strong> The periosteum forms the outer surface of bone, and the endosteum lines the medullary cavity.[\/caption]\n\nFlat bones, like those of the cranium, consist of a layer of\u00a0diplo\u00eb<\/strong>\u00a0(spongy bone), lined on either side by a layer of compact bone (Figure 10.3.3). The two layers of compact bone and the interior spongy bone work together to protect the internal organs. If the outer layer of a cranial bone fractures, the brain is still protected by the intact inner layer.\n\n[caption id=\"attachment_588\" align=\"aligncenter\" width=\"934\"]\"Diagram Figure 10.3.3. Anatomy of a flat bone.<\/strong> This cross-section of a flat bone shows the spongy bone (diplo\u00eb) lined on either side by a layer of compact bone.[\/caption]\n

    Bone Markings<\/h2>\nThe surface features of bones vary depending on the function and location in the body.\u00a0Table 10.3.1\u00a0describes the bone markings, which are illustrated in (Figure 10.3.4). There are three general classes of bone markings: (1) articulations, (2) projections and (3) holes. As the name implies, an\u00a0articulation<\/strong>\u00a0is where two bone surfaces come together (articulus = \u201cjoint\u201d). These surfaces tend to conform to one another, such as one being rounded and the other cupped, to facilitate the function of the articulation. A\u00a0projection<\/strong>\u00a0is an area of a bone that projects above the surface of the bone. These are the attachment points for tendons and ligaments. In general, their size and shape are an indication of the forces exerted through the attachment to the bone. A\u00a0hole<\/strong>\u00a0is an opening or groove in the bone that allows blood vessels and nerves to enter the bone. As with the other markings, their size and shape reflect the size of the vessels and nerves that penetrate the bone at these points.\n\nTable 10.3.1<\/strong> Bone Markings\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
    Marking <\/strong><\/td>\nDescription <\/strong><\/td>\nExample <\/strong><\/td>\n<\/tr>\n
    Articulations<\/td>\nWhere two bones meet<\/td>\nKnee joint<\/td>\n<\/tr>\n
    Canal<\/td>\nPassage in bone<\/td>\nAuditory canal<\/td>\n<\/tr>\n
    Condyle<\/td>\nRounded surface<\/td>\nOccipital condyles<\/td>\n<\/tr>\n
    Crest<\/td>\nRidge<\/td>\nIliac crest<\/td>\n<\/tr>\n
    Facet<\/td>\nFlat surface<\/td>\nVertebrae<\/td>\n<\/tr>\n
    Fissure<\/td>\nSlit through bone<\/td>\nAuricular fissure<\/td>\n<\/tr>\n
    Foramen<\/td>\nHole through bone<\/td>\nForamen magnum in the occipital bone<\/td>\n<\/tr>\n
    Fossa<\/td>\nElongated basin<\/td>\nMandibular fossa<\/td>\n<\/tr>\n
    Fovea<\/td>\nSmall pit<\/td>\nFovea capitis on the head of the femur<\/td>\n<\/tr>\n
    Head<\/td>\nProminent rounded surface<\/td>\nHead of femur<\/td>\n<\/tr>\n
    Holes<\/td>\nHoles and depressions<\/td>\nForamen (holes through which blood vessels can pass through)<\/td>\n<\/tr>\n
    Line<\/td>\nSlight, elongated ridge<\/td>\nTemporal lines of the parietal bones<\/td>\n<\/tr>\n
    Meatus<\/td>\nOpening into canal<\/td>\nExternal auditory meatus<\/td>\n<\/tr>\n
    Process<\/td>\nProminence feature<\/td>\nTransverse process of vertebra<\/td>\n<\/tr>\n
    Projections<\/td>\nRaised Markings<\/td>\nSpinous process of the vertebrae<\/td>\n<\/tr>\n
    Protuberance<\/td>\nProtruding<\/td>\nChin<\/td>\n<\/tr>\n
    Sinus<\/td>\nAir-filled space in bone<\/td>\nNasal sinus<\/td>\n<\/tr>\n
    Spine<\/td>\nSharp process<\/td>\nIschial spine<\/td>\n<\/tr>\n
    Sulcus<\/td>\nGroove<\/td>\nSigmoid sulcus of the temporal bones<\/td>\n<\/tr>\n
    Tubercle<\/td>\nSmall, rounded process<\/td>\nDeltoid tuberosity<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n[caption id=\"attachment_588\" align=\"aligncenter\" width=\"935\"]\"Diagram Figure 10.3.4. Bone features.<\/strong> The surface features of bones depend on their function, location, attachment of ligaments and tendons, or the penetration of blood vessels and nerves.[\/caption]\n

    Bone Cells and Tissue<\/h2>\nBone contains a relatively small number of cells entrenched in a matrix of collagen fibres that provide a surface for inorganic salt crystals to adhere. These salt crystals form when calcium phosphate and calcium carbonate combine to create hydroxyapatite, which incorporates other inorganic salts like magnesium hydroxide, fluoride and sulphate as it crystallises, or calcifies, on the collagen fibres. The hydroxyapatite crystals give bones their hardness and strength, while the collagen fibres give them flexibility so that they are not brittle.\n\nAlthough bone cells compose a small amount of the bone volume, they are crucial to the function of bones. Four types of cells are found within bone tissue: osteoblasts<\/strong>, osteocytes, osteogenic cells<\/strong> and osteoclasts<\/strong> (Figure 10.3.5).\n\n[caption id=\"attachment_588\" align=\"aligncenter\" width=\"940\"]\"Diagram Figure 10.3.5. Bone cells.<\/strong> Four types of cells are found within bone tissue. Osteogenic cells are undifferentiated and develop into osteoblasts. When osteoblasts get trapped within the calcified matrix, their structure and function changes, and they become osteocytes. Osteoclasts develop from monocytes and macrophages and differ in appearance from other bone cells.[\/caption]\n\nThe\u00a0osteoblast\u00a0is the bone cell responsible for forming new bone and is found in the growing portions of bone, including the periosteum and endosteum. Osteoblasts, which do not divide, synthesise and secrete the collagen matrix and calcium salts. As the secreted matrix surrounding the osteoblast calcifies, the osteoblast become trapped within it; as a result, it changes in structure and becomes an\u00a0osteocyte, the primary cell of mature bone and the most common type of bone cell. Each osteocyte is located in a space called a\u00a0lacuna\u00a0and is surrounded by bone tissue. Osteocytes maintain the mineral concentration of the matrix via the secretion of enzymes. Like osteoblasts, osteocytes lack mitotic activity. They can communicate with each other and receive nutrients via long cytoplasmic processes that extend through\u00a0canaliculi<\/strong>\u00a0(singular = canaliculus), channels within the bone matrix.\n\nIf osteoblasts and osteocytes are incapable of mitosis, then how are they replenished when old ones die? The answer lies in the properties of a third category of bone cells\u2014the\u00a0osteogenic cell. These osteogenic cells are undifferentiated with high mitotic activity and they are the only bone cells that divide. Immature osteogenic cells are found in the deep layers of the periosteum and the marrow. They differentiate and develop into osteoblasts.\n\nThe dynamic nature of bone means that new tissue is constantly formed, and old, injured, or unnecessary bone is dissolved for repair or for calcium release. The cell responsible for bone resorption, or breakdown, is the\u00a0osteoclast. They are found on bone surfaces, are multinucleated, and originate from monocytes and macrophages, two types of white blood cells, not from osteogenic cells. Osteoclasts are continually breaking down old bone while osteoblasts are continually forming new bone. The ongoing balance between osteoblasts and osteoclasts is responsible for the constant but subtle reshaping of bone.\u00a0Table 10.3.2\u00a0reviews the bone cells, their functions and locations.\n\nTable 10.3.2. <\/strong>Bone Cells\n\n\n\n\n\n\n\n
    Cell type<\/strong><\/td>\nFunction<\/strong><\/td>\nLocation<\/strong><\/td>\n<\/tr>\n
    Osteogenic cells<\/td>\nDevelop into osteoblasts<\/td>\nDeep layers of the periosteum and the marrow<\/td>\n<\/tr>\n
    Osteoblasts<\/td>\nBone formation<\/td>\nGrowing portions of bone, including periosteum and endosteum<\/td>\n<\/tr>\n
    Osteocytes<\/td>\nMaintain mineral concentration of matrix<\/td>\nEntrapped in matrix<\/td>\n<\/tr>\n
    Osteoclasts<\/td>\nBone resorption<\/td>\nBone surfaces and at sites of old, injured or unneeded bone<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

    Compact and Spongy Bone<\/h2>\nThe differences between compact and spongy bone are best explored via their histology. Most bones contain compact and spongy osseous tissue, but their distribution and concentration vary based on the bone\u2019s overall function. Compact bone is dense so that it can withstand compressive forces, while spongy (cancellous) bone has open spaces and supports shifts in weight distribution.\n

    Compact Bone<\/h2>\nCompact bone is the denser, stronger of the two types of bone tissue (Figure 10.3.6). It can be found under the periosteum and in the diaphyses of long bones, where it provides support and protection.\n\n[caption id=\"attachment_588\" align=\"aligncenter\" width=\"936\"]\"Diagram Figure 10.3.6. Diagram of compact bone.<\/strong> (a) This cross-sectional view of compact bone shows the basic structural unit, the osteon. (b) In this micrograph of the osteon, you can clearly see the concentric lamellae and central canals. LM \u00d7 40. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012).[\/caption]\n\nThe microscopic structural unit of compact bone is called an\u00a0osteon<\/strong>, or Haversian system. Each osteon is composed of concentric rings of calcified matrix called lamellae (singular = lamella). Running down the centre of each osteon is the\u00a0central canal<\/strong>, or Haversian canal, which contains blood vessels, nerves, and lymphatic vessels. These vessels and nerves branch off at right angles through a\u00a0perforating canal<\/strong>, also known as Volkmann\u2019s canals, to extend to the periosteum and endosteum.\n\nThe osteocytes are located inside spaces called lacunae<\/strong> (singular = lacuna), found at the borders of adjacent lamellae. As described earlier, canaliculi connect with the canaliculi of other lacunae and eventually with the central canal. This system allows nutrients to be transported to the osteocytes and wastes to be removed from them.\n

    Spongy (Cancellous) Bone<\/h2>\nLike compact bone,\u00a0spongy bone<\/strong>, also known as cancellous bone, contains osteocytes housed in lacunae, but they are not arranged in concentric circles. Instead, the lacunae and osteocytes are found in a lattice-like network of matrix spikes called\u00a0trabeculae<\/strong>\u00a0(singular = trabecula) (Figure 10.3.7). The trabeculae may appear to be a random network, but each trabecula forms along lines of stress to provide strength to the bone. The spaces of the trabeculated network provide balance to the dense and heavy compact bone by making bones lighter so that muscles can move them more easily. In addition, the spaces in some spongy bones contain red marrow, protected by the trabeculae, where haematopoiesis occurs.\n\n[caption id=\"attachment_588\" align=\"aligncenter\" width=\"936\"]\"Diagram Figure 10.3.7. Diagram of spongy bone.<\/strong> Spongy bone is composed of trabeculae that contain the osteocytes. Red marrow fills the spaces in some bones.[\/caption]\n

    Ageing and the Skeletal System: Paget\u2019s Disease<\/h2>\nPaget\u2019s disease usually occurs in adults over age 40. It is a disorder of the bone remodelling process that begins with overactive osteoclasts. This means more bone is resorbed than is laid down. The osteoblasts try to compensate but the new bone they lay down is weak and brittle and therefore prone to fracture.\n\nWhile some people with Paget\u2019s disease have no symptoms, others experience pain, bone fractures, and bone deformities (Figure 10.3.8). Bones of the pelvis, skull, spine, and legs are the most affected. When occurring in the skull, Paget\u2019s disease can cause headaches and hearing loss.\n\n[caption id=\"attachment_588\" align=\"aligncenter\" width=\"993\"]\"Normal Figure 10.3.8. Paget\u2019s disease.<\/strong> Normal leg bones are relatively straight, but those affected by Paget\u2019s disease are porous and curved.[\/caption]\n\nWhat causes the osteoclasts to become overactive? The answer is still unknown, but hereditary factors play a role. Some scientists believe Paget\u2019s disease is due to an as-yet-unidentified virus.\n\nPaget\u2019s disease is diagnosed via imaging studies and lab tests. X-rays may show bone deformities or areas of bone resorption. Bone scans are also useful. In these studies, a dye containing a radioactive ion is injected into the body. Areas of bone resorption have an affinity for the ion, so they will light up on the scan if the ions are absorbed. In addition, blood levels of an enzyme called alkaline phosphatase are typically elevated in people with Paget\u2019s disease.\n\nBisphosphonates, drugs that decrease the activity of osteoclasts, are often used in the treatment of Paget\u2019s disease. However, in a small percentage of cases, bisphosphonates themselves have been linked to an increased risk of fractures because the old bone that is left after bisphosphonates are administered becomes worn out and brittle. Still, most doctors feel that the benefits of bisphosphonates more than outweigh the risk; the medical professional must weigh the benefits and risks on a case-by-case basis. Bisphosphonate treatment can reduce the overall risk of deformities or fractures, which in turn reduces the risk of surgical repair and its associated risks and complications.\n

    Blood and Nerve Supply<\/h2>\nThe spongy bone and medullary cavity receive nourishment from arteries that pass through the compact bone. The arteries enter through the\u00a0nutrient foramen<\/strong>\u00a0(plural = foramina), small openings in the diaphysis. The osteocytes in spongy bone are nourished by blood vessels of the periosteum that penetrate spongy bone and blood that circulates in the marrow cavities. As the blood passes through the marrow cavities, it is collected by veins, which then pass out of the bone through the foramina.\n\nIn addition to the blood vessels, nerves follow the same paths into the bone where they tend to concentrate in the more metabolically active regions of the bone. The nerves sense pain, and the nerves also play roles in regulating blood supplies and in bone growth, hence their concentrations in metabolically active sites of the bone.\n\n[caption id=\"attachment_588\" align=\"aligncenter\" width=\"744\"]\"Diagram Figure 10.3.9. Diagram of blood and nerve supply to bone.<\/strong> Blood vessels and nerves enter the bone through the nutrient foramen.[\/caption]\n\n
    \n

    Section Review<\/strong><\/p>\n\n<\/header>\n

    \n\nA hollow medullary cavity filled with yellow marrow runs the length of the diaphysis of a long bone. The walls of the diaphysis are compact bone. The epiphyses, which are wider sections at each end of a long bone, are filled with spongy bone and red marrow. The epiphyseal plate, a layer of hyaline cartilage, is replaced by osseous tissue as the organ grows in length. The medullary cavity has a delicate membranous lining called the endosteum. The outer surface of bone, except in regions covered with articular cartilage, is covered with a fibrous membrane called the periosteum. Flat bones consist of two layers of compact bone surrounding a layer of spongy bone. Bone markings depend on the function and location of bones. Articulations are places where two bones meet. Projections stick out from the surface of the bone and provide attachment points for tendons and ligaments. Holes are openings or depressions in the bones.\n\nBone matrix consists of collagen fibres and organic ground substance, primarily hydroxyapatite formed from calcium salts. Osteogenic cells develop into osteoblasts. Osteoblasts are cells that make new bone. They become osteocytes, the cells of mature bone, when they get trapped in the matrix. Osteoclasts engage in bone resorption. Compact bone is dense and composed of osteons, while spongy bone is less dense and made up of trabeculae. Blood vessels and nerves enter the bone through the nutrient foramina to nourish and innervate bones.\n\n<\/div>\n<\/div>\n
    \n

    Review Questions<\/strong><\/p>\n\n<\/header>\n

    \n\n[h5p id=\"288\"]\n\n<\/div>\n<\/div>\n
    \n

    Critical Thinking Questions<\/strong><\/p>\n\n<\/header>\n

    \n\n[h5p id=\"289\"]\n\n[h5p id=\"290\"]\n\n<\/div>\n<\/div>\nClick the drop down below to review the terms learned from this chapter.\n\n[h5p id=\"291\"]","rendered":"
    \n
    \n

    Learning Objectives<\/strong><\/p>\n<\/header>\n

    \n

    By the end of this section, you will be able to:<\/p>\n

      \n
    • Identify the anatomical features of a bone<\/li>\n
    • Define and list examples of bone markings<\/li>\n
    • Describe the histology of bone tissue<\/li>\n
    • Compare and contrast compact and spongy bone<\/li>\n
    • Identify the structures that compose compact and spongy bone<\/li>\n
    • Describe how bones are nourished and innervated<\/li>\n<\/ul>\n<\/div>\n<\/div>\n

      Bone tissue (osseous tissue) differs from other tissues in the body. Bone is hard and many of its functions depend on that characteristic hardness. Later discussions in this chapter will show that bone is also dynamic in that its shape adjusts to accommodate stresses. This section will examine the gross anatomy of bone first and then move on to its histology.<\/p>\n

      Gross Anatomy of Bone<\/h2>\n

      The structure of a long bone allows for the best visualisation of all the parts of a bone (Figure 10.3.1). A long bone has two parts: the\u00a0diaphysis<\/strong>\u00a0and the\u00a0epiphysis<\/strong>. The diaphysis is the tubular shaft that runs between the proximal and distal ends of the bone. The hollow region in the diaphysis is called the\u00a0medullary cavity<\/strong>, which is filled with yellow marrow. The walls of the diaphysis are composed of dense and hard\u00a0compact bone<\/strong>.<\/p>\n

      \"Diagram
      Figure 10.3.1. Anatomy of a long bone.<\/strong> A typical long bone shows the gross anatomical characteristics of bone.<\/figcaption><\/figure>\n

      The wider section at each end of the bone is called the epiphysis<\/strong> (plural = epiphyses), which is filled with spongy bone<\/strong>. Red marrow fills the spaces in the spongy bone. Each epiphysis meets the diaphysis at the metaphysis, the narrow area that contains the\u00a0epiphyseal plate<\/strong>\u00a0(growth plate), a layer of hyaline (transparent) cartilage in a growing bone. When the bone stops growing in early adulthood (approximately 18\u201321 years), the cartilage is replaced by osseous tissue and the epiphyseal plate becomes an epiphyseal line.<\/p>\n

      The medullary cavity has a delicate membranous lining called the\u00a0endosteum<\/strong>\u00a0(end- = \u201cinside\u201d; oste- = \u201cbone\u201d), where bone growth, repair, and remodelling occur. The outer surface of the bone is covered with a fibrous membrane called the\u00a0periosteum<\/strong>\u00a0(peri\u2013 =\u00a0\u201caround\u201d or \u201csurrounding\u201d). The periosteum contains blood vessels, nerves, and lymphatic vessels that nourish compact bone. Tendons and ligaments also attach to bones at the periosteum. The periosteum covers the entire outer surface except where the epiphyses meet other bones to form joints (Figure 10.3.2). In this region, the epiphyses are covered with\u00a0articular cartilage<\/strong>, a thin layer of cartilage that reduces friction and acts as a shock absorber.<\/p>\n

      \"Periosteum
      Figure 10.3.2. Periosteum and endosteum.<\/strong> The periosteum forms the outer surface of bone, and the endosteum lines the medullary cavity.<\/figcaption><\/figure>\n

      Flat bones, like those of the cranium, consist of a layer of\u00a0diplo\u00eb<\/strong>\u00a0(spongy bone), lined on either side by a layer of compact bone (Figure 10.3.3). The two layers of compact bone and the interior spongy bone work together to protect the internal organs. If the outer layer of a cranial bone fractures, the brain is still protected by the intact inner layer.<\/p>\n

      \"Diagram
      Figure 10.3.3. Anatomy of a flat bone.<\/strong> This cross-section of a flat bone shows the spongy bone (diplo\u00eb) lined on either side by a layer of compact bone.<\/figcaption><\/figure>\n

      Bone Markings<\/h2>\n

      The surface features of bones vary depending on the function and location in the body.\u00a0Table 10.3.1\u00a0describes the bone markings, which are illustrated in (Figure 10.3.4). There are three general classes of bone markings: (1) articulations, (2) projections and (3) holes. As the name implies, an\u00a0articulation<\/strong>\u00a0is where two bone surfaces come together (articulus = \u201cjoint\u201d). These surfaces tend to conform to one another, such as one being rounded and the other cupped, to facilitate the function of the articulation. A\u00a0projection<\/strong>\u00a0is an area of a bone that projects above the surface of the bone. These are the attachment points for tendons and ligaments. In general, their size and shape are an indication of the forces exerted through the attachment to the bone. A\u00a0hole<\/strong>\u00a0is an opening or groove in the bone that allows blood vessels and nerves to enter the bone. As with the other markings, their size and shape reflect the size of the vessels and nerves that penetrate the bone at these points.<\/p>\n

      Table 10.3.1<\/strong> Bone Markings<\/p>\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
      Marking <\/strong><\/td>\nDescription <\/strong><\/td>\nExample <\/strong><\/td>\n<\/tr>\n
      Articulations<\/td>\nWhere two bones meet<\/td>\nKnee joint<\/td>\n<\/tr>\n
      Canal<\/td>\nPassage in bone<\/td>\nAuditory canal<\/td>\n<\/tr>\n
      Condyle<\/td>\nRounded surface<\/td>\nOccipital condyles<\/td>\n<\/tr>\n
      Crest<\/td>\nRidge<\/td>\nIliac crest<\/td>\n<\/tr>\n
      Facet<\/td>\nFlat surface<\/td>\nVertebrae<\/td>\n<\/tr>\n
      Fissure<\/td>\nSlit through bone<\/td>\nAuricular fissure<\/td>\n<\/tr>\n
      Foramen<\/td>\nHole through bone<\/td>\nForamen magnum in the occipital bone<\/td>\n<\/tr>\n
      Fossa<\/td>\nElongated basin<\/td>\nMandibular fossa<\/td>\n<\/tr>\n
      Fovea<\/td>\nSmall pit<\/td>\nFovea capitis on the head of the femur<\/td>\n<\/tr>\n
      Head<\/td>\nProminent rounded surface<\/td>\nHead of femur<\/td>\n<\/tr>\n
      Holes<\/td>\nHoles and depressions<\/td>\nForamen (holes through which blood vessels can pass through)<\/td>\n<\/tr>\n
      Line<\/td>\nSlight, elongated ridge<\/td>\nTemporal lines of the parietal bones<\/td>\n<\/tr>\n
      Meatus<\/td>\nOpening into canal<\/td>\nExternal auditory meatus<\/td>\n<\/tr>\n
      Process<\/td>\nProminence feature<\/td>\nTransverse process of vertebra<\/td>\n<\/tr>\n
      Projections<\/td>\nRaised Markings<\/td>\nSpinous process of the vertebrae<\/td>\n<\/tr>\n
      Protuberance<\/td>\nProtruding<\/td>\nChin<\/td>\n<\/tr>\n
      Sinus<\/td>\nAir-filled space in bone<\/td>\nNasal sinus<\/td>\n<\/tr>\n
      Spine<\/td>\nSharp process<\/td>\nIschial spine<\/td>\n<\/tr>\n
      Sulcus<\/td>\nGroove<\/td>\nSigmoid sulcus of the temporal bones<\/td>\n<\/tr>\n
      Tubercle<\/td>\nSmall, rounded process<\/td>\nDeltoid tuberosity<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
      \"Diagram
      Figure 10.3.4. Bone features.<\/strong> The surface features of bones depend on their function, location, attachment of ligaments and tendons, or the penetration of blood vessels and nerves.<\/figcaption><\/figure>\n

      Bone Cells and Tissue<\/h2>\n

      Bone contains a relatively small number of cells entrenched in a matrix of collagen fibres that provide a surface for inorganic salt crystals to adhere. These salt crystals form when calcium phosphate and calcium carbonate combine to create hydroxyapatite, which incorporates other inorganic salts like magnesium hydroxide, fluoride and sulphate as it crystallises, or calcifies, on the collagen fibres. The hydroxyapatite crystals give bones their hardness and strength, while the collagen fibres give them flexibility so that they are not brittle.<\/p>\n

      Although bone cells compose a small amount of the bone volume, they are crucial to the function of bones. Four types of cells are found within bone tissue: osteoblasts<\/strong>, osteocytes, osteogenic cells<\/strong> and osteoclasts<\/strong> (Figure 10.3.5).<\/p>\n

      \"Diagram
      Figure 10.3.5. Bone cells.<\/strong> Four types of cells are found within bone tissue. Osteogenic cells are undifferentiated and develop into osteoblasts. When osteoblasts get trapped within the calcified matrix, their structure and function changes, and they become osteocytes. Osteoclasts develop from monocytes and macrophages and differ in appearance from other bone cells.<\/figcaption><\/figure>\n

      The\u00a0osteoblast\u00a0is the bone cell responsible for forming new bone and is found in the growing portions of bone, including the periosteum and endosteum. Osteoblasts, which do not divide, synthesise and secrete the collagen matrix and calcium salts. As the secreted matrix surrounding the osteoblast calcifies, the osteoblast become trapped within it; as a result, it changes in structure and becomes an\u00a0osteocyte, the primary cell of mature bone and the most common type of bone cell. Each osteocyte is located in a space called a\u00a0lacuna\u00a0and is surrounded by bone tissue. Osteocytes maintain the mineral concentration of the matrix via the secretion of enzymes. Like osteoblasts, osteocytes lack mitotic activity. They can communicate with each other and receive nutrients via long cytoplasmic processes that extend through\u00a0canaliculi<\/strong>\u00a0(singular = canaliculus), channels within the bone matrix.<\/p>\n

      If osteoblasts and osteocytes are incapable of mitosis, then how are they replenished when old ones die? The answer lies in the properties of a third category of bone cells\u2014the\u00a0osteogenic cell. These osteogenic cells are undifferentiated with high mitotic activity and they are the only bone cells that divide. Immature osteogenic cells are found in the deep layers of the periosteum and the marrow. They differentiate and develop into osteoblasts.<\/p>\n

      The dynamic nature of bone means that new tissue is constantly formed, and old, injured, or unnecessary bone is dissolved for repair or for calcium release. The cell responsible for bone resorption, or breakdown, is the\u00a0osteoclast. They are found on bone surfaces, are multinucleated, and originate from monocytes and macrophages, two types of white blood cells, not from osteogenic cells. Osteoclasts are continually breaking down old bone while osteoblasts are continually forming new bone. The ongoing balance between osteoblasts and osteoclasts is responsible for the constant but subtle reshaping of bone.\u00a0Table 10.3.2\u00a0reviews the bone cells, their functions and locations.<\/p>\n

      Table 10.3.2. <\/strong>Bone Cells<\/p>\n\n\n\n\n\n\n\n
      Cell type<\/strong><\/td>\nFunction<\/strong><\/td>\nLocation<\/strong><\/td>\n<\/tr>\n
      Osteogenic cells<\/td>\nDevelop into osteoblasts<\/td>\nDeep layers of the periosteum and the marrow<\/td>\n<\/tr>\n
      Osteoblasts<\/td>\nBone formation<\/td>\nGrowing portions of bone, including periosteum and endosteum<\/td>\n<\/tr>\n
      Osteocytes<\/td>\nMaintain mineral concentration of matrix<\/td>\nEntrapped in matrix<\/td>\n<\/tr>\n
      Osteoclasts<\/td>\nBone resorption<\/td>\nBone surfaces and at sites of old, injured or unneeded bone<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

      Compact and Spongy Bone<\/h2>\n

      The differences between compact and spongy bone are best explored via their histology. Most bones contain compact and spongy osseous tissue, but their distribution and concentration vary based on the bone\u2019s overall function. Compact bone is dense so that it can withstand compressive forces, while spongy (cancellous) bone has open spaces and supports shifts in weight distribution.<\/p>\n

      Compact Bone<\/h2>\n

      Compact bone is the denser, stronger of the two types of bone tissue (Figure 10.3.6). It can be found under the periosteum and in the diaphyses of long bones, where it provides support and protection.<\/p>\n

      \"Diagram
      Figure 10.3.6. Diagram of compact bone.<\/strong> (a) This cross-sectional view of compact bone shows the basic structural unit, the osteon. (b) In this micrograph of the osteon, you can clearly see the concentric lamellae and central canals. LM \u00d7 40. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012).<\/figcaption><\/figure>\n

      The microscopic structural unit of compact bone is called an\u00a0osteon<\/strong>, or Haversian system. Each osteon is composed of concentric rings of calcified matrix called lamellae (singular = lamella). Running down the centre of each osteon is the\u00a0central canal<\/strong>, or Haversian canal, which contains blood vessels, nerves, and lymphatic vessels. These vessels and nerves branch off at right angles through a\u00a0perforating canal<\/strong>, also known as Volkmann\u2019s canals, to extend to the periosteum and endosteum.<\/p>\n

      The osteocytes are located inside spaces called lacunae<\/strong> (singular = lacuna), found at the borders of adjacent lamellae. As described earlier, canaliculi connect with the canaliculi of other lacunae and eventually with the central canal. This system allows nutrients to be transported to the osteocytes and wastes to be removed from them.<\/p>\n

      Spongy (Cancellous) Bone<\/h2>\n

      Like compact bone,\u00a0spongy bone<\/strong>, also known as cancellous bone, contains osteocytes housed in lacunae, but they are not arranged in concentric circles. Instead, the lacunae and osteocytes are found in a lattice-like network of matrix spikes called\u00a0trabeculae<\/strong>\u00a0(singular = trabecula) (Figure 10.3.7). The trabeculae may appear to be a random network, but each trabecula forms along lines of stress to provide strength to the bone. The spaces of the trabeculated network provide balance to the dense and heavy compact bone by making bones lighter so that muscles can move them more easily. In addition, the spaces in some spongy bones contain red marrow, protected by the trabeculae, where haematopoiesis occurs.<\/p>\n

      \"Diagram
      Figure 10.3.7. Diagram of spongy bone.<\/strong> Spongy bone is composed of trabeculae that contain the osteocytes. Red marrow fills the spaces in some bones.<\/figcaption><\/figure>\n

      Ageing and the Skeletal System: Paget\u2019s Disease<\/h2>\n

      Paget\u2019s disease usually occurs in adults over age 40. It is a disorder of the bone remodelling process that begins with overactive osteoclasts. This means more bone is resorbed than is laid down. The osteoblasts try to compensate but the new bone they lay down is weak and brittle and therefore prone to fracture.<\/p>\n

      While some people with Paget\u2019s disease have no symptoms, others experience pain, bone fractures, and bone deformities (Figure 10.3.8). Bones of the pelvis, skull, spine, and legs are the most affected. When occurring in the skull, Paget\u2019s disease can cause headaches and hearing loss.<\/p>\n

      \"Normal
      Figure 10.3.8. Paget\u2019s disease.<\/strong> Normal leg bones are relatively straight, but those affected by Paget\u2019s disease are porous and curved.<\/figcaption><\/figure>\n

      What causes the osteoclasts to become overactive? The answer is still unknown, but hereditary factors play a role. Some scientists believe Paget\u2019s disease is due to an as-yet-unidentified virus.<\/p>\n

      Paget\u2019s disease is diagnosed via imaging studies and lab tests. X-rays may show bone deformities or areas of bone resorption. Bone scans are also useful. In these studies, a dye containing a radioactive ion is injected into the body. Areas of bone resorption have an affinity for the ion, so they will light up on the scan if the ions are absorbed. In addition, blood levels of an enzyme called alkaline phosphatase are typically elevated in people with Paget\u2019s disease.<\/p>\n

      Bisphosphonates, drugs that decrease the activity of osteoclasts, are often used in the treatment of Paget\u2019s disease. However, in a small percentage of cases, bisphosphonates themselves have been linked to an increased risk of fractures because the old bone that is left after bisphosphonates are administered becomes worn out and brittle. Still, most doctors feel that the benefits of bisphosphonates more than outweigh the risk; the medical professional must weigh the benefits and risks on a case-by-case basis. Bisphosphonate treatment can reduce the overall risk of deformities or fractures, which in turn reduces the risk of surgical repair and its associated risks and complications.<\/p>\n

      Blood and Nerve Supply<\/h2>\n

      The spongy bone and medullary cavity receive nourishment from arteries that pass through the compact bone. The arteries enter through the\u00a0nutrient foramen<\/strong>\u00a0(plural = foramina), small openings in the diaphysis. The osteocytes in spongy bone are nourished by blood vessels of the periosteum that penetrate spongy bone and blood that circulates in the marrow cavities. As the blood passes through the marrow cavities, it is collected by veins, which then pass out of the bone through the foramina.<\/p>\n

      In addition to the blood vessels, nerves follow the same paths into the bone where they tend to concentrate in the more metabolically active regions of the bone. The nerves sense pain, and the nerves also play roles in regulating blood supplies and in bone growth, hence their concentrations in metabolically active sites of the bone.<\/p>\n

      \"Diagram
      Figure 10.3.9. Diagram of blood and nerve supply to bone.<\/strong> Blood vessels and nerves enter the bone through the nutrient foramen.<\/figcaption><\/figure>\n
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      Section Review<\/strong><\/p>\n<\/header>\n

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      A hollow medullary cavity filled with yellow marrow runs the length of the diaphysis of a long bone. The walls of the diaphysis are compact bone. The epiphyses, which are wider sections at each end of a long bone, are filled with spongy bone and red marrow. The epiphyseal plate, a layer of hyaline cartilage, is replaced by osseous tissue as the organ grows in length. The medullary cavity has a delicate membranous lining called the endosteum. The outer surface of bone, except in regions covered with articular cartilage, is covered with a fibrous membrane called the periosteum. Flat bones consist of two layers of compact bone surrounding a layer of spongy bone. Bone markings depend on the function and location of bones. Articulations are places where two bones meet. Projections stick out from the surface of the bone and provide attachment points for tendons and ligaments. Holes are openings or depressions in the bones.<\/p>\n

      Bone matrix consists of collagen fibres and organic ground substance, primarily hydroxyapatite formed from calcium salts. Osteogenic cells develop into osteoblasts. Osteoblasts are cells that make new bone. They become osteocytes, the cells of mature bone, when they get trapped in the matrix. Osteoclasts engage in bone resorption. Compact bone is dense and composed of osteons, while spongy bone is less dense and made up of trabeculae. Blood vessels and nerves enter the bone through the nutrient foramina to nourish and innervate bones.<\/p>\n<\/div>\n<\/div>\n

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      Review Questions<\/strong><\/p>\n<\/header>\n

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