5 The Skeletal System: Osseous Tissue and Skeletal Structure

Introduction The skeletal system is made of: Skeletal bones Cartilage Ligaments Connective tissue to stabilize the skeleton Bones are dynamic organs, which consist of several tissue types 2

Introduction Functions of the skeletal system Support Provides the framework for the attachment of other organs Storage of minerals Calcium ions: 98% of the body’s calcium ions are in the bones Phosphate ions Blood cell production Bone marrow produces erythrocytes, leukocytes, and platelets 3

Introduction Functions of the skeletal system (continued) Leverage Muscles pull on the bones to produce movement Protection Ribs protect heart and lungs Skull protects the brain Vertebrae protect the spinal cord Pelvic bones protect the reproductive organs 4

Anatomy of Skeletal Elements There are seven broad categories of bones according to their shapes Sutural bones Irregular bones Short bones Pneumatized bones Flat bones Long bones Sesamoid bones 5

Classification of Bones Long Bones: longer than they are wide, upper and lower limbs. Short Bones: Broad as they are long, cube shaped, round, i.e. tarsals & carpals Flat Bones: Thin, flat and curved, skull (parietal), sternum, scapulae. Irregular Bones: Don’t fit in the others, face, vertebrae Sesamoid: Sesame seed, patella, small and flat Sutural bones: Wormian bones, borders like a jigsaw puzzle.

Figure 5.11 Shapes of Bones 7 Sutural Bones Pneumatized Bones Flat Bones Sutures Parietal bone External table Sutural bone Internal table Diploë (spongy bone) Ethmoid Air cells Irregular Bones Long Bones Vertebra Humerus Short Bones Sesamoid Bones Carpal bones Patella 7

Structure of Bone Bones (osseous tissue) Supporting connective tissue Specialized cells Solid matrix Outer lining Called the periosteum Inner lining Called the endosteum 8

Structure of Bone The Histological Organization of Mature Bone The matrix Calcium phosphate eventually converts to hydroxyapatite crystals Hydroxyapatite crystals resist compression 9

Structure of Bone The Histological Organization of Mature Bone Collagen fibers Make up 2/3 of the bone matrix Contribute to the tensile strength of bones Collagen and hydroxyapatite make bone tissue extremely strong Bone cells Contribute only 2% of the bone mass 10

Structure of Bone The Cells of Mature Bone Osteocytes Osteoblasts Mature bone cells Maintain the protein and mineral content of the matrix Osteoblasts Immature bone cells Found on the inner and outer surfaces of bones Produce osteoid, which is involved in making the matrix Osteoblasts are involved in making new bone. This is a process called osteogenesis Osteoblasts can convert to osteocytes 11

Structure of Bone The Cells of Mature Bone (continued) Osteoprogenitor cells Found on the inner and outer surfaces of bones Differentiate to form new osteoblasts Heavily involved in the repair of bones after a break Osteoclasts Secrete acids, which dissolve the bones thereby causing the release of stored calcium ions and phosphate ions into the blood This process is called osteolysis 12

Figure 5.1a Histological Structure of a Typical Bone Canaliculi Osteocyte Matrix Endosteum Osteoprogenitor cell Medullary cavity Osteocyte: Mature bone cell that maintains the bone matrix Osteoprogenitor cell: Stem cell whose divisions produce osteoblasts Osteoblast Osteoid Matrix Osteoclast Matrix Medullary cavity Osteoblast: Immature bone cell that secretes organic components of matrix Osteoclast: Multinucleate cell that secretes acids and enzymes to dissolve bone matrix The cells of bone 13

Structure of Bone The Osteon It is the basic unit of skeletal bones Consists of: Central canal Canaliculi Osteocytes Lacunae Lamellae 14

Figure 5.1c Histological Structure of a Typical Bone Canaliculi Concentric lamellae Central canal Osteon Lacunae Osteons LM  220 A thin section through compact bone; in this procedure the intact matrix and central canals appear white, and the lacunae and canaliculi are shown in black. 15

Figure 5.1d Histological Structure of a Typical Bone Canaliculi Concentric lamellae Central canal Osteon Lacunae Osteon LM  343 A single osteon at higher magnification 16

Figure 5.1b Histological Structure of a Typical Bone Osteon Lacunae Central canals Lamellae Osteons SEM  182 A scanning electron micrograph of several osteons in compact bone 17

Structure of Bone Two types of osseous tissue Compact bone (dense bone) Compact bones are dense and solid Forms the walls of bone outlining the medullary cavity Medullary cavity consists of bone marrow Spongy bone (trabecular bone) Open network of plates 18

Figure 5.2a-c The Internal Organization in Representative Bones Concentric lamellae Spongy bone Blood vessels Collagen fiber orientation Compact bone Central canal Medullary cavity Endosteum Endosteum The organization of collagen fibers within concentric lamellae Periosteum Compact bone Spongy bone Medullary cavity Capillary Small vein Circumferential lamellae Gross anatomy of the humerus Concentric lamellae Osteons Periosteum Interstitial lamellae Trabeculae of spongy bone Artery Vein Perforating canal Central canal Diagrammatic view of the histological organization of compact and spongy bone 19

Figure 5.2d The Internal Organization in Representative Bones Trabeculae of spongy bone Endosteum Lamellae Canaliculi opening on surface Location and structure of spongy bone. The photo shows a sectional view of the proximal end of the femur. 20

Structure of Bone Structural Differences Compact bone Spongy bone Consists of osteons Makes up the dense, solid portion of bone Spongy bone Trabeculae are arranged in parallel struts Trabeculae form branching plates Trabeculae form an open network Creates the lightweight nature of bones 21

Structure of Bone Functional Differences Compact bone Spongy bone Conducts stress from one area of the body to another area of the body Generates tremendous strength from end to end Weak strength when stress is applied to the side Spongy bone Trabeculae create strength to deal with stress from the side 22

Figure 5.3a Anatomy of a Representative Bone Spongy bone Epiphysis Articular surface of head of femur Metaphysis Compact bone Diaphysis (shaft) Medullary cavity Metaphysis Epiphysis Posterior view Sectional view The femur, or thigh bone, in superficial and sectional views. The femur has a diaphysis (shaft) with walls of compact bone and epiphyses (ends) filled with spongy bone. A metaphysis separates the diaphysis and epiphysis at each end of the shaft. The body weight is transferred to the femur at the hip joint. Because the hip joint is off center relative to the axis of the shaft, the body weight is distributed along the bone so that the medial portion of the shaft is compressed and the lateral portion is stretched. 23

Figure 5.3b Anatomy of a Representative Bone An intact femur chemically cleared to show the orientation of the trabeculae in the epiphysis 24

Figure 5.3c Anatomy of a Representative Bone Articular surface of head of femur Trabeculae of spongy bone Cortex Medullary cavity Compact bone A photograph showing the epiphysis after sectioning 25

Structure of Bone Organization of Compact and Spongy Bone Epiphysis Each end of the long bones Diaphysis Shaft of the long bones Metaphysis Narrow growth zone between the epiphysis and the diaphysis 26

Figure 5.3a Anatomy of a Representative Bone Spongy bone Epiphysis Articular surface of head of femur Metaphysis Compact bone Diaphysis (shaft) Medullary cavity Metaphysis Epiphysis Posterior view Sectional view The femur, or thigh bone, in superficial and sectional views. The femur has a diaphysis (shaft) with walls of compact bone and epiphyses (ends) filled with spongy bone. A metaphysis separates the diaphysis and epiphysis at each end of the shaft. The body weight is transferred to the femur at the hip joint. Because the hip joint is off center relative to the axis of the shaft, the body weight is distributed along the bone so that the medial portion of the shaft is compressed and the lateral portion is stretched. 27

Structure of Bone The Periosteum and Endosteum Periosteum Outer surface of the bone Isolates and protects the bone from surrounding tissue Provides a route and a place for attachment for circulatory and nervous supply Actively participates in bone growth and repair Attaches the bone to the connective tissue network of the deep fascia 28

Structure of Bone The Periosteum and Endosteum Periosteum and Tendons Tendons are cemented into the lamellae by osteoblasts Therefore, tendons are actually a part of the bone 29

Figure 5.4c Anatomy and Histology of the Periosteum and Endosteum Zone of tendonbone attachment Tendon Periosteum Medullary cavity Endosteum Spongy bone of epiphysis Epiphyseal cartilage LM  100 A tendonbone junction 30

Structure of Bone The Periosteum and Endosteum Endosteum Inner surface of bone Lines the medullary cavity Consists of osteoprogenitor cells Actively involved in repair and growth 31

Figure 5.4ab Anatomy and Histology of the Periosteum and Endosteum Circumferential lamellae Joint capsule Cellular layer of periosteum Cellular layer of periosteum Fibrous layer of periosteum Fibrous layer of periosteum Canaliculi Lacuna Endosteum Compact bone Osteocyte Perforating fibers The periosteum contains outer (fibrous) and inner (cellular) layers. Collagen fibers of the periosteum are continuous with those of the bone, adjacent joint capsules, and attached tendons and ligaments. Bone matrix Giant multinucleate osteoclast Endosteum Osteoprogenitor cell Osteocyte Osteoid Osteoblasts The endosteum is an incomplete cellular layer containing osteoblasts, osteoprogenitor cells, and osteoclasts. 32

Bone Development and Growth Before six weeks of development, the skeleton is cartilage Cartilage cells will be replaced by bone cells This is called ossification Osteogenesis Bone formation Calcification The deposition of calcium ions into the bone tissue 33

Bone Development and Growth There are two types of ossification Intramembranous ossification Involved in the development of clavicle, mandible, skull, and face Endochondral ossification Involved in the development of limbs, vertebrae, and hips 34

Bone Development and Growth Intramembranous ossification Mesenchymal cells differentiate to form osteoblasts Osteoblasts begin secreting a matrix Osteoblasts become trapped in the matrix Osteoblasts differentiate and form osteocytes More osteoblasts are produced, thus move outward Eventually, compact bone is formed 35

Figure 5.5 Histology of Intramembranous Ossification Mesenchymal cells aggregate, differentiate into osteoblasts, and begin the ossification process. The bone expands as a series of spicules that spread into surrounding tissues. As the spicules interconnect, they trap blood vessels within the bone. Over time, the bone assumes the structure of spongy bone. Areas of spongy bone may later be removed, creating medullary cavities. Through remodeling, spongy bone formed in this way can be converted to compact bone. Osteocyte in lacuna Bone matrix Osteoblast Osteoid Embryonic connective tissue Blood vessel Osteocytes in lacunae Blood vessels Osteoblast layer Mesenchymal cell Blood vessel Blood vessel Osteoblasts Spicules LM  32 LM  32 36

Bone Development and Growth Endochondral ossification The developing bone begins as cartilage cells Cartilage matrix grows inward Interstitial growth Cartilage matrix grows outward Appositional growth Blood vessels grow around the cartilage 37

Bone Development and Growth Endochondral ossification (continued) Perichondrial cells convert to osteoblasts Osteoblasts develop a superficial layer of bone around the cartilage Blood vessels penetrate the cartilage Osteoblasts begin to develop spongy bone in the diaphysis This becomes the primary center of ossification 38

Enlarging chondrocytes within calcifying matrix Figure 5.7a Anatomical and Histological Organization of Endochondral Ossification (Part 1 of 2) As the cartilage enlarges, chondrocytes near the center of the shaft increase greatly in size. The matrix is reduced to a series of small struts that soon begin to calcify. The enlarged chondrocytes then die and disintegrate, leaving cavities within the cartilage. Blood vessels grow around the edges of the cartilage, and the cells of the perichondrium convert to osteoblasts. The shaft of the cartilage then becomes ensheathed in a superficial layer of bone. Enlarging chondrocytes within calcifying matrix Epiphysis Diaphysis Bone formation Hyaline cartilage Steps in the formation of a long bone from a hyaline cartilage model 39

Figure 5.7a Anatomical and Histological Organization of Endochondral Ossification (Part 2 of 2) Blood vessels penetrate the cartilage and invade the central region. Fibroblasts migrating with the blood vessels differentiate into osteoblasts and begin producing spongy bone at a primary center of ossification. Bone formation then spreads along the shaft toward both ends. Remodeling occurs as growth continues, creating a medullary cavity. The bone of the shaft becomes thicker, and the cartilage near each epiphysis is replaced by shafts of bone. Further growth involves increases in length (Steps 5 and 6) and diameter (see Figure 5.9). Medullary cavity Medullary cavity Blood vessel Primary ossification center Superficial bone See Figure 5.9 Spongy bone Metaphysis Steps in the formation of a long bone from a hyaline cartilage model 40

Bone Development and Growth Endochondral ossification (continued) The cartilage near the epiphysis converts to bone Blood vessels penetrate the epiphysis Osteoblasts begin to develop spongy bone in the epiphysis Epiphysis becomes the secondary center of ossification 41

Figure 5.7b Anatomical and Histological Organization of Endochondral Ossification (Part 1 of 2) Capillaries and osteoblasts migrate into the epiphyses, creating secondary ossification centers. Soon the epiphyses are filled with spongy bone. An articular cartilage remains exposed to the joint cavity; over time it will be reduced to a thin superficial layer. At each metaphysis, an epiphyseal cartilage separates the epiphysis from the diaphysis. Hyaline cartilage Articular cartilage Epiphysis Spongy bone Metaphysis Epiphyseal cartilage Periosteum Compact bone Diaphysis Secondary ossification center 42

Bone Development and Growth Epiphyseal plate Area of cartilage in the metaphysis Also called the epiphyseal cartilage Cartilage near the diaphysis is converted to bone The width of this zone gets narrower as we age 43

Epiphyseal cartilage matrix Cartilage cells undergoing division Figure 5.7b Anatomical and Histological Organization of Endochondral Ossification (Part 2 of 2) Epiphyseal cartilage matrix Cartilage cells undergoing division Zone of proliferation Zone of hypertrophy Medullary cavity Osteoblasts Osteoid Epiphyseal cartilage LM  250 Light micrograph showing the zones of cartilage and the advancing osteoblasts at an epiphyseal cartilage 44

Figure 5.7 Anatomical and Histological Organization of Endochondral Ossification As the cartilage enlarges, chondrocytes near the center of the shaft increase greatly in size. The matrix is reduced to a series of small struts that soon begin to calcify. The enlarged chondrocytes then die and disintegrate, leaving cavities within the cartilage. Blood vessels grow around the edges of the cartilage, and the cells of the perichondrium convert to osteoblasts. The shaft of the cartilage then becomes ensheathed in a superficial layer of bone. Blood vessels penetrate the cartilage and invade the central region. Fibroblasts migrating with the blood vessels differentiate into osteoblasts and begin producing spongy bone at a primary center of ossification. Bone formation then spreads along the shaft toward both ends. Remodeling occurs as growth continues, creating a medullary cavity. The bone of the shaft becomes thicker, and the cartilage near each epiphysis is replaced by shafts of bone. Further growth involves increases in length (Steps 5 and 6) and diameter (see Figure 5.9). Capillaries and osteoblasts migrate into the epiphyses, creating secondary ossification centers. Soon the epiphyses are filled with spongy bone. An articular cartilage remains exposed to the joint cavity; over time it will be reduced to a thin superficial layer. At each metaphysis, an epiphyseal cartilage separates the epiphysis from the diaphysis. Hyaline cartilage Epiphyseal cartilage matrix Cartilage cells undergoing division Articular cartilage Epiphysis Spongy bone Zone of proliferation Enlarging chondrocytes within calcifying matrix Epiphysis Metaphysis Zone of hypertrophy Epiphyseal cartilage Medullary cavity Periosteum Blood vessel Medullary cavity Primary ossification center Compact bone Diaphysis Diaphysis Superficial bone See Figure 5.9 Spongy bone Medullary cavity Osteoblasts Osteoid Bone formation Metaphysis Epiphyseal cartilage LM  250 Hyaline cartilage Secondary ossification center Light micrograph showing the zones of cartilage and the advancing osteoblasts at an epiphyseal cartilage Steps in the formation of a long bone from a hyaline cartilage model 45

Figure 5.8 Epiphyseal Cartilages and Lines X-ray of the hand of a young child. The arrows indicate the locations of the epiphyseal cartilages. X-ray of the hand of an adult. The arrows indicate the locations of epiphyseal lines. 46

Figure 5.6 Fetal Intramembranous and Endochondral Ossification Temporal bone Parietal bone Mandible Intramembranous ossification produces the roofing bones of the skull Clavicle Frontal bone Scapula Humerus Metacarpal bones Phalanges Ribs Radius Endochondral ossification replaces cartilages of embryonic skull Vertebrae Ulna Cartilage Hip bone (ilium) Primary ossification centers of the diaphyses (bones of the lower limb) Femur Fibula Tibia Phalanx Future hip bone Metatarsal bones At 10 weeks the fetal skull clearly shows both membrane and cartilaginous bone, but the boundaries that indicate the limits of future skull bones have yet to be established. At 16 weeks the fetal skull shows the irregular margins of the future skull bones. Most elements of the appendicular skeleton form through endochondral ossification. Note the appearance of the wrist and ankle bones at 16 weeks versus at 10 weeks. 47

Bone Development and Growth Enlarging the diameter of bone Called appositional growth Blood vessels that run parallel to the bone becomes surrounded by bone cells “Tunnels” begin to form Each “tunnel” has a blood vessel in it 48

Bone Development and Growth Enlarging the diameter of bone Osteoblasts begin to produce matrix, thus creating concentric rings As osteoblasts are laying down more bone material, osteoclasts are dissolving the inner bone, thus creating the marrow cavity 49

Figure 5.9a Appositional Bone Growth (Part 1 of 2) Bone formation at the surface of the bone produces ridges that parallel a blood vessel. The ridges enlarge and create a deep pocket. The ridges meet and fuse, trapping the vessel inside the bone. Ridge Periosteum Perforating canal Artery Three-dimensional diagrams illustrate the mechanism responsible for increasing the diameter of a growing bone. 50

Figure 5.9a Appositional Bone Growth (Part 2 of 2) Bone deposition proceeds inward toward the vessel, beginning the creation of a typical osteon. Additional circumferential lamellae are deposited and the bone continues to increase in diameter. Osteon is complete with new central canal around blood vessel. Second blood vessel becomes enclosed. Periosteum Circumferential lamellae Central canal of new osteon Three-dimensional diagrams illustrate the mechanism responsible for increasing the diameter of a growing bone. 51

Figure 5.9b Appositional Bone Growth Bone resorbed by osteoclasts Bone deposited by osteoblasts Infant Child Young adult Adult A bone grows in diameter as new bone is added to the outer surface. At the same time, osteoclasts resorb bone on the inside, enlarging the medullary cavity. 52

Bone Development and Growth There are four major sets of blood vessels associated with the long bones Nutrient vessels Enter the diaphysis and branch toward the epiphysis Re-enter the compact bone, leading to the central canals of the osteons Metaphyseal vessels Supply nutrients to the diaphyseal edge of the epiphysis 53

Bone Development and Growth Four major sets of blood vessels (continued) Epiphyseal vessels Supply nutrients to the medullary cavities of the epiphysis Periosteal vessels Supply nutrients to the superficial osteons 54

Figure 5.10 Circulatory Supply to a Mature Bone Articular cartilage Epiphyseal artery and vein Branches of nutrient artery and vein Metaphyseal artery and vein Periosteum Periosteum Compact bone Periosteal arteries and veins Connections to superficial osteons Medullary cavity Nutrient artery and vein Nutrient foramen Metaphyseal artery and vein Metaphysis Epiphyseal line 55

Bone Development and Growth Factors Regulating Bone Growth Nutrition Calcium ions Phosphate ions Magnesium ions Citrate Carbonate ions Sodium ions Vitamins A, C, D (calcitriol) 56

Bone Development and Growth Factors Regulating Bone Growth (continued) Hormones: Parathyroid gland Releases parathyroid hormone Stimulates osteoclasts Stimulates osteoblasts Increases calcium ion absorption from the small intestine to the blood 57

Bone Development and Growth Factors Regulating Bone Growth (continued) Hormones: Thyroid gland Releases calcitonin Inhibits osteoclasts Removes calcium ions from blood and adds it to bone 58

Bone Development and Growth Factors Regulating Bone Growth (continued) Hormones: Thyroid gland Releases thyroxine (T4) Maintains normal activity of the epiphyseal cartilage 59

Bone Development and Growth Factors Regulating Bone Growth (continued) Hormones: Pituitary gland Releases growth hormone (somatotropin) Maintains normal activity of the epiphyseal cartilage 60

Bone Maintenance, Remodeling, and Repair Aging and the Skeletal System When we’re young, osteoblast activity balances with osteoclast activity When we get older, osteoblast activity slows faster than osteoclast activity When osteoclast activity is faster than osteoblast activity, bones become porous Estrogen keeps osteoclast activity under control 61

Bone Maintenance, Remodeling, and Repair Aging and the Skeletal System As women age, estrogen levels drop Osteoclast control is lost Osteoclasts are overactive Bones become porous This is osteoporosis 62

Bone Maintenance, Remodeling, and Repair Injury and Repair When a bone is broken, bleeding occurs A network of spongy bone forms Osteoblasts are overly activated, thus resulting in enlarged callused area This area is now stronger and thicker than normal bone 63

Clinical Note 5.3 Fractures and Their Repair (Part 3 of 4) Repair of a fracture Fracture hematoma Dead bone Bone fragments Spongy bone of external callus Periosteum Immediately after the fracture, extensive bleeding occurs. Over a period of several hours, a large blood clot, or fracture hematoma, develops. An internal callus forms as a network of spongy bone units the inner edges, and an external callus of cartilage and bone stabilizes the outer edges. 64

Clinical Note 5.3 Fractures and Their Repair (Part 4 of 4) External callus Internal callus External callus The cartilage of the external callus has been replaced by bone, and struts of spongy bone now unite the broken ends. Fragments of dead bone and the areas of bone closest to the break have been removed and replaced. A swelling initially marks the location of the fracture. Over time, this region will be remodeled, and little evidence of the fracture will remain. 65

Anatomy of Skeletal Elements Bone markings include: Projections Depressions Fissures Foramina Canals (meatuses) 66

Figure 5.12a Examples of Bone Markings (Surface Features) Trochanter Head Neck Facet Tubercle Condyle Femur 67

Figure 5.12b Examples of Bone Markings (Surface Features) Fissure Process Ramus Foramen Skull, anterior view 68

Figure 5.12c Examples of Bone Markings (Surface Features) Canal Sinuses Meatus Skull, sagittal section 69

Figure 5.12d Examples of Bone Markings (Surface Features) Tubercle Head Sulcus Neck Tuberosity Fossa Trochlea Condyle Humerus 70

Figure 5.12e Examples of Bone Markings (Surface Features) Crest Spine Fossa Line Foramen Ramus Pelvis 71

Table 5.1 Common Bone Marking Terminology 72