1 Anatomy and Physiology IBIOL 2401 Chapter 6 The Skeletal System -Bone Tissue

2 Functions of the Skeletal SystemSupport – forms the framework of the body. Protection – encloses vital organs (heart, brain, lungs). Movement – along with the muscles, helps move the body. Mineral storage and homeostasis – stores calcium, phosphate. Blood cell formation (hemopoiesis) – houses red bone marrow that has stem cells to form RBC, WBC and platelets. Fat storage – bones store fat as yellow bone marrow. 2

3 The Skeletal System – Bone TissueSkeletal system is made of: Bone tissue Cartilage Dense connective tissue Adipose tissue Blood/vascular connective tissue Nervous tissue 3

4 Bone Shapes Short bone: small, like cubes Bones of the wrist and ankleFlat bone: like plates Bones of the cranium, hip, ribs Long bone: long and thin Bones of the arms and legs Irregular bone: irregular, complex shape Vertebrae, facial bones Sesamoid bone: bones wrapped inside a tendon – patella Sutural bone: bones found between cranial bones 4

5 Bone Markings Bone markings are surface features thathelp in naming of the bone parts provide information on muscle and ligament attachments, passage of blood vessels and nerves, and bone articulations used as landmarks by physicians to locate some of the internal structures 5

6 Bone Markings 6

7 Bone Structure Diaphysis: the shaft of a long boneMade of mostly compact bone tissue Epiphysis: the two ends of a long bone Made of mostly spongy bone tissue Spaces filled with red bone marrow Metaphysis: the area between diaphysis and epiphysis Has the cartilagenous epiphyseal plate before puberty Medullary cavity: a cavity in the diaphysis that is filled with yellow bone marrow 7

8 Bone Structure Periosteum: dense connective tissue membrane covering the outer surface of the long bone - for muscle attachment, help with the growth and repair of the bone Endosteum: dense connective tissue lining the medullary cavity – to help with the growth and repair of the bone 8

9 Bone Tissue Bone tissue is connective tissue.It is composed of dispersed cells with solid extracellular matrix. 9

10 Bone Tissue Four types of bone cells:Osteoprogenitor cells: dividing stem cells that differentiate into osteoblasts. Located in the endosteum, deeper portions of periosteum. Osteoblasts: immature bone cells that secrete extracellular matrix (collagen fibers and calcium salts). Osteocytes: mature bone cells formed by osteoblasts trapped inside solid extracellular matrix; function to maintain bone tissue. Osteoclasts: modified white blood cells – fight infections and break down bone tissue (bone resorption) releasing calcium into blood. 10

11 Bone Tissue Extracellular matrix: Collagen fibers for flexibilitySalts for strength – calcium phosphate and calcium carbonate 11

12 Types of Bone Tissue Two types: Compact Spongy Compact bone tissue:80% of bone - Makes up most of the diaphysis of long bone Supports movement and weight of the body Characterized by its dense arrangement 12

14 Types of Bone Tissue Spongy bone:Composed of a network of plates called trabeculae. Trabeculae are made of concentric rings called lamellae. Scattered between the lamellae are the lacunae with osteocytes. Canaliculi connect lacunae to the surface of the trabeculae. Between trabeculae are spaces that give the bone a “spongy” appearance and contain red bone marrow involved in hemopoiesis (blood cell formation), blood vessels, nerves. 14

15 Blood Supply to a Bone 15

16 Bone Formation Ossification: Bone formation.Begins in fetus with mesenchymal tissue that will undergo transformation to form bones. Two process for the formation of bone: Intramembranous ossification involves deposition of osseous tissue within mesenchymal membranes Endochondral ossification involves the replacement of a hyaline cartilage “mold” with osseous tissue. 16

17 Bone Formation – Intramembranous OssificationForms the flat bones of the skull. Starts in a fetus…..newborn skull still contains areas where process is not complete -- soft spots (fontanels) Development Involves the following processes: 1. Ossification center development: mesenchymal cells aggregate and differentiate into osteogenic cells. They then transform into osteoblast cells that produce bony matrix 2. Calcification: osteoblasts become completely surrounded by extracellular matrix, become osteocytes in lacunae, and canaliculi form. Calcification of extracellular matrix occurs with the accumulation of calcium and mineral salts. 3. Trabeculae formation: extracellular matrix join up to form trabeculae of spongy bone. Spaces between trabeculae are penetrated by blood vessels and red bone marrow is established. 4. Periosteum development: remaining mesenchymal cells at the periphery differentiate into the periosteum. Compact bone tissue develops replacing the spongy bone tissue. 17

18 Bone Formation – Intramembranous Ossification18

19 Bone Formation – Endochondral OssificationForms most bones in the body. Consists of the following steps: Cartilage model development: mesenchymal cells differentiate into chondroblast that lay down cartilage matrix forming a cartilage mold of the future bone. Surrounding membrane becomes the perichondrium. Cartilage model growth: interstitial growth occurs---chondrocytes divide and secrete cartilage matrix causing linear growth of the model. Increased matrix at the periphery from chondrocytes deep to the perichondrium causes appositional growth (widening). Chondrocytes hypertrophy and matrix begins to calcify. Some chondrocytes die and leave behind large lacunae. Primary ossification center development: Nutrient artery penetrates perichondrium. Osteogenic cells differentiate into osteoblast; begin to produce bony matrix. Perichondrium now a periosteum. Primary ossification center forms as osteoblast deposit bony matrix over calcified cartilage matrix forming spongy bone. Medullary cavity formed: osteoclast brought in by nutrient artery begin to hollow out spongy bone at the middle of the bone forming medullary cavity. Formation of spongy bone continues towards the epiphyses. Spongy bone will be replaced by compact bone. 19

20 Bone Formation – Endochondral Ossification20

21 Bone Formation – Endochondral OssificationEndochondral ossification contd……. 5. Secondary Ossification centers development: branches of epiphyseal arteries penetrate into the epiphyseal areas of model and stimulate the formation of secondary ossification centers. Osteogenic cells differentiate into osteoblast laying down bony matrix. Spongy bone formation proceeds outward towards the periphery. No formation of medullary cavity 6. Articular cartilage formation: at the surface of the epiphyses, hyaline cartilage will remain as articular cartilage for the bone. Areas between the epiphyses and the diaphysis of bone will remain as epiphyseal plates containing hyaline cartilage that will allow the bone to grow in length. 21

22 Bone Formation – Endochondral Ossification22

23 Bone Growth Interstitial growth for increasing length.Appositional growth for increasing thickness (width). Growth takes place in fetus, early childhood years and during puberty. At the end of puberty, interstitial growth stops while appositional growth may continue. 23

24 Bone Growth Interstitial bone growthEpiphyseal plates between diaphysis and the two epiphyses are made of hyaline cartilage. Chondrocytes in the epiphyseal plates divide and secrete extracellular matrix, increasing the length of the bone. At the same time, the ossification in the diaphysis expands. Continues to grow until the age of 18 for females and 21 for males. Hormones (decreasing hGH and increasing sex hormones) cause the cartilage in the epiphyseal plates to growing and becomes completely ossified. An epiphyseal line develops in the region where epiphyseal plate existed. Presence of epiphyseal line indicates that linear bone growth is no longer possible. Premature ossification of epiphyseal plate of one bone in the extremities may result in unequal bone lengths. 24

25 Bone Growth Appositional growth for increasing thickness (width).Process includes: Differentiation of cells at the periosteum into osteoblasts that begin to deposit collagen fibers and extracellular matrix; forming bony ridges. Osteoblast become osteocytes as they become surrounded by bony matrix. Extracellular matrix ridges surround periosteal blood vessels. Osteoclast in the endosteum break down tissue to create larger medullary cavity. 25

26 Bone Remodeling Bone continually undergoes changes - remodelingBone resorption via osteoclast --- dissolve bone Osteoclast release enzymes and acids Bone deposition via osteoblasts ----deposit bone matrix Bone remodeling is the continuous process of resorption and deposition that occurs in bone Allows for healing of broken bone More stress means thicker bones (more deposition) 26

27 Factors that Affect Bone Growth & RemodelingMinerals: Typically calcium and phosphorous; smaller amounts of magnesium, fluoride, iron and manganese needed for remodeling and growth Vitamins: Vitamin C needed for normal production of collagen. Folic Acid and Vitamin B for protein production. Vitamin A stimulates osteoblasts Hormones: Human growth hormone produced by anterior pituitary gland stimulates osteoblast, increase protein synthesis, and cell division at the epiphyseal plate and periosteum. Thyroid hormones (T3 and T4) promote bone growth via osteoblast stimulation Sex hormones (estrogen and androgens) initially stimulate bone growth but as their secretion increase, epiphyseal plates become ossified to form epiphyseal lines. 27

28 Bone Fractures Fracture: Any break in the bone.Closed (simple)- simple bone break; bones do not protude out of the muscles and skin. Open (compound)- fracture where bone punctures skin Comminuted- ends of broken bones splinter leaving fragments between broken ends Greenstick- bones “bend” rather than breaking cleanly; seen in children Impacted- broken end of one bone driven into another bone Pott’s fracture- fracture of distal fibula, usually resulting in damage to medial aspect of ankle joint Colle’s fracture- fracture of distal radius often seen upon falling on outstretched arm. Stress fractures- microscopic fractures seen with repeated stress to bones. 28

29 Bone Fractures 29

30 Bone Fracture Repair Involves the following processes:Production of fracture hematoma: fracture cause damage of blood vessels and nerves Bleeding; blood clot (fracture hematoma). Pain. Inflammation; edema (swelling); phagocytes and osteoclasts begin to clean damaged/dead tissue. Formation of fibrocartilage callus: fibroblasts move from the periosteum to the fracture site; deposit collagen fibers. Osteoblast cells in the periosteum, begin to produce fibrocartilage callus that provides support. Bony callus produced: osteoblasts begin to deposit bony matrix forming spongy bone. Bony callus replaces fibrocartilage callus. 4. Bone remodeling: spongy bone is replaced with compact bone at the periphery. Damaged tissue is removed by osteoclasts. 30

31 Bone Fracture Repair 31

32 Calcium Homeostasis Blood calcium levels important for normal body functioning. Bone is the storage site for most of the calcium in the body Calcium is involve in blood coagulation, nerve conduction, muscle contraction and certain enzymatic reaction 32

33 Calcium Homeostasis Regulation via Parathyroid hormone (PTH):Produced by parathyroid gland, increases blood calcium levels Stimulates osteoclasts activity resulting in bone resorption. Calcitonin (CT): Produced by parafollicular cells of thyroid gland Functions to decrease blood calcium levels Inhibites osteoclast Stimulates bone deposition and calcium uptake 33

34 Calcium Homeostasis 34

35 Bone Aging Bone deposition outpaced by resorption at middle-age.With increased age demineralization of bone and brittleness occurs Demineralization: the loss of bone mass resulting from the loss of calcium and other minerals. Menopausal women are at greater risk for demineralization due to the rapidly decreased levels of estrogen. Males at lower risk due to continued production of testosterone late into life (levels decrease but not to the same extent as the hormonal loss seen in women) Decreased protein synthesis results in brittleness of bone 35

36 Osteoporosis OsteoporosisResults from decrease bone mass; bones more prone to fracture Predominantly seen in menopausal women due to decreased estrogen levels Risk increases with family history, thin/small build, diet poor in calcium or vitamin D, sedentary life style, smoking, drinking alcohol and Caucasian or Asian ethnicity. Treatment Hormone replacement therapy in menopausal women (must consider risks involved with hormone therapy) and calcium supplementation Prevention: diet containing adequate calcium (especially when young) and weight bearing exercise 36

37 Clinical ApplicationsGiantism: overproduction of human growth hormone during early childhood years. Pituitary dwarfism: underproduction of human growth hormone during early childhood years. Acromegaly: overproduction of human growth hormone after puberty leading to selective thickening of facial, hand and feet bones. Marfan’s syndrome: mutation that causes excessive cartilage in the epiphyseal plates; disproportionate, longer arms and legs; associated cardiovascular problems. Achondroplasia: mutation that expresses as lack of cartilage growth that leads to unusually short arms and legs. Osteogenesis imperfecta: genetic disorder that causes deficiency of fibers in the bone matrix; bones become brittle and fracture easily. Osteomalacia: a disorder that causes deficiency of minerals in the bone matrix; bones become weak and bend easily. Osteosarcoma: bone cancer. Osteomyelitis: bacterial infection of the bone. 37