Lecture 6- BONE (PDF)

1  ILOs By the end of this lecture the student should be able to: Describe the general structure of bone. Enumerate bone cells, describe their structure and correlated function. Describe bone matrix and relate the structure to function. Interpret the changes i...

1  ILOs By the end of this lecture the student should be able to: Describe the general structure of bone. Enumerate bone cells, describe their structure and correlated function. Describe bone matrix and relate the structure to function. Interpret the changes in bone matrix that occurs in different diseases. Correlate these changes in bone to different diseases. list sites of compact and cancellous bone. Describe the structure and correlated function of compact bone. Describe osteon & its constituents. Identify EM photo of osteon & DESCRIBE its components. Describe the structure and correlated function of compact bone. Describe different zones of growing end of long bone. Describe bone growth and remodeling. Enumerate cells involved in bone remodeling. Interpret the effect of aging on the skeletal turnover. 2  BONE It is a special type of connective tissue. Hardest, vascular, weight bearing type of connective tissue. Characterized by solid or stony hard matrix. 3  Structure of bone It is formed of:- Periosteum: covers external surface of bone. Endosteum: covers internal surface of bone. Bone cells: (osteoblasts, osteocytes and osteoclasts). Bone matrix, formed of calcified organic material which is: A) Organic (collagen type- I and GAG)=osteoid. B) Inorganic (mineral salts: calcium phosphate). 4  Bone Matrix -Organic: collagen type- I and proteoglycans. - Inorganic: hydroxyapatite(CaPO4 crystals). If mineral is removed, bone is too bendable 5 If collagen is removed, bone is too brittle  Bone matrix 1.Organic portion 2.Inorganic portion - Osteoid tissue - hydroxyapatite (35%) crystal (65%): -Type I collagen (90 % of osteoid) Calcium - Sulphated GAG Phosphate. -proteoglcans -Hco3, Mg, Fe, Na. -Glycoprotein:e.g. osteonectin. 6  Bone cells – Osteoblasts: bone forming cell, synthesizes the organic component of the matrix. – Osteocytes: are found in cavities (lacunae) between layers of bone (lamellae). – Osteoclasts: bone resorption (bone eating) & remodelling. 7  Bone cells osteoblasts Bone forming cell. They are protein secreting cell??: secretes collagen type –I, proteoglycan, glycoprtein. Mineralize the matrix with ca po4. They are cubical and covers all bone surfaces. Incorporate in the matrix and change to osteocytes. 8  Bone cells: Osteocytes - Osteocytes are present SINGLY in spaces called lacunae. - They are flat but have processes (branches). - The processes are present in spaces called canaliculi. - Surrounded by matrix, Can make small amount of matrix to maintain it so their death is followed by matrix resorption. -Lacunae: spaces occupied by osteocyte cell body. -Canaliculi: canals occupied by osteocyte cell processes. 9  Bone cells: Osteocytes – Processes of adjacent cells connect via gap junction. – Then can transfer nutrients from one cell to the next through gap junctions. – Processes act as a sensor for mechanical stresses. – Lack of exercise (e.g astronauts) lead to decreased bone density. 10 Bone cells: Osteoclasts Osteoclasts: Resorption of bone – Large motile cell with many nuclei (Multinucleated) and arise from fusion of many (blood cells) monocytes. – They lie in cavities called resorption bay or (Howship’s lacunae) in endosteum. 11  E.M: Osteoclast 1.Ruffled border {finger like projections OR irregular folds}: site of active bone resorption. 2.Clear zone: site of adhesion to the bone matrix. 3.Vesicular zone: a. Exocytotic vesicles {lysosomal enzymes extruded to the space} b. Endocytotic vesicles {bone fragments are taken to the cell} 4.Basal zone:{ organelles & nuclei} 12 Function: Bone Resorption:- They pump H ions across membrane, and form acid, TO decalcify bone (release Ca and Ph.) It secretes enzymes (e. g. collagenase) to degrade collagen and GAG of the matrix. 13 Periosteum & endosteum: they are layers of vascularized connective tissue and bone forming cells covering external & internal surfaces of bones Periosteum: covers the external surface of bone. Endosteum. covers the internal surfaces. 14  Periosteum It is formed of:- 1.Outer fibrous layer: collagenous fibers TYPE–I + fibroblast + blood vessels. 2.Inner layer: contains osteoblasts + osteoprogenitor cells). 15  Endosteum: thin layer containing osteoblasts and osteoclasts that lines all internal spaces including spaces in cancellous bone. Function of periosteum & endosteum: 1.Nutrition of bone. 2.Growth & repair of bone. 16 Long Bone Structure Diaphysis – Shaft – Compact bone Epiphysis – End of the bone – Cancellous bone Epiphyseal plate: growth plate – Hyaline cartilage; present until growth stops Epiphyseal line: when bone stops growing in length Medullary cavity: in the middle 17 of the bone and has bone marrow.  Types of bones According to gross observation:- 1.Compact (cortical) (lamellar) bone: dense without cavities. 2.Cancellous (spongy) (trabecular) bone: bone with many interconnecting cavities. 18  Compact Bone Sites: 1.shaft of long bone 2.outer&inner table of flat bone of skull (plates) 3.outer covering of vertebrae &ribs Structure:- 1.periosteum 2.Endosteum 3.Bone cells 4.Matrix 19  Microscopic Structure of Compact Bones 20 Figure 6.6a, b  Microscopic Structure of Compact Bones It has multiple layers of calcified matrix called bone lamellae so it is called lamellar bone. The lamellae are well organized: parallel to each other or around vascular canal. 21  Organization of Lamellae 1. External circumferential lamellea: immediately under the periosteum. 2. Inner circumferential lamellea: around the marrow cavity 3. Haversian system: central canal, concentric lamellae and osteocytes 4. Interstitial lamellea: inbetween 22 the osteon  Haversian system(Osteon) Definition: structural unit of compact bone. Consisted of:- 1.Haversian (central) canal: -parallel to long axis of bone -It is lined by osteoblasts -It contains blood vessels & nerves. Perforating or Volkmann’s canal: Perpendicular to long axis. Also contain blood vessels. 2. Concentric lamellea They are calcified matrix arranged in concentric layer around Haversian canal 3. Osteocytes 23  Organization of Lamellae Concentric lamellae Interstitial Outer circumferential lamellae lamellae Inner circumfe- rential lamellae Volkman’s canal 24 Periosteum Endosteum Central canal 25  Cancellous Bone Site: 1.Epiphysis of long bone 2.All flat bones 3.All irregular bones 4.Shafts of the ribs Structure:-Bone trabeculea {branching &anastomosing} + bone marrow cavities 26  Trabecular (Cancellous) Bone Structure: Thin and delicate bone trabeculae which are interconnecting rods ( branching & anastomosing) inclosing inbetween cavities (that contain bone marrow). Trabeculae are covered with endosteum. 27  Histogenesis of bone A. Bone formation:- 1.Intramembranous 2.Endochondral B. Bone growth: 1.Growth in length 2.Growth in width C. Bone remodeling: 28  Intramembranous ossification It is the process by which most of flat bones form (most of the skull, mandible & maxilla) 1. The process begins in embryonic mesenchymal membrane. 2. Involves formation of ossification centers. 3. Fontanelles? Areas in skull of newborn = CT not yet ossified. 29  Endochondral bone formation Process by which long bones are formed: Segment of hyaline cartilage (model of bone) bone) Occurs in most of body bones (long bones). Primary ossification center appears in first trimester. Secondary ossificaton center appears later Cartilage remains only in:- 1.Articular surface 2.Epiphyseal plate (responsible for bone growth & disappear in adults. 30 31  Bone growth The bone increase in size at birth to adulthood 1.Growth in length 2.Growth in width. 32  Bone growth in length 1.Zone of resting cartilage. 2.Zone of proliferation:cells in number 3.Zone of hypertrophy & maturation: Cells in size. 4.Zone of calcification 5.Zone of ossification & spongy bone. 6.stage of remodeling &compact bone. 33  Bone remodelling It means subperiosteal bone deposition +bone resorption in the other region It is due to the coordination of the activity of 2 cells: 1.Osteoblasts: synthesizing new osteoid. 2.Osteoclasts: eroding formed bone. It is a continuous process throughout life. It is under the control of hormonal and physical factors. 34  Bone remodelling In healthy adults, each year, about10% of the bone is replaced by remodelling. Every10 years, the entire skeletal system is replaced It is high in children and babies (to cope for demands.e.g. walking). It is low in adults but increased in certain conditions: e.g. Increased physical activity, repair of fractures. Bone remodeling aims at: Maintain bone structure. Structural modification needed to meet 35 increased demands.  Joints Bones are connected to each other via joints. Joints are either: 1. Joints with limited mobility (synarthrosis). 2. Joints with free movement (synovial joints) or diarthrosis. e.g. Elbow, Knee joint. Synovial joint: Both bone ends are covered with hyaline cartilage (with no perichondrium). Bones are separated from each other by synovial fluid. Covered by a fibrous capsule. The capsule is lined internally by synovial membrane. 36 Synovial fluid: Contains: hyalouronic acid, lubricin. Function: 1. lubricant for the joint. 2. Supply oxygen and nutrients to chondrocytes of articular cartilage. Synovium (synovial membrane) It is the internal lining of joint capsule. It is formed of: Cells: type A cells (macrophage like): phagocytic. type B cells(fibroblast like): secrete hyalouronic acid, lubricin. 37  Some bone disorders 38 Osteogenesis imperfecta Defect in osteoid collagen (collagen type-I). Leads to bone fragility, multiple fractures & deformities. 39 Osteomalacia: Is a disease occuring due to defective minera- lization either due to: insufficient Ca or Po4 ions. There is soft bones. Osteomalacia in growing children leads to rickets with deformity of the soft bones. 40 Osteoporosis: Means progressive loss of bone density, thinning of bone tissue and increased vulnerability to fractures. Occurs in immobilized patients and postmenopausal (old) women. Examination: DXA scans (Dual energy x-ray absorptiometry) 41  Paget’s disease: Due to increased osteoclast activity- bone resorption. Osteoblast attempt to fill remaining erosion New bone deposition is haphazard (irregular). Repaired bone is prone to fractures. 42  Function of Bones Support: provides hard framework Protection of vital organs: like brain &heart. Movement: skeletal muscles use bones as levers Mineral storage: reservoir for important minerals like calcium. Blood-cell formation: bone contains red bone marrow which forms blood cells. 43 44