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Chapter 6 Skeletal System: Bones and Bone Tissue 6-1 Skeletal System • Consists of bones, cartilage, tendons, ligaments • Functions: – – – – – Support Protection Movement Storage Blood cell production 6-2 Hyaline Cartilage •Most bones develop from it • Chondroblasts produce matrix; become chondrocytes • Matrix contains collagen fibers (strength) and proteoglycans (trap water) • Contains no blood vessels or nerves • Perichondrium – Dense connective tissue membrane that surrounds cartilage Cartilage most intimately associated with bone 6-3 Hyaline Cartilage (cont’d) • Articular Cartilage: thin layer of hyaline cartilage covering ends of bones at moveable joints •Has no perichondrium, no blood vessels, no nerves •Growth: •Appositional: chondroblasts in perichondrium secrete new matrix; add new chondrocytes to outside of tissue •Interstitial: chondrocytes within cartilage divide; add more matrix from inside existing cartilage 6-4 Bone Shapes • Long – Upper and lower limbs • Short – Carpals and tarsals • Flat – Ribs, sternum, skull, scapulae • Irregular – Vertebrae, facial, hip 6-5 Long Bone Structure • Diaphysis – Shaft – Compact bone • Epiphysis – End of the bone – Cancellous bone • Epiphyseal plate – Hyaline cartilage – Bone growth in length • Epiphyseal line – When bone stops growing in length, becomes ossified 6-6 Long Bone Structure • Medullary cavity – Red marrow: site of blood cell formation – Yellow marrow: adipose tissue • Periosteum – Glistening, white outer surface of bone – Bone growth in diameter • Endosteum – Lines bone cavities 6-7 6-8 6-9 Flat, Short, Irregular Bones • Flat Bones – No diaphyses, epiphyses – Outer covering of compact bone that surrounds cancellous bone • Short and Irregular Bone – Compact bone that surrounds cancellous bone center – No diaphyses and not elongated 6-10 Bone Histology • Bone matrix – Organic: Collagen (flexibility) and proteoglycans (traps water) – Inorganic: Hydroxapatite: calcium phosphate (compression) • Bone cells – Osteoblasts-produce collagen and proteoglycans in matrix » Bone-forming cells (ossification or osteogenesis) – Osteocytes- mature cells » Produce substances needed to maintain bone matrix – Osteoclasts-responsible for resorption (breakdown) of bone – Stem cells or osteochondral progenitor cells: become osteoblasts or chondroblasts • Woven bone: Collagen fibers randomly oriented • Lamellar bone: Mature bone in thin layers, called lamellae • Cancellous(spongy) bone: Trabeculae • Compact bone: Denser: fewer spaces than cancellous bone 6-11 Bone Matrix •Mineral gives weight-bearing strength •Collagen gives flexible strength 6-12 Bone Cells • Osteoblasts – Formation of bone through ossification or osteogenesis • Osteocytes – Mature bone cells – Lacunae: Spaces – Canaliculi: Cell processes • Osteoclasts – Responsible for bone resorption (breakdown) 6-13 Woven and Lamellar Bone • Woven bone – Collagen fibers randomly oriented in many direction – Formed: • During fetal development • During fracture repair • Remodeling – Removing old bone (osteoclasts) and adding new (osteoblasts) • Lamellar bone – Mature bone in sheets called lamellae 6-14 Cancellous Bone •Consists of trabeculae: interconnecting rods or plates of bone; spaces filled with bone marrow, blood vessels •Oriented along lines of stress; provide structural strength 6-15 Compact Bone •Blood vessels enter bone itself •Lamellae oriented around blood vessels • Central or haversian canals: Parallel to long axis • Lamellae: Concentric, circumferential, interstitial • Osteon or haversian system: Central canal, contents, associated concentric lamellae and osteocytes • Perforating or Volkmann’s canal: Perpendicular to long axis 6-16 Bone Development • Intramembranous ossification – Takes place in connective tissue membrane – Some skull bones, part of mandible, diaphysis of clavicles – Begins at 8 weeks, ends at 2 years • Endochondral ossification – Takes place in cartilage – Most bones develop from cartilage – Begins at 8 weeks; some begins at 18-20 years • Both methods of ossification – Produce woven bone that is remodeled – After remodeling, formation cannot be distinguished as one or other 6-17 Intramembranous Ossification 6-18 Endochondral Ossification 6-19 Endochondral Ossification 6-20 Endochondral Ossification 6-21 6-22 Growth in Bone Length • Appositional growth – New bone on old bone or cartilage surface • Epiphyseal plate zones – – – – Resting cartilage Proliferation Hypertrophy Calcification 6-23 Growth in Bone Length •Epiphyseal plate closure occurs between 12 and 25 years of age 6-24 Growth in Bone Width 6-25 Factors Affecting Bone Growth • Nutrition – Vitamin D • Necessary for absorption of calcium from intestines • Insufficient causes rickets and osteomalacia – Vitamin C • Necessary for collagen synthesis by osteoblasts • Deficiency results in scurvy • Hormones – Growth hormone from anterior pituitary – Thyroid hormone required for growth of all tissues – Sex hormones as estrogen and testosterone • Females stop growing earlier than males: estrogen causes quicker closure of epiphyseal plate than testosterone 6-26 Bone Remodeling • Converts woven bone into lamellar bone • Old bone constantly removed by osteoclasts and new bone formed by osteoblasts • Involved in bone growth, changes in bone shape, adjustment of bone to stress, bone repair, calcium regulation 6-27 Bone Repair •4 Steps: 6-28 Calcium Homeostasis • Bone is the major storage site for calcium in the body – Calcium moves into bone as osteoblasts build new bone – Calcium moves out of bone as osteoclasts break down bone • Osteoclast activity increases when blood calcium levels are too low; decreases when blood calcium too high – When osteoclast and osteoblast activity is balanced, the movement of calcium in and out is equal 6-29 Calcium Homeostasis Cont’d • Parathyroid hormone (PTH) from parathyroid glands is the major regulator of blood calcium levels • PTH increases blood calcium levels by: – Increasing bone breakdown (increased numbers of osteoclasts) – Increasing calcium absorption from small intestine (by promoting formation of Vitamin D) – Reabsorption of calcium from urine • Tumors that secrete large amounts of PTH cause so much bone breakdown that bones become weak and fracture easily • Calcitonin from thyroid gland decreases blood calcium by decreasing bone breakdown (decreasing osteoclast activity) 6-30 Calcium Homeostasis 6-31 Effects of Aging on Skeletal System • Bone Matrix decreases (less collagen; matrix formation by osteoblasts slower than matrix breakdown by osteoclasts) and becomes more brittle • Bone Mass decreases (after age 35) • Increased bone fractures • Bone loss causes deformity, loss of height, pain, stiffness – Stooped posture – Loss of teeth 6-32 Bone Fractures 6-33