Skeletal Tissue

1. Skeletal Tissue Chapter 7

2. Types of Bone Tissue • Compact bone • Dense or solid appearance • Cancellous or spongy bone • Open spaces filled with needle-like bone structures

3. Long bones typically longer than wide Have shaft w/ heads at both ends contains mostly compact bone Ex: femur, humerus, ulna, radius Short bones Cube or box-shaped Mostly spongy bone Ex: carpals & tarsals Flat bones Thin, flattened Usually curved surface Thin layer of compact bone covering spongy bone Ex: ribs, skull, sternum, scapulae Bone marrow aspirations occur here Irregular bones Various shapes/sizes Do not fit into other categories Ex: vertebrae, facial bones Sesamoid bones: occur singularly (ex: patella) Structural Classification of Bones

5. Long Bones – In More Detail • Diaphysis • Shaft of the bone • Hollow, compact bone • Epiphyses • Ends of the long bones • Points of muscle attachment • Stability to joints • Spongy bone filled with red marrow • Epiphyseal plate: area between diaphysis and epihyses (“growth plate”)

6. Long Bones – In More Detail • Articular cartilage • Hyaline cartilage that covers joint surfaces • Periosteum • Dense, white fibrous membrane that covers bone (excepts joint surfaces) • Tendon fibers interlace with these fibers creating a firm attachment • Medullary (marrow) cavity • Hollow space in diaphysis of long bones • Filled with yellow marrow (CT rich in fat) • Endosteum • Epithelial membrane that lines the medullary cavity

8. Bone Tissue • Connective tissue • Consists of cells, fibers, extracellular matrix • Matrix predominates • Matrix hard and calcified • High content of collagen fibers

9. Composition of Bone Matrix Extracellular bone matrix can be subdivided into two components: • Inorganic salts • Organic matrix • Inorganic salts • Hydroxyapatite – specialized chemical crystals of calcium and phosphate • Needle-like; found btwn spaces of collagen fibers; oriented to reduce stress • Deposition of these chemicals = calcification • Mg, Na, sulfate, F are also found in bone

10. Composition of Bone Matrix 2. Organic matrix • Collagenous fibers • Ground substance • Protein & polysaccharide mixture • Provides support & adhesion for cellular & fibrous elements • Necessary for growth & repair • Chondroitin sulfate – important component of ground substance • Glucosamine and chondroitin sulfate required for bone & cartilage repair and maintenance

11. Microscopic Structure of Compact Bone • Structural unit of compact bone = osteon or Haversian system • Each osteon surrounds a cannal & runs lengthwise • Bone cells “cemented” within these units • Structure permits delivery and removal of nutrients and wastes • Structures within each osteon: • Lamellae, lacunae, canaliculi, Haversian canal

12. Osteon structure • Lamellae: concentric, cylinder-shaped layers of calcified matrix • Lacunae (“little lakes”): small spaces filled with tissue fluid which hold bone cells • Canaliculi: very small canals connecting lacunae together • Haversian canal: extend lengthwise through the center of an osteon • contains blood vessels, lymphatic vessels & nerves • Volkmann’s canal: (not part of osteon) transverse canals containing nerves and blood vessels • Carry blood from exterior surface of bone to osteons

15. Microscopic Structure of Cancellous (spongy) Bone • No osteons • Needle-like bony spicules – trabeculae • Bone cells found within the trabeculae • Spongy bone usually lies between two layers of compact bone • Diploe • Ex: skull bones • Bony spicules arranged along lines of stress • Varies for different types of bones

17. Types of Bone Cells • Osteoblasts – bone-forming cells • synthesize & secrete a specialized organic matrix = osteoid; important part of ground substance • Osteoid serves as the framework for calcium & phosphate deposits = accumulation of mineralized bone • Osteoclasts – bone-reabsorbing cells • Giant multinucleated cells • Large number of mitochondria and lysosomes • Responsible for erosion of bone minerals • Break down bone matrix for remodeling & release of calcium • Osteocytes – mature bone cells • Mature, nondividing osteoblasts • Lie within lacunae

18. Bone Marrow • Bone marrow = myeloid tissue • Specialized, soft CT • Site of blood cell production • Found within medullary cavities of long bones & spongy bone • Red marrow • In infant and children – red marrow dominates • RBC production • In adults - Ribs, bodies of vertebrae, ends of long bones, pelvis still contain red marrow • During times of blood loss – yellow  red

19. Bone Marrow • During aging red marrow becomes yellow • Marrow cells become saturated with fat • Inactive during blood cell production

20. Functions of Bone • Support • Shape, alignment, positioning of body parts • Protection • Protects organs • Movement • Bones and joints act as levers • Muscles attached to bones produce movement of joints • Mineral storage • Calcium and phosphorous • Homeostasis of blood calcium levels • Hematopoiesis • Blood cell formation

21. Regulation of Blood Calcium Levels • Bones store approx 98% of body’s calcium • Roles of calcium in the body: • Transmission of nerve signals • Skeletal & cardiac muscle contractions • Osteoblasts – remove calcium from blood • Osteoclasts – release calcium into blood

22. Mechanisms of Calcium Homeostasis • ***Parathyroid Hormone*** – parathyroid glands • Ca levels below homeostatic “set point”: • Osteoclast activity stimulated • Calcium absorbed from urine in renal system • Vitamin D synthesis stimulated  increases absorption of calcium in intestine • Calcitonin – thyroid gland • High blood Ca levels  • Stimulate osteoblast activity and inhibit osteoclast activity • Miacalcin: calcitonin nasal spray; used in osteoporosis pts

23. Development of Bone • Infant – cartilage  replaced by calcified bone matrix • Requires osteoblast & osteoclast activity • Osteogenesis • Intramembranous ossification: process by which most flat bones are formed within fibrous connective tissue membranes • Ex: skull bones (flat bones) & irregular bones

24. Endochondral Ossification (fig 7-8 pg. 198) • Bones formed from cartilage model • Occurs from center to ends • Typical of long bones • Cartilage model • Periosteum  ring or collar of bone • Primary ossification center and entrance of blood vessels • Medullary cavity; thickening/lengthing of collar • Secondary ossification center in epiphyseal cartilage • Enlargement of secondary centers; bone growth moves toward diaphysis • Epiphyseal plates close

27. Bone Growth & Resorption • Bones grow in diameter • Osteoclasts enlarge diameter of medullary cavity • Osteoblasts in the periosteum build new bone from the outside • Grow in length (at epiphyseal plates) • Childhood/adolescence – growth > resorption • Adulthood – growth = resorption • > 35-40 yrs – growth < resorption

28. Bone Remodeling • Formation of new Haversian systems (osteons) • Osteoclasts erode surface of bone  grooves • Blood vessels lie in these grooves • New bone covers these blood vessels (osteoblast activity) • Grooves  canals • New lamallae develops

29. Bone Fracture Repair • Fracture = break in the continuity of bone • Vascular damage initiates repair sequence • Vascular damage  hemorrhage  blood clot (fracture hematoma) • Hematoma resorbed  callus • Specialized repair tissue that binds the broken ends of the fracture together • Callus replaced by normal bone tissue • Proper alignment and immobilization necessary

30. Questions • Name the two types of bone tissue. • List the six structural components of a typical long bone visible to the naked eye. • What are the two principal chemical components of bone matrix? • What disease is characterized by loss of bone mineral density? • List and briefly describe the four structures found within an osteon. • Name and briefly describe the three major bone cell types. • What are the 5 functions of bone? • Briefly describe the steps in endochondral ossification.

31. Clarification Resorption = destruction Role of osteoclasts is bone resorption (destruction); erodes bone tissues releasing calcium into the blood