The Skeletal System Bone Tissue Chapter 6 The

The Skeletal System: Bone Tissue Chapter 6

The Skeletal System: Bone Tissue Functions of Bone and Skeletal System n Structure of Bone n Histology of Bone Tissue n Blood and Nerve Supply of Bone n Bone Formation n Bone’s Role in Calcium Homeostasis n Exercise and Bone Tissue n Aging and Bone Tissue n

Functions of Bone and Skeletal System n Support Protection n Assistance in Movement n Mineral Homeostasis n Blood Cell Production n Triglyceride Storage n

Functions of Bone and Skeletal System n Support q Structural framework of the body n n n Supports soft tissues Provides attachment points for tendons of skeletal muscle Protection q Protects important internal organs n n n Cranium protects brain Vertebrae protects spinal cord Ribs protect lungs and heart

Functions of Bone and Skeletal System n Assistance in Movement q Skeletal muscle attaches to bone n n Skeletal muscle contraction pulls on bone producing movement Mineral Homeostasis q Bone tissue stores several minerals n Acts to serve as a reservoir of critical minerals q q Calcium (99% of body’s content) Phosphorus

Functions of Bone and Skeletal System n Blood Cell Production q Red bone marrow produces (Hemopoiesis) n n Red blood cells White blood cells Platelets Triglyceride Storage q Yellow bone marrow n Triglycerides stored in adipose cells q Serves as a potential chemical energy reserve

Structure of Bone n q q q Long Bone Anatomy (Humerus) Diaphysis Epiphysis Metaphysis n q Articular cartilage n q q q Epiphyseal growth plate Perforating fibers Periosteum Medullary cavity Endosteum

Histology of Bone Tissue n Extracellular matrix surrounding widely separated cells q Matrix n n 15% water 30% collagen fibers 55% crystallized mineral salts The most abundant mineral salt is calcium phosphate

Histology of Bone Tissue n A process called calcification is initiated by bone-building cells called osteoblasts n Mineral salts are deposited and crystallize in the framework formed by the collagen fibers of the extracellular matrix n Bone’s flexibility depends on collagen fibers

Histology of Bone Tissue n n Four types of cells are present in bone tissue Osteogenic cells q n Osteoblasts q q n Undergo cell division (stem cells); the resulting cells develop into osteoblasts Bone-building cells Synthesize extracellular matrix of bone tissue Osteocytes q q Mature bone cells Exchange nutrients and wastes with the blood

Histology of Bone Tissue n Osteoclasts q q Release enzymes that digest the mineral components of bone matrix (resorption) Regulate blood calcium level

Histology of Bone Tissue n Bone may be categorized as: q q Compact Spongy

Histology of Bone Tissue n Compact Bone q q q Resists the stresses produced by weight and movement Components of compact bone arranged into repeating structural units called osteons or Haversian systems Osteons consist of a central (Haversian) canal with concentrically arranged lamellae, lacunae, osteocytes, and canaliculi

Histology of Bone Tissue n Osteon q q Central canals run longitudinally through bone Around the central canals are concentric lamellae n q q Rings of calcified matrix (like the rings of a tree trunk) Between the lamellae are small spaces called lacunae which contain osteocytes Radiating in all directions from the lacunae are tiny canaliculi filled with extracellular fluid and the plasma membrane extensions of the osteocyte

Histology of Bone Tissue n Osteon q Canaliculi connect lacunae, forming a system of interconnected canals n q Providing routes for nutrients and oxygen to reach the osteocytes The organization of osteons changes in response to the physical demands placed on the skeleton

Histology of Bone Tissue n Spongy Bone q q Lacks osteons Lamellae arranged in a lattice of thin columns called trabeculae n n n Spaces between the trabeculae make bones lighter Trabeculae of spongy bone support and protect the red bone marrow Hemopoiesis (blood cell production) occurs in red bone marrow of spongy bone

Histology of Bone Tissue n Spongy Bone q q Within each trabecula are lacunae that contain osteocytes Osteocytes are nourished from the blood circulating through the trabeculae within the red bone marrow. Interior bone tissue is made up primarily of spongy bone The trabeculae of spongy bone are oriented along lines of stress n helps bones resist stresses without breaking while keeping them light.

Blood and Nerve Supply of Bone n n Bone is richly supplied with blood q Periosteal arteries accompanied by nerves supply the periosteum and compact bone q Epiphyseal veins carry blood away from long bones Nerves accompany the blood vessels that supply bones q The periosteum is rich in sensory nerves sensitive to tearing or tension

Bone Formation n n The process by which bone forms is called ossification Bone formation occurs in four situations: q 1) Formation of bone in an embryo q 2) Growth of bones until adulthood q 3) Remodeling of bone q 4) Repair of fractures

Bone Formation n Formation of Bone in an Embryo q Bone formation follows one of two patterns n Intramembranous ossification q q n Flat bones of the skull and mandible are formed in this way “Soft spots” that help the fetal skull pass through the birth canal later become ossified forming the skull Endochondral ossification q q The replacement of cartilage by bone Most bones of the body are formed in this way including long bones

Bone Formation n Formation of Bone in an Embryo q Cartilage formation and ossification occurs during the sixth week of embryonic development

Bone Formation - Intramembranous ossification

Bone Formation - Endochondral ossification

Bone Growth During Infancy, Childhood and Adolescence n n Growth in Length The growth in length of long bones involves two major events: q 1) Growth of cartilage on the epiphyseal plate q 2) Replacement of cartilage by bone tissue in the epiphyseal plate

Bone Growth During Infancy, Childhood and Adolescence n Osteoclasts dissolve the calcified cartilage, and osteoblasts invade the area laying down bone matrix n The activity of the epiphyseal plate is the way bone can increase in length n At adulthood, the epiphyseal plates close and bone replaces all the cartilage leaving a bony structure called the epiphyseal line

Bone Growth During Infancy, Childhood and Adolescence n Growth in Thickness q n Bones grow in thickness at the outer surface Remodeling of Bone q q q Bone forms before birth and continually renews itself The ongoing replacement of old bone tissue by new bone tissue Old bone is continually destroyed and new bone is formed in its place throughout an individual’s life

Bone Growth During Infancy, Childhood and Adolescence n A balance must exist between the actions of osteoclasts and osteoblasts q q q If too much new tissue is formed, the bones become abnormally thick and heavy Excessive loss of calcium weakens the bones, as occurs in osteoporosis Or they may become too flexible, as in rickets and osteomalacia

Factors Affecting Bone Growth and Bone Remodeling n n n Normal bone metabolism depends on several factors Minerals q Large amounts of calcium and phosphorus and smaller amounts of magnesium, fluoride, and manganese are required for bone growth and remodeling Vitamins q Vitamin A stimulates activity of osteoblasts q Vitamin C is needed for synthesis of collagen q Vitamin D helps build bone by increasing the absorption of calcium from foods in the gastrointestinal tract into the blood q Vitamins K and B 12 are also needed for synthesis of bone proteins

Factors Affecting Bone Growth and Bone Remodeling n Hormones q During childhood, the hormones most important to bone growth are growth factors (IGFs), produced by the liver and bone tissue (under influence of h. GH) n q q IGFs stimulate osteoblasts, promote cell division at the epiphyseal plate, and enhance protein synthesis Thyroid hormones (T 3 and T 4) also promote bone growth by stimulating osteoblasts Insulin promotes bone growth by increasing the synthesis of bone proteins

Factors Affecting Bone Growth and Bone Remodeling n Hormones q Estrogen and testosterone cause a dramatic effect on bone growth n n n q Cause of the sudden “growth spurt” that occurs during the teenage year Promote changes in females, such as widening of the pelvis Ultimately-Shut down growth at epiphyseal plates Parathyroid hormone, calcitriol, and calcitonin are other hormones that can affect bone remodeling

Bone’s Role in Calcium Homeostasis n Bone is the body’s major calcium reservoir n Levels of calcium in the blood are maintained by controlling the rates of calcium resorption from bone into blood and of calcium deposition from blood into bone q Both nerve and muscle cells depend on calcium ions (Ca 2+) to function properly q Blood clotting also requires Ca 2+ q Many enzymes require Ca 2+ as a cofactor

Bone’s Role in Calcium Homeostasis n Actions that help elevate blood Ca 2+ level q Parathyroid hormone (PTH) regulates Ca 2+ exchange between blood and bone tissue n n n PTH increases the number and activity of osteoclasts (i. e. , resorption) PTH acts on the kidneys to decrease loss of Ca 2+ in the urine PTH stimulates formation of calcitriol (active form of Vitamin D) a hormone that promotes absorption of calcium from foods in the gastrointestinal tract

Bone’s Role in Calcium Homeostasis

Bone’s Role in Calcium Homeostasis n Actions that work to decrease blood Ca 2+ level q q q The thyroid gland (i. e. , parafollicular cells) secretes calcitonin (CT) which inhibits activity of osteoclasts The result is that CT promotes bone formation and decreases blood Ca 2+ level See Table 6. 2

$Fracture and Repair of Bone n Fracture Types q Open (compound) fracture n q$

Fracture and Repair of Bone n Fracture Types q Open (compound) fracture n q Closed (simple) fracture n q Fracture of the fibula, with injury of the tibial articulation Colles’ fracture n q One end of the fractured bone is forcefully driven into another Pott’s fracture n q A partial fracture in which one side of the bone is broken and the other side bends Impacted fracture n q The bone is splintered, crushed, or broken into pieces Greenstick fracture n q Does not break the skin Comminuted fracture n q The broken ends of the bone protrude through the skin A fracture of the radius in which the distal fragment is displaced Stress fracture n A series of microscopic fissures in bone

Fracture and Repair Ø Anatomical appearance – like breaking a green twig Greenstick

Fracture and Repair n Anatomical appearance – the distal part is shoved up into the proximal part. Impacted

Fracture and Repair n Anatomical appearance – though not seen here, one or both bones are “open” to the outside. Open (compound)

$Fracture and Repair n Eponyms – Colles’ is a fracture of the distal radius$

Fracture and Repair n Eponyms – Colles’ is a fracture of the distal radius ± ulna. Colles’

Fracture and Repair of Bone n n Calcium and phosphorus needed to strengthen and harden new bone after a fracture are deposited only gradually and may take several months The repair of a bone fracture involves the following steps q q 1) Formation of fracture hematoma n Blood leaks from the torn ends of blood vessels, a clotted mass of blood forms around the site of the fracture 2) Fibrocartilaginous callus formation n Fibroblasts invade the fracture site and produce collagen fibers bridging the broken ends of the bone 3) Bony callus formation n Osteoblasts begin to produce spongy bone trabeculae joining portions of the original bone fragments 4) Bone remodeling n Compact bone replaces spongy bone

Osteon Periosteum Compact bone Spongy bone Fracture hematoma Blood vessel Fibroblast Phagocyte Fracture hematoma Red blood cell Bone fragment Osteocyte 1 Fibrocartilaginous callus Osteoblast Collagen fiber Chondroblast Cartilage Formation of fracture hematoma Bony callus Osteoblast Spongy bone 2 Fibrocartilaginous callus formation New compact bone Osteoclast Osteocyte 3 Bony callus formation 4 Bone remodeling

$Fracture and Repair n Once a bone is fractured, repair proceeds in a predictable$

Fracture and Repair n Once a bone is fractured, repair proceeds in a predictable pattern: The first step, which occurs 6 -8 hours after injury, is the formation of a fracture hematoma, as a result of blood vessels breaking in the periosteum and in osteons.

Fracture and Repair n The second and third steps involve the formation of a callus (takes a few weeks, to as many as six months). q Phagocytes remove cellular debris and fibroblasts deposit collagen to form a fibrocartilaginous callus. . .

Fracture and Repair q . . . which is followed by osteoblasts forming a bony callus of spongy bone.

Fracture and Repair n The final step takes several months and is called remodeling : q q Spongy bone is replaced by compact bone. The fracture line disappears, but evidence of the break remains.

Exercise and Bone Tissue n Bone tissue alters its strength in response to changes in mechanical stress q q n n Under stress, bone tissue becomes stronger through deposition of mineral salts and production of collagen fibers by osteoblasts and fibroblasts Unstressed bones diminishes because of the loss of bone minerals and decreased numbers of collagen fibers The main mechanical stresses on bone are those that result from the pull of skeletal muscles and the pull of gravity Weight-bearing activities help build and retain bone mass

Aging and Bone Tissue n n The level of sex hormones diminishes during middle age, especially in women after menopause q A decrease in bone mass occurs q Bone resorption by osteoclasts outpaces bone deposition by osteoblasts Female bones generally are smaller and less massive than males q Loss of bone mass in old age has a greater adverse effect in females

Aging and Bone Tissue n There are two principal effects of aging on bone tissue: q 1) Loss of bone mass n n q Results from the loss of calcium from bone matrix The loss of calcium from bones is one of the symptoms in osteoporosis 2) Brittleness n n n Results from a decreased rate of protein synthesis Collagen fibers gives bone its tensile strength The loss of tensile strength causes the bones to become very brittle and susceptible to fracture

Compact Bone

Spongy Bone (Cancellous)

Hyaline Cartilage

Adipose Tissue