THE SKELETAL SYSTEM
BONE TISSUE –6
FUNCTIONS OF THE SKELETAL SYSTEM:
1.
Support
 Structural framework
 Support soft tissues
 Attachments for tendons of most skeletal muscles
2.
Protection
 Cranium protects the brain
 Vertebrae protect the spinal cord
 Ribcage protects the heart and lungs
3.
Assistance in movement
 Bones provide a lever system used by the muscles
4.
Mineral Homeostasis
 Bone stores calcium, phosphorous and other minerals
 Bone acts as a “mineral bank” and can release minerals into the blood to maintain
homeostasis
5.
Blood cell production
 Red bone marrow
-Hematopoiesis -process of making blood cells
*RBC, WBC, platelets
-tissue is a network of reticular fibers with developing blood cells
-other cells present:
*adipocytes, macrophages, fibroblasts
-all bone marrow is red in the new born, then decreases with age
Triglyceride storage
 Yellow bone marrow
-the major portion of bone marrow in the adult
-primarily adipocytes with a few blood cells
6.
ANATOMY OF A BONE:
1.
Diaphyses
 Long, cylindrical main part of the bone
2.
Epiphyses
 Proximal and distal ends of the bone
3.
Metaphyses
 Region in mature bone where diaphysis joins the epiphysis
 Growing bone
 Epiphyseal plate (growth plate)
-allows the diaphysis to grow in length but not in width
-point where cartilage is replaced by bone
4.
Articular Cartilage
 Thin layer of hyaline cartilage covering the epiphysis
 Located in articulations –bones join other bones
 Absorbs shock
 Reduces friction
5.
6.
7.
Periosteum
 Dense irregular CT
 Surrounds the bone (except at articular cartilages)
 Contains bone forming cells
 Allows bone to increase in diameter
 Protects bone
 Assists in fracture repairs
 Nourishes bone tissue
 Attachment point for tendons and ligaments
Medullary Cavity
 Or marrow cavity
 Space within diaphysis that contains the fatty yellow marrow
Endosteum
 Membrane that contains bone forming cells
 Lines medullary cavity
HISTOLOGY OF BONE TISSUE:
1.
Matrix
 25% water
 25% protein
 50% crystallized mineral salts
-hydroxyapatite (calcium phosphate)
-calcium carbonate
-other minerals
-bone hardness due to mineralized salts
-deposited in the collagen fiber framework
-bone flexibility due to collagen fibers
-calcification is initiated by osteoblasts
2.
Types of bone cells
 Osteogenic cells
-mesenchymal stem cells
-the only bone cells to undergo cell division
-daughter cells become osteoblasts
-found along the inner layer of the periosteum
-endosteum and the bone canals that contain blood vessels

Osteoblasts
-bone building cells
-synthesize and secrete collagen fibers
-initiate calcification

Osteocytes
-mature bone cells
-principal cells of bone tissue
-osteoblasts that have become trapped in the matrix
-no longer secrete matrix
-maintain bone

Osteoclasts
-huge cells
-derived from the fusion of many (50) monocytes
-concentrated in the endosteum
-digest protein and mineral components of the bone
3.
4.
Compact Bone Tissue
 80% of the skeleton
 external layer of all bones
 diaphyses of long bones
 provides protection and support
 tissue arranged in units called osteons
 blood and lymph vessels and nerves enter bone through perforating canals
 central canal
 concentric lamellae
 lacunae
 canaliculi
 Osteons aligned with the lines of stress of the bone
 Arrangement of osteons responds to activity
Spongy Bone Tissue
 20% of bone
 does not contain true osteons
 lamellae arranged in an irregular lattice –Trabeculae
 macroscopic spaces between trabeculae are filled with red bone marrow
 Each trabecula has an osteocyte that lies in the lacunae
 Light and reduces overall bone weight
 Trabeculae support and protect red bone marrow
 Found in: hip bones, ribs, breastbone, backbones, and the ends of long bones
 Site of hematopoiesis in the adult
BLOOD AND NERVE SUPPLY OF BONE:
1.
Periosteal arteries
 Accompanied by nerves
 Enter diaphysis through perforating canals
2.
nutrient artery
 large artery in the center of the diaphysis
 divides into proximal and distal branches that supply the diaphysis as far as the
epiphyseal plates
3.
nutrient foramen
 hole that the nutrient artery goes through into the medullary canal
4.
5.
6.
7.
8.
9.
10.
metaphyseal arteries
 enter metaphyses and supply red bone marrow with the nutrient artery
epiphyseal arteries
 supply the red bone marrow and tissue of the epiphyses
nutrient veins
 take blood away
epiphyseal veins
metaphyseal veins
periosteal veins
lots of sensory nerves in the periosteum
BONE FORMATION
Basics:
 embryo skeleton
-fibrous connective tissue membranes
-hyaline cartilage templates of future bones
-ossification begins 6th or 7th week of life
-two patterns of ossification

INTRAMEMBRANOUS OSSIFICATION
-fewest steps
-formation of flat bones of the skull and mandible

ENDOCHONDRAL OSSIFICATION
Replacement of cartilage by bone –most of the bones of the body
1)
Development of the cartilage model
2)
Growth of the cartilage model
3)
Development of the primary ossification center
4)
5)
Development of secondary ossificaiton centers
Formation of articular cartilage and epiphyseal plate
BONE GROWTH
Growth in Length
Epiphyseal Plate has 4 zones
1.
Zone of resting cartilage
2.
Zone of proliferating cartilage
3.
Zone of hypertrophic cartilage
4.
Zone of calcified cartilage
5.
Growth in Thickness:
Bone grows in thickness only by apposotional growth.
FACTORS AFFECTING BONE GROWTH
1.
Aequate diet of minerals and vitamins
 Calcium and phosphorus
 Fluorid
 Magnesium, iron, manganese
 Vitamin C
-synthesis of collagen
-differentiation of osteoblasts into
osteocytes
 Vitamins K and B12 -protein synthesis
 Vitamin A stimulates osteoblast activity
2.
Hormones
 IGFs –insulin growth factors
-Stimulate cell division at epiphyseal plate and in the periosteum
-Stimulate protein synthesis for new bone
 hGH
 Thyroid hormones -T3 and T4
 Insulin
 Sex Steriods
-at puberty the secretion of these hormones
-cause growth spurt
-promote typical skeletal changes for each of the sexes
-ultimately close epiphyseal plates –stop growth
BONE FRACTURES AND REPAIRS
Definition: a fracture is any break in a bone
Types of Fractures:
1.
Open (compound)
 Broken ends protrude through the skin
2.
Simple (closed)
 Does not break the skin
3.
Comminuted
 Bone splinters on impact leaving fragments between two major fragments
4.
Greenstick
 Occurs in children
 A partial fracture
 One side of the bone breaks
 The other side of the bone bends
5.
Impacted
 One end of the fractured bone is forcefully driven into the other
6.
Stress
 Fracture not visible
 Series of microscopic fissures
REPAIR OF A BONE FRACTURE:
1. Formation of fracture hematoma
 Blood vessels break at fracture line
 Clot forms at fracture site 6-8 hours post injury
 Bone cells at fracture site die (no blood supply)
 Swelling and inflammation occur in response to dead bone cells
 Phagocytic cells and osteoclasts remove dead and damaged tissue
 May last up to several weeks
2.
Fibrocartilaginous callus formation
 Infiltration of new blood vessels into fracture hematoma
 Organization of granulation tissue into procallus
 Fibroblasts invade the procallus
 Fibroblasts produce collagen fibers
 Connect broken ends of the bone
 Phagocytes continue to remove debris
 Osteogenic cells develop into chondrocytes
 Chondrocytes produce fibrocartilage
 Procallus transformed into fibrocartilaginous callus
 Broken ends of bone bridged
 Fibrocartilaginous callus lasts 3 weeks
3.
Bony callus formation
 In time fibrocartilage is converted to spongy bone
 Becomes a bony callus
 Bony callus lasts 3-4 months
4.
Bone remodeling
 Dead portions of bone fragments are resorbed by osteoclasts
 Compact bone replaces spongy bone around periphery of the fracture
BONE’S ROLE IN CALCIUM HOMEOSTASIS
Calcium facts:
 99% of body calcium is in the bone
 nerve cells depend on calcium
 many enzymes require calcium as a cofactor
 calcium is required for blood clotting
 blood calcium range is between 9-11 mg/100 mL
Hormone Regulation of Blood Calcium
1.
PTH -Parathyroid Hormone
 Secreted by the parathyroid glands
 Negative feedback system to adjust blood calcium
 If blood calcium falls –receptors in the parathyroid gland detect the change
 Cyclic AMP turns on synthesis of PTH
 PTH release into the blood
 PTH increases the number and activity of osteoclasts
 Increases bone resorption
 Calcium and phosphorus ions are released from the bone into the blood
 Calcium level returns to the normal range
2.
Calcitonin
 When blood calcium levels are too high calcitonin is secreted by the parafollicular
cells in the thyroid gland.
 Calcitonin inhibits osteoclasts
 Speeds up calcium uptake by bone
 Accelerates calcium deposition in bone
 Promotes bone formation
 When bone is placed under stress calcitonin production increases
AGING AND BONE TISSUE:




Decrease of sex steroids with middle age results in a decrease in bone mass
In old age the loss of bone through resorption occurs more rapidly than bone gain.
Women’s bones are relatively smaller –so they are more susceptible.
Two principal effects:
1.
Loss of bone mass
 Demineralization
 Begins after age 30 in women and accelerates after 45 due to decrease in
estrogen
 As much as 30% of bone calcium can be lost by age 70
 8% of bone is lost every 10 years
 males calcium loss begins after age 60
 in males only 3% of calcium is lost every 10 years
2.
Brittleness
 Results from a lower rate of protein synthesis


3.
Organic portion of the matrix diminishes
Collagen fibers
Osteoporosis
 Bone resorption outpaces bone deposition
 Bone mass becomes so depleted that bones fracture from stress of daily living
 1,000,000 fractures / year
 affects the entire skeletal system
 shrinkage of vertebrae, height loss, hunched backs, bone pain
 Affects:
-post menopausal Caucasian or Asian women
-some genetic correlation
-low calcium or vitamin D diet
-inactive life style
-smoking

Therapy
-Calcium supplements
-Fosemax -inhibits osteoclasts
-Hormone replacement

Prevention
-adequate calcium intake
-weight bearing exercise