The Skeletal System: Bone TissueChapter 6

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

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

Functions of Bone and Skeletal System Support

Structural framework of the body Supports soft tissues Provides attachment points for tendons of skeletal

muscle

Protection Protects important internal organs

Cranium protects brain Vertebrae protects spinal cord Ribs protect lungs and heart

Functions of Bone and Skeletal System Assistance in Movement

Skeletal muscle attaches to bone Skeletal muscle contraction pulls on bone producing

movement

Mineral Homeostasis Bone tissue stores several minerals

Acts to serve as a reservoir of critical minerals Calcium (99% of body’s content) Phosphorus

Functions of Bone and Skeletal System Blood Cell Production

Red bone marrow produces (Hemopoiesis) Red blood cells White blood cells Platelets

Triglyceride Storage Yellow bone marrow

Triglycerides stored in adipose cells Serves as a potential chemical energy reserve

Structure of Bone

Diaphysis Epiphysis Metaphysis

Epiphyseal growth plate Articular cartilage

Perforating fibers Periosteum Medullary cavity Endosteum

Long Bone Anatomy (Humerus)

Histology of Bone Tissue Extracellular matrix surrounding widely

separated cells Matrix

15% water 30% collagen fibers 55% crystallized mineral salts

The most abundant mineral salt is calcium phosphate

Histology of Bone Tissue A process called calcification is initiated

by bone-building cells called osteoblasts

Mineral salts are deposited and crystallize in the framework formed by the collagen fibers of the extracellular matrix

Bone’s flexibility depends on collagen fibers

Histology of Bone Tissue Four types of cells are present in bone tissue Osteogenic cells

Undergo cell division (stem cells); the resulting cells develop into osteoblasts

Osteoblasts Bone-building cells Synthesize extracellular matrix of bone tissue

Osteocytes Mature bone cells Exchange nutrients and wastes with the blood

Histology of Bone Tissue Osteoclasts

Release enzymes that digest the mineral components of bone matrix (resorption)

Regulate blood calcium level

Histology of Bone Tissue Bone may be categorized as:

Compact Spongy

Histology of Bone Tissue Compact Bone

Resists the stresses produced by weight and movement

Components of compact bone are 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 Osteon

Central canals run longitudinally through bone Around the central canals are concentric

lamellae 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

Osteon Canaliculi connect

lacunae, forming a system of interconnected canals 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 Spongy Bone

Lacks osteons Lamellae are arranged in a lattice of thin

columns called trabeculae 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 Spongy Bone

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 helps bones resist stresses without breaking

while keeping them light.

Blood and Nerve Supply of Bone

Bone is richly supplied with blood Periosteal arteries

accompanied by nerves supply the periosteum and compact bone

Epiphyseal veins carry blood away from long bones

Nerves accompany the blood vessels that supply bones The periosteum is

rich in sensory nerves sensitive to tearing or tension

Bone Formation The process by which bone forms is

called ossification Bone formation occurs in four situations:

1) Formation of bone in an embryo 2) Growth of bones until adulthood 3) Remodeling of bone 4) Repair of fractures

Bone Formation Formation of Bone in an Embryo

Bone formation follows one of two patterns Intramembranous ossification

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 The replacement of cartilage by bone Most bones of the body are formed in this way including

long bones

Bone Formation Formation of Bone in an Embryo

Cartilage formation and ossification occurs during the sixth week of embryonic development

Bone Formation - Intramembranous ossification

Bone Formation - Intramembranous ossification

Bone Formation - Intramembranous ossification

Bone Formation - Intramembranous ossification

Bone Formation - Endochondral ossification

Bone Formation - Endochondral ossification

Bone Formation - Endochondral ossification

Bone Formation - Endochondral ossification

Bone Formation - Endochondral ossification

Bone Formation - Endochondral ossification

Bone Growth During Infancy, Childhood and Adolescence Growth in Length The growth in length of

long bones involves two major events: 1) Growth of cartilage

on the epiphyseal plate

2) Replacement of cartilage by bone tissue in the epiphyseal plate

Bone Growth During Infancy, Childhood and Adolescence

Osteoclasts dissolve the calcified cartilage, and osteoblasts invade the area laying down bone matrix

The activity of the epiphyseal plate is the way bone can increase in length

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 Growth in Thickness

Bones grow in thickness at the outer surface Remodeling of Bone

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 A balance must exist between the actions of

osteoclasts and osteoblasts

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 Normal bone metabolism depends on several factors Minerals

Large amounts of calcium and phosphorus and smaller amounts of magnesium, fluoride, and manganese are required for bone growth and remodeling

Vitamins Vitamin A stimulates activity of osteoblasts Vitamin C is needed for synthesis of collagen Vitamin D helps build bone by increasing the

absorption of calcium from foods in the gastrointestinal tract into the blood

Vitamins K and B12 are also needed for synthesis of bone proteins

Factors Affecting Bone Growth and Bone Remodeling Hormones

During childhood, the hormones most important to bone growth are growth factors (IGFs), produced by the liver and bone tissue (under influence of hGH) IGFs stimulate osteoblasts, promote cell division at the

epiphyseal plate, and enhance protein synthesis

Thyroid hormones (T3 and T4) 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 Hormones

Estrogen and testosterone cause a dramatic effect on bone growth 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 Bone is the body’s major calcium reservoir 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 Both nerve and muscle cells depend on

calcium ions (Ca2+) to function properly Blood clotting also requires Ca2+

Many enzymes require Ca2+ as a cofactor

Bone’s Role in Calcium Homeostasis Actions that help elevate blood Ca2+ level

Parathyroid hormone (PTH) regulates Ca2+ exchange between blood and bone tissue PTH increases the number and activity of

osteoclasts (i.e., resorption) PTH acts on the kidneys to decrease loss of

Ca2+ 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 Actions that work to decrease blood Ca2+

level

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 Ca2+ level

See Table 6.2

Fracture and Repair of Bone Fracture Types

Open (compound) fracture The broken ends of the bone protrude through the skin

Closed (simple) fracture Does not break the skin

Comminuted fracture The bone is splintered, crushed, or broken into pieces

Greenstick fracture A partial fracture in which one side of the bone is broken and the other side

bends Impacted fracture

One end of the fractured bone is forcefully driven into another Pott’s fracture

Fracture of the fibula, with injury of the tibial articulation Colles’ fracture

A fracture of the radius in which the distal fragment is displaced Stress fracture

A series of microscopic fissures in bone

Fracture and Repair Anatomical appearance – like breaking a green twig

Greenstick

Fracture and Repair Anatomical appearance – the distal part is

shoved up into the proximal part.

Impacted

Fracture and Repair Anatomical appearance – though not seen

here, one or both bones are “open” to the outside.

Open (compound)

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

radius ± ulna.

Colles’

Fracture and Repair of Bone 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 1) Formation of fracture hematoma

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 Fibroblasts invade the fracture site and produce collagen fibers

bridging the broken ends of the bone 3) Bony callus formation

Osteoblasts begin to produce spongy bone trabeculae joining portions of the original bone fragments

4) Bone remodeling Compact bone replaces spongy bone

Compact boneSpongy bone

Periosteum

Fracture hematoma

Fracturehematoma

BonefragmentOsteocyte

Red bloodcell

Blood vessel

Formation of fracture hematoma

Phagocyte

Osteon

1

Phagocyte

Osteoblast

Fibroblast

Fibrocartilaginouscallus

Collagen fiberChondroblastCartilage

Fibrocartilaginous callus formation2

Compact boneSpongy bone

Periosteum

Fracture hematoma

Fracturehematoma

BonefragmentOsteocyte

Red bloodcell

Blood vessel

Formation of fracture hematoma

Phagocyte

Osteon

1

Bony callus

Spongy bone

Osteoblast

Bony callus formation

Osteocyte

3

Compact boneSpongy bone

Periosteum

Fracture hematoma

Fracturehematoma

BonefragmentOsteocyte

Red bloodcell

Blood vessel

Formation of fracture hematoma

Phagocyte

Osteon

1

Phagocyte

Osteoblast

Fibroblast

Fibrocartilaginouscallus

Collagen fiberChondroblastCartilage

Fibrocartilaginous callus formation2

Spongy bone

OsteoblastOsteoclast

New compactbone

Bony callus formation Bone remodeling

Osteocyte

3 4

Compact boneSpongy bone

Periosteum

Fracture hematoma

Fracturehematoma

BonefragmentOsteocyte

Red bloodcell

Blood vessel

Formation of fracture hematoma

Phagocyte

Osteon

1

Phagocyte

Osteoblast

Fibroblast

Fibrocartilaginouscallus

Collagen fiberChondroblastCartilage

Fibrocartilaginous callus formation2

Bony callus

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

The second and third steps involve the formation of a callus (takes a few weeks, to as many as six months). Phagocytes remove cellular debris and fibroblasts

deposit collagen to form a fibro-cartilaginous callus...

Fracture and Repair

Fracture and Repair ... which is followed by osteoblasts forming a bony

callus of spongy bone.

Fracture and Repair The final step takes several months and is

called remodeling : Spongy bone is replaced by

compact bone. The fracture line

disappears, but evidence of the breakremains.

Exercise and Bone Tissue Bone tissue alters its strength in response to

changes in mechanical stress 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 The level of sex hormones diminishes during

middle age, especially in women after menopause A decrease in bone mass occurs Bone resorption by osteoclasts outpaces bone

deposition by osteoblasts Female bones generally are smaller and less

massive than males Loss of bone mass in old age has a greater

adverse effect in females

Aging and Bone Tissue There are two principal effects of aging on bone tissue:

1) Loss of bone mass Results from the loss of calcium from bone matrix The loss of calcium from bones is one of the symptoms in

osteoporosis 2) Brittleness

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