Chapter 6

The Skeletal System: Bone Tissue

 

Lecture Outline

INTRODUCTION            

•      Bone is made up of several different tissues working together: bone, cartilage, dense connective tissue, epithelium, various blood forming tissues, adipose tissue, and nervous tissue.

•      Each individual bone is an organ; the bones, along with their cartilages, make up the skeletal system.

Chapter 6
The Skeletal System:Bone Tissue

•       Dynamic and ever-changing throughout life

•       Skeleton composed of many different tissues

–    cartilage, bone tissue, epithelium, nerve, blood forming tissue, adipose, and dense connective tissue

Functions of Bone

•      Supporting & protecting soft tissues

•      Attachment site for muscles making movement possible

•      Storage of the minerals, calcium & phosphate -- mineral homeostasis

•      Blood cell production occurs in red bone marrow (hemopoiesis)

•      Energy storage in yellow bone marrow

Anatomy of a Long Bone

•       diaphysis = shaft

•       epiphysis = one end of a long bone

•       metaphyses are the areas between the epiphysis and diaphysis and include the epiphyseal plate in growing bones.

 

•       Articular cartilage over joint surfaces acts as friction reducer & shock absorber

•       Medullary cavity = marrow cavity

Anatomy of a Long Bone

•       Endosteum = lining of marrow cavity

•       Periosteum = tough membrane covering bone but not the cartilage

–    fibrous layer = dense irregular CT

–    osteogenic layer = bone cells & blood vessels that nourish or help with repairs

Histology of Bone

•       A type of connective tissue as seen by widely spaced cells separated by matrix

•       Matrix of 25% water, 25% collagen fibers & 50% crystalized mineral salts

•       4 types of cells in bone tissue

HISTOLOGY OF BONE TISSUE

•      Bone (osseous) tissue consists of widely separated cells surrounded by large amounts of matrix.

•      The matrix of bone contains inorganic salts, primarily hydroxyapatite and some calcium carbonate, and collagen fibers.

•      These and a few other salts are deposited in a framework of collagen fibers, a process called calcification or mineralization.

–   The process of calcification occurs only in the presence of collagen fibers.

–   Mineral salts confer hardness on bone while collagen fibers give bone its great tensile strength.

bone cells.(Figure 6.2)

•         Osteogenic cells undergo cell division and develop into osteoblasts.

•         Osteoblasts are bone-building cells.

•         Osteocytes are mature bone cells and the principal cells of bone tissue.

•         Osteoclasts are derived from monocytes and serve to break down bone tissue.

Cells of Bone

•       Osteoprogenitor cells ---- undifferentiated cells

–    can divide to replace themselves & can become osteoblasts

–    found in inner layer of periosteum and endosteum

•       Osteoblasts--form matrix & collagen fibers but can’t divide

•       Osteocytes ---mature cells that no longer secrete matrix

•       Osteoclasts---- huge cells from fused monocytes (WBC)

–    function in bone resorption at surfaces such as endosteum

Cells of Bone

Osteoblasts       Osteocytes                   Osteoclasts

Matrix of Bone

•      Inorganic mineral salts provide bone’s hardness

–   hydroxyapatite (calcium phosphate) & calcium carbonate

•      Organic collagen fibers provide bone’s flexibility

–   their tensile strength resists being stretched or torn

–   remove minerals with acid & rubbery structure results

 

•      Bone is not completely solid since it has small spaces for vessels and red bone marrow

–   spongy bone has many such spaces

–   compact bone has very few such spaces

Compact Bone

•      Compact bone is arranged in units called osteons or Haversian systems (Figure 6.3a).

•      Osteons contain blood vessels, lymphatic vessels, nerves, and osteocytes along with the calcified matrix.

•      Osteons are aligned in the same direction along lines of stress. These lines can slowly change as the stresses on the bone changes.

Compact or Dense Bone

•      Looks like solid hard layer of bone

•      Makes up the shaft of long bones and the external layer of all bones

•      Resists stresses produced by weight and movement

Histology of Compact Bone

•      Osteon is concentric rings (lamellae) of calcified matrix surrounding a vertically oriented blood vessel

•      Osteocytes are found in spaces called lacunae

•      Osteocytes communicate through canaliculi filled with extracellular fluid that connect one cell to the next cell

•      Interstitial lamellae represent older osteons that have been partially removed during tissue remodeling

Spongy Bone

•      Spongy (cancellous) bone does not contain osteons. It consists of trabeculae surrounding  many red marrow filled spaces (Figure 6.3b).

•      It forms most of the structure of short, flat, and irregular bones, and the epiphyses of long bones.

•      Spongy bone tissue is light and supports and protects the red bone marrow.

The Trabeculae of Spongy Bone

•       Latticework of thin plates of bone called trabeculae oriented along lines of stress

•       Spaces in between these struts are filled with red marrow where blood cells develop

•       Found in ends of long bones and inside flat bones such as the hipbones, sternum, sides of skull, and ribs.

Blood and Nerve Supply of Bone

•      Periosteal arteries

–   supply periosteum

•      Nutrient arteries

–   enter through nutrient foramen

–   supplies compact bone of diaphysis & red marrow

•      Metaphyseal & epiphyseal aa.

–   supply red marrow & bone tissue of epiphyses

 

BONE FORMATION

•      All embryonic connective tissue begins as mesenchyme.

•      Bone formation is termed osteogenesis or ossification and begins when mesenchymal cells provide the template for subsequent ossification.

•      Two types of ossification occur.

–   Intramembranous ossification is the formation of bone directly from or within fibrous connective tissue membranes.

–   Endochondrial ossification is the formation of bone from hyaline cartilage models.

Intramembranous

•      Intramembranous ossification forms the flat bones of the skull and the mandible (Figure 6.5).

–   An ossification center forms from mesenchymal cells as they convert to osteoblasts and lay down osteoid matrix.

–   The matrix surrounds the cell and then calcifies as the osteoblast becomes an osteocyte.

–   The calcifying matrix centers join to form bridges of trabeculae that constitute spongy bone with red marrow between.

–   On the periphery the mesenchyme condenses and develops into the periosteum.

 

 

 Intramembranous Bone Formation

Endochondrial

•      Endochondrial ossification involves replacement of cartilage by bone and forms most of the bones of the body (Figure 6.6).

•      The first step in endochondrial ossification is the development of the cartilage model.

Endochondral Bone Formation

•       Development of Cartilage model

–    Mesenchymal cells form a cartilage model of the bone during development

•       Growth of Cartilage model

–     in length by chondrocyte cell division and matrix formation ( interstitial growth)

–    in width by formation of new matrix on the periphery by new chondroblasts from the perichondrium (appositional growth)

–    cells in midregion burst and change pH triggering calcification and chondrocyte death

Endochondral Bone Formation

•      Development of Primary Ossification Center

–   perichondrium lays down periosteal bone collar

–   nutrient artery penetrates center of cartilage model

–   periosteal bud brings osteoblasts and osteoclasts to center of cartilage model

–   osteoblasts deposit bone matrix over calcified cartilage forming spongy bone trabeculae

–   osteoclasts form medullary cavity

Endochondral Bone Formation

•       Development of Secondary Ossification Center

–    blood vessels enter the epiphyses around time of birth

–    spongy bone is formed but no medullary cavity

•       Formation of Articular Cartilage

–    cartilage on ends of bone remains as articular cartilage.

 

Bone Scan

•      Radioactive tracer is given intravenously

•      Amount of uptake is related to amount of blood flow to the bone

•      “Hot spots” are areas of increased metabolic activity that may indicate cancer, abnormal healing or growth

•      “Cold spots” indicate decreased metabolism of decalcified bone, fracture or bone  infection

BONE GROWTH

Growth in Length

•      To understand how a bone grows in length, one needs to know details of the epiphyseal or growth plate (Figure 6.7).

•      The epiphyseal plate consists of four zones: (Figure 6.7b)

–   zone of resting cartilage,

–   zone of proliferation cartilage,

–   zone of hypertrophic cartilage, and

–   zone of calcified cartilage The activity of the epiphyseal plate is the only means by which the diaphysis can increase in length.

•      When the epiphyseal plate closes, is replaced by bone, the epiphyseal line appears and indicates the bone has completed its growth in length.

Bone Growth in Length

•      Epiphyseal plate or cartilage growth plate

–   cartilage cells are produced by mitosis on epiphyseal side of plate

–   cartilage cells are destroyed and replaced by bone on diaphyseal side of plate

•      Between ages 18 to 25, epiphyseal plates close.

–   cartilage cells stop dividing and bone replaces the cartilage (epiphyseal line)

•      Growth in length stops at age 25

Zones of Growth in Epiphyseal Plate

•      Zone of resting cartilage

–   anchors growth plate to bone

•      Zone of proliferating cartilage

–   rapid cell division (stacked coins)

•      Zone of hypertrophic cartilage

–   cells enlarged & remain in columns

•      Zone of calcified cartilage

–   thin zone, cells mostly dead since matrix calcified

–   osteoclasts removing matrix

–   osteoblasts & capillaries move in to create bone over calcified cartilage

Growth in Thickness

•      Bone can grow in thickness or diameter only by appositional growth (Figure 6.8).

•      The steps in thes process are:

–   Periosteal cells differentiate into osteoblasts which secrete collagen fibers and organic molecules to form the matrix.

–   Ridges fuse and the periosteum becomes the endosteum.

–   New concentric lamellae are formed.

–   Osetoblasts under the peritsteum form new circumferential lamellae.

Bone Growth in Width

•       Only by appositional growth at the bone’s surface

•       Periosteal cells differentiate into osteoblasts and form bony ridges and then a tunnel around periosteal blood vessel.

•       Concentric lamellae fill in the tunnel to form an osteon.

Factors Affecting Bone Growth

•      Nutrition

–   adequate levels of minerals and vitamins

•   calcium and phosphorus for bone growth

•   vitamin C for collagen formation

•   vitamins K and B12 for protein synthesis

•      Sufficient levels of specific hormones

–   during childhood need insulinlike growth factor

•   promotes cell division at epiphyseal plate

•   need hGH (growth), thyroid (T3 &T4) and insulin

–   sex steroids at puberty

–   At puberty the sex hormones, estrogen and testosterone, stimulate sudden growth and modifications of the skeleton to create the male and female forms.

Hormonal Abnormalities

•      Oversecretion of hGH during childhood produces giantism

•      Undersecretion of hGH or thyroid hormone during childhood produces short stature

•      Both men or women that lack estrogen receptors on cells grow taller than normal

–   estrogen is responsible for closure of growth plate

BONES AND HOMEOSTASIS

Bone Remodeling

•      Remodeling is the ongoing replacement of old bone tissue by new bone tissue.

–   Old bone is constantly destroyed by osteoclasts, whereas new bone is constructed by osteoblasts.

–   In orthodontics teeth are moved by brraces.  This places stress on bone in the sockets causing osteoclasts and osteablasts to remodel the sockets so that the teeth can be properly aligned (Figure 6.2)

–   Several hormones and calcitrol control bone growth and bone remodeling (Figure 6.11)

Bone Remodeling

•      Ongoing since osteoclasts carve out small tunnels and osteoblasts rebuild osteons.

–   osteoclasts form leak-proof seal around cell edges

–   secrete enzymes and acids beneath themselves

–   release calcium and phosphorus into interstitial fluid

–   osteoblasts take over bone rebuilding

•      Continual redistribution of bone matrix along lines of mechanical stress

–   distal femur is fully remodeled every 4 months

 

 

 

Fracture and Repair of Bone

A fracture is any break in a bone.

•      Fracture repair  (Figure 6.10)involves formation of a clot called a fracture hematoma, organization of the fracture hematoma into granulation tissue called a procallus (subsequently transformed into a fibrocartilaginous [soft] callus), conversion of the fibrocartilaginous callus into the spongy bone of a bony (hard) callus, and, finally, remodeling of the callus to nearly original form.

 

Fracture & Repair of Bone

•      Healing is faster in bone than in cartilage due to lack of blood vessels in cartilage

•      Healing of bone is still slow process due to vessel damage

•      Clinical treatment

–   closed reduction = restore pieces to normal position by manipulation

–   open reduction = realignment during surgery

Fractures

•      Named for shape or position of fracture line

•      Common types of fracture

–   greenstick -- partial fracture

–   impacted -- one side of fracture driven into the interior of other side

Fractures

•      Named for shape or position of fracture line

•      Common types of fracture

–   closed -- no break in skin

–   open fracture --skin broken

–   comminuted -- broken ends of bones are fragmented

Fractures

•      Named for shape or position of fracture line

•      Common types of fracture

–   Pott’s -- distal fibular fracture

–   Colles’s -- distal radial fracture

–   stress fracture -- microscopic fissures from repeated strenuous activities

Repair of a Fracture

Repair of a Fracture

•       Formation of fracture hematoma

–    damaged blood vessels produce clot in 6-8 hours, bone cells die

–    inflammation brings in phagocytic cells for clean-up duty

–    new capillaries grow into damaged area

•       Formation of fibrocartilagenous callus formation

–    fibroblasts invade the procallus & lay down collagen fibers

–    chondroblasts produce fibrocartilage to span the broken ends of the bone

Repair of a Fracture

•       Formation of bony callus

–    osteoblasts secrete spongy bone that joins 2 broken ends of bone

–    lasts 3-4 months

•       Bone remodeling

–    compact bone replaces the spongy in the bony callus

–    surface is remodeled back to normal shape

 

 

 

Calcium Homeostasis & Bone Tissue

•      Skeleton is a reservoir of Calcium & Phosphate

•      Calcium ions involved with many body systems

–   nerve & muscle cell function

–   blood clotting

–   enzyme function in many biochemical reactions

•      Small changes in blood levels of Ca+2 can be deadly (plasma level maintained 9-11mg/100mL)

–   cardiac arrest if too high

–   respiratory arrest if too low

Hormonal Influences

•       Parathyroid hormone (PTH) is secreted if Ca+2 levels falls

–    PTH gene is turned on & more PTH is secreted from gland

–    osteoclast activity increased, kidney retains Ca+2 and produces calcitriol

•       Calcitonin hormone is secreted from parafollicular cells in thyroid if Ca+2 blood levels get too high

–    inhibits osteoclast activity

–    increases bone formation by osteoblasts

EXERCISE AND BONE TISSUE

•      Within limits, bone has the ability to alter its strength in response to mechanical stress by increasing deposition of mineral salts and production of collagen fibers.

–   Removal of mechanical stress leads to weakening of bone through demineralization (loss of  bone minerals) and collagen reduction.

•   reduced activity while in a cast

•   astronauts in weightless environment

•   bedridden person

–   Weight-bearing activities, such as walking or moderate weightlifting, help build and retain bone mass.

Development of Bone Tissue

•       Both types of bone formation begin with mesenchymal cells

•       Mesenchymal cells transform into chondroblasts which form cartilage

 OR

•       Mesenchymal cells become osteoblasts which form bone

 Developmental Anatomy

5th Week =limb bud appears as mesoderm covered with ectoderm

6th Week = constriction produces hand or foot plate

     and skeleton now totally cartilaginous

7th Week = endochondral ossification begins

8th Week = upper & lower limbs appropriately named

AGING AND BONE TISSUE

•      Of two principal effects of aging on bone, the first is the loss of calcium and other minerals from bone matrix (demineralization), which may result in osteoporosis.

–   very rapid in women 40-45 as estrogens levels decrease

–   in males, begins after age 60

 

•      The second principal effect of aging on the skeletal system is a decreased rate of protein synthesis

–   decrease in collagen production which gives bone its tensile strength

–   decrease in growth hormone

–    bone becomes  brittle & susceptible to fracture

 

Osteoporosis

•      Decreased bone mass resulting in porous bones

•      Those at risk

–   white, thin menopausal, smoking, drinking female with family history

–   athletes who are not menstruating due to decreased body fat & decreased estrogen levels

–   people allergic to milk or with eating disorders whose intake of calcium is too low

•      Prevention or decrease in severity

–   adequate diet, weight-bearing exercise, & estrogen replacement therapy (for menopausal women)

–   behavior when young may be most important factor

 

Disorders of Bone Ossification

•      Rickets

•   calcium salts are not deposited properly

•   bones of growing children are soft

•   bowed legs, skull, rib cage, and pelvic deformities result

•      Osteomalacia

•   new adult bone produced during remodeling fails to ossify

•   hip fractures are common

 

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