6-1

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

6-2

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 (called hematopoiesis)

• Energy storage in yellow bone marrow

6-3

Anatomy of a Long Bone• Diaphysis = shaft

• Epiphysis = one end of a long bone, expanded portion, forms a joint (articulates) with another bone

• Metaphysis = growth plate region

• Articular cartilage covers joint surfaces, acts as friction & shock absorber

• Medullary cavity = marrow cavity

• Endosteum = lining of marrow cavity

• Periosteum = tough membrane covering entire bone, except for epiphyses but not the cartilage

• Projections of bones (called processes) provide sites for the attachment of ligaments and tendons

• Grooves and openings in the bone serve as passageways for blood vessels and nerves

6-4

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 (hydroxy apatite)

• 4 types of cells in bone

tissue

6-5

Cell Types of Bone

• Osteoprogenitor cells ---- undifferentiated cells (stem 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 (build

new bone)

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

• Osteoclasts---- huge multinucleated cells

– function in bone resorption at surfaces such as endosteum (break down bone)

6-6

Matrix of Bone• Inorganic mineral salts provide bone’s hardness and make it

resistant to crushing

– hydroxyapatite (calcium phosphate) & calcium carbonate

• Organic collagen fibers provide bone’s strength and

flexibility

– their tensile strength resists being stretched or torn

– remove minerals with acid & rubbery structure results

• Calcification is hardening of tissue when mineral crystals

deposit around collagen fibers

• 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

6-7

Types of Bone in a Long Bone

• The wall of the diaphysis of a long bone is

composed of tightly packed tissue called

compact bone.

• The epiphyses are composed of spongy

bone with thin layers of compact bone on

the surface.

6-8

Histology of Compact Bone• Osteon consists of concentric rings (lamellae) of calcified matrix

surrounding a vertically oriented blood vessel in a passage called the osteonic canal . Osteons give bone its substance.

• Osteocytes 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

6-9

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.

No true Osteons.

6-10

Bone Formation or Ossification

• All embryonic connective tissue begins as

mesenchyme.

• Intramembranous bone formation = formation

of bone directly from mesenchymal cells.

• Endochondral ossification = formation of bone

from hyaline cartilage.

6-11

Intramembranous Bone Formation

• Mesenchymal cells become osteoprogenitor cells then osteoblasts.

• Osteoblasts surround themselves with matrix to become osteocytes.

• Matrix calcifies into trabeculae with spaces holding red bone marrow.

• Mesenchyme condenses as periosteum at the bone surface.

• Superficial layers of spongy bone are replaced with compact bone.

6-12

Endochondral Bone Formation (1)

• 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

– in width by formation of new matrix

on the periphery by new chondroblasts

from the perichondrium

– cells in midregion burst and change

pH triggering calcification and

chondrocyte death

6-13

Endochondral Bone Formation (2)

• Development of Primary Ossification Center– nutrient artery penetrates center of

cartilage model

– perichondrium changes into periosteum

– osteoblasts and osteoclasts migrate to center of cartilage model

– osteoblasts deposit bone matrix forming spongy bone trabeculae

– Osteoclasts destroy trabeculae forming medullary cavity. Later compact bone forms

6-14

Endochondral Bone Formation (3)

• Development of Secondary Ossification Center

– blood vessels enter the epiphyses around time of birth

– spongy bone is formed but no medullary cavity

• Articular Cartilage

– cartilage on ends of bone remains as articular cartilage.

6-15

Bone Growth in Length

• Epiphyseal plate or cartilage growth plate

(exists between primary and secondary

ossification centers)

– Are bands of cartilage. As these cartilage

cells grow and divide, the bone grows in

length.

– cartilage cells are produced by mitosis on

epiphyseal plate

– cartilage cells are destroyed and replaced

gradually by bone

– Between ages 18 to 25, epiphyseal plates

close( bone replaces the cartilage, epiphyseal

line) and bone growth stops.

• Growth in length stops at age 25

6-16

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.

6-17

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

• vitamin D for calcium absorption

• 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

• growth spurt and closure of the epiphyseal growth plate

• estrogens promote female changes -- wider pelvis

6-18

Types of Bone Marrow

1. Red marrow: forms red blood cells, white blood cells,and platelets. Blood cell formation is called hematopoiesis.

• Is red due to the presence of the oxygen carrying pigment, hemoglobin

• Is most numerous in infants. As infants get older, it is replaced by yellow marrow.

2. Yellow marrow: functions in fat storage

6-19

Hormonal Abnormalities

• Oversecretion of hGH during childhood

produces gigantism

• 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 responsible for closure of growth plate

6-20

Bone Remodeling

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

– Osteoclasts dissolve bone matrix and release calcium and phosphorus into interstitial fluid(bone resorption)

– osteoblasts take over bone rebuilding (bone formation)

• Continual redistribution of bone matrix along lines of mechanical stress

– distal end of femur is fully remodeled every 4 months

6-21

Calcium Homeostasis & Bone Tissue

• Skeleton is 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. If too high, osteoblasts use excess to build new bone.

– respiratory arrest if too low. If too low, osteoclasts break down bone to replace blood levels of calcium.

6-22

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.

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

– inhibits osteoclast activity

– increases bone formation by osteoblasts

6-23

Exercise & Bone Tissue

• Pull on bone by skeletal muscle and gravity is

mechanical stress .

• Stress increases deposition of mineral salts &

production of collagen (calcitonin prevents bone

loss)

• Lack of mechanical stress results in bone loss

– reduced activity while in a cast

– astronauts in weightlessness

– bedridden person

• Weight-bearing exercises build bone mass (walking or weight-lifting)

6-24

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