Bone Tissue

the skeletal system

206 bones form the structural framework for the body

the bones, along with their cartilages, make up this system

bone

living tissue, with each individual bone is an organ

skeletal system function

bones support the soft tissues

provide attachment the sites for muscles, to assist them in movement

protect internal organs

stores and release of minerals (calcium, phosphorus)

triglyceride storage (yellow marrow)

hemopoiesis

skeletal tissue

compact bone

spongy bone

ligaments

tendons

compact bone

dense connective tissue

white

smooth solid

spongy bone

internal to compact bone

houses bone marrow

ligaments

dense regular connective tissue

bone to bone

tendons

dense regular connective tissue

bone to muscle

classification of bones

long bones

short bones

flat bones

irregular bones

long bones

length > width

most common

vary in size

short bones

length is approx equal to width

carpals and tarsals

includes sesamoid bones (embedded within a tendon)

flat bones

flat and slightly curved

scapular and skull bones

surface for muscle attachment

irregular bones

elaborate complex shapes

vertebrae

gross anatomy of long bones

diaphysis

epiphysis

metaphysis

epiphyseal plate

epiphyseal line

periosteum

outer fibrous layer

inner cellular layer

articular cartilage (hyaline cartilage)

medullary cavity

endosteum

diaphysis

provide leverage and weight support

epiphysis

end of long bone

resists stress from many direction

forms joints

houses spongy bone

has a proximal and distal end

metaphysis

between diaphysis and epiphysis and contains

epiphyseal plate

epiphyseal line

epiphyseal plate

less than 21yo

thin layer of hyaline cartilage for continued length growth

epiphyseal line

greater than 21yo remnant of plate

periosteum

covers outer surface except areas covered by articular cartilage

two layers

outer fibrous layer

inner cellular layer

outer fibrous layer

dense irregular connective tissue

protects bone

anchors blood vessels and nerves

attaches tendons and ligaments

inner cellular laye

composed of osteoblasts and osteoclasts

articular cartilage (hyaline cartilage)

covers joint surface of epiphysis

reduces friction

absorbs shock

medullary cavity

hollow cylindrical space

houses red and yellow bone marrow

endosteum

lining the medullary cavity, an incomplete layer of cells

vascular membrane of connective tissue

red bone marrow

hemopoietic

where red blood cells (RBCs), white blood cells, and platelets are produced

also has reticular connective tissue and fat

yellow bone marrow

replaces red bone marrow in long bones

contains fat (adipose tissue)

can serve as source of energy

blood supply and innervation

bones are highly vascularized, with vessels enter from periosteum

nutrient foramen allows entry of a nutrient artery and vein

nerves accompany the blood vessels to innervate the bone, periosteum, endosteum, marrow

short, flat, and irregular bones

external surface is still compact bone

interior is entirely spongy bone (diploe)

no medullary cavity

ossificaiton

formation and development of bone tissue

8th though 12th week of embryonic development

2 types:

intramembranous ossification

endochondral ossification

intramembranous ossification

primarily in flat bones of skulls

begins when mesenchyme becomes thicker and condensed

intramembranous ossification steps

1. ossification centers form

2. osteoid undergoes calcification

3. formation of trabeculae

   1. development of the periosteum

endochondral ossification

development of most bones including long and short bones, the bones of the axial (ribs and vertebrae), and the appendicular skeleton (e.g. upper and lower limbs)

requires the use of a cartilage model

endochondral ossification steps

1. fetal hyaline cartilage model develops

2. growth of the cartilage model

3. development of the primary ossification center

4. development of medullary cavity

5. development of secondary ossification centers

   1. formation of articular cartilage and epiphyseal plate

factors affecting bone length change

genetic factors determine bone shape and size

factors that alter the mineralization process or the production of organic matrix

hormones

hormones that promote bone growth

growth hormone: epiphyseal plate

insulin like growth factors: epiphyseal plate

thyroid hormone

sex hormones

thyroid hormone

increase metabolic rate of bone cells

sex hormones (estrogen, testosterone)

increase cartilage growth and bone formation—epiphyseal plate

hormones that inhibit bone growth

glucocorticoids

serotonin

glucocorticoids

impairs growth at epiphyseal plate

serotonin

osteoprogenitor cells stop differentiating

parathyroid hormone (PTH)

secreted by the parathyroid galnds

PTH increases blood Ca2+ level by activating osteoclast activity

PTH stimulates formation of calcitriol (active form of vitamin D), a hormone that promotes absorption of Ca2+ from GIT into the blood

calcitonin (CT)

secreted by the parafollicular cells of the thyroid glands

CT inhibit the activity of osteoclasts

promote Ca2+ deposition into bones thus decrease blood Ca2+ level

bone cell types

osteogenic cell

osteocyte

osteoblast

osteoclast

bone connective tissue

composed of both cells and extracellular matrix

osteogenic cell (osteoprogenitor cell)

stem cells derived from mesenchyme

produce osteoblasts

found in periosteum and endosteum

mesenchyme

embryonic connective tissue

osteoblasts

side-by-side on periosteum

abundant rough ER and golgi apparatus

secretes osteiod (semisolid bone matrix)

become entrapped from crystal deposition

differentiates into osteocytes

no mitosis

osteocytes

mature bone cells that maintain bone matrix

detects mechanical stress on bones

signals to osteoblasts to make new bone matrix at surface

no mitosis

osteoclasts

found on bone surface

phagocytic cells—multinuclear

from cells similar to monocytes and macrophages

break down bones for resorption

resorption lacuna

resorption lacuna

depression or pit on bone surfaces where osteoclasts are located

bone matrix

the nonliving, mineralized extracellular substance that forms the structural framework of bone tissue

made of organic and inorganic portions

organic

osteoids from osteoblasts

inorganic

salt crystals

osteoid

proteins secreted by osteoblasts

osteoid composition

collagen

semi-solid ground substance

• proteoglycans (chondroitin surface)

• glycoproteins

osteoid function

components five tensile strength

resist stretching and twisting

flexibility

inorganic portion

salt crystals

deposit along long axis of collagen fibers

rigidity and inflexibility—compressional strength

salt crystals

primarily calcium phosphate

calcium hydroxide 

hydroxyapatite (contains calcium and phosphate)

sodium, magnesium, sulfate, fluoride

bone formation

requires vitamin D and C

1. osteoblast secretes osteoid

2. calcification

3. ions precipitate out of solution

   1. osteoblasts then become trapped, turning into ostecytes

calcification

osteoid hardens as hydroxyapatite crystals deposit in bone matrix (in and around collagen)

bone reabsorption

the balance between osteoblast and osteoclast activity is what causes subtle bone remodelling

1. bone matrix destroyed

2. osteoclasts secrete proteolytic enzymes digests collagen proteoglycans

3. hydrochloric acid dissolved mineral parts

   1. calcium and phosphate enter blood

bone remodeling

breakage

resorption

reversal

formation

osteoporosis

low bone density, “porous bones”

over bony reabsorption, with inadequate formation of new bone

found in elderly, or postmenopause due to low estrogen levels

paget’s disease of the bone

disordered bony remodeling

first overactive osteoclasts

then osteoblasts lay down large, weak, brittle bones prone to fracture

osteons (haversian system)

basic functional and structural unit

parallel to diaphysis of long bone

allows for great resistance to stresses applied in many directions

central canal

center of osteon

parallel to osteon

contains blood vessels and nerves

concentric lamellae

rings of bone connective tissue

collagen fibers at 90 degree angles

strength and resilience

osteocytes

cell that maintains bone matrix

found in small spaces between adjacent lamellae

space is called lacunae

canaliculi are interconnecting channels—extend from each lacunae

connect lacunae and central canal

house osteocyte cytoplasmic projections

permit intercellular contact and communication

perforating canals

contain blood vessels and nerve similar to central canal

perpendicular to central canal

connects central canals

interstitial lamellae

between osteons

no central canal

spongy (cancellous) bone

no osteons

trabeculae

parallel lamellae

trabeculae

open lattice of narrow rods and plates

bone marrow fills in between

cartilage

strong, flexible, and semi-rigid supporting tissue

avascular, nourished by long ranged diffusion

2 types of growth

• interstitial

• appositional

certain bone growth depends on cartilage growth

interstitial cartilage growth

chondrocytes lay down matrix in existing tissue to increase length

mainly in childhood and adolescence

appositional cartilage growth

chondroblasts lay down new matrix on surface to increase width

bone fractures

transverse (nondisplaced)

transverse (displaced)

compound

oblique

comminuted

greenstick

transverse (nondisplaced

transverse (displaced)

compound

oblique

comminuted

greenstick

hematoma formation

blood vessels in the broken bone tear and hemorrhage, resulting in the formation of clotted blood, or a hematoma, at the site of the break

the severed blood vessels at the broken ends of the bone are sealed by the clotting process

bone cells deprived of nutrients begin to die

0-2 weeks

fibrocartilaginous callus formation

within days of the fracture, capillaries grow into the hematoma, while phagocytic cells begin to clear away the dead cells

fibroblasts and osteoblasts enter the area and begin to reform bone

the repair tissue between the broken bone ends, the fibrocartilaginous callus, is composed of both hyaline and fibrocartilage

some bone spicules may also appear at this point

2-3 weeks

fibroblasts

produce collagen fibers that connect the broken bone ends, while osteoblasts start to form spongy bone

bony callus formation

the fibrocartilaginous callus is converted into a bony callus of spongy bone

it takes about two months for the broken bone ends to be firmly joined together after the fracture

similar to the endochondral formation of bone when cartilage becomes ossified;

osteoblasts, osteoclasts, and bone matrix are present

3-6 weeks

bony remodelling

the bony callus is then remodeled by osteoclasts and osteoblasts, with excess material on the exterior of the bone and within the medullary cavity being removed

compact bone is added to create bone tissue that is similar to the original, unbroken bone

this remodeling can take many months

the bone may remain uneven for years

8 weeks-2 years