The Skeletal System: Bone Tissue

1
Chapter 6

• The Skeletal System Bone Tissue
• Lecture Outline

2
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.

3
The Skeletal SystemBone 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

4
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

5
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

6
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

7
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

8
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.

9
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.

10
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
  cant 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

11
Cells of Bone

• Osteoblasts Osteocytes Osteoclasts

12
Matrix of Bone

• Inorganic mineral salts provide bones hardness
• hydroxyapatite (calcium phosphate) calcium
  carbonate
• Organic collagen fibers provide bones
  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

13
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.

14
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

15
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

16
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.

17
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.
18
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

19
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.

20
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.

21
Intramembranous Bone Formation

• Mesenchymal cells become osteoprogenitor cells
  then osteoblasts.
• Osteoblasts surround themselves with matrix to
  become osteocytes.

22
Intramembranous Bone Formation (cont.)

• 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.

23
Intramembranous Bone Formation
24
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.

25
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

26
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

27
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.

28
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

29
BONE GROWTH
30
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.

31
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

32
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

33
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.

34
Bone Growth in Width

• Only by appositional growth at the bones 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.

35
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.

36
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

37
BONES AND HOMEOSTASIS
38
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)

39
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

40
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.

41
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

42
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

43
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

44
Fractures

• Named for shape or position of fracture line
• Common types of fracture
• Potts -- distal fibular fracture
• Colless -- distal radial fracture
• stress fracture -- microscopic fissures from
  repeated strenuous activities

45
Repair of a Fracture
46
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

47
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

48
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 Ca2 can be
  deadly (plasma level maintained 9-11mg/100mL)
• cardiac arrest if too high
• respiratory arrest if too low

49
Hormonal Influences

• Parathyroid hormone (PTH) is secreted if Ca2
  levels falls
• PTH gene is turned on more PTH is secreted from
  gland
• osteoclast activity increased, kidney retains
  Ca2 and produces calcitriol
• Calcitonin hormone is secreted from
  parafollicular cells in thyroid if Ca2 blood
  levels get too high
• inhibits osteoclast activity
• increases bone formation by osteoblasts

50
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.

51
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

Mesenchymal Cells
52
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

53
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

54
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

55
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

56

• end