1. The Skeletal System: Bone Tissue
Chapter 6
The Skeletal System: Bone Tissue

2. INTRODUCTION
Bone is made up of several different tissues working together: bone, cartilage, dense connective tissue, epithelium, various types of 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.
Dynamic and continuously changing throughout life.

3. Functions of Bone Supporting and protecting soft tissues
Attachment site for muscles making movement possible
Storage of the minerals, calcium and phosphate - mineral homeostasis
Blood cell production occurs in the red bone marrow (hemopoiesis)
Energy storage in the yellow bone marrow

4. 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 and shock absorber
Medullary cavity  marrow cavity

5. Anatomy of a Long Bone-continued
Endosteum  lining of marrow cavity
Periosteum  tough membrane covering bone but not the cartilage
fibrous layer  dense irregular connective tissue
osteogenic layer  bone cells and blood vessels that nourish or help with repairs

6. HISTOLOGY OF BONE TISSUE
Bone (osseous) tissue (a type of connective 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.
Matrix of ~25% water, ~25% collagen fibers and ~50% crystalized mineral salts
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.
4 types of cells in bone tissue

7. Cells of Bone Osteoprogenitor cells undifferentiated cells
can divide to replace themselves and can become osteoblasts
found in inner layer of periosteum and endosteum
Osteoblastsform matrix and collagen fibers but can’t divide
Osteocytes mature cells that no longer secrete matrix
Osteoclastslarge cells from fused monocytes (white blood cells-WBC);
function in bone resorption at surfaces such as endosteum

8. Matrix of Bone
Inorganic mineral salts provide bone’s hardness
hydroxyapatite (calcium phosphate) and calcium carbonate
Organic collagen fibers provide bone’s flexibility
their tensile strength resists being stretched or torn
remove minerals with acid and rubbery structure results
Bone is not completely solid since it has small spaces for blood vessels and red bone marrow
spongy bone has many spaces
compact bone has very few spaces

9. Compact Bone or Dense Bone
Makes up the shaft of long bones and the external layer of all bones
Compact bone is arranged in units called osteons or Haversian systems.
The 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 stress on the bone changes due to weight and movement.

10. Histology of Compact Bone
The osteon is made up of concentric rings (lamellae) of a calcified matrix surrounding a vertically oriented blood vessel
Osteocytes are found in spaces called lacunae
Osteocytes communicate through canaliculi that arefilled with extracellular fluid that connects one cell to the next cell
Interstitial lamellae represent older osteons that have been partially removed during tissue remodeling

11. Spongy Bone
Spongy (cancellous) bone does not contain osteons. It consists of trabeculae surrounding many red marrow filled spaces. This latticework (trabeculae) of thin plates are oriented along lines of stress.
Spaces in between these areas are filled with red marrow where blood cells develop.
Spongy bone tissue is light and supports and protects the red bone marrow.
It is found in ends (epiphyses) of long bones and inside flat bones such as the hipbones, sternum, sides of skull and ribs.

12. Blood and Nerve Supply of Bone
Periosteal arteries
supply periosteum
Nutrient arteries
enter through nutrient foramen
supplies compact bone of diaphysis and red marrow
Metaphyseal and epiphyseal arteries.
supply red marrow and bone tissue of epiphyses

13. 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.

14. Intramembranous Bone Formation
Intramembranous ossification forms the flat bones of the skull and the mandible.
An ossification center forms from mesenchymal cells as they convert to osteoprogenitor cells then to osteoblasts and lay down the osteoid matrix.
The matrix surrounds the cell which 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.
-Superficial layers of spongy bone are replaced with compact bone.

15. Endochondral Bone Formation
Endochondrial ossification involves replacement of cartilage by bone and forms most of the bones of the body.
Development of the cartilage model
Mesenchymal cells form a cartilage model of the bone during development
The growth of the cartilage model in length is through chondrocyte cell division and matrix formation ( interstitial growth). The growth in width is due to the formation of new matrix on the periphery by new chondroblasts from the perichondrium (appositional growth)
cells in midregion burst and causes a change in pH triggering calcification and death of the chondrocyte

16. Endochondral Bone Formation-continued
Development of Primary Ossification Center
perichondrium lays down the periosteal bone collar
the nutrient artery penetrates center of cartilage model
The periosteal bud brings osteoblasts and osteoclasts to the center of cartilage model
osteoblasts deposit the bone matrix over calcified cartilage forming spongy bone trabeculae
the osteoclasts form the medullary cavity

17. Endochondral Bone Formation-continued
Development of Secondary Ossification Center
blood vessels enter the epiphyses around the time of birth
spongy bone is formed but a medullary cavity is not
Formation of Articular Cartilage
the cartilage on ends of bone remains as articular cartilage.

18. Bone Growth in Length
Growth of bone in length, depends on the epiphyseal or growth plate.
The epiphyseal plate consists of four zones:
When the epiphyseal plate closes,
it is replaced by bone, the epiphyseal line appears and indicates the bone has completed its growth in length.
Zone of resting cartilage
anchors growth plate to bone
Zone of proliferating cartilage
rapid cell division by mitosis (stacked coins) responsible for growth
Zone of hypertrophic cartilage
cells enlarged and remain in columns
Zone of calcified cartilage
thin zone, cells mostly dead since matrix calcified
osteoclasts removing matrix
osteoblasts and capillaries move in to create bone over calcified cartilage
Between ages ~18 to 25, the epiphyseal plates close.
cartilage cells stop dividing and bone replaces the cartilage (epiphyseal line)
Growth in length stops at age ~25

19. Growth in Thickness
Bone can grow in thickness or diameter only by appositional growth.
The steps in these 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.

20. 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 insulinlike growth factor is needed to promote cell division at epiphyseal plate. Other hormones hGH (human growth hormone), thyroid (T3 andT4) and insulin are also required.
At puberty the sex hormones, estrogen and testosterone, stimulate sudden growth and modifications of the skeleton to create the male and female forms.

21. Bone Remodeling
Remodeling -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.
Ongoing process since osteoclasts carve out small tunnels and osteoblasts rebuild osteons.
osteoclasts form leak-proof seals around cell edges
osteoclasts secrete enzymes and acids underneath themselves
osteoclasts release calcium and phosphorus into the interstitial fluid
osteoblasts take over bone rebuilding
Continual redistribution of bone matrix along lines of mechanical stress
for eample, the distal part of the femur is fully remodeled every ~ 4 months
Several other hormones and calcitrol regulate bone growth and bone remodeling.

22. Fractures A fracture is any break in a bone
Named for shape or position of the fracture line
Common types of fractures:
closed - no break in skin
open fracture - skin broken
comminuted - broken ends of bones are fragmented
greenstick-partial fracture with one side of the bone broken and the other side bends (children only)
impacted-one end of bone forced into the anterior of the other
Pott’s-distal end of the lateral leg bone (fibula) with injury to the distal tibial articulation
Colles’- fracture of the distal end of the lateral forearm bone with the distal fragment displaced posteriorly
stress fracture-microscopic fissures in bone without evidence of injury to other tissues

23. Fracture and Repair of Bone
Healing is faster in bone than in cartilage due to lack of blood vessels in cartilage
Healing of bone is still a slow process due to the damage of blood vessels
Fracture repair involves formation of a clot called a fracture hematoma
This is followed by the organization of the fracture hematoma into granulation tissue called a procallus
Which is subsequently transformed into a fibrocartilaginous [soft] callus)
The fibrocartilaginous callus is converted into the spongy bone of a bony (hard) callus
Finally, this bony callus is remodeled to the original form of bone

24. Repair of a Fracture Formation of fracture hematoma
damaged blood vessels produce a clot in ~6-8 hours, as the bone cells diein the area
inflammation brings in phagocytic cells to remove debris or harmful agents
new capillaries begin to grow into damaged area
Formation of fibrocartilagenous callus formation
fibroblasts invade the procallus and collagen fibers are deposited
chondroblasts produce fibrocartilage to span the broken ends of the bone

25. Repair of a Fracture (Continued
Formation of bony callus
osteoblasts secrete spongy bone that joins two broken ends of bone
lasts ~ 3-4 months
Bone remodeling
compact bone replaces the spongy in the bony callus
the surface is remodeled back to normal shape

26. Calcium Homeostasis and Bone Tissue
The skeleton is a reservoir of calcium and phosphate
Calcium ions are involved with many body systems
nerve and muscle cell function
blood clotting
enzyme activities in many biochemical reactions
Small changes in blood levels of Ca+2 can be fatal
(plasma levels are maintained at ~ 9-11mg/100mL)
cardiac arrest if too high
respiratory arrest if too low

27. Hormonal Influences
Parathyroid hormone (PTH) is secreted by the parathyroid gland if Ca+2 levels decrease
The PTH gene is turned on and more PTH is secreted from gland
This increases the activity of the osteoclasts
The kidney retains Ca+2 and produces calcitriol
Calcitonin hormone is secreted from the parafollicular cells in thyroid gland if Ca+2 blood levels become elevated
inhibits osteoclast activity
increases bone formation by the osteoblasts