1. Functions of Bone Rigid skeleton supports the body
Provides sites for attachment of muscles and organs
Protective cover for soft organs
Hemopoiesis (marrow cavities)
Reservoir for body’s calcium supply
Calcium used for muscle contraction, blood clotting, cell membrane permeability, transmission of nerve impulses

2. Bone Connective tissue with a mineralized matrix (calcium phosphate)
Matrix is dense and rigid, contains fibers of mainly Type I collagen.
Both the collagen and the ground substance become mineralized in bone.

3. Bone Matrix
Calcified matrix of fibers (Type I collagen) and ground substance (proteoglycans with chondroitin sulfate and keratan sulfate side chains, glycoproteins)
Organic component (osteoid)= type I collagen, highly cross-linked
Inorganic (mineral) component = crystals of calcium hydroxyapatite (calcium and phosphorus)
Hard – nutrients and metabolites cannot diffuse through the calcified matrix as done in cartilage.
Highly vascularized:
Canaliculi –small channels or canals between lacunae that allow osteocytes to exchange wastes and bring metabolites in; osteocytes send out cytoplasmic processes into the canaliculi and contact neighboring cells through gap junctions.

4. Structure of Bones Diaphysis – shaft Epiphysis – articular ends
Periosteum – covers external surfaces;
- consists of an outer layer of dense, irregular collagenous (fibrous) CT and an inner layer of osteoprogenitor and osteoblast cells
Marrow – in central cavity of bone, hemopoietic tissue, red or yellow

5. Epiphysis of an adult long bone
Spongy
(cancellous)
bone
Compact bone
Compact bone

6. Spongy (Cancellous) Bone
- composed of spicules or trabeculae of bone;
- forms network around bone marrow.

7. Compact Bone dense bone no cavities;
found along the outer surface of shafts of long bone.;

8. Compact bone has a lamellar structure
circumferential lamellae (on surfaces)
Haversian lamellae
interstitial lamellae.

9. Compact Bone – Low Power highly vascularized
Haversian canal - the
space running
through an osteon;
- contains blood vessels, nerves, osteoblasts and osteoprogenitor cells.
Haversian canal
Volkmann’s
canal
Volkmann’s canals – connect Haversian canals of adjacent osteons

10. Ground Bone – higher power canaliculus
lacuna

11. Bone Marrow fat cells Blood-forming cells
Osteogenic (osteoprogenitor ) cells – line inner layer of endosteum
periosteum, and haversian canals;
- undifferentiated stem cells that give rise to osteoblasts

12. Bone Formation – 2 types
Intramembranous ossification – formation of bone directly from mesenchyme (embryonic CT) with no cartilage precursor
Endochondral – formation of bone on cartilage
scaffolding;
- bone eventually replaces cartilage.

13. Occurs in flat bones of skull, mandible, maxilla.
Intramembranous
Ossification
Occurs in flat bones of skull, mandible, maxilla.
Osteoblasts develop from CT mesenchyme cells.
Osteoblasts secrete osteoid (uncalcified bone matrix).

14. Intramembranous Ossification
Bone matrix is first formed as a network of spicules or trabeculae.
Osteoblasts – immature bone cells; - - secrete osteoid (uncalcified bone matrix
Osteocytes – mature bone cells; trapped within lacunae of bone matrix
osteocytes
osteoblasts

15. Bones are dynamic structures! Continually being remodeled
Osteoclasts – large, multinucleate cells;
at the surfaces of bone
- resorb & remodel bone in response to hormone and calcium levels
Howships lacunae – depressions made by osteoclasts as they etch away the bone.

16. Osteoclast activity is regulated by hormones
Parathyroid hormone – increases blood calcium levels; stimulates activity of osteoclasts and increases their numbers.
Calcitonin (from thyroid gland) – decreases blood calcium levels; reduces the numbers and activity of osteoclasts.

17. Endochondral Ossification
Occurs in long and short bones, vertebrae
Bone is proceeded by temporary hyaline cartilage model that serves as a structural scaffold.
Bone tissue REPLACES the cartilage
Cartilage begins to calcify and causes chondrocytes to hypertrophy.
Cartilage calcifies so diffusion of nutrients to chondrocytes stops and they die.
Fragmented pieces of calcified cartilage serve as the framework to deposit the bone.
Osetoprogenitor cells and blood vessels from surrounding connective tissue penetrate and invade degenerating cartilage model.
Cartilage of diaphysis replaced by bone except for at epiphyseal plate.

18. Endochondral Ossification – bone replaces a cartilage model

19. Endochondral Ossification
cartilage
bone replacing cartilage

20. Endochondral Ossification
bone replacing cartilage

21. Summary of Bone Cell Types

22. Primary vs. Secondary Bone
Primary – immature or “woven” bone
First bone formed in fetal development or bone repair
Abundant osteocytes
Irregular bundles of collagen (Type I)
Secondary bone –
mature or lamellar
bone

23. Methods to Make Slides of Bone
Decalcified bone sections
Acid solution removes calcium salts and softens bone.
Bone can then be embedded and sectioned.
Osteocytes are distorted.
Ground bone sections
Saw bone in slices.
Grind piece down until thin enough to put on a slide.
Cells are destroyed and lacunae and canaliculi filled with bone debris.