1. Lecture # 13: Bone Tissue (Chapter 7) Objectives:
1- Describe the gross anatomical features of a typical long bone and a typical short bone.
2- Discuss the microscopic structure of compact bone and compare it to that of cancellous bone.
3- Define ossification and distinguish between intramem- branous and endochondral ossification.
4- Explain the effects of vitamin D and various hormones on bone physiology and plasma Ca++.

2. The Shapes of Bones Irregular bones Flat bones Short bones Long bones
They have elaborate shapes that don’t fit into the other categories
Flat bones
They protect soft organs. They are
curved but wide & thin
Short bones
They are equal in length and width.
They glide across one another in multiple directions
Long bones
Longer than wide. They serve as rigid levers that are acted upon by skeletal muscles to produce body movements

3. General Features of Bones
Spongy bone
Articular cartilage
Proximal epiphysis
Distal epiphysis
Cortex or cortical bone (compact bone)
Epiphyseal line
Articular cartilage
Diaphysis
Medullary cavity
Compact bone
Spongy bone
Periosteum
Compact bone
Endosteum
Yellow bone marrow
Compact bone
Periosteum
Perforating (Sharpey’s fibers)
Nutrient arteries

4. Osseous Tissue Matrix Bone Cells A- Inorganic matter
B- Organic matter
Calcium phosphate (2/3 of the weight of the bone)
Hydroxyapatite and other calcium salts
They withstand compression but they are inflexible
Collagen fibers (1/3 of the weight of the bone)
Osseous Tissue
They are remarkably strong and they can easily tolerate twisting and bending
The collagen fibers and the hydroxyapatite form a protein-crystal combination that allow the bone to be strong, somewhat flexible, and highly resistant to shattering
1- Osteogenic (Osteoprogenitor ) cell
2- Osteoblast
3- Osteocyte
4- Osteoclast

5. Bone Cells 3- Osteocyte 2- Osteoblast 1- Osteogenic cell 4- Osteoclast
Matrix
Osteoid
3- Osteocyte
2- Osteoblast
1- Osteogenic cell
4- Osteoclast
-They are in the inner layer of the periosteum and in the endosteum
-They are in pits called resorption bays
They are located in the lacunae interconnected by the canaliculi
-They are in a single layer in the bone surface under the endosteum
Functions:
Functions:
Functions:
Functions:
- They contribute to the homeostatic maintenance of the surrounding matrix
-They are bone-forming cells. They produce the soft new bone matrix (osteoid) in a process called osteogenesis
-They are stem cells that divide to produce the osteoblasts
-They are bone-dissolving cells that secret acids and proteolytic enzymes that release the stored calcium (osteolysis)

7. Compact Bone Osteocyte Concentric lamellae Osteon or Haversian system
Circumferential lamellae
Concentric lamellae
Central or Haversian canal
Central or Haversian canal
Lacuna
Canaliculus
Periosteum
Perforating or Volkmann’s canal
Perforating or Sharpey’s fibers
Blood vessel

9. Spongy Bone It has a sponge-like appearance
It provides strength with minimal weight. The trabeculae develop along bone’s lines of stress
The spongy bone consists of:
- slivers of bone called spicules
- thin plates of bone called trabeculae
- spaces filled with red bone marrow
- few osteons and no central canals
- all osteocytes close to bone marrow

10. Ossification occurs during the whole life
Bone Development
Ossification or Osteogenesis is the formation of bone
During ossification, deposit of calcium salts occurs (calcification), but calcification occurs also in other tissues. When calcification occurs in tissues other than bones, the result is calcified tissue that does not resemble bone.
Ossification occurs during the whole life
1- Embryo: Formation of bonny skeleton
2- Until early adulthood: Bone growth
3- Adulthood: Remodeling and repair

11. Formation of the Bonny Skeleton
Fibrous membrane
Hyaline cartilage
Embryo skeleton before week 6
After week 6: Ossification begins
Ossification
Intramembranous Ossification
Endochondral Ossification
(It produces the flat bones of the skull and clavicles)
(It produces most bones of the body)

12. Intramembranous Ossification
Condensation of mesenchyme into soft sheet permeated with blood capillaries 1 2
Deposition of osteoid tissue by osteoblasts
on mesenchymal surface; entrapment of first
osteocytes; formation of periosteum
Honeycomb of bony trabeculae formed by continued mineral deposition; creation of spongy bone 3 4
Surface bone filled in by bone deposition,
converting spongy bone to compact bone.
Persistence of spongy bone in the middle layer.

13. Endochondral Ossification

14. Endochondral Ossification3
Vascular invasion, formation of primary marrow cavity, and appearance of the secondary ossification center 1
Early cartilage model 2
Formation of the primary ossification center, bony collar, and periosteum

15. Endochondral Ossification6
Adult bone with a single marrow cavity and closed
epiphyseal plate
Bone at birth, with enlarged primary marrow cavity and appearance of the secondary marrow cavity in one epiphysis 4 5
Bone of child, with epiphyseal plate at distal end

16. Endochondral Ossification Intramembranous Ossification
All bones of the skeleton below the base of the skull, except the clavicles
Intramembranous Ossification
Cranial bones of the skull, mandible, and clavicles

17. Calcium Homeostasis
Calcium is used for much more than bone structure. Calcium is needed in:
1- Neuron communication
2- Muscle contraction
3- Blood clotting
4- Exocytosis
Hypocalcemia:
It is a blood calcium deficiency
Hypocalcemia causes an excess of excitability of muscle, tremors, spasms or tetany (inability to relax)
Hypocalcemia can result from: vitamin D deficiency, diarrhea, thyroid tumors, underactive parathyroids, pregnancy and lactation, accidental removal of parathyroid glands during thyroid surgery
Hypercalcemia:
It is a blood calcium excess
Hypercalcemia causes a deficiency of excitability of nerve and muscle (sluggish reflexes, depression)

18. Calcium Homeostasis
Calcium homeostasis is maintained by three hormones:
1- Parathyroid Hormone
It is produced by the parathyroid glands when the concentration of calcium in the blood falls bellow normal
2- Calcitonin
It is produced by the thyroid gland when the concentration of calcium in the blood rises above normal
3- Calcitriol (it is a form of vitamin D)
It is an steroid hormone produced by the kidneys, which increases the concentration of calcium in the blood when it falls bellow normal

19. First Hormonal Mechanism Second Hormonal Mechanism
Calcium Homeostasis
First Hormonal Mechanism
Blood concentration of calcium
Blood concentration of calcium
Calcitonin hormone is released by thyroid gland
It inhibits the osteoclast activity and increases the osteoblast activity
Hypercalcemia
Second Hormonal Mechanism
It increases the osteoclast activity and inhibits the osteoblast activity. It also reduces the urinary calcium excretion
Parathyroid hormone is released by parathyroid gland
Blood concentration of calcium
Blood concentration of calcium
Hypocalcemia

20. Calcitonin

21. 1- Calcitonin
It is produced by the thyroid gland when the concentration of calcium in the blood rises above normal (Hypercalcemia)
Correction for Hypercalcemia
Ca+2

22. 2- Parathyroid Hormone (PTH)
It is produced by the parathyroid glands when the concentration of calcium in the blood falls bellow normal (Hypocalcemia)
Calcitriol
Ca+2
Ca+2
Ca+2
Ca+2

23. Correction for Hypocalcemia

24. Third Hormonal Mechanism
3- Calcitriol:
It is a form of vitamin D produced by the sequential action of the skin, liver, and kidneys.
Calcitriol behaves as a hormone that raises blood calcium concentration:
1- Increases calcium absorption by small intestine.
2- Increases calcium resorption from the skeleton (increases stem cell differentiation into osteoclasts which liberates calcium and phosphate from bone).
3- Promotes kidney reabsorption of calcium ions, so less lost in urine
Calcitriol is necessary for bone deposition, which also needs adequate calcium and phosphate
Abnormal softness of bones (rickets) in children and (osteomalacia) in adults without adequate vitamin D

25. Calcitriol Synthesis and Action

26. 3- Calcitriol
It is produced by the sequential action of the skin, liver, and kidneys when the concentration of calcium in the blood falls bellow normal.
Calcitriol
Calcitriol
Ca+2
Ca+2
Ca+2
Ca+2
Ca+2
Calcitriol