Lecture # 13: Bone Tissue(Chapter 7)

Objectives:

4- Explain the effects of vitamin D and various hormones on bone physiology and plasma Ca++.

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.

The Shapes of Bones

Long bonesLonger than wide. They serve as rigid levers that are acted upon by skeletal muscles to produce body movements

Short bones They are equal in length and width.They glide across one another in multiple directions

Flat bonesThey protect soft organs. They are curved but wide & thin

Irregular bonesThey have elaborate shapes that don’t fit into the other categories

Spongy bone

Compact bone

Epiphyseal line

Periosteum

Articular cartilage

Articular cartilage

Periosteum

Endosteum

Yellow bone marrow

Compact bone

Perforating (Sharpey’s

fibers)

Nutrient arteries

Medullary cavity

Spongy bone Compact bone

Diaphysis

Proximal epiphysis

Distal epiphysis

Cortex or cortical bone (compact bone)

General Features of Bones

Osseous Tissue

Calcium phosphate (2/3 of the weight of the bone)Hydroxyapatite and other calcium salts

A- Inorganic matter

B- Organic matterCollagen fibers (1/3 of the weight of the bone)

They withstand compression but they are inflexible

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

Matrix

Bone Cells 3- Osteocyte

2- Osteoblast

4- Osteoclast

1- Osteogenic (Osteoprogenitor ) cell

Bone Cells

3- Osteocyte 2- Osteoblast 1- Osteogenic cell 4- Osteoclast

- 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 in a single layer in the bone surface under the endosteum

Functions:

They are located in the lacunae interconnected by the canaliculi

Functions:

Matrix Osteoid

Functions:

-They are in pits called resorption bays

Functions:-They are bone-dissolving cells that secret acids and proteolytic enzymes that release the stored calcium (osteolysis)

-They are in the inner layer of the periosteum and in the endosteum

Osteon or Haversian system

Concentric lamellae

Circumferential lamellae

Central or Haversian

canal

Perforating or Volkmann’s

canal

Periosteum

Perforating or Sharpey’s

fibersBlood vessel

Concentric lamellae

Central or Haversian

canal

Lacuna

Osteocyte

Canaliculus

Compact Bone

Spongy Bone

It has a sponge-like appearance 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

It provides strength with minimal weight. The trabeculae develop along bone’s lines of stress

Bone Development

1- Embryo: Formation of bonny skeleton

2- Until early adulthood: Bone growth

3- Adulthood: Remodeling and repair

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

Formation of the Bonny Skeleton

After week 6: Ossification begins

Intramembranous Ossification

Endochondral Ossification

(It produces the flat bones of the skull and clavicles)

(It produces most bones of the body)

Embryo skeleton before week 6

Fibrous membraneHyaline cartilage

Ossification

Intramembranous Ossification

Condensation of mesenchyme into soft sheet permeated with blood capillaries

1 2 Deposition of osteoid tissue by osteoblastson mesenchymal surface; entrapment of firstosteocytes; 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.

Endochondral Ossification

1Early cartilage model

2Formation of the primary ossification center, bony collar, and periosteum

3Vascular invasion, formation of primary marrow cavity, and appearance of the secondary ossification center

Endochondral Ossification

Bone at birth, with enlarged primary marrow cavity and appearance of the secondary marrow cavity in one epiphysis

4 5Bone of child, with epiphyseal plate at distal end

6Adult bone with a single marrow cavity and closedepiphyseal plate

Endochondral Ossification

Endochondral Ossification

Intramembranous Ossification

Cranial bones of the skull, mandible, and clavicles

All bones of the skeleton below the base of the skull, except the clavicles

Calcium HomeostasisCalcium is used for much more than bone structure. Calcium is needed in:1- Neuron communication 2- Muscle contraction3- Blood clotting 4- ExocytosisHypocalcemia:

It is a blood calcium deficiencyHypocalcemia causes an excess of excitability of muscle, tremors, spasms or tetany (inability to relax)

It is a blood calcium excessHypercalcemia:

Hypercalcemia causes a deficiency of excitability of nerve and muscle (sluggish reflexes, depression)

Hypocalcemia can result from: vitamin D deficiency, diarrhea, thyroid tumors, underactive parathyroids, pregnancy and lactation, accidental removal of parathyroid glands during thyroid surgery

Calcium Homeostasis

Calcium homeostasis is maintained by three hormones:

1- Parathyroid Hormone

2- Calcitonin

It is produced by the parathyroid glands when the concentration of calcium in the blood falls bellow normal

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

First Hormonal Mechanism

Blood concentration of calcium

Calcitonin hormone is released by thyroid gland

It inhibits the osteoclast activity and increases the osteoblast activity

Blood concentration of calcium

Hypercalcemia

Calcium Homeostasis

Blood concentration of calcium

Parathyroid hormone is released by parathyroid gland

It increases the osteoclast activity and inhibits the osteoblast activity. It also reduces the urinary calcium excretion

Blood concentration of calcium

Hypocalcemia

Second Hormonal Mechanism

Calcitonin

1- CalcitoninIt is produced by the thyroid gland when the concentration of calcium in the blood rises above normal (Hypercalcemia)

Ca+2

Correction for Hypercalcemia

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

Ca+2

Ca+2

Ca+2

Ca+2Calcitriol

Correction for Hypocalcemia

3- Promotes kidney reabsorption of calcium ions, so less lost in urine

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).

Abnormal softness of bones (rickets) in children and (osteomalacia) in adults without adequate vitamin D

Calcitriol is necessary for bone deposition, which also needs adequate calcium and phosphate

Third Hormonal Mechanism

Calcitriol Synthesis and Action

Calcitriol Calcitriol

3- Calcitriol

Ca+2 Ca+2Ca+2

Ca+2

Ca+2

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