bone tissue

8th lecture

December 10, 2015
Specialized Connective Tissue [Bone (Osseous) Tissue]

Bone:

Bone is a specialized connective tissue composed of calcified intercellular material, the bone matrix and cells.

The main function of bone tissue are:

• bone tissue provides solid support for the body, protects vital organs such as those in the cranial and thoracic cavities, and harbors cavities containing bone marrow where blood cells are formed.
• Bone (or osseous) tissue also serves as a reservoir of calcium, phosphate, and other ions that can be released or stored in a controlled fashion to maintain constant concentrations in body fluids.
• In addition, bones form a system of levers that multiply the forces generated during skeletal muscle contraction and transform them into bodily movements.
• This mineralized tissue therefore confirms mechanical and metabolic functions to the skeleton.

Classification of Bone by Shape

• Long
• Short
• Flat
• Irregular

Bone Matrix

Solid ground is made of mineral crystals
2/3 of bone matrix is calcium phosphate, Ca3(PO4)2:
reacts with calcium hydroxide, Ca(OH)2 to form crystals of hydroxyapatite, Ca10(PO4)6(OH)2 which integrates other calcium salts and ions

Bone Matrix

Matrix Proteins- 1/3 of bone matrix is protein fibers (collagen type I)
Mineral salts make bone rigid and compression resistant but would be prone to break down
Collagen fibers add extra tensile strength but mostly add torsional flexibility to resist break down

There are four major types of cells

in matrix only

endosteum only
periosteum + endo

1. Osteoblasts

Immature bone cells that secrete matrix compounds (osteogenesis)
Eventually become surrounded by calcified bone and then they become osteocytes

Figure 6–3 (2 of 4)

2.Osteocytes
Mature bone cells that maintain the bone matrix

Figure 6–3 (1 of 4)

Osteocytes
Live in lacunae
Found between layers (lamellae) of matrix
Connected by cytoplasmic extensions through canaliculi in lamellae (gap junctions)
Do not divide
Maintain protein and mineral content of matrix
Help repair damaged bone

3. Osteoprogenitor Cells

Mesenchyme stem cells that divide to produce osteoblasts
Are located in inner, cellular layer of periosteum
Assist in fracture repair

4. Osteoclasts

Secrete acids and protein-digesting enzymes

Figure 6–3 (4 of 4)

Osteoclasts
Giant, mutlinucleate cells
Dissolve bone matrix and release stored minerals (osteolysis)
Often found lining in endosteum lining the marrow cavity
Are derived from stem cells that produce macrophages

Homeostasis

Bone building (by osteocytes and -blasts) and bone recycling (by osteoclasts) must balance:
more breakdown than building, bones become weak
exercise causes osteocytes to build bone

Bone membranes

Periosteum:
covers outer surfaces of bones
consist of outer fibrous and inner cellular layers
Contains osteblasts responsible for bone growth in thickness

Functions of Periosteum

• Isolate bone from surrounding tissues
• Provide a route for circulatory and nervous supply
• Participate in bone growth and repair

Endosteum

An incomplete cellular layer:

lines the marrow cavity
covers trabeculae of spongy bone
lines central canals
Contains osteoblasts, osteoprogenitor cells, and osteoclasts
Is active in bone growth and repair

Gross Anatomy of Bones: Bone Textures

Compact bone – dense outer layer
Spongy bone – honeycomb of trabeculae filled with yellow bone marrow

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Flat Bones of the Skull (compact bone)
Two layers of compact bone
Inner table
Outer table
Region of spongy bone sandwiched between them
Called the diploe
Both layers of compact bone are covered by periosteum


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Osteon

The basic structural unit of mature compact bone
Osteon = Osteocytes arranged in concentric lamellae around a central canal containing blood vessels
Lamella – weight-bearing, column-like matrix tubes composed mainly of collagen

Compact Bone

Figure 6–5

Three Lamellae Types
Concentric Lamellae
Circumferential Lamellae
Lamellae wrapped around the long bone line tree rings
Binds inner osteons together
Interstitial Lamellae
Found between the osteons made up of concentric lamella
They are remnants of old osteons that have been partially digested and remodeled by osteoclast/osteoblast activity


Compact Bone

Figure 6–5

Microscopic Structure of Bone: Compact Bone
Figure 6.6a

Microscopic Structure of Bone: Compact Bone

Figure 6.6b

Microscopic Structure of Bone: Compact Bone

Figure 6.6c

Spongy Bone Tissue

Makes up most of the bone tissue in short, flat, and irregularly shaped bones, and the head (epiphysis) of long bones; also found in the narrow rim around the marrow cavity of the diaphysis of long bone


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Spongy Bone Microanatomy
No osteons
In trabeculae:
Parallel lamellae
Osteocytes in lacunae
canaliculi

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