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

1. Isolate bone from surrounding tissues2. Provide a route for circulatory and nervous

supply3. Participate in bone growth and repair

Endosteum

An incomplete cellular layer:lines the marrow cavitycovers trabeculae of spongy bonelines central canals

Contains osteoblasts, osteoprogenitor cells, and osteoclastsIs 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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