1- Is the highest differentiation of CT

2-Form the skeleton

3-sites for attachment of the muscles and

tendons essential for locomotion.

4-Supports and protects vital organs

5- Harbors bone marrow, fat store

6- Acts as a reservoir of Calcium & Phosphate

BoneBone

Gross anatomy of bones

Compact bone Spongy

(trabecular) bone

Blood vessels Medullary

cavity Membranes

PeriosteumEndosteum

Cartilage BoneWater content: ~70%Collagen II: ~40% of organic content.

Grows interstitially and by apposition.

Avascular

Water content: 25% Collagen I: 90% of organic

content.

Other Ground Substance

Osteonectin: anchor collagen to bone mineral.

Osteocalcin: Calcium binding protein involved in bone calcification.

Osteopontin: Binding of osteoblasts and osteoclasts to bone.

Grows only by apposition.

Highly vascular

Types of BoneTypes of Bone(Flat, long, short and irregular bones)(Flat, long, short and irregular bones)

Flat bones

Spongy bone :

Compact + spongy Have bone marrow

Long bones

Diaphysis= shaft Epiphyses at the ends: covered with “articular”

(=joint) cartilage Epiphyseal line in adults

Kids: epiphyseal growth plate (disc of hyaline cartilage that grows to lengthen the bone)

Blood vessels Nutrient arteries and veins through nutrient foramen

Cells Fibers

Matrix1. Osteogenic cells2. Osteoblasts3. Osteocytes4. Osteoclasts

Type 1 Collagen (90%)

Structure of bone

Organic1.GAGs: Hyaluronan, Chondroitin & Keratan Sulfate2.Proteoglycans

InorganicHydroxyapatite crystals

[Ca10(PO4)6(OH)2]Calcium phosphate

1. Osteogenic or osteoprogenitor cells:

- stem cells derived from mesenchyme. - They are spindle-shaped with elongated nuclei.- present in:

• inner layer of periosteum • endosteum • lining the vascular canals of compact bone.

Function: differentiation into osteoblast cells.

Cells of Bone

2. Osteoblasts: - young bone cells, located at the surfaces .- known as bone-forming cells.- cuboidal cells with short, slender processes.- has a large nucleus + prominent nucleolus.- The cytoplasm: basophilic, well-developed Golgi

apparatus, mitochondria.- can not divide. Functions: synthesis of the organic component and

deposition of inorganic components of bone matrix.

3. OsteocytesOnce the osteoblast cells surrounded by matrix:1. The cytoplasmic processes become more evident. 2. The cells become flattened.3. lacunae and canaliculi appear

osteoblasts are referred to as Osteocytes. - Processes make contact via gap junctions (?????)- can not divide.- faint basophilic cytoplasm.- Dark nucleus.- Few RER & Golgi. Function:maintenance of the bony matrix.

4. Osteoclasts:- Bone-eating cells. - found in depressions known as Howship’s lacunae.- derived from the fusion of blood monocytes.- very large cells (20 – 100 µm).- multinucleated (5 –50).- acidophilic cytoplasm. -The surface facing the bone matrix is irregular.- contain a well-developed Golgi + great number of

lysosomes.Function:secrete acid collagenase, and proteolytic enzymes that

attack the bone matrix and liberate the calcified ground substance.

Bone cells

Osteoblasts Active Inactive

Osteocytes Osteoclasts

Howship’s lacuna (resorption bay)

Osteocytes, Lacunae and Canaliculi

Source Undetermined

Types of bone

Compact Bone

Cancellous bone

Compact (dense) and Spongy (cancellous) Bone

cancellous bone

Compact bone

• found in diaphysis (shaft) of long bones.

• dense, hard, with no cavities.

• found in epiphyses (bulbous ends) of long bones, flat (Skull) & irregular (vertebrae) bone

• with cavities.

.

1.Compact boneCompact bone is a solid mass composed of:

1. Haversian system.2. Circumferential lamellae.3. Interstitial lamellae (O&I). 4. Periostem 5. Endosteum

1. Haversian system (osteon):the structural unit of compact bone, composed of:1. Haversian canal: it runs parallel to the long axis of bone. It

contains loose connective tissue rich in blood vessels and nerves and lined with osteogenic cells.

2. Bone lamellae: they are calcified osteoid tissue arranged in concentric layers (4-20 layers).

3. Osteocytes: they are present in their lacunae between the bone lamellae.

2. Circumferential lamellae:bone lamellae under the periosteum (outer circumferential lamellae)

or adjacent to the endosteum (inner circumferential lamellae).3. Interstitial lamellae: lamellae between Haversian system

Volkmann`s canal: transverse or oblique canals that connect haversian canals together.

Haversian system (osteon), Haversian canal (HC) and Volkmann’s canal (VC)

Transverse section Longitudinal section

Osteon

VC

HC

VC

HC

Compact bone

Osteons

•Nutrients diffuse from vessels in central canal•Alternating direction of collagen fibers

increases resistance to twisting forces

Isolated osteon:

• Canaliculi

• Tiny canals

• Radiate from the central canal to lacunae

• Form a transport system

Figure 5.3

Dark spots are called ‘lacunae’ and would contain osteocytes in living bone

Central canal containing an artery, vein, lymph vessel and nerves.

Haversian system (osteon) /Harversian canal

HC

HC

Periosteum: Connective tissue membrane Covers entire outer surface of bone except at epiphyses Two sublayers

1. Outer fibrous layer 2. Inner (deep) cellular osteogenic layer (Osteoblasts: bone

depositing cells, also osteoclasts: bone destroying cells) Secured to bone by perforating fibers (Sharpey’s fibers)

Endosteum: Lines the internal surfaces of bones - one cell thick Is also osteogenic

• Important roles of the periosteum and endosteum: 1. nutrition 2. histogenesis and repair.

Periosteum and Endosteum

Endosteum

Source Undetermined (Both images)

Cancellous Bone

1- NO Haversian system.2- Irregularly arranged bone. 3- Irregular marrow spaces.4- Periostem and endosteum.

*

Cancellous bone

Compact and Cancellous Bone

Michigan Medical School Histology Slide Collection