BONE BONE The most amazing story of bone histology!

Functions of Bone 1-Supports soft tissue 2-Protects vital organs (cranium, thoracic cavity) 3-Contains bone marrow 4-Reservoir of Ca++, PO4 to maintain constant concentrations in body fluids 5-Allows body to move

General classifications of bones A- Anatomical classification B-Histological classification C-According to the technique used on 1-Decalcified bone: 2-Ground bone:

A- Anatomical classification 1-Long bones : e.g. Femur ,humerus, ulna and tibia 2-Short bones: e.g. Bones of the hand and foot 3-Flat bone: e.g. Scapula ,sternum,skull and ribs. 4-Irregular bones : e.g. Pelvic bone and vertebrae.

Classification of Bones by Shape Figure 6.1

B-Histological classification 1-Compact, regular, or non-branched. 2-Cancellous, irregular, spongy and branched.

Basic structures of the bone I- CONNECTIVE TISSUE COVERINGS Such coverings are either: -Periosteum from the external bone surface. -Endosteum from the interior of the bone.

PERIOSTEUM -It consists of two main layers: -It is a fibrous sheath that envelops the surface of the bone except the articular surfaces. -It consists of two main layers: 1-Outer fibrous layer and is considered the site of muscle attachment. 2-Inner osteogenic layer which contains the bone forming cells.

Periosteum Mesenchyme Fibroblasts Osteoprogenitor cells

II- BONE CELLS - There are four types of bone cells: 1-Osteoprogenitor cell. 2-Bone immature cell (0steoblast). 3-Bone mature cell (Osteocyte) 4-Bone eating cell (Osteoclast)

1-Osteoprogenitor cell. -It is the main stem cell that gives origin to the other types of bone cells. -It is derived from the mesenschyme and characterized by its great mitotic activity. -Similar to the UMC (spindle in shape,elongated nucleus and pale basophilic cytoplasm).

The Histology of Compact Bone Figure 6.3a

Osteogenic cells:

2-Bone immature cell (0steoblast). -Responsible for the formation of both mature bone cell and the organic components of the bone matrix (Collagen type I, Proteoglycan and glycoprotein ). -They arise from osteogenic cells, and when imprisoned in lacunae they are called osteocytes. -Osteoblast is situated mainly in the inner layer of the periosteum

-By the LM It is a branched cell with basophilic cytoplasm which contains large round nucleus.

Osteoblasts

3-Bone mature cell (Osteocyte) -It is the mature bone cell. -By LM it is branched and situated inside bony cavity called Lacuna, while its processes lies inside a cylindrical bony canaliculi. -Only one cell is located inside each lacuna, each two cells are in contact together by gap junctions.

Canaliculi between Osteocytes

4-Bone eating cell (Osteoclast) -Such cell is derived from the mononuclear cell Monoctye in the bone marrow. -It is the bone eating or resorption cell and situated at bone surface. -It is a large cell and present inside a bony cavity called Howship’s lacuna.

-It contains multiple nuclei which are central in postion. -By LM -It is characterized by being large having acidophilic cytoplasm and serrated border. -The border facing the bone matrix is rough and contains micovilli and called Ruffled border. -It contains multiple nuclei which are central in postion.

Osteoclasts

III- BONE MATRIX A-Inorganic matter (65% of bone weight) -It is formed mainly of crystals of calcium Phosphate and carbonate. B-Organic matter (35% of bone weight) -It is in the form of collagenous bundles type I and ground substance. - the matrix are arranged in special patterns: Circumferential. Concentric. Interstitial .

Types of bone Histologically two types of bone are known; the compact and the cancellous bone. However, bone is classified anatomically into:

I-Compact bone It is the solid ivory type of bone, which is formed of regularly arranged lamellae. Sites: 1-Diaphyses of long bones. 2-Inner and outer tables of flat bones. 3-Outer covering of irregular short bones.

Compact Bone:

II-Cancellous or Spongy Bone It is formed of irregularly shaped trabeculae and specules containing few osteocytes. Bone marrow fills the spaces between bone trabeculae. Sites: 1-The middle layer of flat bones 2-Ends of long bones ( epiphyses ) and short irregular bones.

Spongy Bone