05/02/2023

HISTOLOGY OF BONEBy: Dr.Najeebullah TareenBolan Medical College Quetta

BONE

BONE• bone is a specialized form of dense connective tissue.

• The most hard of all connective tissues.

• Bone is the main component of the skeleton in the adult human.

•  Bone gives the skeleton the necessary rigidity to function as attachment and lever for muscles and supports the body against gravity.

HISTOLOGY

HISTOLOGY• Histos means Tissue logy means study

• Histology ---- Microscopic Anatomy

• Histology is mainly the study of microscopic structures like cells and tissues.

HISTOLOGY OF BONE

• Like other Connective Tissue, the bone is composed of cells and matrix.

BONE CELLS

BONE CELLS• Four types of cells are found in Bone.

1. Osteoprogenitor cells (or stem cells of bone)

2. Osteoblasts (Bone Forming Cells)

3. Osteocytes ( Mature Bone Cells)

4. Osteoclasts (Bone Resorbing Cells)

OSTEOPROGENITOR CELLS• A stem cell that is the precursor of an osteoblast.

• Osteogenic cells  are the only bone cells that divide.

• Osteogenic cells differentiate and develop into OSTEOBLASTS which, in turn, are responsible for forming new bone.

• Osteoprogenitor cells are located in the inner cellular layer of the periosteum and the endosteum.

OSTEOBLASTS

• Osteoblasts are bone forming cells.• They originate from osteoprogenitor cells.• They synthesize unmineralized bone matrix (Osteoid) and Alkaline

Phosphatase that bring about mineralization of osteoid.• Active Osteoblasts are large cells and posses cytoplasmic

processes which make contact with those of other cells.• Inactive osteoblasts are fusiform cells.• they become trapped in their own secretions and differentiate into

Osteocytes.

OSTEOCYTES• Osteocytes are mature bone cells and they are derived from Osteoblasts.

• Osteocytes are star shaped cells with fine cytoplasmic process.

• Osteocytes have an average half life of 25 years.

• Osteocytes lie within small cavities called lacunae.

• Although osteocytes are relatively inert cells Osteocytes synthesize sclerostin, a secreted protein that inhibits bone formation by binding to LRP5/LRP6 coreceptors.

OSTEOCLASTS• Osteoclasts are bone resorbing cells.

• They are multinucleated giant cells containing 30 or more nuclei.

• Found in close association with bone surface located in shallow grooves called Howship’s Lacunae.

• Osteoclasts secret several organic acids and lysosomal enzymes (cathepsin C, beta-glycerophosphatase, beta-glucuronidase). Acids dissolve mineral component while enzymes destroy organic matrix.

BONE MATRIX

BONE MATRIX• Bone matrix consist of :1. Organic component • Constitutes 35% of dry weight of bone.• Consist of (i) Collagen Fibers & (ii) amorphous ground substance.• Collagen fibers are type-I, making 95% of organic mass.2. Inorganic Component • Constitutes 65% of dry weight of bone.• Calcium phosphate is the chief bone mineral deposited as crystal of

calcium hydroxyapatite over collagen fibers

MICROSCOPIC ARRANGEMENT OF BONE CELLS AND BONE

MATRIX

MICROSCOPIC ARRANGEMENT

• The bone matrix is arrange in layers know as Lamellae.• There are three type of lamellae based on their arrangement.1. Concentric lamellae (surrounds Haversian canal)2. Interstitial lamellae (present between two Haversian systems)3. Circumferential lamellae (Present at periphery) • The bone cells are present within spaces called lacunae between the

lamellae.• The Haversian canal is a longitudinal canal, around which lamellae are

arranged in a regular manner. • Some small canals pierces the bone from periosteum and endosteum, these

channels are called Volkmann's canals.• Periosteum: thick fibrous sheath covers the bone at outer side except

articular surfaces. • Endosteum: Thin layer of connective tissue that lines the medullary cavity.

Q#1: THE ONLY BONE OF THE BODY WHICH IS VISIBLE TO

EVERYONE?

Q#2: THE ONLY BONE THAT FULLY GROWN AT BIRTH ???

STAPES

Q#3: THE ONLY JOINTLESS BONE IN OUR BODY IS???

HYOID BONE