Microscopic anatomy of bone

Bone cells

1. Osteoprogenitor cells

2. Osteoblasts

3. Osteocytes

4. Osteoclasts

Osteoprogenitor cells

• Location: Positioned within the cellular layers of the periosteum (covering external bone surfaces) and the endosteum (lining internal cavities)

• Function: Function as stem cells derived from mesenchyme that undergo cell division to differentiate into osteoblasts

Osteoblasts

• Location: Arranged along active inner and outer bone surfaces within the endosteum and preiosteum

• Function: Serve as bone forming cells that synthesize and secrete osteoid (the initial organic bone matrix). As mineral deposition calcifies the matrix around them, osteoblasts become more entrapped within spaces called lacunae and differentiate into mature osteocytes

Osteocytes

• Location: Housed individually inside small matrix chambers termed lacunae, with their cellular processes extending through tiny channels called canaliculi

• Functions: Function as mature bone cells that continuously monitor and maintain the structural integrity of the surrounding calcified bone matrix

Osteoclasts

• Location: Situated along bone surfaces undergoing matrix breakdown, resting in shallow depressions called resorption lacunae along the endosteum and periosteum

• Function: Large, multi nucleated cells formed from the fusion of bone marrow stem cells that perform bone resorption (breaking down calcified matrix) using lysosomes and specialized ruffled border during bone growth and remodeling

Osteon

An individual osteon consists of concentric layers of calcified matrix organized around central canal

Structural organization of an osteon (Central canal (Haversian canal))

A cylindrical channel located at the center of the osteon that carries blood vessels (arteries and veins) and nerves to deliver oxygen, nutrients, and sensory coverage to bone tissue

Structural organization of an osteon (Concentric Lamellae)

Ring like layers of calcified bone matrix arranged concentrically around the central canal. Alternating collagen fiver orientations between adjacent lamellae help resist twisting (torsional) forces

Structural organization of an osteon (lacunae)

• Small matrix spaces located between adjacent concentric lamellae that house mature bone cells

Structural organization of an osteon (cancaliculi)

Tiny, hair like interconnecting channels radiating through the calcified matrix. They connect neighboring lacunae to one another and to the central canal, allowing osetocyte cellular processes to transport nutrients and signals

Structural organization of an osteon (perforating canals (Volkmaann canals)

Channels running perpendicular to the central canals that connect the vascular and nerve supply of central canals to the external periosteum and internal endosteum

Compact bone

Basic unit of compact bone is the osteon

• Also known as Haversian system

• Cylindrical structures

• Run parallel to the diaphysis

Spongy bone

• Does not have Osteons

• Trabeculae contains parallel lamellae

• Distributes stress from mechanical forces across framework

Anatomical location of compact bone

• Forms the dense, solid external outer shell covering all bones beneath the periosteum

• Forms the entire thick wall of the elongated shaft (diaphysis) in longs bones

Anatomical location of spongy bone

Located internally, deep to the outer compact bone layer

Fills the expanded, bulbous ends (epiphyses) of long bones

Forms the internal core (sandwiched layer) of flat, short, and irregular bones)

Ossification

Is the formation of bone connective tissue

Two patterns

• Intramembranous ossification

• Endochondral ossification

Intramembranous ossification

• Produces flat bones of the skull, some facial bones, mandible, and central portion of clavicle

• Bones develop from mesenchyme membrane

• Calcification entraps osteoblasts within lacunae in bone matrix

• Osteoblasts become osteocytes

Endochondral ossification

Formation of most bones of skeleton: upper/lower limbs, pelvis, vertebrae, ends of clavicles

Bones develop from hyaline cartilage model

• The fetal hyaline cartilage model develops

• Cartilage calcifies, and periosteal bone collar forms

• The primary ossification center forms in the diaphysis

• Secondary ossification centers from in the epiphyses

• Bone replaces cartilage, except the articular cartilage and epiphyseal plates

• Epiphyseal plates ossify and from epiphyseal lines

A long bone growth in length

Interstitial growth occurs in epiphyseal plate

Growth in a bones diameter

Appositional growth, occurs in periosteum