BONE

Bone

Is a specialized connective tissue and is one of the hardest tissues of the body

Second only to cartilage in its ability to withstand stress

Functions of bone

Provides mechanical support Provides levers for the action of

muscles/help in locomotion Serves as site for storage of calcium

in the body Protects vital organs Encloses the blood forming elements

of the bone marrow

Structural Elements of Bone (composition) Bone cells

Osteoprogenitor cells – stem cells Osteoblasts – forms bone Osteocytes – maintains bone Osteoclasts – resorbs bone

Bone matrix Sharpey’s fibers Blood vessels, nerves, lymph vessels

(Haversian canals)

BONE CELLS

Osteoprogenitor cells

Stem cells derived from the mesenchyme Possess mitotic potential and the ability to

differentiate into mature bone cells Resemble mesenchymal cells and are

spindle-shaped, with pale-staining elongated nuclei and sparse cytoplasm (small amount of rough ER and poorly developed golgi complex)

Mostly found in the inner portion of the periosteum, in the endosteum, and within vascular canals of compact bone

Types of Osteoprogenitor cells Preosteoblasts

Possess some endoplasmic reticulum and gives rise to the osteoblasts

Preosteoclasts Has more

mitochondria and free ribosomes and give rise to the osteoclasts

Osteoblasts

Associated with bone formation

Found where osseous matrix is being deposited

Vary in shape, some being cuboidal and others pyramidal, and are frequently in continuous layer suggestive of an epithelial arrangement

Osteoblasts

Have numerous finger-like cytoplasmic processes that extend into the developing bone matrix to contact cell processes from neighboring osteoblasts

Functions of the osteoblasts Contain ribose nucleoprotein

concerned with the synthesis of the organic components of bone matrix, i.e. collagen and glycoproteins

Contain the enzyme alkaline phosphatase concerned not only with the elaboration of matrix but also with its calcification

Osteocytes

Principal bone cells Are osteoblasts which have become

imprisoned within the bone matrix and are lodged inside cavities called LACUNA

Have faintly basophilic cytoplasm which can be shown to contain fat droplets, some glycogen and fine granules similar to those of or are present within osteoblasts

Osteocytes

Has fine cytoplasmic processes which extend for some distance and are called radiating canaliculi. These form an extensive continuous communication system among osteocytes

Functions of the radiating canaliculi

Passageway of nutrients and metabolites (intracellular transport)

Intercommunication system – interconnection to other osteocytes

For storage of calcium

Osteoclasts Bone destroyers Are multinucleated giant cells (5-50 nuclei),

varying in size and shape with serrated borders and vacuolated cytoplasm

Found to lie within enzymatically etched depressions in the matrix known as HOWSHIP’S LACUNA

Under the EM, the surface of the osteoclasts facing the matrix has numerous cytoplasmic projections and microvilli – this is described as a RUFFLED BORDER

Osteoclasts Derived from the mononuclear cells

(monocytes) of hemopoietic bone marrow Secrete acid, collagenase, and other

proteolytic enzymes that attack the bone matrix and liberate the calcified ground substance

FUNCTIONS: Responsible for bone resorption

activities Engaged in elimination of debris formed

during bone resorption

DISTINGUISHING PARTS OF BONES

Periosteum

The outermost covering of bones Made up of a dense fibrous CT that

covers all portions of the compact bone except those covered b articular cartilages

Important for growth and regeneration Consists of two layers: the outer and

inner layer (osteogenetic layer or cambium)

Layers of the Periosteum OUTER LAYER – consists of collagen fibers

and fibroblasts. Made up of loose/areolar CT, blood vessels and nerves

INNER LAYER – OSTEOGENETIC LAYER OR CAMBIUM. It is where regeneration occurs. It is more cellular, composed of flattened cells or osteoprogenitor cells with potential to divide by mitosis and differentiate into osteoblasts. Presence of bundles of Sharpey’s fibers – penetrate the bone matrix and serving to bind the periosteum to the bone

Endosteum Lines all internal

surfaces of cavities within the bone. Composed of a single layer or flattened osteoprogenitor cells and a very small amount of CT. Considerably thinner than the periosteum

Principal Functions of the Periosteum and Endosteum For nutrition of osseous tissue Provision of a continuous

supply of new osteblasts for repair or growth of bone

Bone Matrix

Composed of concentrically arranged bony lamellae Types of layering (lamella)

Circumferential lamella – external, internal

Concentric/Haversian lamella Interstitial or Intermediate lamella

External Circumferential lamella or Periosteal lamella

Continuous uninterrupted layer found immediately beneath the periosteum around the circumference of the bone

Internal Circumferential Lamella or Endosteal Lamella Continuous uninterrupted

lamellae immediately surrounding the central medullary cavity

Concentric or Haversian Lamella

Layers of bone surrounding the Haversian canal

Varies from 5 – 20 layers

Interstitial or Intermediate lamella Found in between

haversian systems Triangular or

irregularly shaped groups of parallel lamella

May represent lamella left by former haversian systems destroyed during growth and remodelling of bone

Haversian system

Composed of small central canal (HAVERSIAN CANAL) which is in constant communication with the blood vessels

Surrounding the canal are concentric bony lamella called haversian lamella

Haversian system

A bright thin line called CEMENTING LINE separates one haversian system from another

Haversian systems or OSTEONES (OSTEON) are found underneath the periosteal lamella

Volkmann’s canal

Canals that are not surrounded by bony lamella

Traverse the bone in a direction perpendicular or oblique to the lamella

Larger than haversian canals

STRUCTURAL CLASSIFICATION OF BONE

Macroscopic Approach

1. Spongy or Cancellous bone

2. Compact bone

Spongy or Cancellous bone

Composed of anastomosing bone trabeculae or spicules

Has a simple and less organized architecture

Has a lattice-work pattern with numerous small cavities

Found internal to compact bone Has no haversian system

Compact bone

Composed of dense and concentrically arranged bony trabeculae or lamella

More solid with fewer cavities Found external to spongy bone Presence of haversian system

Microscopic Approach1. Immature bone /Fibrous

bone: These have more cells & fibers in them. In humans they are found only in fetus, sockets of alveolar bone, sutures of the skull

2. Mature bone /Lamellar bone: The type of bone which are composed of thin plates (lamellae) of bony tissue. Most mature human bones are lamellar bones.

Types of Immature bone1. Bundle bone

Have thick bundles of collagen fibers which lie parallel to each other with osteocytes between them

2. Woven bone or coarsely bundled bone

Bundles of collagen fibers of its matrix run in various directions

According to the Manner of Development

1. Intramembranous or Membrane bone

2. Endochondral or Substitution bone

Intramembranous/Membrane bone Develop directly from the

mesenchyme without the intervention of the cartilage

Examples are: flat bones of the skull and face (part of the maxilla and mandible)

Endochondral/Substitution bone Develop within fetal hyaline

cartilages Pass cartilage development first then

calcifies to bones Examples are: all bones except the

flat bones of the skull and face