Lecture 1

Bone Morphology

There are several different types of bones

1. Long bones

   Majority of the bones in the body are long bones

2. Short bones

   Mainly found in the carpal and tarsal regions

3. Flat bones

   Mainly found in the skull

4. Irregular bones

Anatomical Terminology of Bone Structures

In long bones, the following terms are used to refer to specific anatomic locations:

• diaphysis (shaft) – exclusively cortical bone surrounding marrow cavity

• epiphysis at each end – mostly trabecular surrounded by a thin shell of cortical bone

• metaphysis between diaphysis and epiphyses – transition from mostly cortical to mostly trabecular

Based on organisation of tissue, osseous tissue can be categorised as either cortical (aka compact) or trabecular (aka cancellous or spongy) bone. All bones are invested in a connective tissue covering outer surface (except at articular surfaces) which is called periosteum. Periosteum has an outer fibrous layer and an inner cambrial layer that contains osteoprogenitor cells.

The inner aspect of bones where bone tissue faces bone marrow a single layer of cells cover all bony surfaces which referred to as the endosteum.

Bone Blood Supply

Blood is essential for survival, maintenance and functionality of bone tissue

A typical long bone is supplied by multiple sources:

• Nutrient artery and vein enter the diaphysis via a nutrient foramen to supply marrow and cortical bone → when reaching into the marrow can be called medullary vessels

• Penetrating periosteal vessels that supply most of epiphysis (epiphyseal vessels) and metaphysis (metaphyseal vessels) in mature bones as well as a third of the thickness of the diaphyseal cortex

• Non-penetrating periosteal vessels that form a capillary plexus in the periosteum

Periosteal vessels penetrate around 1/3 of the outer cortex whilst the inner cortex is supplied by the branches of the medullary vessels and makes up 2/3

Bone Cells

Remember that bone comprises of a matrix in which cells reside in that matrix

Osteoblasts

= round or cuboidal cells found on bone surfaces → usually found close together

Secrete osteoid (mixture of bone matrix proteins) which then becomes mineralised

Secrete enzymes and factors that initiate the mineralisation of bone matrix

Become osteocytes as they are embedded in the very matrix they secrete

Osteocytes

= small round cells within lacunae surrounded by bone matrix

Possess fine processes extending into canaliculi (ultramicroscopic canals in bone matrix)

• Processes form junctions with processes from other osteocytes, bone lining cells and osteoblasts

• Attachments between two neighbouring osteoblasts remain even as matrix is forming → leads to the cellular processes found between osteocytes

  • Need these processes to obtain nutrients etc.

Play role in the maintenance of bone matrix

Sense mechanical strain (mechanoreception) and convert them into chemical signals

Initiate chemotaxis to attract osteoclasts in response to injury

Bone lining cells

= flattened cells covering resting bone surfaces

• Sometimes considered to be quiescent osteoblasts

Capable of differentiating into osteoblasts to initiate bone formation processes form junctions with processes from other bone lining cells, osteocytes and osteoblasts

Think of a bone lining cell as an osteocyte that has been left out of the bone matrix fully exposed on the surface!

Osteoclasts

= large multinucleate cells

Sparsely scattered on bone surfaces

Responsible for bone resorption:

• adhere to bone through a sealing zone, creating microenvironment between cell and bone surface

• secrete hydrogen ions and lysosomal enzymes to degrade bone matrix

• Forms pits called Howship lacuna

Bone Matrix

Proteinaceous/Organic Phase

A major component of the organic phase is type I collagen. Several additional glycoproteins also exist in bone matrix with many able to bind calcium

Mineral Phase

The mineral phase consists chiefly of hydroxyapatite crystals (mainly calcium and phosphate).

Magnesium and other minerals are also present but at considerably lower levels than calcium and phosphorus

Bone Organisation

Woven Bone

• Haphazardness arrangement

• Seen when rapid expansion of bone volume is necessary such as during foetus growth or bone growth healing

Lamellar Bone

• Well organised tissue arrangement

• Arranged in sheet like layers

• Seen in mature bones

• Found majority in body

• Determines how you see the bone macroscopically (such as in trabecular)

Compact Bone (=Cortical Bone)

Macroscopic arrangement of lamellae in cylindrical structures

• Lamallae are concentric rings that surround the haversian canal

Compact bone is lamellar in structure with its lamellae organised into Haversian systems (osteons).

• Osteons are longitudinal cylinders consisting of concentric bone lamellae surrounding a central blood vessel and nerve branches (vasomotor and sensory) within a ‘Haversian canal’.

  • Osteons are oriented in a long axis format due to the mechanical forces applied to the bone. They are in line with the external forces → better resists the forces

• Blood vessels within neighbouring Haversian canals are linked to each other through transverse ‘Volkmann’s canals’ (=perforating canal)

Trabecular Bone

It comprises a network (lattice or mesh) of bone plates and rods interspersed with spaces containing bone marrow and blood vessels.

Due to its vast surface area, trabecular bone is highly metabolically active and responsive to changes in microenvironment especially those caused by mechanical forces and ageing

Depending of the organisation of collagen fibres, trabecular bone could be woven or lamellar

Bone Development

Intramembranous ossification

The process of de novo (new) bone formation directly from osteoblastic precursors

This mode of ossification is responsible for the development of most flat skull bones.

Starts by condensation of embryonic mesenchyme into cell clusters (bone anlagen), followed by differentiation into osteoblasts and secretion of osteoid to form spicules of woven bone, gradually forming a network of trabeculae.

Endochondral ossification

There is a transient cartilage phase is first generated before being replaced by bone

Forms majority of the long bones

Examples

Types of Bones

1. Long bones

   • Femur, radius, ulna, humerus etc.

2. Short bones

   • Carpal bones

3. Flat bones

   • Skull bones

4. Irregular bones

   • Vertebrae

Definitions

Howship lacuna - refers to the resorption pits created by osteoclasts

Interstitial lamellae - lamellae layers in between the osteons

Outer circumferential lamellae - outer layers of lamellae located beneath the periosteum and surrounding the outside of the entire