Bone Response to Damage

BACK TO BONES

Introduction

• Bone is a hard, highly specialised connective tissue
• Consists of interconnected cells embedded in a calcified, collagenous matrix
• Living, dynamic, responsive tissue, growing and remodelling throughout life
• Pathogenesis of many bone diseases is complex
  • May involve genetic defects, diet or infection or a combination of these
• Function:
  • Support/protection
  • Movement
  • Stem cell storage
  • Mineral storage

Normal structure

• Cells
  • Osteoblasts
    • Mesenchymal cells
    • Arise from bone marrow stroma
    • Produce bone matrix = osteoid
    • Cell membranes are rich in alkaline phosphatase (ALP)

• • Osteocytes
    • Osteoblasts that have become surrounded by mineralised bone matrix
    • Occupy cavities called lacunae

• • Osteoclasts
    • Multinucleated cells
    • Derived from haematopoietic stem cells
    • Responsible for bone resorption (have a brush border for this)

• Matrix
  • Type I collagen forms the backbone of the matrix
  • Mineral – accounts for 65% of bone and includes Ca, P, Mg, Mn, Zn, Cu, Na

Bone organisation

Osteoblasts deposit collagen in different patterns

1. Woven bone: • This is a “random weave” which is only a normal feature in the foetus • In adults it is a sign of a pathological condition (e.g. fracture, inflammation, neoplasia)

2. Lamellar bone: Orderly layers which are much stronger than woven bone There are two main types: i) Compact bone (cortical) which forms 80% of total bone mass and forms the shell of long bone shafts; this contains Haversian systems

ii) Cancellous bone (spongy or trabecular) occurs in vertebrae, flat bones and epiphyses of long bones; it contains no Haversian systems

Fig 1. Long bone anatomy

Periosteum and blood supply

The periosteum is a specialised sheath of connective tissue covering bone except at the articular surfaces. The inner layer merges with the outer layer of bone and contains osteoblasts and osteoprogenitor stem cells.

The blood supply to the mature bone enters via the periosteum. Damage to the periosteum invokes a hyperplastic reaction of the inner layer.

Bone development There are two main types of bone development: • Endochondral ossification (cartilage model) o Long bones mainly o Cartilage model is vascularised and centres of ossification develop. The latter are primary (diaphyseal) and secondary (epiphyseal)

• Intramembranous ossification o Flat bones mainly (e.g. skull) o Mesenchymal cells differentiate into osteoblasts. There is no cartilage precursor template

Physis • The cartilage model remains only at the junction of the diaphyseal and epiphyseal centres to become the physis or growth plate. This is an important site since many congenital or nutritional bone diseases in the growing animal are manifest at this site.

• In neonates and growing animals, the growth plate is “open”, i.e. chondrocyte proliferation balances cell maturation and death (Fig 2). The growth plate “closes” and ossifies at maturity.

Fig 2. Growth plate cartilage is divided into zones

Bone resorption • Mediated by two hormones o Parathyroid hormone (PTH) which is produced by chief cells in the parathyroid glands in response to decreased serum calcium

o Calcitonin which is produced by C-cells in the thyroid glands in response to increased serum calcium

    PTH

↓ Osteoclasts increase in number ↓ Osteoclasts attach to bone and resorb mineralised matrix

Osteoclasts do not have receptors for PTH so how do they do this? Answer:

LOW CALCIUM → INDUCES PTH SECRETION → BONE RESORPTION → CALCIUM 

CALCITONIN HAS THE OPPOSITE EFFECT - IT INHIBITS OSTEOCLASTS

Bone dynamics

Bone growth and maintenance of normal structure are directly related to mechanical forces which generate bioelectrical potentials (piezoelectricity). These potentials strengthen bone and inactivity reduces them, thereby leading to bone loss. In neonates, bone growth predominates and modelling is important. In adults, formation of bone is balanced by resorption; this is known as remodelling. It continues in a subtle but active way throughout life under the influence of hormones and mechanical pressure. Bone resorption may exceed formation in pathological states (hormonal, trauma, nutritional) or in old age and disuse.