Bone Physiology

components of bone percentages

1.       Collagen type I scaffold makes up about 15% of the weight of the bone

2.       Organic matrix is 10% of the bone with growth factors

3.       Mineral component of hydroxyapatite is the cement to fill in the holes and makes up 75% of the weight of bone to make it hard and rigid and not easily compressed, and this contains both calcium and phosphate in a Ca:P ratio

4 types of bone cell

osteoprogenitor/osteogenic cells

osteoclasts

osteoblasts

osteocytes

function of osteoblasts

Synthesise and secrete collagen and ground substance as osteoid, which is then mineralised and this embeds the osteoblasts in mineralised bone around them and become osteocytes

Calcium phosphate crystals are formed by osteoblasts which mineralise the bone. The production of calcium phosphate crystals is catalysed by alkaline phosphatase

name of osteocyte projections and location

stellate porcesses in canaliculi

describe an osteon

Bone is made of individual units called osteons, which are made of lamellae which are hollow tubes and arranged like tree trunk rings. Where lamellae meet there are small gaps called lacunae that are filled with osteocytes for communication between canaliculi which maintain the bone and control calcium homeostasis. Within each osteon are collagen fibres running in alternating directions, with bone salts in between. This allows compact bone to withstand torsion.

In the centre of each osteon is the Haversian canal containing vessels and nerves, which run parallel to the long axis of the bone, and then there are Volkmann’s canals runninng between them to allow blood to pass from the outer to inner areas

what causes osteopetrosis

too much osteoblastic activity leading to dense bones

spongy bone differences

made of trabeculae and outer endosteum with bone marrow in between

no Haversian canals

osteoclast “sorry” action

As osteoclasts resorb bone, they leave behind mitogens which are growth factors to encourage osteoprogenitor cells or osteogenic cells to mature to osteoblasts and lay down matrix forming a cement or reversal line that forms the border of the newly remodelled bone unit. Therefore the cells are useful in repairing issues in the bone

how do osteoclasts demineralise bone

They acidify their subcellular space to dissolve CaPO4 and enzymatically digest collagen and other matrix proteins with collagenase to break down the matrix further. Then osteoclasts absorb the end products of degradation and release them into the interstitial fluid

How is the primary ossification formed in endochondral ossification

1.       During foetal development, the skeleton consists of hyaline cartilage, and then ossification starts in the middle of long bones in the diaphysis. The cartilage is made up of chondrocytes, and has an outer layer called the perichondrium which is a protective layer

2.       Then chondrocytes start to grow and calcify to osteoprogenitor cells and the blood supply developing at the edges causes the ossification of the perichondrium to periosteum.

Then the capillaries from the outer blood supply start to penetrate into the bone, and this forms the primary ossification centre

function of primary ossification centre

osteoprogenitor cells can differentiate to osteoblasts and produce osteoid which is mineralised to form the bone collar under the periosteum to eventually form cortical bone

when does growth end - species differences

Growth continues in the growth plate during infancy as there is hyaline cartilage present between the metaphysis and epiphysis which allows bone to lengthen as the rest of the body grows. The plate will close at puberty at epiphyseal closure where they meet and stop growing as we have run out of cartilage

Various growth plates close at different times depending on adult size of the breed

-          E.g. toy breeds are skeletally mature at 8 months whereas giant breeds take until 14 months, and cats are only mature at 20-24 months so grow much slower

what happens in the pre-hypertrophic zone

·       cells produce a matrix causes the cartilage matrix to expand from within and this pushes the epiphysis away from the diaphysis an elongates the bone

give all the zones of bone lengthening

·       Resting zone/zone of reserve cartilage

·       Proliferative zone

·       Pre-hypertrophic zone

·       Hypertrophic zone

·       Zone of endochondral ossification

what happens in the hypertrophic zone

This contains larger cells at 10x bigger due to each division making bigger cells, which helps to increase the amount of ECM production. However this layer contains no cell division as the cells just swell and enlarge which compresses the matrix into linear bands between the columns of hypertrophied cells, and then the hypertrophied cells are further removed from the epiphyseal plate and become active to calcify the cartilage to bone by alkaline phosphatases. Then chondrocytes die by apoptosis due to a lack of blood supply

intermembranous ossification process

In the fibrous connective tissue membrane, selected centrally located mesenchymal stem cells cluster and differentiate into osteoblasts, forming an ossification centre.

Bone matrix (osteoid) is secreted within the fibrous membrane by osteoblasts which is mineralised within a few days. Trapped osteoblasts in the mineralised osteoid become osteocytes

There are two layers that form bone between them to form two parallel plates of bone that become the two cortices of the flat bone

state wolff’s law

bone adapts to the load placed upon it

what forms in orientation with the largest forces

trabecular struts