Histology of Bone

Question 1

Where is the origin of bone in the bone?

Answer 1

The mesenchyme - primitive embroyonic Connective Tissue (stem cells)

Question 2

What is the composition of bone?

Answer 2

Rigid connective tissue (few cells, large extracellular matrix, many fibres)

Bones ECM is made from organic and inorganic components which are embedded specialised bone cells

Organic= composed of glycoprotein, ground substance and collagen fibres

Inorganic= Inorganic salts (calcium hydroxyapatite crystals, which mineralise the organic matrix)

Specialised bone cells= osteoblasts, osteocytes, osteoclasts

Question 3

Bones Extracellular matrix is composed of?

Answer 3

60-70% inorganic salts

30-40% collagen

Around 5% protein and carbohydrates

Composition changes with age and metabolic status - dynamic tissue

Question 4

What is the organic component (pre-mineralised) of matrix called?

Answer 4

Osteoid/woven bone - this is only found in adults where bone remodelling is occuring

Question 5

What is the organic component called when inorganic components have been added?

Answer 5

Mineralised/mature bone

Calcium hydroxyapatite crystal

Rickets is a lack of mineralised bone

Question 6

What type of collagen is found in the extracellular matrix of bone?

Answer 6

Modified Type 1 collagen fibres

Different to other tissues as molecular structure more strongly cross-linked and gaps within the fibres are larger - these allow space for bone salt crystals

(50% of Calcium hydroxy. crystals are within fibres)

Question 7

Whats the Periosteum?

Answer 7

A membrane that covers the outer surface of all bones, except at the joints of long bones; containing oesteoprogenitor cells (OSTEOBLASTS)

Question 8

What is the Endosteum?

Answer 8

lines the inner surface of the medullary cavity of all long bones; containing osteoprogenitor cells

Question 9

What are Osteoblasts?

Answer 9

Immature bone cells, created from osteoprogenitor cells in the Periosteum/Endosteum.

Question 10

Whats the morphology of Osteoblasts?

Answer 10

Cuboidal/polygonal in shape

Question 11

What do Osteoblasts produce?

Answer 11

The organic bone matrix; known as OSTEOID (primary/woven/unmineralised bone)

Question 12

When do Osteoblasts change into Osteocytes?

Answer 12

Once they are surrounded by mineralised bone

Question 13

What is the process of Bone mineralisation?

Answer 13

1. Osteoblasts secrete Osteoid components (type 1 collagen, proteoglycans)
2. Matrix vesicles containing enzymes such as alkaline phosphatase and pyrophosphatase strip phosphate ions off other molecules
3. Phosphate ions accumulate in the vesicles alongside calcium ions - the raw materials of new hydroxyapatite
4. Matrix vesicles thus form the focus for the deposition of hydroxyapatite crystals
5. Crystals continue to accrete and spread into the surrounding osteoid tissue

Question 14

What are the characteristics of Osteocytes?

Answer 14

Mature bone cells, occupy a space called a lacunae = a pocket sandwiched between layers of matrix

The layers are called Lamellae. Osteocytes cannot replicate so a lacunae will always contain one cell

Question 15

How do Osteocytes communicate and get nutrients?

Answer 15

Via narrow passageways through the lamelae called CANALICULI. They are supplied by blood vessels in the central canal

Question 16

What are the 2 key functions of Osteocytes? (1)

Answer 16

1. Maintain matrix

They maintain the protein and mineral content of the matrix. There is a continuous turnover of matrix components via osteocyte chemical secretion which causes matrix breakdown.

Question 17

What are the 2 key functions of Osteocyts? (2)

Answer 17

They help repair damaged bone. If released from their lacunae, osteocytes can convert to a less specialised cell type, such as osteoblasts or an osteogenic cell.

Question 18

What are the characteristics of Osteoclasts?

Answer 18

Multinucleate cells derived from the macrophage/monocyte system in blood

Large cells with 50+ nuclei, numerous mitochondria, golgi and lysosomes = highly active cells

Question 19

What is the function of Osteoclasts?

Answer 19

Absorb and remove bone matrix

Acids and protein-digesting enzymes secreted by Osteoclasts dissolve the matrix and release the stored minerals = OSTEOLYSIS

Question 20

What are Osteoclasts important for?

Answer 20

Calcium homeostasis

Parathyroid hormone stimulates osteoclast activity whilst Calcitonin depresses it

Question 21

What are Resorption bays?

Answer 21

When Osteoclasts start to degrade the bone, they create pits/grooves in the matrix.