Histology of Bone Flashcards
Where is the origin of bone in the bone?
The mesenchyme - primitive embroyonic Connective Tissue (stem cells)
What is the composition of bone?
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
Bones Extracellular matrix is composed of?
60-70% inorganic salts
30-40% collagen
Around 5% protein and carbohydrates
Composition changes with age and metabolic status - dynamic tissue
What is the organic component (pre-mineralised) of matrix called?
Osteoid/woven bone - this is only found in adults where bone remodelling is occuring
What is the organic component called when inorganic components have been added?
Mineralised/mature bone
Calcium hydroxyapatite crystal
Rickets is a lack of mineralised bone
What type of collagen is found in the extracellular matrix of bone?
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)
Whats the Periosteum?
A membrane that covers the outer surface of all bones, except at the joints of long bones; containing oesteoprogenitor cells (OSTEOBLASTS)
What is the Endosteum?
lines the inner surface of the medullary cavity of all long bones; containing osteoprogenitor cells
What are Osteoblasts?
Immature bone cells, created from osteoprogenitor cells in the Periosteum/Endosteum.
Whats the morphology of Osteoblasts?
Cuboidal/polygonal in shape
What do Osteoblasts produce?
The organic bone matrix; known as OSTEOID (primary/woven/unmineralised bone)
When do Osteoblasts change into Osteocytes?
Once they are surrounded by mineralised bone
What is the process of Bone mineralisation?
- Osteoblasts secrete Osteoid components (type 1 collagen, proteoglycans)
- Matrix vesicles containing enzymes such as alkaline phosphatase and pyrophosphatase strip phosphate ions off other molecules
- Phosphate ions accumulate in the vesicles alongside calcium ions - the raw materials of new hydroxyapatite
- Matrix vesicles thus form the focus for the deposition of hydroxyapatite crystals
- Crystals continue to accrete and spread into the surrounding osteoid tissue
What are the characteristics of Osteocytes?
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
How do Osteocytes communicate and get nutrients?
Via narrow passageways through the lamelae called CANALICULI. They are supplied by blood vessels in the central canal
What are the 2 key functions of Osteocytes? (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.
What are the 2 key functions of Osteocyts? (2)
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.
What are the characteristics of Osteoclasts?
Multinucleate cells derived from the macrophage/monocyte system in blood
Large cells with 50+ nuclei, numerous mitochondria, golgi and lysosomes = highly active cells
What is the function of Osteoclasts?
Absorb and remove bone matrix
Acids and protein-digesting enzymes secreted by Osteoclasts dissolve the matrix and release the stored minerals = OSTEOLYSIS
What are Osteoclasts important for?
Calcium homeostasis
Parathyroid hormone stimulates osteoclast activity whilst Calcitonin depresses it
What are Resorption bays?
When Osteoclasts start to degrade the bone, they create pits/grooves in the matrix.
