Bone tissue formation Flashcards
What is bone?
Specialised connective tissue
Complex organ with multiple functiones
Vasculatised - constant exchange with systemic cell population, GF and hormones
What are the roles of bones?
Protection and support
Locomotion
Calcium homeostasis and mineral storage
Stem cell niche and hosting of haematopoiesis
Storage of energy
How many bones does an adult skeleton have?
206 bones
80 - axial
126 - appendicular
Macroscopic structure of bones
Different bones have different specialised shapes according to their function
Living matrix - constant remodelling and adaptation
Variety of forms but similar architecture with varying degrees of specialisation according to mechanical requirements
What are the different types of macroscopic structures bones can have?
Flat
Sutural
Short
Irregular
Sesamoid
Long
What does the structure and composition of bones maximise?
Maximises strength, stiffness and flexibility of bones
What are the components of long bones?
Cortical and trabecular
Mantains stiffness in the longitudinal direction
What are bones covered by?
Periosteum
Highly vascularised connective tissue
Describe the cortical component of long bones
Dense layer of compact bone around the bone marrow cavity
Found in the middle section of the bone (diaphysis)
Describe the trabecular component of long bones
Mesh of intrconnected arches
Highly porous, spongy bone
Found at both ends of the bone (Epiphysis)
What are the epiphyses?
The two ends of the long bone
What ensures optimal load bearing in long bones?
Combination of cortical, lamellar bone and trabecullae bone
Microscopic structure of bone
Dynamic tissue with living composite material
Specialised cells and ECM
What do the organic and structural components of the ECM provide the bone?
Tensile strength
Around 10 kg/mm2
What do the inorganic and biomineral components of the ECM provide the bone?
Compressive strength
Aroung 15 kg/mm2
Describe the microscopic structure of compact bone
Osteon unit
Concentric organisation of lamellar bone
Blood vessels run though Canaliculi, Haversian canal and Volkmann’s canal
What is an osteon unit?
Fundamental functional unit of compact bone
What is the Haversian canal?
Contains the bone’s nerve and blood supplies
Found inside the osteon
What are bone canaliculi?
Microscopic canals that run through bone lacunae (gaps) which carries a structure
What are Volkmann’s canals?
Small channels that transmit bloof vessels from the periosteum into the bone
Describe microscopic structure of Trabecular bone
Greater surface area that cortical
Suitable for metabolic activity - exchange of calcium
Highly vascular
Frequently contains red bone marrow
Traberculae are aligned towards the distribution of mechanical load
What process happens in the red bone marrow?
Haematopoiesis
What cells make up the bone structure?
Osteobalsts
Osteocytes
Osteoclasts
What is the origin of osteoblasts?
Mesenchymal stem cells
What are the biomarkers for osteoblasts?
Alkaline-phosphatase
Osteocalcin
What are the roles of osteoblasts?
Bone forming
Secrete organic bone matrix
Responsible for biomineralisation
Respond to multiple signaling cues and regulate osteoclasts
What is another name for bone matrix?
Osteoid
Why is it important that osteoblasts regulates osteoclasts?
To balance bone resorption and formation
What is the appearance of osteoblasts?
Epithelia-like cell sheet at site of matrix deposition
Basophilic
What is the source of osteocytes?
Osteoblasts
Terminally differentiated osteoblasts form osteocytes
Where are osteocytes found?
Reside in lacunae
What are characteristics of osteocytes?
Long-lived
Non-dividing
Project cytoplasmic processes in canaliculi
These cell-cell contacts allow communication + nutrient and waste exchange through gap junctions
What is the role of osteocytes?
Resorb, synthesize AND mineralise matrix
At lower rate than osteoblasts and osteoclasts
Contribute to bone turnover
The contribution of osteocytes to bone remodelling is at a lower rate than osteoblasts and osteoclasts
TRUE or FALSE
TRUE
What triggers osteocyte bone turnover?
Osteocytes sense mechanical forces
They then respond and transduce signals
Affect bone structure and cellular components
What is the source of osteoclasts?
Myeloid origin
Fused from precursor cells in bone marrow
Why are osteoclasts multi-nucleated cells?
Formed by the fusion of precursor cells in the bone marrow
Structural features of osteoclasts
Large
Motile
Macrophage-like - drive from the same precursors as macrophages
Polar
Eosinophilic
Acid cytoplasm
Highly secretory
What are biomarkers for osteoclasts?
TRAP-positive
Tartate-resistant alkaline phosphatease
What is the role of osteoclasts?
As new bone material is added peripherally from the internal surface of the periosteum, there is a hollowing out of the internal region to form the bone marrow cavity
Enter the bone through blood vessels
Dissolve both the inorganic anc protein portions of the bone matrix
How do osteoclasts resorb bone?
Ruffled border secretes protons and matrix degrading enzymes
What is the advantage of the ruffled border?
Increases resorptive surface area
What is the pH of the resorptive pit?
pH≤4.5
What do osteoclasts form when they resorb bone?
Howship lacunae - pits of eroded bone
What triggers resorption of the bone by osteoclasts?
Osteoclasts respnond to multiple hormonal, growth factor and cell-cell contact mediated signals
Which cell is the associated cellular components of the bone tissue?
Mesenchymal stem cells/ bone marrow stromal cells
Haematopoietic stem cells
What is the bone marrow?
The medullary cavity - stem cell niche for haematopoiesis
What is the medullary cavity replaced with as we age?
Adipocytes containing yellow marrow for energy storage
Starts in long bones
What two lineages arise from the medullar cavity?
Myeloid lineage
Lymphoid lineage
What are stromal cells of the bone marrow?
Additional cells
Support haematopoiesis structurally and physiologically
Characteristics of mesenchymal stem cells
Sub-population of stroma
Trilineage potential: bone, cartilage, adipose tissue
Highly proliferative and regenerative capacity for musculoskeletal tissue regeneration
What is haematopoiesis?
Formation of blood cellular components
Bone marrow’s primary function
What is the origin of mesenchymal stem cells?
No consensus has been reached
Can be found in multiple tissues
Ongoing questions about mesenchymal stem cells
Do they represent a bone fide stem cell population with discrete properties?
What are the definitive markers of MSCs?
Are they an activated form of pericytes?
Are they transfdifferentitating fibroblasts?
What is the condition caused by too many osteoclasts?
Osteoporosis
What is the condition caused by too many osteoblasts?
Osteopetrosis
