MSS: Cellular Structure of Bone Flashcards
List some functions of bone.
Structure - give structure and shape to the body
Mechanical - sites for muscle attachment - if bones were missing, would not be able to move
Protective - vital organs and bone marrow e.g. skull protects brain. Bone marrow makes red blood cells, white blood cells, platelets
Metabolic - reserve of calcium and other minerals. Bone is also an endocrine organ and has a role in metabolism.
Describe bone composition.
Partly inorganic and partly organic.
Inorganic - makes up 65% of bone
- consists of crystals of calcium hydroxyapatite (Ca10(PO4)6(OH)2)
- inorganic component is storehouse for 99% of Calcium in the body
- and 85% of the Phosphorus, 65% Sodium, Magnesium
Organic - makes up 35% of bone
- bone cells and extracellular protein matrix (mainly different collagens e.g. collagen type 1 as well as other proteins)
List the different anatomical bone types.
- Flat e.g. sternum, skull - protective function of specific
organs - Long - limb bones e.g. femur, humerus, ulnar, radius - allow us to make large movements
- Short/cuboid e.g. carpals in wrists, tarsals in feet - provide stability in joins and allows us to make smaller, more precise movements
- Irregular - specific complex shapes that allow us to protect organs e.g. pelvis protecting reproductive organs, vertebrae protecting spinal chord
- Sesamoid - embedded in muscles or tendons. Reduce stresses and strains on muscles. Most very small and found in feet and hands - one large on is the patella (kneecap in leg)
Describe the macroscopic structure of bone.
Trabecular bone - also called cancellous/ spongy bone. Thin ‘honeycomb-like’ bone - highly metabolically active and also adds strength to the bone
- Cortical bone - also called compact bone, thick bone that makes up the shaft of the bone
Describe the microscopic structure of bone.
- Lamellar bone - mature bone
- Woven bone - immature bone - occurs during initial bone development
What are some differences between cortical and trabecular bone?
Cortical (thick bones that makes up the external tube):
- long bones (predominant type of bone found in long bones)
- 80% of skeleton
- Appendicular (bones of appendages e.g. arms/legs)
- 80-90% calcified
- Mainly has structural, mechanical, and protective function
Trabecular (spongy bone criss-cross on internal diameter):
- vertebrae and pelvis
- 20% of skeleton
- axial (bones of body’s long axis e.g. vertebrae)
- 15-25% calcified
- mainly metabolic (highly metabolically active due to large surface area)
- large surface area (due to criss-cross nature)
Describe the structure of a bone.
- Cortical/compact bone on the outside of the tube
- Trabecular/spongy bone in the middle.
- Crossing the Medullary cavity - the space enclosed by the cortical bone.
- The main shaft in the middle of the bone is called the Diaphysis.
- The ends of the bones are called the Epiphysis.
- Epiphysis and diaphysis are separated by growth plate
- Region below the growth plate separating the epiphysis and diaphysis is called the Metaphysis.
How is bone development completed?
Bones develop throughout growth and into adulthood
- Growth plate fusion and ossification completes development
- Clavicles growth plates fuse at around 20 years old
What are the two different processes by which bone develops?
Intramembranous ossification (inside the membranous tissue)
- Direct differentiation of osteoblasts (bone forming cells) from mesenchyme precursors in the mesenchymal connective tissue and begin to lay down layers of bone
- Flat bones typically go through this process
Endochondral ossification (within the cartilage) - Bones form from a pre-existing cartilage model - Long bones mainly go through this process e.g. femur/limb bones (This process is what allows us to have rapid linear growth during childhood and adolescence)
What happens in endochondral ossification?
Early foetal development - cartilage scaffold of long bones forms within limb buds.
By birth - scaffold has expanded in size, has an issue with vasculature within the cartilage model. Centre of the cartilage model becomes hypoxic. To correct this, blood vessels then invade the scaffold and bring the precursors of the bone cells which convert the centre of the cartilage scaffold into bone. Known as the primary ossification centre. Ossification then expands out of the ossification centre in the diaphysis, moving gradually towards the epiphysis of the bone.
Childhood - bone continues to increase in size, a secondary ossification centre will form in the epiphyses. This remains separate from the primary ossification centre via a cartilage structure known as the growth plate. This allows rapid linear growth and increase height.
Growth then finally finishes and bone development is complete when the cartilage growth plates finally becomes ossified. This fuses the primary and secondary ossification centres together.
Describe the distinct zones in the chondrocytes.
Reserve zone - though to contain the stem cell population in growth plates. Proliferate very slowly and maintain their population. Close to the blood supply in the secondary ossification centre.
Proliferative zone - chondrocytes highly proliferative and form distinct column-like structures as they divide. Cells move further away from the epiphysis and the blood supply. Chondrocytes undergo hypertrophic
differentiation.
Hypertrophic zone - chondrocytes expand in size and produce a specific collagen called collagen 10. The further into the hypertrophic zone the cell is, the larger it will be as well as it is further from the blood supply. Eventually they apoptose,
Calcified cartilage zone - left behind after the apoptosis
Ossification zone - calcified cartilage is then converted into bone
List three specialised bone cells?
Osteocytes - Mature bone cells. Make up over 90% of all cells found in bone tissue. They are a mechanosensory network embedded in mature bone.
Osteoclasts - Bone breaking cells. Multi nuclear cells that resorb/remove (break down) bone.
Osteoblasts - Immature bone cells. Produce new bone via the secretion of osteoid
Describe osteocytes.
- Embedded in lacunae in mature bone
- Connected together via ‘dendritic-like’ processes moving through canalicular channels
- Form a mechanosensory network throughout bone
- Network can detect pressures and strains going through bone and signal if it needs other cells to come in and make any necessary changes to the tissue
Describe osteoclasts.
- Bone breaking cells
- Initially formed from fusion of monocyte macrophage precursors.
- Osteoclasts seal off a portion of bone beneath them from the rest of the environment via an actin ring
- They then secrete acids and enzymes onto the bone to ‘resorb’ the sealed off portion of bone
Describe osteoblasts.
- Form new bone
- Secretes osteoid - contains organic component of new bone
- Osteoid mineralised over time, incorporating the inorganic portion of bone tissue to become mature bone
- Some osteoblasts are embedded in new bone (osteoids) during this process and differentiate into mature bone cells known as osteocytes. This is how the osteocyte network is maintained in newly forming bone.