3.6 Bone Physiology Flashcards
What is osteogenesis
Bone tissue formation
What are the different stages of osteogenesis and when do they occur
ossification - from the 2nd month of embryonic development until birth
postnatal bone growth - from birth until early adulthood
bone remodelling and repair - lifelong
What are chondrocytes
Cartilage cells
What is ossification
The primary formation of bone tissue
What are the 2 types of ossification
Intramembranous ossification and Endochondral ossification
What is intramembranous ossification
The development of flat bones (such as skull, mandible or clavicles) from the mesenchyme. It takes place in the fibrous membrane.
Describe the stages of intramembranous ossification
The mesenchymal cells differentiate into clusters of osteoblasts called the ossification centre in the fibrous connective tissue. They then secrete the bone matrix before it calcifies. Some of the osteoblasts are then trapped in the matrix and become osteocytes The mesenchyme then condenses to form the periosteum. Trabeculae then forms as woven bone starts to develop. Lamellar bone then gradually replaces the woven bone, forming a the compact bone structure. Trabeculae of the spongey bone can then form between the lamellar layers of the compact bone with red bone marrow forming in the trabeculae spaces
What is endochondral ossification
The development of the rest of the skeleton from hyaline cartilage
In endochondral ossification, what happens at week 9 of embryonic development
The bone collar forms around the diaphysis (shaft) of the hyaline cartilage model, forming the primary ossification center. The hyaline cartilage at the centre of the diaphysis calcifies and then develops cavities, these cavities are necessary for blood vessels to enter and spongey bone to form.
In endochondral ossification, what happens at month 3 of embryonic development
The periosteal bud invades the cavities in the cartilage, this periosteal bud has blood vessels/nerves/red marrow elements/osteogenic cells/osteoclasts, spongy bone can then form in the center of the cartilage.
In endochondral ossification, what happens at birth
The diaphysis continues to elongate and a medullary cavity forms. In the epiphyses (ends of the bone) secondary ossification centres form.
In endochondral ossification, what happens from childhood to adolescence
The epiphyses ossify, completing the transition from cartilage to bone. Hyaline cartilage only remains the epiphyseal plates (growth plates) and articular cartilages (for joints).
Give an example of an area of research being looked into in ossification
There are gels that can facilitate endochondral ossification that could potentially be useful in repairing bone defects.
During ossification of the bone matrix, what happens to chondorcytes and osteocytes
Chondrocytes die as the cant receive nutrients. But osteocytes stay alive as they are connected by a canaliculi pathway
What are the 2 types of postnatal bone growth
Interstitial growth
Appositional growth
Describe interstitial growth
Interstitial growth increases the length of long bones. It occurs in the epiphyseal (growth) plates which remain open until 18-25
Describe the different zones of an epiphyseal plate
- the resting zone at the top with a store of inactive chondrocytes
- the proliferation zone is where chondrocytes undergo mitosis, this pushes older cells downwards
- the hypertrophic zone, the older chondrocytes enlarge, pushing the plate outwards
- the calcification zone, the matrix calcifies and starts to deteriorate, this kills the chondrocytes
- the ossification zone, osteoblasts lay down new bone tissue on the remnants of the old cartilage matrix
Describe appositional growth
An increase in the thickness and remodelling of all bones via osteoblasts and osteoclasts on bone surfaces. It occurs in the periosteum where the lamellae grow inwards to form a new osteon. The periosteal ridges form, then periosteal blood vessels are enclosed, then new lamellae are formed before a new osteon (functional unit of compact bone).
For what reasons are bones constantly being remodelled in adulthood
-adjust strength in proportion to stress
-rearrange its shape to accommodate mechanical forces in response to stress patterns
-renew old bone that has become too weak and brittle
-remove damaged areas
-release minerals such as calcium
What proportion of bone tissue is replaced annually
20%
Describe the stages in the bone remodelling process
Activation- preosteoclasts are stimulated an differentiate under the influence of cytokines and growth factors into mature active osteoclasts
Resorption- osteoclasts digest the old bone mineral matrix
Reversal- the end of resorption
Formation- osteoblasts synthesise new bone matrix
Quiescence- the osteoblasts then become resting bone lining cells on the surface of the newly formed bone
Give some factors that regulate bone growth and remodelling
Genetics, hormones, nutrition, physical activity and environmental factors
What is Wolff’s law
A bone’s mechanical loading environment determines its mass and trabecular architecture
What is mechanostat
A feedback loop where strain thresholds can activate of depress bone modelling leading to asymmetry
Describe how nutrition impacts bone formation
Bone formation requires a diet rich in protein, vit A C D, ca, phosphorus, magnesium, manganese etc.
-vit D important in regulation of Ca and Phos levels, our primary source of vit D is made by our skin when exposed to sunlight
-vit C for collagen synthesis and differentiation of osteoblasts into osteocytes
-vit A controls activity and distribution of osteoblasts and clasts during development
-vit K and b12 required for protein synthesis
Describe the different hormones involved in regulating bone formation
insulin like growth factors (IGFs) - produced by the bone and the liver, promotes cell division at epiphyseal plate and periosteum, enhances synthesis of bone proteins, production is stimulated by hGH
human growth hormone (hGH) - produced by anterior pituitary, stimulates epiphyseal plate activity, stimulates osteoblast proliferation and activity, stimulates osteoclast differentiation and activity, modulated by thyroid hormone
parathyroid hormone (PTH) - stimulates osteoclasts to degrade bone matrix and release Ca, stimulates synthesis of vit d
calcitonin - released by parafollicular cells of thyroid, stimulates osteoblasts to deposit ca, inhibits osteoclast activity
sex hormones (androgens and oestrogens) - at puberty aid osteoblast activity and promote new bone, promote and end growth spurts (at epiphyseal plate)
What is osteoporosis and what are ways to treat it.
Osteoporosis is the loss of bone mass, it is common in old age and makes bones more likely to fracture. It can be treated with hormone replacement therapy, whole body vibration therapy and exercise.
