Bone Development Flashcards
What is meant by ossification?
The process by which bone is formed
What are the 2 types of ossification, and what do they depend on?
Intramembranous ossification - flat bones
Endochondrial ossification - long bones
What is the first step in intramembranous ossification?
Mesenchymal cells in the centre of the bone cluster and differentiate into osteoblasts, forming ossification centres.
What type of bone is formed by intramembranous ossification?
Flat bones - ribs, scapula, skull and pelvis
Following the formation of ossification centres, how are trabeculae formed during intramembranous ossification?
Osteoid is secreted by osteoblasts. Trapped osteoblasts become osteocytes. The osteoid becomes calcified and forms needle-like structures of spongy bone called spicules. Spicules aggregate to form beam-like structures called trabeculae.
What is ‘woven-bone’?
Woven bone is a mechanically weak, spongy bone which is formed from a random interconnected network of thickend trebaculae
How and when is a periosteum formed in intramembranous ossification?
After trebaculae and woven bone formation, blood vessels on the outside of the spongy bone condense to form the periosteum, a fibrous sheath that covers bone.
How is lamellar bone different from woven-bone?
Lamellar bone forms after woven-bone,surrounding it. It is compact and contains condensed osteoid and regular, parallel sheets of collagen making it much stronger than spongy bone.
What are the 2 types of bone and how do they differ?
Spongy/cancellous bone forms an irregular network of fine bony columns, spaces are filled with bone marrow.
Cortical bone- forms external services, regular parallel sheets of collagen = strong.
What are haversian and volkmann’s canals?
Haversians run veritally in the centre of the osteoid, volkmann’s canals run horizontally between haversian canals.
They carry blood vessels, lymph vessels and nerves.
How does the arrangement of osteocytes differ in immature and mature bone of compact bone?
Mature bone has osteocytes arranged in concentric lamellae of osteons.
Immature bone has osteocytes which are fairly randomly arranged.
Which plane do resorption canals run?
In parallel with the osteons long axis
What is the function of cytoplasmic processes on the osteocytes?
Reach out to adjacent osteocytes, connecting via canaliculi to allow passage of nutrients between cells.
How does the structure of spongy bone differ to compact bone?
The internal structure of trabeculae is similar to compact bone, as osteocytes lie between lamellae. No osteons, No haversian or volkmann’s canals.
Outline the process of bone remodelling.
Cutting cut resorbs bone through the action of osteoclasts (release H+ ions and lysosomal enzymes) and synthesis new bone by osteoblasts.
What is the composition of bone?
65% Mineral
23% Collagen
10% Water
2% Non- collagen proteins
What do chondrocytes secrete?
Collagen and elastin to give strength and flexibility
What is bone marrow the site for?
Haematopoiesis
Outline the process that leads to osteoclast bone resorption.
Osteoclasts anchor to the bone surface and create a micro-environment underneath cell (‘sealed zone’) where it creates an acidic environment that dissolves bone mineral content. Enzymes released remove the remaining collagen.
What factors influence the process of bone remodelling ?
Nutrition and physical activity - strengthen bone
Glucocorticoids - promote osteoclast activity
Outline the processes of fracture repair.
- Hematoma formation
- Fibrocartilaginous callus formation (soft)
- Bony callus formation (hard)
- Bone remodelling
Define osteoporosis.
Decrease in bone mass, making bones more fragile and susceptible to fracture.
Define osteomalacia.
Inadequate mineralisation of bone due to vitamin D deficiency. Mineral bone loss.
What is osteoporosis with aging characterised by?
Outer surfaces of trabeculae are regularly remodelled by osteoclasts and osteoblasts. Osteoporosis associated with aging results from incomplete filling of osteoclast resorption bays.
What are the 2 types of osteoporosis?
Type 1- Post menopausal women. Increased osteoclast activity as a result of oestrogen withdrawal.
Type 2- In elderly of both sexes due to attenuated osteoblast function.
What are risk factors for osteoporosis?
genetic
Insufficient calcium intake
Lack of exercise
Smoking has been linked
Describe the process of endochondral ossification.
- Periosteum differentiates into osteoblasts which form the BONY COLLAR on the diaphysis.
- The central cavity calcifies
- Periosteal bud invades the cavity, supplying osteogenic cells which begin to form spongy bone. = primary ossification centre
- Secondary ossification centres form at epiphysis and begin to make spongy bone after birth.
Outline what happens in step 1 of bone fracture repair.
- Hematoma
Blood clot forms and bone cells on fracture edge die
What is the second step of fracture repair?
Fibrocartilaginous callus is formed.
Fibroblasts secrete collagen, which span the break.
Angiogenesis
What is the third step of fracture repair?
Bony callus formation.
Trabeculae develop as the fibrocartilaginous callus is converted to a hard bony callus of cancellous bone.
What time of bone formation is involved in the formation of a bony callus?
Both endochondral and intramembranous
What is the final step of fracture repair?
Bone remodelling
The callus of spongy bone is remodelled into compact bone.
Name the 5 zones in epiphyseal plate growth.
- Zone of reserve
- Zone of proliferation
- Zone of hypertrophy
- Zone of calcification
- Zone of resorption
What happens in the zone of reserve?
Chondrocytes resting, no proliferation or matrix production
What happens in the zone of proliferation?
Chondrocytes divide, forming columns and secreting matrix
What happens in the zone of hypertrophy?
Chondrocytes grow in size, compressing the matrix into linear bands between the columns
What happens in the zone of calcification?
Matrix becomes calcified and chondrocytes degenerate
What happens in the zone of resorption?
The calcified matrix is in direct contact with the marrow cavity.
Small blood vessels and connective tissue invade the region occupied by dying chondrocytes, leaving calcified cartilage spicules. Bone is laid down on these spicules.
