Bone development and ossification Flashcards
What is ossification (bone formation)?
removal of cartilage by osteoclasts to make space for bone marrow
intramembranous ossification - how is bone formed
formed directly from mesenchymal cell condensation in the membrane
intramembranous ossification - what bones are formed in this way?
skull, cortical bone shafts of long bones
intramembranous ossification - stages of formation
- embryonic CT/ mesenchyme proliferates
- differentiates
- directly to osteoprogenitor cells
- form osteoblasts
- osteoblasts synthesise matrix (osteoid)
then : matrix deposition, formation of periosteum, vascularisation, and continued growth and remodelling
intramembranous ossification - matrix deposition and mineralisation
- collagen fibres randomly arranged in irregular bundles and there are large and numerous osteocytes = woven bone
intramembranous ossification - vascularisation details
bone formation continues to form a network of trabecular bone
- haematopoietic bone marrow forms between trabeculae following incorporation of blood vessels
intramembranous ossification -continued growth and remodelling details
later woven bone is remodelled by osteoclasts and replaced by lamellar bone
endochondral ossification - how is bone formed
cartilage is deposited by chondroblasts in the shape of the required bone and is surrounded by the perichondrium
what are chondrocytes?
chondroblasts embedded In their own matrix
endochondral ossification - stages of formation
- cells of the perichondrium differentiate to osteoprogenitor cells and osteoblasts - they lay down the bony collar around diaphysis (cartilage model)
- impermeable bony collar shuts supply of nutrients to chondrocytes- degenerate and cartilage matrix becomes calcified - restricts diffusion further
- capillaries invade the periosteum and bony collar and bring osteoclasts with them- they resorb calcified cartilage to excavate a central cavity (marrow space)
at the same time, osteoprogenitor cells migrate in with blood vessels to form osteoblasts - lay down bone matrix around the cartilage core that remains
endochondral ossification - what occurs at the epiphyses?
chondrocytes undergo rapid proliferation, forming longitudinal column cells - increases the length of developing bone
endochondral ossification - what happens when chondrocytes age?
get further from extending epiphyseal surface
they hypertrophy and begin to calcify the cartilage matrix - restricts nutrients and cells die
endochondral ossification - more on formation of bone
end walls between empty lacunae break down, opening up the longitudinal tunnels in the calcified cartilage, and blood vessels bring more osteoblasts to lay down bone on top of the remaining calcified cartilage
- early spongy bone is remodelled by osteoclasts and osteoblasts to give mature lamellar bone
where are primary ossification centres found?
mid shaft
where are secondary ossification centres found?
epiphyses
