Bone Formation Flashcards
Intramembranous ossification
Bone is formed from clusters of mesenchymal cells; responsible for the formation of most flat bones, the mandible, and the clavicle; takes place in highly vascularized tissue.
Endochondral ossification
Bone is deposited on a preexisting cartilage matrix; responsible for formation of long and short bones (limbs)
*Growth of hyaline cartilage model occurs by both appositional and interstitial growth
Function of proteoglycans in bone matrix
Bind growth factors like TGFb, BMP, and Ihh
Function of glycoproteins in bone matrix
Important for holding things together and for the mineralization process
Steps of intramembranous ossification
- Mesenchymal cells migrate and cluster in locations where bone is meant to form
- Clustered mesenchymal cells differentiate into osteoprogenitor cells
- Osteoprogenitor cells differentiate into osteoblasts, which secrete components of the bone matrix (type I collagen to make osteoid), facilitate mineralization of the matrix (osteocalcin and alkaline phosphatase), and secrete glycoproteins (osteonectin) to bind matrix components to minerals.
- Osteoblasts surround w/ matrix –> osteocytes
- Osteoblasts congregate on edges of spicules to make trabecular network
- If spongy bone then remaining mesenchyme among trabeculae gives rise to the bone marrow cells. If compact bone then the trabeculae thicken until they obliterate the intervening vascular spaces.
Immature bone remodeled into mature.
Fontanelles
Soft spots on babies’ skulls where bone has not formed yet
Steps of endochondral ossification
- Formation of cartilage model; perichondrial cells in the midregion of the cartilage model stop producing chondroblasts
- Growth of the cartilage model (appositional at edge and interstitial at middle)
- Formation of bone collar when perichondrial cells produce osteoblasts, which secrete osteoid.
- Chondrocytes hypertrophy
- Hypertrophic cells secrete type X collagen (mineralization of matrix), alkaline phosphatase (enzyme for mineral deposition), VEGF (stimulates vascular invasion)
- Death of chondrocytes and disintegration of matrix by matrix metalloproteases. Remnants of calcified cartilage at end of cavity = epiphyseal cartilage.
- Blood vessels from periosteum invade cavity, bringing HSCs that will form cells of bone marrow.
- Osteoprogenitor cells migrate from periosteum, differentiate into osteoblasts, and start secreting osteoid to form mixed spicules, which are eventually removed by osteoclasts and replaced by bone.
- Secondary ossification centers form in epiphysis.
Zones of epiphyseal plate
- Zone of reserve (resting zone)
- Zone of proliferation
- Zone of hypertrophy
- Zone of calcification
- Zone of ossification
Resting zone
Of epiphyseal plate; typical hyaline cartilage
Zone of proliferation
Of epiphyseal plate; chondrocytes divide rapidly and form columns of cells
Zone of hypertrophy
Of epiphyseal plate; large chondrocytes whose cytoplasm has accumulated glycogen; narrow areas of matrix between lacunae; cells secrete type X collagen, alkaline phosphatase, and VEGF
Zone of calcification
Of epiphyseal plate; matrix becomes calcified; chondrocytes die via apoptosis
Zone of ossification
Of epiphyseal plate; osteoprogenitor cells invading and differentiating into osteoblasts, which secrete bone matrix onto the calcified cartilage matrix.
Epiphyseal plate stimulated by what hormone?
Somatotropin; released by anterior pituitary. Bone grows until epiphyseal closure. Upon closure forms epiphyseal line.
Increases in bone width
Appositional growth; due to expansion of marrow cavity. Bone produced by osteoprogenitor cells in the periosteum on the external surface. Bone removed from the internal surface, causing cavity to grow.
***Intramembranous ossification
