Bone Development + Growth

what are the types of bone growth and when do they occur

in utero:

• intramembranous ossification

• endochondral ossification

after birth:

• appositional growth

• longitudinal growth

what is intramembranous ossification and what does it make

uses fibrous membranes as a template to form most flat bones 

what is endochondral ossification and what does it make

uses hyaline cartilage to form all other bones (including flat bones) 

what is appositional growth and what does it do

uses epiphyseal plate to grow long bones 

what is longitudinal growth and what does it do

uses periosteum and endosteum to grow all bones and make them thicker 

intramembranous ossification: 1st step

1. ossification center (start of bone formation) will form when the mesenchymal stem cells differentiate into osteoblasts that will secrete fibers and osteoids

intramembranous ossification: 2nd step

the osteoids secreted by the osteoblasts trap the osteoblasts inside a now mineralized ECM matrix and become osteocytes

intramembranous ossification: 3rd step

the osteoids are laid down between blood vessels randomly, making trabeculae. the surrounding mesenchyme becomes the periosteum and endosteum. Then the external trabeculae are replaced with compact bone and the spongy bone stays inside and is filled with red bone marrow (hematopoietic stem cells)

endochondral ossification: 1st step

chondroblasts make hyaline cartilage, making a bone collar around the diaphysis and prevents nutrients from going to the cartilage, thus making the cartilage die

endochondral ossification: 2nd step

the cartilage in the center of the diaphysis calcifies and develops cavities

endochondral ossification: 3rd step

periosteal bud (a blood vessel) invades the internal cavity and brings osteoblasts and osteoclasts, creating spongy bone

endochondral ossification: 4th step

Then, the diaphysis elongates and a medullary cavity forms. More ossification appears in the epiphyses where basically the same steps happen in the epiphyses

Longitudinal growth: where does it happen

in the epiphyseal plate (remaining hyaline cartilage)

what are the 4 zones of the epiphyseal plate

• Proliferation zone – the cartilage cells undergo hyperplasia 

• Hypertrophic zone – older cartilage cells grow by hypertrophy 

• Calcification zone – matrix becomes calcified and the cartilage cells begin to die 

• Ossification zone – new bone cells are forming from the calcification zone 

how do the zones inside the epiphyseal plate allow for bone growth?

These zones inside the epiphyseal plate allow the space between the epiphysis and diaphysis to expand by slowly moving the ossification zone further up until there's no more epiphyseal plate and there's just a line, making the bone grow longitudinally

what is the relation between osteoclast and osteoblast activity in the periosteum and endosteum

More osteoblast activity at the periosteum, and osteoclast activity at the endosteum to balance the size of bone so it's not too heavy 

what do growth hormones and thyroxine do for bone growth

stimulates epiphyseal plate activity

what do testosterone and estrogen do for bone growth

• Promote osteoblast activity 

• Inhibit osteoclast activity 

• Promote calcification of bone 

• End growth by epiphyseal plate closure 

how does cartilage grow?

through

• appositional growth (similar to appositional growth for bone) - uses periochondrium to make it thicker
  interstitial growth - cartilage cells grow through hyperplasia and hypertrophy

how does cartilage get their nutrients if they’re asvascular and not innervated?

from diffusion by the surrounding perichondrium, bone, or synovial fluid

is cartilage calcification good? why or why not?

not good. it shld only calcify in the epiphyseal plate. leads to

• osteoarthritis (wear and tear)

• chondrocytes will secrete calcium containing crystals