Ossification and Joints Flashcards
What is ossification?
The replacement of connective tissue by bone.
What happens when mineralisation fails?
Osteomalacia
How does mineralisation fail?
Lack of Ca2+ in the diet or malabsorption from the small intestinal disease results in poor mineralisation of bones
How are bones formed in the foetus?
The replacement of pre-existing CT by bone
What are the two types of ossification?
Intramembranous and endochondral
What type of bones does intramembranous ossification form?
flat bones
Where does intramembranous ossification occur?
Ossification occurs within a “membrane” of primitive mesenchymal cells
Describe stage one of intramembranous ossification/
Mesenchymal cells differentiate into osteoblasts
A cluster of osteoblasts form an ossification centre and lay down osteoid
Describe stage 2 of intramembranous ossification
Mineralisation of osteoid occurs within a few days to produce an island of bone
Trapped osteoblasts become osteocytes
Describe stage 3 of intramembranous ossification?
Islands of bone join to form trabeculae of woven bone with blood vessels
Mesenchyme cells condense at edges to form periosteum
Describe stage 4 of intramembranous ossification
Increased bone formation occurs on outer and inner surfaces to produce plates of bone
Spongy bone persists internally and red marrow develops in the trabecular spaces
What is endochondral ossification?
Involves the ossification of a hyaline cartilage template
Hyaline is deposited in the shape of the required bone within a mass of mesenchymal tissue
When does endochonral ossification occur?
Begins in the second month of foetal development
What is endochondral ossification used for?
Forms the long bones of the body
What is the epiphysis?
The club shaped ends of the “bone” templates
What is the diaphysis?
The shaft of the bone template
What happens in stage 1 of endochondral ossification
Cartilage cells in the central diaphysis die to leave cavities
Perichondrium produces osteoblasts which lay down a bone collar by intramembranous ossification
What happens in stage 2 of endochondral ossification
Cartilage matrix in centre continues to deteriorate
Diaphysis diameter increases by bone deposition on the outside and resporption on the inside
Chondrocytes in the rest of the bone multiply increasing the length and breadth
What happens in stage 3 of endochondral ossification
Capillaries penetrate the diaphysis carrying osteoprogenitor cells from the periosteum with them
Osteoprogenitor cells transform to osteoblasts and set up a primary ossification centre
Bone is laid down in replacement of lost cartilage in centre
What happens in stage 4 of endochondral ossification
At birth capillaries penetrate the epiphyses
Osteoprogenitor cells are dragged in, differentiate into osteoblasts and produce secondary ossification centres where bone is laid down in place of cartilage
Formation of medullary cavity
What happens in stage 5 of endochondral ossification
Ossification of the epiphyses is complete, cartilage remains only in the
Epiphyseal growth plate
Articular cartilage
How does the bone lengthen?
Through the epiphyseal growth plate
Chondrocytes died
Matrix becomes calcified
Osteoblasts come in and start to deposit bone tissue in its place
Cartilage is replaced by bone and growth plate lengthening bone
What is post natal growth?
From constant growth of cartilage particularly at the epiphyseal plate
What is a joint?
An articulation or union between two or more bones
Weakest part of the sekelton
What are the functions of joints?
Allow mobility of the skeleton
Hold the skeleton together
What are the three structural classification of joints?
Fibrous
Cartilaginous
Synovial
What are the features fibrous joints?
Bones joined by dense irreagular CT
Typically immovable
No joint cavity
What are examples of a fibrous joint?
Suture
Gomphosis
What are the features of a cartilaginous joint?
Connected entirely by cartilage
Lack of a joint cavity
some movement possible
What are the features of synovial joints
Freely movable
Articular cartilage coats bone surfaces
Joint cavity exists containing synovial fluid to lubricate
Enclosed by capsular ligament lined synobvial membrane
