Bone And Ossification Flashcards
2 ways in which bone is formed
Endochondral ossification
Intramembranous ossificatiom
Endochondral ossification
Transformation of a pre-existing cartilaginous model or proformer
Intramembranous ossification
Direct ossification of a primitive mesenchymal connective tissue
What is the first stage of long bone formation
Creation of cartilaginous models or proformers
Subsequently converted to bone
What are proformers made of
Hyaline cartilage
Hyaline cartilage of proformers synthesis
By chondroblasts
Produce a glassy amorphous looking matrix within which the cells become trapped causing them to transform into less synthetically active chondrocytes
Formation of chondroblasts
Differentiate from progenitor cells within embryonic connective tissue (mesenchyme)
What encapsulates hyaline cartilage
Fibrous perichondrium that contains both progenitor cells and differentiated chondroblasts
Major component of osteoid
Type 1 collagen
Primary bone
First type of bone to be produced in a developing bone
Structure of primary bone
Randomly woven collagen
Calcium is amorphous not crystalline
What type of bone is present at birth
Primary bone
Structure of secondary bone
Collagen organised into regularly arranged laminae
Calcium deposited as hydroxyapatite with the long axes of the crystals parallel to the collagen fibres
Characteristics of secondary bone
Light
Strong
How can secondary bone occur
Woven bone (open framework)
Compact lamellae
Osteons
Osteoclasts
Occur frequently in developing bone
Multinucleated cells
Digest bone and play a part in remodelling
Original of osteoblasts
Differentiate from precursor cells contained within capsule of the proformer or that arrive in the cavity of developing bone via the blood stream
Which cells produce bone
Osteoblasts
Osteocytes
When the osteoid is ossified osteoblasts become trapped within the matrix where they regress into a less active form
How is contact between osteocytes maintained
By minute canaliculi within the bone through which spidery processes from the osteocytes make contact with each ither
When are long bones first created
2nd trimester
How do long bones start in utero
Hyaline cartilage proformers
Which type of cartilage is particularly responsive to growth and sex hormones
Epiphyseal cartilage
Growth plates
Continue to exist beyond puberty and are largely responsible for the lengthening of bones during childhood and adolescence
1st stage of endochondral ossification
Formation of hyaline cartilage proformers
2nd stage of endochondral ossification
The shaft (diaphysis) of the proformer becomes calcified and a bony collar is established around it
3rd stage of endochondral ossification
The bony collar is penetrated by blood vessels that bring in more osteoprogenitor cells that differentiate into osteoblasts and lay down bone
4th stage of endochondral ossification
A primary centre of ossification is established- involves the erosion of the proformer cartilage and formation of primary bone
6th stage of endochondral ossification
Later secondary centres of ossification of formed in the heads (epiphyses) of the bones
7th stage of endochondral ossification
As the primary and secondary centres expand the amount of cartilage is reduced so that it becomes restructure to the epiphyseal growth plates that separate the diaphysis from the epiphyses and to the articular surfaces of the bones
Epiphyses
Heads of the bines
Diaphysis
Shaft of the proformer
Where is a primary centre of ossification established
Bony collar around diaphyssi
Where is secondary centre of ossification established
Epiphyses
Zone of hyperplasia
Chondrocytes of resting cartilage first divide creating columns of progeny
Zone of hypertrophy
Newly formed cells grow, eroding the cartilage matrix as they enlarge
What remains after zone of hypertrophy
Thin spicules of cartilage
The chondrocytes themselves having been released from the matrix and resorbed
Zone of ossification
Remnant strands of cartilage become calcified and serve as a substrate to which osteoblasts become attached
Osteoblasts then lay down primary bone
Appositional growth
Bone is eroded and re-deposited in the outside of the shaft of the bone
How does the length of a long bone grow
At a epiphyseal growth plate
Zone of hyperplasia
Zone of hypertrophy
Zone of ossification
How does the girth of a long bone increase
Appositional growth
Where does intramembranous ossification occur
Flat bones of the skull
When does intramembranous ossification start
Second trimester
Which type of bone formation requires a proformer
Endochondral ossification
1st stage of intramembranous ossification
Small clusters of progenitor cells within a primitive mesenchyme transform into osteoblasts and start to deposit isolated islands of bone
2nd stage of intramembranous ossification
As the islands enlarge they coalesce to create an open network of bone
3rd stage of intramembranous ossification
Osteoblasts continue to deposit bone on the mesh work until the holes become filled in thereby creating a plate of primary bone
4th stage of intramembranous ossification
Primary bone is eroded and replaced with denser more organised lamellar secondary bone
What is the end product of osteogenesis
Mature secondary bone
Composition of mature bone
5% hydroxyapatite
How is bone examined
Native bone is ground into thin slithers with grinding paste
Why is the bone specimen not stained
Contrast is created by refraction caused by the crystalline nature of the tissue
How is bone treated for examination
Treated with acid and/or chelating agents
-procedure removes most of the calcium but organic components are retained
What colour does compact lamellar secondary bone stain
Purple/pink
What part of the compact lamellar bone stains strongly
The collagen matric
What surrounds bone
Periosteum
Periosteum
Outer fibrous layer
Inner cellular layer - paler staining
Site of osteoprogenitor cells
Inner cellular layer of periosteum
Contains rows of dark blue staining osteoblasts
How does hyaline cartilage normally obtain its oxygen and nutrients
Small pieces of cartilage do not have a direct blood supply, the oxygen and nutrients diffuse through the matrix from the blood vessels in the perichondrium
Large masses of cartilage have a conventional blood supply via a capillary bed- during osteogenesis the diaphysis and epiphyses of bone proformers are invaded by blood vessels
In which part of a long bone does ossification first occur
Diaphysis
Approximately what proportion of primary centres of ossification have formed by the time of birth
Almost all have started to ossify and some are well advanced by this time
Carpal and tarsal bones have not begun to ossify
Which long bones only have a single epiphyseal growth plate
Distal phalanges of fingers and toes
Which are the last sets of bones to begin the process of ossification
Carpal and tarsal bone - begin at age 7/8
Why does the cytoplasm of osteoblasts stain purple blue
Cells synthesise large quantities of osteoid
Contain a profuse endoplasmic reticulum with large numbers of ribosomes attached to it
Ribosomes and DNA stain dark blue
What are Volkmann’s canals
Cross connections between the Haversian canals of adjacent osteons
Where on a long bone does cartilage persist even after the closure of the growth plate
At the ends as articular cartilage
Which hormone promotes the breakdown of bone
Parathormone
Howship’s lacunae
Depressions in the surface of bone formed by osteoclasts
How to describe bone by origin
Primary (woven)
Secondary (lamellar)
How to describe bone by mode of formation
Membranous
Endochondrial
How to describe bone by gross appearance
Spongy/cancellous
Compact
How to describe bone by formed
Osteons
Appositinal
Osteoid
Extracellular matrix rich in type 1 collagen, glycosaminoglycans and proteoglycans
Osteons
Central Haversian canal containing blood vessel
Surrounded by concentric rings of lamellar bone with osteocytes
Secondary bine
Synovium
Lines the inside of joint capsule
1-4 layers of synovial cells
Type A - phagocytes
Type B - rich in rER
Variable shapes- squamous to cuboidal
Richly vascular
Highly innervated
Type A synovial cells
Phagocytes
Type B synovial cells
Secrete synovial fluid
Rich in rER
Osteoclasts:
Are derived from megakaryocytes
Are multinucleated
Differentiate into osteoprogenitor cells
Lie in Haversian canals
Synthesise osteoid
Multinucleated
Osteocytes:
Are dead cells
Do not communicate with each other
Have more endoplasmic reticulum than osteoblasts
Lie on the surface of bones
May resorb formed bone matrix
May resorb formed bone matrix
Which of these bones is formed by intramembranous ossification?
Clavicle
Femur
Humerus
Ischium
Scaphoid
Clavicle
Why bones are formed by intermembranous ossification
Flat bones of skull
Clavicle
Mandible
Which of the following can be found in Haversian canals?
Blood vessels
Haematopoietic marrow
Osteoclasts
Osteocytes
Sharpey’s fibres
Blood vessels
