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37 Cards in this Set

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What are the 2 ways bone can be formed?

1. Formation of bone onto a temporary cartilage model or proformer (endochondral ossification)




2. Formation of bone directly onto fibrous connective tissue. There is no intermediate cartilage stage. Directossification of a primitive mesenchymal connective tissue (intramembranous ossification).

What is the first stage in the process of long bone formation?

Creation of cartilaginous models or proformers.

These are subsequently converted into bone.

What is the hyaline cartilage of the proformers synthesised by?

Chondroblasts that differentiate from progenitor cells with the embryonic connective tissue (mesenchyme).

What do the proformers produce?

A glassy matrix - the cells become trapped causing them to transform into less synthetically active chondrocytes.

What is the hyaline cartilage encapsulated by?

Fibrous perichondrium

What does the fibrous perichondrium contain?

1. The differentiated chondroblasts (that differentiate from the progenitor cells and synthesise hyaline cartilage proformers)




2. The progenitor cells

How does hyaline cartilage obtain its oxygen and nutrients?

1. Small pieces of cartilage do not have a direct blood supply, the oxygen and nutrientsdiffuses through the matrix from the blood vessels in the perichondrium.




2. Largemasses of cartilage have a conventional blood supply via a capillary bed.

What is bone produced by?

Osteoblasts that differentiate from precursor cells.

Where are the precursor cells contained?

Within the capsule of the proformer or they arrive via the blood stream.

What do osteoblasts first synthesise?

The organic component of bone - osteoid - made up mainly from Type 1 collagen

What do the osteoids turn in to?

Osteocytes (osteoids become calcified)

What connects the osteocytes?

Canaliculi within the bone

What are osteoclasts?

They are different to osteoblasts and osteocytes.




- In developing bone


- Multinucleate


- Digest bone


- Remodel bone in growth process

What is the difference between primary and secondary bone?

Primary


- First type formed


- Collagen from osteoid is randomly woven


- Calcium is amophous rather than crystalline


- Most primary bone formed at birth




Secondary


- Open framework or as compact lamellae

What happens to the shaft of the proformer in endochondral ossification I, and what does this lead to?

- Shaft of proformer becomes calcified


- Bony collar established around it


- Collar penetrated by blood vessels that bring in more osteoprogenitor cells > osteoblasts > bone

What is then established here? What happens?

Primary centre of ossification.


- Proformer cartilage eroded


- Primary bone formed

What is later formed in the heads of the bones?

Secondary centres of ossification

What are growth plates?

The epiphyseal growth plates are where the cartilage becomes restricted to as the primary and secondary centres expand.


It is largely responsible for lengthening bones during growth.

What is epiphyseal cartilage responsive to?

Growth and sex hormones.

At the end of a growing long bone, what can be seen in endochondral ossification II?

- The chondrocytes of the resting cartilage first divide creating columns of progeny (zone ofhyperplasia).


- Next, they cells grow, eroding the cartilage matrix as theyenlarge (zone of hypertrophy).


- Eventually all that remains are thin spicules of cartilage, thechondrocytes themselves having been released from the matrix and resorbed.

What then happens to the remnant strands of cartilage?

They become calcified and serve as a substrate to whichosteoblasts become attached. These osteoblasts then lay down primary bone (zone ofossification).


This is contributing to lengthening of the bone by adding new bone to the epiphyseal plates.

What is happening at the same time to widen the girth of the bone?

Bone is eroded and redeposited on the outside of the shaft by appositional growth.

In which part of a long bone does ossification first occur?

Invariably in the diaphysis.

Approximately what proportion of primary centres of ossification have formed by thetime of birth?

Almost all bones have started to ossify by the time of birth and some are well advanced by this time. Some do not have secondary centres at this stage and the carpal and tarsal bones have not even begun to ossify.

Which long bones have only a single epiphyseal growth plate?

The distal phalanges of the fingers and toes

Which are the last (sets) of bones to begin the process of ossification?

The carpal and tarsal bones, the last of which starts to ossify at the age of 7 or 8 years of age.

What happens in intramembranous ossification?

- Bone is deposited directly in a primitive embryonic mesenchyme(connective tissue).


- No cartilaginous proformer is involved.


- Initially small clusters of progenitor cells within a primitive mesenchyme transform intoosteoblasts and start to deposit isolated islands of bone.


- As these islands enlarge theycoalesce to create an open meshwork of bone. The osteoblastscontinue to deposit bone on this meshwork until the holes become filled in thereby creating aplate of primary bone.


- Later, this primary bone is eroded and replaced with denser, moreorganised lamellar secondary bone.

Why does the cytoplasm of osteoblasts stain purple/blue?

These cells synthesise large quantities of osteoid. The cells contain a profuseendoplasmic reticulum with large numbers of ribosomes attached to it. It is these thatstain dark blue, along with the DNA of the nucleus.

What is the end product of osteogenesis? What does it contain?

Mature secondary bone. Mature bone normally containsabout 5% hydroxyapatite, a hard crystalline calcium salt. This renders bone too tough to slice

What are Volkmann's canals?

These are the cross connections between the Haversian canals of adjacent osteons.

What is bone surrounded by?

Periosteum

What are the 2 layers of periosteum?

An outer fibrous layer and aninner more cellular (and paler staining) layer. The latter contains rows of dark blue stainingosteoblasts that are in the process of laying down new bone by a process referred to asappositional growth.

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 and which cell type is particularlyresponsive to its action?

Parathormone to which osteoclasts respond.

What is compact mineralised bone made up from?

Osteons, each with a golden coloured Haversian canal at its centre

What do osteoblasts do?

Lay down an unmineralised connective tissue (osteoid) rich in type 1 collagen, that will become mineralized to form bone. Osteoclasts resorb/remodel bone.

Where would hyaline cartilage from the head of a long bone be from more specifically? Describe it's appearance histologically.

The epiphyseal growth plate.




It has an amorphous matrix and the dark-staining chondrocytes trapped within it.