LECTURE 5 (Bones Part II) Flashcards
Describe Bone remodelling in compact bone
Osteoclasts remove old bone and form small, tunnel-like cavities -> Tunnels are quickly invaded by OSTEOPROGENITOR CELLS from the ENDOSTEUM or PERIOSTEUM and sprouting loops of capillaries -> OSTEOBLASTS develop, line the wall of the tunnels, and begin to secrete OSTEOID in a cyclic manner, forming a new OSTEON with concentric lamellae of bone and trapped OSTEOCYTES
Describe the diagnostic technique where Tetracycline is used
1) Tetracycline is administered twice to patients with an interval of 11-14 days
2) A bone biopsy is performed
3) Bone is sectioned without decalcification and examined
What is the importance of Tetracycline?
Fluorescent molecules that binds newly deposited OSTEOID MATRIX during MINERALISATION with high affinity and specifically labels new bone under the UV microscope
[allows for measuring rate of BONE GROWTH -> diagnosis of bone disorders]
What does bone that is labelled with Tetracycline look like?
Bone formed while tetracycline was present appears as FLUORESCENT LAMELLAE and DISTANCE between the LABELED LAYERS is proportional to the RATE of bone appositional growth
Which diseases is Tetracycline useful in diagnosing?
- OSTEOMALACIA = mineralisation is impaired
- OSTEITIS FIBROSA CYSTICA = increased osteoclast activity results in removal of bone matrix and fibrous degeneration
Describe the two processes in which Bone development/Osteogenesis occurs
- INTRAMEMBRANOUS OSSIFICATION = osteoblasts differentiate directly from mesenchyme and begin secreting osteoid
- ENDOCHONDRAL OSSIFICATION = preexisting matrix of hyaline cartilage is eroded and invaded by osteoblasts which begin osteoid production
What happens before both processes of Osteogenesis?
In both processes, Woven bone is produced first and is soon replaced by stronger lamellar bone
Describe Osteogenesis Imperfecta
Also known as “Brittle bone disease”, are a group of congenital disorders in which osteoblasts produce deficient amounts of type I collagen or defective type I collagen due to genetic mutations
- All characterised by fragility of bones
- Fragility reflects the deficit in normal collagen which normally reinforces and adds a degree of resiliency to the mineralised bone matrix
Where does Intramembranous ossification take place?
Within condensed sheets “membranes” of embryonic mesenchymal tissue
Which bones are formed by intramembranous ossification?
- Skull & Jaws
- Scapula
- Clavicle
- Most flat bones
Describe what happens in Intramembranous Ossification
Condensed mesenchyme bone formation begins in OSSIFICATION CENTERS -> OSTEOPROGENITOR CELLS arise, proliferate and form incomplete layers of osteoblasts around a network of developing capillaries -> OSTEOID secreted by OSTEOBLASTS calcifies forming small irregular areas of woven bone with osteocytes in LACUNAE and CANALICULI -> Continued matrix secretion and calcification enlarges area & leads to fusion of neighbouring ossification centres
What happens when the Woven bone gradually gets replaced by Compact bone?
Compact bone encloses a region of cancellous bone with marrow and larger blood vessels
What happens to mesenchymal regions that do not undergo ossification?
Give rise to ENDOSTEUM and PERIOSTEUM of the new bone
Describe the bone formation in cranial flat bones
Lamellar bone formation predominates over BONE RESORPTION at both the internal and external surfaces -> Internal and external plates of compact bone arise while the central portion maintains its CANCELLOUS NATURE
[FONTANELLES or “soft spots” on the heads of newborns are areas of the skull where membranous tissue has not yet OSSIFIED]
Where does Endochrondal Ossification take place?
Within hyaline cartilage shaped as a small version or model of the bone to be formed
What is important in Endochrondal ossification?
- Forms most bones of the body
- Well studied in developing long bones
What are the steps of Endochrondal Ossification ?
1) Fetal hyaline cartilage model develops
2) Late in FIRST TRIMESTER, Cartilage calcifies and a PERIOSTEAL BONE COLLAR forms around DIAPHYSIS causing CHRONDOCYTE HYPERTROPHY in underlying cartilage
3) PRIMARY OSSIFICATION CENTRE forms in the DIAPHYSIS - Osteoid is deposited by new osteoblasts, undergoes calcification into woven bone and is remodelled as compact bone
4) SECONDARY OSSIFICATION CENTRE forms in EPIPHYSES
5) Bone replaces cartilage (except the articular cartilage and epiphyseal plates)
6) EPIPHYSEAL PLATES ossify and form EPIPHYSEAL LINES
What does Chrondocyte hypertrophy do?
Compresses the surrounding matrix and initiates its calcification by releasing OSTEOCALCIN and ALKALINE PHOSPHATASE
What happens when hypertrophic chrondocytes die?
They create empty spaces within the calcified matrix -> One or more blood vessels from the PERICHONDRIUM penetrate the bone collar bringing OSTEOPROGENITOR CELLS to the POROUS CENTRAL REGION -> Newly formed osteoblasts move into available spaces and produce woven bone -> Remnants of CALCIFIED CARTILAGE at this stage are BASOPHILIC and NEW BONE is ACIDOPHILIC
What happens when hypertrophic chrondocytes die?
They create empty spaces within the calcified matrix -> One or more blood vessels from the PERICHONDRIUM penetrate the bone collar bringing OSTEOPROGENITOR CELLS to the POROUS CENTRAL REGION -> Newly formed osteoblasts move into available spaces and produce woven bone -> Remnants of CALCIFIED CARTILAGE at this stage are BASOPHILIC and NEW BONE is ACIDOPHILIC
What are the two regions of cartilage?
- ARTICULAR CARTILAGE = within the joints between long bones which persist through adult life
- SPECIALLY ORGANISED EPIPHYSEAL CARTILAGE = connects each epiphysis to the diaphysis and allows longitudinal bone growth
What is the function of epiphyseal cartilage?
Responsible for the growth in bone length and disappears upon completion of bone development in adulthood (epiphyseal closure)
What does the epiphyseal growth plate show?
Distinct regions of cellular activity
What are the different locations and zones of Epiphyseal activity?
- ZONE OF RESERVE (OR RESTING) CARTILAGE = composed of typical hyaline cartilage
- PROLIFERATIVE ZONE
- ZONE OF HYPERTROPHY
- ZONE OF CALCIFIED CARTILAGE
- ZONE OF OSSIFICATION