Lecture 43: Biochemistry of Bone Formation

Question 1

what are the 4 phases of skeletal development

Answer 1

-migration
- interaction with epithelial cells
-interaction leads to mesenchymal condensation
-followed by differentiation to chondroblasts or osteoblasts

Question 2

what are the types of bone formation

Answer 2

endochondral bone formation and intramembranous bone formation

Question 3

describe what happens in endochondral bone formation

Answer 3

indirect- mesenchyme forms cartilage template first which is later replaced by bone

Question 4

where does endochondral bone formation occur

Answer 4

-in most bones of the skeleton especially bones that bear weight and have joints
- fracture repair sites

Question 5

describe intramembranous bone formation

Answer 5

direct transformation of mesenchymal cells to osteoblasts

Question 6

where does intramembranous bone formation occur

Answer 6

• cranial vault, some facial bones, parts of mandible and clavicle
  -fracture repair sites

Question 7

how do blood vessels invade the forming bone

Answer 7

VEGF produced by hypertrophic chondrocytes

Question 8

when does the secondary ossification center appear

Answer 8

around the time of birth

Question 9

what is the mechanism by which intramembranous ossification works

Answer 9

-mesenchymal cells condense to produce osteoblasts which deposit osteoid
- osteoid matrix calcifies/osteoblasts become arranged along calcified region of the matrix
- some osteoblasts trapped in bone matrix become osteocytes

Question 10

what is the first type of bone produced developmentally

Answer 10

woven bone

Question 11

when is woven bone produced

Answer 11

when osteoblasts need to form bone rapidly such as in embryonic development, fracture healing, and disease states

Question 12

what is immature woven bone remodeled and replaced with

Answer 12

lamellar bone

Question 13

describe woven bone

Answer 13

-disorganized structure
-randomly oriented collagen fibrils
-increased cell density
-reduced mineral content

Question 14

describe lamellar bone

Answer 14

-highly organized
-bone lamellae concentrically arranged around central canal containing blood vessels and nerves
-parallel oriented collagen fibrils
-mechanically stronger

Question 15

what is lamellar bone further classified into

Answer 15

compact (cortical) bone and cancellous (spongy) bone

Question 16

what is another name for woven bone? lamellar bone?

Answer 16

woven - primary
lamellar - secondary

Question 17

what are they 3 types of bone cells

Answer 17

osteoclasts, osteoblasts, osteocytes

Question 18

what do osteoblasts, chondrocytes, myoblasts and adipocytes all differentiate from

Answer 18

a common mesenchymal progenitor

Question 19

where are osteoblasts located

Answer 19

cuboidal cells of bone forming surfaces

Question 20

what do osteoblasts produce

Answer 20

large amounts of ECM proteins (especially type 1 collagen) called osteoid which then mineralized

Question 21

what is the lifespan of osteoblasts

Answer 21

weeks

Question 22

what are transcription factors for osteoblasts

Answer 22

RUNX2 and Osterix

Question 23

what are the ECM proteins that make up osteoblasts

Answer 23

type 1 collagen, osteopontin, osteocalcin, bone sialoprotein (BSP)

Question 24

what pathway do RUNX2 and Osterix work in

Answer 24

beta catenin

Question 25

what is RUNX2 essential for

Answer 25

bone and tooth development

Question 26

what does a heterozygous mutation of RUNX2 result in

Answer 26

cleidocranial dysplasia

Question 27

what are the symptoms of cleidocranial dysplasia

Answer 27

-autosomal dominant - delayed ossification of midline structures - clavicles partly or completely missing -late closing of fontanelle - supernumerary teeth -prognathic mandible due to hypoplasia of maxilla

Question 28

what transcription factor does RUNX2 induce

Answer 28

Osterix

Question 29

what osteoblast genes does osterix control expression of

Answer 29

-type 1 collagen -osteocalcin -osteopontin

Question 30

what are mutations in osterix associated with

Answer 30

osteogenesis imperfecta type 7

Question 31

what are some key signaling pathways that regulate osteoblast differentiation

Answer 31

BMPs, WNT/Beta catenin,

Question 32

what are BMPs

Answer 32

originally purified from bone extracts that induce bone formation when implanted in muscle

Question 33

what are BMPs required for

Answer 33

skeletal development/maintenance of adult bone homeostasis

Question 34

what do BMPs do

Answer 34

-promote differentiation from early osteoprogenitor cells -fracture healing

Question 35

what do mutations in BMPs lead to

Answer 35

skeletal defects/disorders

Question 36

what is fibrodysplasia ossificans progressiva (FOP)

Answer 36

heterotrophic bone formation (bone forming in soft tissues)

Question 37

what happens in FOP

Answer 37

-ribbons, sheets, plates of bone in extra skeletal sites - fuses joints, ribs - bone forms in response to tissue trauma

Question 38

what mutation is involved in FOP

Answer 38

BMP type 1 receptor

Question 39

what does a mutation in the BMP type 1 receptor in FOP do

Answer 39

causes mild constitutive activation and overactivation with BMP ligand binding -acquired responsiveness to activin A

Question 40

what is the Wnt/ beta catenin pathway important in

Answer 40

determining bone mass

Question 41

what do activating mutations in Lrp5 lead to

Answer 41

high bone mass in humans

Question 42

what do inactivation mutations in Lrp5 lead to

Answer 42

low bone mass

Question 43

what is the first phase of mineralization initiated by

Answer 43

matrix vesicle

Question 44

what does the second phase of mineralization involve

Answer 44

propagation of mineralization on collagen fibers

Question 45

what are matrix vesicles

Answer 45

extracellular membrane bound vesicles produced by osteoblasts

Question 46

what do matrix vesicles contain

Answer 46

transporters and enzymes involved with the generation of phosphate and uptake of calcium and phosphate into the vesicle to reach high enough concentrations to initiate mineralization

Question 47

where is alkaline phosphatase highly expressed

Answer 47

in osteoblasts and odontoblasts

Question 48

what does alkaline phosphatase do

Answer 48

hydrolyzes pyrophosphate which is a natural inhibitor of mineralization- thereby increasing local phosphate concentration, promoting mineralization

Question 49

what are mutations in the alkaline phosphatase gene associated with

Answer 49

hypophosphatasia

Question 50

what is hypophosphatasia

Answer 50

rare heritable rickets/osteomalacia

Question 51

what is hypophosphatasia caused by

Answer 51

mutation in human alkaline phosphatase gene -TNSALP

Question 52

what is HPP characterized by

Answer 52

impaired mineralization of skeleton/dentition, leg bowing, rachitic rosary, early tooth loss, waddling gait, muscle weakness, seizures

Question 53

what are ENPP1 mutations associated with

Answer 53

syndrome of spontaneous infantile arterial and periarticular calcification

Question 54

a mutation in what components will affect mineralization

Answer 54

ENPP1, TNSALP, Anx5, PHOSPHO1

Question 55

what are osteocytes

Answer 55

terminally differentiated osteoblasts embedded in bone matrix

Question 56

what percentage of bone cells are osteocytes

Answer 56

over 90%

Question 57

what is the lifespan of osteocytes

Answer 57

decades

Question 58

are osteocytes quiescent cells

Answer 58

no they are active cells with key functions in bone

Question 59

what is the transcription factor of osteocytes

Answer 59

Mef2c

Question 60

what are the early osteocyte markers

Answer 60

-E11/gp38/podoplanin -dentin matrix protein-1 (DMP1) - matrix extracellular phosphoglycoprotein (MEPE) - phosphate regulating endopeptidase homolog, X linked (PHEX)

Question 61

what is the late osteocyte marker

Answer 61

sclerostin (SOST)

Question 62

what are some potential functions of osteocytes

Answer 62

-mechanosensors -control bone resorption and bone formation -regulate mineralization -regulators of mineral homeostasis- both calcium and phosphorus -regulate activity of osteoblasts and osteoclasts

Question 63

what is sclerostin expressed in

Answer 63

mature osteocytes, cementocytes, odontoblasts

Question 64

what does sclerostin do

Answer 64

antagonizes Wnt/beta catenin signaling pathway , limits bone formation

Question 65

what does the deletion or mutation of SOST gene result in

Answer 65

sclerosteosis or Van Buchem's disease

Question 66

what happens in Sclerosteosis

Answer 66

increased bone mass, especially obvious in craniofacial skeleton

Question 67

what genes do osteocytes express that are important in phosphate homeostasis

Answer 67

FGF23 DMP1 PHEX

Question 68

what endocrine role do osteocytes play

Answer 68

regulation of phosphate homeostasis