Lecture 16; Post Natal Growth 2 Flashcards
What is the process of bone growth known as?
Endochondral ossification
What do bones start from in endochondral ossification?
Cartilaginous structures
Describe endochondral ossification;
- Cartilaginous bone
- Periosteal bone collar
- Primary center of ossification forms
- Growth plate forms between formed diaphysis and secondary center of ossification
- Secondary center gives rise to epiphysis
- Growth plate forms on the metpahysis
Describe the structure of the growth plate at the metaphysis;
- Well these are initially chondrocytes in the cartilage structure
- Resting Zone; Chondrocytes
- Proliferating zone; Chondrocytes is proliferating
- Hypertrophic zone; Chrondrocytes are becoming hypertrophic (cells in columns)
Chondrocytes = stem cells??
What happens to the growth plate?
Bone invade the hypertrophic chondrocytes and the columns give rise to the bone architecture.
Cartilage deposition vs bone absorption rates will determine epiphyseal closure
Describe the growth plate morphology in terms of cell types;
Secondary center of ossification Proliferating Chondrocytes Pre-hypertrophic chondrocytes Hypertrophic chondrocytes Primary center of ossification
LONG BONE FORMS FROM THE ENDS TOWARDS THE MIDDLE (I.E EPIHYSIS TO DIAPHYSIS)
What explains the pathology in bone malformation;
Looking at the signalling
What sort of signalling disruptions can lead to bone pathology in growth;
GH = Essential for chond. prolif. (via IGF1)
FGF = Tonic inhibition
T3,T4 are key integrating factors of chon. hypertrophy. (can double inhibit PThrP to cause Hyp. i.e they inhibit PThrP which would normally inhibit hyp.)
What hormones promote blood vessel invasion of the bone and effect the balance of osteoclast:osteoblasts?
BV invasion; VEGF,
BMPs; Osteoblast
RANKL; Osteoclast
How does GH generate proliferation?
GH appears to stimulate growth by a combination of direct effects and effects mediated by IGFs.
How does GH stimulate IGF?
– It stimulates the production of endocrine (hepatic!) IGF-1 and its major binding protein (IGFBP-3).
What does GH do in bone growth?
– Directly induces the clonal expansion and differentiation of target stem cells (such as prechondrocytes)
How do chondrocytes respond to GH?
– Differentiating cells (chondrocytes) then respond to GH by forming IGF-1 and IGF-1 receptors, which makes them responsive to the growth-promoting effect of both endocrine IGF-1 and IGFs secreted locally (autocrine and paracrine IGFs).
What does IGF and GH receptor KO in rats tell us?
GH has a direct effect on growth
IGF and GH have a much larger effect together than either alone
Where does IGF 1 come from and act on?
• Major post-natal growth promoting factor.
- Majority bound to binding proteins (BP-3).
- Principally produced in liver (endocrine) and bone (paracrine, autocrine). Acts on wide range of tissues.
What is IGF-1 function?
- Insulin-like (promoting glucose, lipid and amino acid uptake).
- Cell proliferation & differentiation.
What does defects in IGF-1 action lead to?
Defects in IGF-1 synthesis or action lead to growth failure in humans and laboratory animals.
What may the effects of IGF-1 depend on?
• The effects of IGF-1 may depend on the tissue of origin.
– Local production of IGF-1GH-induced somatic growth
– circulating IGF-1, mainly from the liver, may not be essential for growth, but provides negative feedback for the GH axis.
What portion of circulating IGF-1 is active?
• The free (unbound) IGF-1 is thought to be the biologically active fraction of circulating IGF-1
What is IGF-1 regulated by?
• IGF-1 is primarily regulated by GH when nutrition is normal.
What is IGF-2 regulated by?
• IGF-2 is produced by cells independently of GH and IGF-1
– Essential for normal fetal growth
– Effects largely through IGF-1 receptor: more severe defects from receptor knockout than loss of either IGF
What do chondrocytes secrete that is essential for their proliferation?
IHH (indian Hedgehog)
• Secreted by chondrocytes transitioning from proliferation to hypertrophy
What does IHH bind to?
• Binds cell receptor patched 1 (Ptch1)
– Inactivates a repressor, Gli3
– Requires normal chondroitin sulphate and intact primary cilium
What major signalling mechanism is present in chondrocytes to stimulate proliferating?
• Multiple WNTs are expressed by growth plate chondrocytes and stimulate proliferation
Write some short notes on how WNTS work;
• A family of secreted glycoproteins that are essential for skeletal development
– receptor Frizzled (Frz) • non-canonical pathway
– + co-receptor Lrp5 or Lrp6 • canonical=b-catenin
What are BMPs a part of?
• Part of transforming growth factor b (TGFb) superfamily
How do BMPs signal??
• BMP receptors are complexes of type I and type II serine/threonine kinase receptors, which when activated phosphorylate receptor-Smads, translocation to the nucleus.
What does BMP signalling do?
• Activation increases chondrocyte PROLIFERATION
What other paracrine factor does BMP increase?
• BMPs induce IHH expression and IHH induces expression of BMPs in growth cartilage… but not absolutely essential!
What paracrine factors are important in bone growth?
IHH
WNT
BMP
What FGF does the human growth plate predominately express?
Human growth plate predominantly expresses FGFs 1, 2, 15, 19
What receptors do FGF function act through and cause?
• through FGF receptor 3 (FGFR3) – inhibiting proliferation
BUT promotes hypertrophy
What mutation causes short limbed dwarfism?
• Achondroplasia = activating mutations in FGFR3
– Most common short-limbed dwarfism in humans
How does the environment influence FGF?
- FGF activity is also modulated by glycosaminoglycans (like IHH)
- FGF-induced repression of chondrocyte proliferation is limited by the desulphation of glycosaminoglycans
- BMP signalling antagonises the inhibition of chondrocyte proliferation due to activation of FGFR3 (dynamic balance between FGF and BMP)
What mediates chondrocyte proliferation?
Cyclins (Control rate of proliferation)
Describe how cyclin activation comes about?
• Chondrocyte proliferation is mediated by cyclins
– Requires activation of E2F transcription factors
– Conversely, FGFR3 inhibits E2F
– Cyclin D1 is required for normal proliferation of growth plate chondrocytes
What do cyclins require?
• Requires transcriptional repressor TRPS1
– Tricho–rhino–phalangeal syndrome: skeletal malformations due to mutations of TRPS1
What regulates chondrocyte hypertrophy?
• Thyroid hormones are a critical systemic regulator of the transition to hypertrophy
What does T3 do in chondrocyte hypertrophy?
• T3 stimulates morphological hypertrophy but not proliferation
What does hypothyroidism lead to in bone growth?
• Hypothyroidism … abnormally thin growth plates with much less hypertrophic chondrocytes — i.e. short limbs
Describe the local effects of thyroxine;
Complex;
– ?Wnt4,Local IGF-1
– + Induces FGFR3
• ie inhibit proliferation plus accelerating hypertrophy
Describe the effects of Thyroxine on PTHrP;
Decreased (PTHrP) and receptor in chondrocytes
– PTHrP suppresses hypertrophy, partly by suppressing RUNX2 (which promotes hypertrophy)
– absence of PTHrP allows IHH to promote hypertrophy
What happens to the growth plate over time?
• The growth plate is constantly converted to bone – The chondrocytes die rapidly in the last row of lacunae
What do hypertrophic chondrocytes release that promotes growth plate invasion?
• The hypertrophic chondrocytes release soluble factors that precisely control the invading blood vessels, osteoclasts and osteoblasts
What causes the shift in chondrocyte proliferation;bone absoprtion (towards closure)?
• As we approach skeletal maturity ossification progresses faster than replacement of chondrocytes
– Regulated by oestrogen (E2)!
– E2 accelerates senesce of the growth plate
• resistance to E2=failure of closure
Also delayed pubertal growth spurt in boys b/c lack of T aromatisation??
How is short stature stratified?
Proportionate
Disproportionate
Give on overview of GH effects;
• Both metabolic and growth effects:
– 1. Inhibits glucose uptake, promotes glycogenolysis
– 2. Stimulates protein synthesis
– 3. Promotes lipolysisinsulin resistance
What stimulates GH release?
• secretion increased by sleep, hypoglycaemia (through grelin), amino acids, malnutrition, exercise, stress, sex steroids
What decreases GH release?
• secretion by obesity, psychosocial depression
Learn the GH axis; describe it here;
GH regulated by SST (-ve) and GHRH (+ive)
GH acts on;
- Bones = IGF production -> IGF1, FFA, blodd glucose elevation = -ve feedback
- liver = IGF1 production which acts on bones and does all the same things mentioned
Nutrients from gut also cause IGF1 production in liver
Nutrients also decrease Grhelin release from gut
Increased blood glucose decreases SST production (GH increases BG)
Ghrelin normally increases SST
Describe GH release;
• Growth hormone (GH) is secreted by somatotrophs in the anterior pituitary gland.
– pulsatile,every 3 to 4 hours and virtually undetectable GH concentrations in between.
– GH secretion greater average daily output in women
– Mainly at night (stagesIII-IV)
– increases in puberty compared to latency childhood
What generates a pulsatile GH release?
Pulses are the result of interaction between the hypothalamic peptides GHRH and somatostatin (SS).
What determines the amplitude of GH release?
– The amplitude of the GH peak is determined by GHRH stimulating the pituitary somatotrophs
• secretion of stored GH • GH gene transcription.
What determines the trough levels of GH?
– Trough levels of GH are determined by SS inhibiting GHRH release from the hypothalamus and GH release from the pituitary.
What determines the timing of the GH pulse?
– Withdrawal of SS determines the timing of a GH pulse.
How does IGF change with nutrition?
Overweight = Increased hepatic IGF1 -> Low GH
Malnourished = Decreased IGF 1 -> Chronically high GH (lost negative feedback) (inc. GH = malnourished diabetes as inc. BG)
