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18 Cards in this Set
- Front
- Back
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What is metamorphosis? |
- post-embryonic developmental event that transforms the larval form of an animal into a radically different juvenile (that only needs to frow and sexually mature to become an adult) |
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Examples of metamorphosis: |
- tadpole to a froglet - bilaterally symmetrical flatfish into an asymmetric juvenile - insect larva into a juvenily fly or butterfly |
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What are the three types of changes that occur during Xenopus metamorphosis? |
- death and resorption of tadpole-specific structures - growth of new, frog-specific structures - remodelling/reprogramming of existing cells and structures |
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Give some examples of structures that die in Xenopus tadpole? |
- resorption of tail - death of skin, notochord and muscle cells, ingestion of cell contents by macrophages - resorption of gills |
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Give an example of new structure that are grown in Xenopus during metamorphosis: |
- formation of limbs |
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Give some examples of remodelling/reprogramming of existing cells and structures: |
- Shortening of gut and folding of gut epithelium - narrowing of head, movement of eyes to top of head - activation within liver cells of genes that encode Urea cycle enzymes |
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What triggers metamorphosis? |
- If young tadpoles are exposed to thyroid hormones they will metamorphose prematurely - if hyroid glands fail to develop normally or are removed - tadpoils fail to metamorphose = Thyroid hormones trigger metamorphosis |
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What are the functional forms of Thyroid hormones and how are they made? |
- the thyroid gland produces T4 - deodination of T4 (mainly in target tissues) turns it into T3 - much more potent, active form of T4 |
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What are the two deiodinase enzymes important in metamorphosis? |
- Type ii deiodinase = T4 -> T3 (highly active) - Type III deiodinase = T4->rT3 (inactive) - D2:D3 ratio determins whether entering T4 will be active or not |
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How do thyroid hormones work? |
- bind thyroid receptors (TR) in nucleus - TRs act as TFs - TRs bind to DNA seq - thyroid response elements - TRE (found close to target genes) |
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When do TRs bind to TREs? |
- whether or not thyroid hormones are present BUT - in the absence of hormone the receptor usually recruits co-repressor proteins, which repress the target gene |
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What happens when thyroid hormones bind to a TR? |
- The receptor exchanges co-repressor proteins for co-activators. - these stimulate expression of the target gene |
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Summary of the TH signalling pathway: |
- Pituitary sends thyroid stimulating hormone to thyroid gland - Thyriod gland releases T4 into bloodstream - depending on D2 and D4, T4 is either inactivated or turned to a highly active form (T3) -T3 binds to Thyroid hormone receptor (TR), which is bound to thyroid response element - target gene is activated |
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How does coordination of different tissue transformations occur? |
- different tissues have different sensitivities to thyroid hormones - e.g. limb formation is triggered by low TH - tail resorption is triggered by high TH - TH accumulate as metamorphosis progresses ensuring that tail resorption does not begin before limbs have formed |
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The developing limbs are more responsive to T3 than the tail. Why is that? |
- the limbs are able to make and respond to T3 throughout metamorphosis - the tail has little capacity to do either - the tail becomes able to respond to T3 late in metamorphosis, because the rising levels of TH induce expression of TRs and D2 |
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TH treatment of tadpolu induces two waves of gene activation and repression. Describe,. |
- 1st wave - within a few hours of treatment, represent genes directly regulated by TH. Many of the genes encode TFs and signalling proteins - 2nd wave - after 48 hours - genes indirectly regulated by TH = genes regulated by 1st wave |
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Give some examples of genes upregulated and downregulated in tail in response to T3: |
- ~ 1500 upregulated - collagenases, proteases, caspases - ~ 600 down regulated - collagens, enzymes involved in glycolysis and Krebs |
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What is Neoteny? |
- organisms become sexually mature whilst still in their larval state - stay that way throughout life - Mexican Axolotl - usually spends entire life as aquatic organism. Treatment with TH = metamorphosis into terrestrial salamander |
