Developmental biology 4 (Prof. Dale) Flashcards
How many cell types are there is then human body ?
Can you cite a few examples ?
~250
Goblet cells & enterocytes, fibroblast, red & white blood cells, pancreas: exocrine & ductal cells, skeletal muscle, neuron, keratinocyte, pancreas: ß-cells (can be stained for insulin)
From which which embryonic tissue is skeletal muscle made ?
How are myoblasts maintained in a mitotic state ?
Which signals triggers myocyte differentiation ?
- sk muscle –> made by myotome region of each somite –> induced by signals from adjacent tissues (Wnt signals from the neural tube)
- induced muscle precursors = myoblasts –> maintained in a mitotic state by FGF signals
- muscle specific transcription factors e.g. MyoD1 (transcription of which is induced by Wnt3a signalling) –> inhibit cell proliferation, promote cell fusion + activate transcription of genes required for differentiation of sk muscle
What is sk muscle made of ?
Large multinucleate cells called myofibres.
How is the contractile apparatus of the muscle arranged ?
What happens during muscle contraction ?
In sarcomeres, with characteristic cross-striations.
The dark band (A) is made of myosin while the light band (I) is made of actin.
Myosin and actin slide past each other during muscle contraction.
Can you cite a few muscle-specific proteins ?
Alpha-Actin, Myosin II, Tropomyosin, Titin, Nebulin, Creatine, Phosphokinase, AChR.
How many protein coding genes does the human genome contain ?
What proportion of these genes are transcribed by all cells ? To what purpose ?
~20,000
Only 6-7% are transcribed by all cells –> housekeeping genes required to maintain basic cellular functions (e.g. metabolism, RNA & protein synthesis, cell cycle control)
What are tissue selective genes ?
Tissue selective genes = genes transcribed by only a few cell-types (sometimes only one):
- globins (⍺ and ß) in erythrocytes
- insulinin ß-islet cells
- ⍺-actins in muscle cells
- crystallins in lens cells
- immunoglobulins in B-lymphocytes
What proportion of tissue selective genes are transcribed in the brain/muscle/liver/kidney/lungs/prostate ?
- ~9% in the brain
- ~5% in muscle
- ~4% in liver
- ~1% in kidney
- ~1% in lung
- ~1% in prostate
Which proteins control tissue specific transcription ?
Where are these proteins situated ?
How do these proteins interact general transcription factors ? To what purpose ?
- Tissue specific transcription –> controlled by distal control elements = enhancers –> which are bound by transcription factors (activators)
- Enhancers –> may be as much as 1 million base pairs away from the promoter + are brought to the promoter by DNA bending proteins
- Tissue specific transcription factors –> now interact w/ general transcription factors + mediator proteins ==> regulation of RNA synthesis by RNAp II
How is gene transcription regulated ?
- through epigenetic mechanisms that affect chromatin structure –> modifications of DNA + associated core histones
- methylation of cytosine in gene promoters = gene silencing
- methylation of core histones = gene activation + silencing
- acetylation of core histones = gene activation
What histone modifications can be seen in transcribed Vs non-transcribed genes ?
- transcribed genes –> H3K4m3 & H3K9ac that create “loose”
chromatin –> accessible to transcription factors - non-transcribed genes –> highly methylated DNA (5mC) + alternative histone modifications (e.g. H3K9m3) –> compact chromatin –> inaccessible to transcription factors
In D. Melanogaster, where are cells that will form the adult cuticle found ? In which form ?
In which form are abdominal segments present ?
When do these differentiate ?
In the larva as undifferentiated discs (w/ discs for the mouth parts, eye antenna, wing, haltere, legs and genitalia)
Abdominal segments –> present as nests of cells = histoblasts –> differentiate during pupal development (blue pigment stain distal elmts, the aristae of the antennal disc + wing blade of the wing disc)
Ernst Hadorn (1902-76) removed imaginal discs from larvae and cut them in two. One half he transplanted to a 3rd instar larva to see what it will form, the other to an adult female. What did he observe after 2 weeks ? What did he then do (and repeat again and again...) ?
- after 2 weeks he removed the imaginal disc and found that it had grown in size
- he cut it in again, transplanting half to a 3rd instar larva + half into a 2nd adult female (repeating this procedure more than 300 times over 12 years)
In most cases –> disc continued to differentiate the same tissues
Occasionally, during Hadorn’s transplant experiments, the transplanted disc formed a different part of the cuticle e.g. a wing instead of a leg.
What were these rare events called ?
Transdeterminations.
Sir John Gurdon FRS destroyed the egg nucleus (UV irradiation) of a X laevis egg and injected a nucleus isolated from a differentiated cell inside it.
What did he observe ?
- small proportion of embryos = form tadpoles + even adults
- most embryos = partial, but nuclei isolated from these and transferred to enucleated eggs –> can form tadpoles and adults
- cells from partial embryos –> can also be grafted to a host embryo + will differentiate into multiple cell types
- up to 30% of intestinal nuclei can form functional non intestinal cell types, demonstrating that the genome of differentiated cells can be reprogrammed
