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Cell Differenciation
Organisation of the Body
| Question | Answer |
|---|---|
| What is differentiation | The development of specialised cell types When cells start to divide they develop into different cell types in the correct location and at the correct time |
| Terminally differenciated cells | All contain the same set of organelles and express some common proteins (housekeeping genes/proteins) with fundamental roles Express cell type specific proteins not expressed in non-differentiated cells, leading to specialised function/structures |
| Some examples of cell type specific proteins | Keratinocyte - keratin - surface protection Erythrocyte - haemoglobin - oxygen transport Melanocyte - melanin - pigment production Pancreatic islet cell - insulin - carbohydrate metabolism Lens cell - crystallin - light transmission |
| Use of a product of specialised cells - Caspase | Caspase 9, produced by apoptotic cells is needed for modelling the nervous system. In mutant mice lacking this protein the brain grows to double the normal size as the normal process of killing off any neurons with incorrect connections does not happen |
| How do different cells take on different identities | Differentiated cell types are characterised by differential protein expression |
| Terminal differentiation and the cell cycle | Terminally differentiated cells enter G0 where they do not divide. This is typically the end point. However, some cells e.g. hepatocytes can re enter the cell cycle. These can dedifferentiate and start to grow again to regenerate the organ |
| Sequential process of differentiation | This occurs throughout development (from the 8>16 cell division). Cell potency becomes more restricted This occurs through gradual specialisation which may lead to terminal differentiation. Cell type specific gene expression begins during this process |
| Cell potency | Totipotent-can make any embryonic/non-embryonic cell type e.g. fertilised egg (nine banded armadillo splits into 4 cells from 1 cell to form 4 identical embryos) Unipotent-fully committed to terminal differentiation Pluripotent-can make most cell types |
| 2 stages of differentiation | Specification - differentiated autonomously but can be reversed if signalled by other cells Determination - differentiates autonomously in all circumstances even if moved in the body |
| Commitment prior to differentiation | Takes place in different developmental transitions and can involve proliferating cells e.g. stem cells, basal layer of skin, germ layers The cell is committed in isolation, but can be reversed by interactions with other cells |
| Formation of germ layers - Xenopus (Inductive development) | 3 different layers: Ectoderm (nervous system), Endoderm (lining of lungs and gut) and Mesoderm (all other structures). Their futures are specified, except if ectoderm moved to contact with endoderm, in which case it forms endodermal tissue. |
| What does formation of germ layers show | Cells talk to each other relative to their positions in the embryo. There is a degree of plasticity in development. It can adapt - cells can communicate and change their specialisations to fit where they are |
| How does the cell cycle link with differentiation | Terminally differentiating cells have typically exited the cell cycle, Pluripotent cells have specialised gene expression programmes but typically can still proliferate. |
| Proof that differentiation does not alter the genome | An intestinal nucleus was taken from one frog and inserted into the egg of another. Many of these eggs from a differentiated nucleus were able to form fully developed adults, showing that the genome must be unaltered, as the nucleus can be reprogrammed |
| How does differentiation affect the genome | The genome of differentiated cells is not normally altered in sequence |
| Transcription factors which maintain muscle specialisation | MyoD is a basic helix-loop-helix protein that regulates muscle-specific genes. When randomly introduced into fibroblasts MyoD is able to trigger skeletal muscle like development (cells begin to fuse and become multinucleated) |
| The myogenic bHLH family and their role | MyoD, Myf5, myogenin and MRF4 can all reprogram cultured cells. MyoD and Myf5 have redundant roles in skeletal muscle . MyoD inhibits G1 cyclin accumulation and proliferation, growth factors inhibit its binding to DNA-regulate cell cycle/differentiation |
| Studies into skeletal muscle differentiation | myoD/myf% double mutant mice have no muscle - not stimulated to differentiate myogenin mutants have no myoblast function mrf4 mutants have no sarcomere structure |
| Stability of differentiated states | Transcription factors show positive autoregulation. Factors which drive differentiation block the cell cycle. Promotors of active genes are frequently methylated and associated histones are deacetylated- heterochromatin is stably inactivated. |
| Induced pluripotent cells - reprogramming somatic cells | Differentiated cells can be reprogrammed by expressing transcription factors normally expressed in embryonic stem cells, which removes epigenetic changes as a result of differentiation. TF with fibroblasts made them differentiate similarly to ESC |
| Role of Pax5 in maintaining B cell differentiation | Genetic removal of Pax5 transcription factor in mature B cells of the immune system produces uncommitted progenitor cells that can differentiate into other immune cell types |
| Gene expression not regulated by transcription | Alternative splicing - RNA spliced differently in different cells e.g. Dscam with 38000 forms RNA editing - single base modifications in Apo-B100 results in sorter Apo-B48 Genome rearrangments |
| Example of when differentiation leads to genome changes | Genome rearrangement occurs in B cells to allow for many different antibodies to be produced. If somatic cloning was performed by a B cell a normal animal would be produced that could only form one type of antibody (genome has changed) |
| What maintains stability of differentiation | Transcription factors and chromatin modification play a key role in specifying and maintaining the differentiated state |
| Types of cell type specific differentiation | In mammals cells are often instructed by other cells to become committed and differentiated Two main types: Inductive development Regulative development |
| Mechanism of Inductive development | Involves cell-cell signalling Diffusible ligands bind to a cell surface or intracellular receptor - paracrine or juxtacrine Cell surface ligand and receptor = paracrine Gap junctions = juxtacrine as cells are next to each other |
| Role of inductive development of skeletal muscle | Somites in the embryo can become muscle, vertebrae, cells in the dermis etc Their differentiation is dependant on spatial organisation. Somites are told what to do by neighbouring cells, so take different forms depending on where they are in the body |
| Other forms of inductive development - hox genes | Hox genes are expressed in the body in the order they are located in the genome. If Drosophila leg hox genes are moved to the facial ones, legs will develop on the face. If mouse Hoxc8 is mutated, extra vertebrae will develop. |
| Hox genes and synpolydactyly | HoxD13 controls where digits develop on the hand. (controls patterning) Mutations in this can lead to the wrong number of digits developing |
| Regulative development in early embryos - sea urchins | When a 4 cell embryo was split, all 4 cells developed into sea urchins. This shows regulative development - the cells told each other where they were and so all developed into full sea urchins |
| Example of non interaction development - mosaic development | When a tunicate 8 cell embryo was split into 4 2 cell embryos (unevenly distributed coloured cytoplasm), the cells continued to develop as expected. These cells already knew what they would become, so did not communicate |
| Mosai development in animals - stem cells | Asymmetric division is controlled by evolutionarily controlled proteins (PAR proteins, Numb etc) These regulate stem cell division in all higher animals and a asymmetrically localised in mammals. Stem cells can divide without cell-cell communication |
| How do cells restrict their fates and progress to differentiated state | Regulative and mosaic development can control differentiation Major role for cell-cell interaction in inductive differentiation |
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