Flashcards in Cellular Adaptations Deck (21):
What is intracrine signalling?
Intracrine signalling is where the signalling molecule is produced and binds to a receptors inside the cell and so is not secreted.
What are the 4 ways you can classify any signal a cell gets?
Either the cell is told to divide, differentiate, survive (resist apoptosis) or die.
What are growth factors and what roles do they have?
Growth factors are polypeptides that are coded by Proto-oncogenes. As well as the obvious growth factors may also have a role in: contractility of the cell, locomotion, differentiation, viability, activation and angiogenesis.
Give 4 examples of important growth factors?
• Epidermal Growth Factor (EGF) – stimulate epithelial cell, hepatocytes and fibroblasts and are released by keratinocytes, macrophages and inflammatory cells. It binds to epidermal growth factor receptor (EGFR)
• Vascular endothelial Growth factor (VEGF) – induces blood vessel development
• Platelet derived growth factor (PDGF) – stored in platelet alpha granules and are released on activation, causes migration and proliferation of macrophages, smooth muscle cells and monocytes
• Granulocytes colony-stimulating growth factor (G-CSF) – stimulates the bone marrow to produce granulocytes, especially neutrophils, used as a treatment to stimulate poorly functioning bone marrow after chemotherapy
What happens in the G0 stage of the cell cycle?
After completion the cell either enters G0 or begins the cycle again. In G0 the cells can under-go terminal differentiation.
What happens to the cell cycle in tissue growth
Growth of a tissue occurs when the cell cycle is shortened in length or quiescent cells are converted to proliferating cells.
What stages does interphase encompass?
G0 to G2 is known as interphase.
Describe the cell cycle
In G1 or Gap1 the cell replicates all its organelles and increases in size, S which is DNA synthesis and G2 or Gap 2 where the cell prepares to divide.
Where are the restriction points in the cell cycle - give an example
There are important checkpoints in interphase where the cell assesses if it should carry on with the cell cycle or enter apoptosis due to damaged DNA. The most important one is towards the end of G1 called the R (restriction) point. If triggered p53 is produced and attempts to repair the DNA and failing that stimulates apoptosis. Other checkpoints take place between G1 and S and between G2/M which check for DNA damage before and after replication.
How is cell cycle progression regulated?
Progression through the cell cycle is regulated by Cyclins and enzymes called cyclin dependant kinases (CDK). These enzymes become active by binding to and complexing with cyclin. These enzymes then phosphorylate proteins driving the cell cycle forward. Growth factors work by activating or inhibiting the production of CDK’s and cyclin.
What are adult stem cells?
Adult stem cells are cells with prolonged proliferative activity and show asymmetric replication, i.e. one of the mitotic daughter cells will be more differentiated than the original whilst the other daughter cell will remain exactly the same.
What are permanent cell populations?
Permanent cell populations – terminally differentiated cells such as in cardiac and skeletal muscle. They may possess a small amount of stem cells but not enough to mount a proliferative response to cell loss.
What are labile cell populations?
Labile (proliferative) cell populations – these tissue have the ability to proliferate because although most of the cells are terminally differentiated they contain enough stem cells. This is because they tend to be tissues that are continually being replaced i.e. epithelial tissue of the skin and gut.
What are stable cell populations?
• Stable cell populations – These are cell populations that are not constantly dividing and so are in G0 but have the capacity to be induced into entering the cell cycle. Examples of this are hepatocytes
What are the names of the 4 mechanisms by which tissue can grow?
1. Proliferate to replace losses
2. Hyperplasia (increase number above normal)
3. Hypertrophy (increase in size)
4. Atrophy (become smaller)
5. Metplasia (be replaced by different cell type)
Describe tissue growth by proliferation
This is the replacement of cells lost by identical cells. Usually the replacement cells are as good as the originals. This process of regeneration is induced upon the cell by: Growth factors, cell to cell communication and electric currents and nervous stimulation. Reconstruction is different to regeneration as it involves a whole body part and doesn’t really take place in humans at all except in rare cases in children.
Describe tissue growth by Hyperplasia
This can take place in labile or stable cell populations and involves an increase in the number of cells present in a tissue above that of normal. It remains under control of the body and is reversible unlike neoplasia. Note that this process also increases the size of the organ above that of normal.
Describe tissue growth by hypertrophy
This is an increase in size of an organ but not from increasing the number of cells but from increasing their size. The cells become bigger due to an increase in components not because of swelling. This process tends to take place in stable cell populations or populations that have little regenerative powers. Like hyperplasia this takes place in response to an increased demand on the cell, hypertrophy and hyperplasia often happen at the same time in response to the same stimulus.
Describe tissue growth by atrophy
This is a reduction in cell size and tissue size by apoptosis. The cells reduce size until they are the smallest they can be and still survive. This happens due to a lack of growth factor, nutrients or stimulation. Osteoporosis type 1 occurs by atrophy of the bone matrix.
Describe tissue growth by metaplasia
Metaplasia is the replacement of one cell by another cell type. Very rarely is it as simple as one cell changing its differentiation. More common is for a cell to die and then be replaced by a new cell of a different specialisation. Metaplasia does not take case across germ layers but only from adult stem cells in the same cellular population. One good examples of metaplasia is that of the pseudostratified cells lining the respiratory tract which can change to stratified squamous when exposed to cigarette smoke.