- On this slide up here, we have a few examples where programmed cell
death is used in a developmental context. On the top here, we can see how
programmed cell death can be used to sculpt fingers and toes during
development. Here, our bodies take the strategy of building the full
structure initially, so right in (A) we can see tissue between the digits of the
hand or foot. It builds the full structure and then uses apoptosis to
specifically kill off the cells in between the digits.
- Another example of this is seen during different stages of the life cycle
here in the frog. There may be a part of the body that is necessary at one
stage but then the organism may dispose of it at a later stage, the removal of
the tadpole tail for example.
- Even in adults as well the regulation of organ size, apoptosis plays a key
role in that. If organs become too large, the body can recognize that and use
apoptosis to trim down their size.
- In addition in adults, programmed cell death plays a key role in protecting
the body against dangerous cells.
- Here is one danger to a cell, right here is excessive production of the
protein Myc. As we learned about last day, Myc will drive forward the cell
cycle promoting S phase and the synthesis of DNA, promoting cell
division. If you have too much Myc, this is oncogenic, it is called an
oncogene, there is more potential for this cell to become cancerous and
- Programmed cell death can help control this. It goes through this pathway:
The cell can detect that there is too much Myc activity going on and then
this feeds into the pathway involving p53 so this detection of excessive
Myc production produces a protein called Arf. This then binds to the
protein Mdm2 and removes it from p53. So you remember that normally,
Mdm2 is actively degrading p53 but now that there is this signal that there
is a danger in the cell, now the p53 is no longer degraded so the p53 will
now act to arrest the cell cycle as we learned about last day and if things
aren’t rectified by arresting the cell cycle, the p53 will then induce
apoptosis. If the cell can’t get control of its cell division, then signals will
be sent out to kill off the cell since it is a danger to the body.
- You can think of this as well in response to DNA damage. We learned last
day that instead of excessive Myc production, that another danger to the
cell would be DNA damage so the cell does not want to go a round of DNA
synthesis, replicating that damaged DNA, replicating any mutations. The
same thing applies here, in that case as well, we learned last day how p53
will arrest the cell cycle to try and repair the damage but if the damage
can’t be repaired, then that cell will just be killed off by p53. So this is an
example of how programmed cell death can kill off dangerous cells.
- Apoptosis is a very regulated and stereotyped process and we contrast this
with necrosis. The cell death is not regulated by the cells themselves but
something outside has damaged them and killed them off accidentally.
- The bursting of the cell is shown in the slide, there is the outline of the
cell and its contents bursting out into the surrounding area. This material
that is normally kept inside the cell is now in the extracellular space. This
can lead to damaging inflammatory reaction, immune cells will come to
deal with this and there will be a lot of swelling in that part of the body.