|| by Donald
Arlington High School
Teacher Instructions to top
This built-in program for self-destruction is also found in other types of cells in adult tissues. Most animal cells are capable of apoptosis using cell-to-cell signalling to rid unwanted cells from the tissues. Reasons for apoptosis include:
1) the ridding of cells no longer functional in development or evolution
(such as the
tail of a tadpole as it becomes a frog or the larval cells of an insect as the larva
develops into an adult during the pupal stage - all of the larval cells of an insect
undergo apoptosis except for the imaginal discs which contain the information for
making the adult);
2) the elimination of cells needed by only one sex (such as the Mullerian duct in
3) the decrease in the number of germ cells able to be passed into the next generation
(such as the 6-7 million possible follicles present at birth decreasing to the 400
follicles usually released in human females);
4) the elimination of cells that migrated to abnormal locations or lacked proper
linkages with extra-cellular substances (such as lymphocytes that were not
activated by specific antigens);
5) cells that were originally produced in excess (such as neurons in vertebrate
There are approximately 100 billion neurons in the average human brain. To appreciate the enormous scale here, consider that there are also an estimated 100 billion trees in the Amazon rain forest. It has been calculated that about 50% of the neurons present at birth die after synapses form in vertebrates. When a target neuron receives a signal from its source across the synapse, it often sends an acknowledgement back across the synapse in the form of a nerve growth factor (NGF). These chemicals fix the course of axon development, facilitating future synaptic transmission, by preventing apoptosis. Neurons not receiving NGF are weeded out to make more efficient neural pathways. Thus the nervous system is continuously selecting for the most competitive cells via Darwinian selection - a sort of "survival of the fittest." This includes cells with the most receptors and cells with the most efficient signal transduction systems. It turns out to be more effective to eliminate less competitive cells than to prevent the proliferation of new cells. Apoptosis in these neurons is analogous to a sculptor removing marble in the creation of a work of art. The synaptic connections between the neurons are more important than the sheer number of the neurons. Specific life experiences will both enable and reinforce the cellular connections in the synaptic pathways of the brain, increasing the plasticity and dynamic nature of the brain. This process helps to explain how memories may develop, change and be stored. It may prevent the accumulation of "memory clutter", explain "photographic memory", and explain those memories that are claimed to be "on the tip of my tongue."
A similar situation occurs in the immune system where various cytokines are used to signal the presence of a foreign antigen amongst the various lymphocytes. Macrophages and Helper T-cells use interleukins in their inter-cellular communication. Normally lymphocytes last for several weeks before their apoptosis mechanisms are activated. When lymphocytes contact a foreign antigen, growth hormones are released by the target cells which turn off the apoptosis death program in the lymphocytes, extending the life of the cells from weeks to years. These lymphocytes act as "memory" cells that are able to recall the prior contact with the molecules of pathogens.
There are other functions for apoptosis in the development and continued maintenance of multicellular organisms. The lens of the embryonic eye consists of apoptotic cells that replace their cytoplasmic contents with the clear protein, crystallin, as the eye develops. Keratinocytes undergo apoptosis to form the temporary protective coat on the outer layer of skin. Sunburn and peeling of skin occur when ultra-violet light causes keratinocytes to undergo premature apoptosis.
How apoptosis works on the cellular level is not well understood , but it is known to take about three hours to complete. The vast majority of the cells are in the Go , nonproliferating, phase of the cell cycle. A specific set of transcription factors is activated from the highly conserved Hox gene family in cells undergoing apoptosis. A plethora of molecular messengers, cytokines and growth factors, are released to enable cell-to-cell signalling to activate the process. Apoptosis begins with the shrinking of the nucleus and cytoplasm of the cell. Then the chromatin and cytoplasm are partitioned off into apoptotic cell fragments at the cell surface, "blebbing" off in a process of exocytosis that resembles boiling. A good view of this is illustrated on the cover of the December 1996 issue of Scientific American. The subcellular fragments are then ingested by neighboring cells or macrophages. This all happens without any leakage of cell contents into the extra-cellular space or bloodstream. This is important because accidentally leaked cytoplasm would trigger an inflammatory response in the organism. Apoptosis is relatively inconspicuous to the immune system, whereas necrosis results in inflammation in the surrounding environment.
Cancers appear when more cells proliferate than die in a given tissue. Many human neoplasms have been found to have a mutation in at least one apoptosis gene. Some tumor suppressor genes control cell proliferation, while others are involved in the induction of apoptosis. A mutation in these genes turn off the apoptosis program, and they appear to "forget" to die. A mutated gene controlling apoptosis could be considered a "selfish gene" because it causes the cell that houses it to take advantage of a survival situation rather than "going gently into that dark night" by suicide for the betterment of the whole organism. The oncogene, c-myc, has been shown to induce apoptosis. The production of the tumor suppressor protein p53 has been shown to lead to prompt activation of programmed cell death. Mammals have the bcl-2 gene that codes for membrane protein that prevents or delays apoptosis in neurons and lymphocytes. The memory lymphocytes in the immune system appear to activate the bcl-2 genes. Other diseases associated with apoptosis include AIDS, progressive brain degeneration such as Huntington's disease and Alzheimer's disease, and autoimmune diseases like rheumatoid arthritis where lymphocytes undergo too little apoptosis.
The Student Activity
(Please note that this is a teacher guideline. No "Activity Sheet" need be given to the students.)
This activity starts with a classroom of students sitting at their desks. They will be asked to model the plasticity of neural pathways for memory in the brain. It requires some advanced preparation. Each student is given a specific card representing a neuron in the brain, some cards are blank and some with colored spots (colored paper clips can be used instead of colored dots). The cards with colored spots should form a linear pattern across the classroom student desks, while some may make a bend or curve.
For example: (Please change this arrangement to suit your classroom needs.)
1) have cards with blue
dots located at student desks in the third row going from the
front of the room to the back;
2) have cards with green
dots located at student desks along a side row from front to
back continuing along the back row to the card with the blue dot at the back of the third
3) have cards with red
dots at student desks located diagonally from front left to back right
the classroom. (See sample seating chart for this example below.)
The spotted cards represent those cells that recognize
and reinforce each other with cytokines or growth factors when stimulated.
That cell-to-cell communication inhibits apoptosis. Those cards lacking
the specific colored dots repesent cells not in the desired pathway, thus
they lack the protective cytokines. Their apoptotic apparatus is
activated, and they will die a rather quiet death taking their genome out
of the evolutionary continuum.
Students in the classroom simulate the cell activity according to the dots on their cards. Students holding cards with dots will recognize each other with a showing of the dots to neighbors with the same dots - they should stand up, smile at each other, give a wink and touch hands (or give a "low-five", as opposed to a "high-five"). Those students without the dots should gently rest their heads on their desks in a simulated apoptotic death.
|YELLOW CARD||blue green||green||green red
1. Try the blue dots sequence first by "stimulating" the student in the front third row to "flash" his/her card. Individual students may randomly stand up and smile while showing off their card, but only the student with the same colored dot seated next to the originally stimulated student will receive a return smile and "touch" from the originator of the signal. Students lacking the proper dot must sit down. Within a short time, all of the students with blue dots should be standing and smiling at each other. Seated students (without the blue dots) undergo apoptosis and rest their heads on their desks - however, as students, they should still alert to the happenings around them. After the process is completed, only those students with the blue dots should be smiling at each other.
2. Next, "enliven" the whole class again, and stimulate one of
the red dots - actually, any red dot will do. Again,
have students randomly show each other their cards. If a student
next to the originally stimulated card/neuron has a red dot, s/he
should smile, stand and touch; if not, the student should sit down and
"die" in good apoptopic fashion. At the end of this simulation, only
those students with the red dots should be standing and smiling
at each other.
3. Now do the same for the green dots. After this, try a combination of both blue and green dots at the same time, linked together.
4. For the next scenerio you should alternate between activating the red dots and the green dots. Students should have recognised their colored dot partners in the pathway from the earlier situations, and memory enables them to quickly stand, smile and touch (representing the cytokines in the cell-to-cell communication). First the "red-dotters" stand, then they sit; then the "green-dotters" stand, then sit; then the "red-dotters" again; and back and forth for a few times. This alternation simulates the changing neural pathways in the brain.
5. For the final simulation, have two specially colored cards (yellow index cards vs white?) located at opposite corners of the classroom. No dots are involved in this example, only the propinquity of the neighbors to the activating card/neuron is required. The object here is to set up the shortest pathway possible between the two colored cards. One of the colored cards initiates a message to the other colored card. A wave of standing/smiling activity should sweep across the room between the two colored cards. The whole class should be standing and smiling. The target colored card then sends a message back to the initiator. The shortest distance between the colored cards is then determined by the class of standing cardholders (neurons). All cardholders that are more than the shortest distance sit down to an apoptotic cell death. This should result in a straight-line diagonal across the classroom.
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