(a) For multicellularity to be successful, individual cells must cooperate

(b) Ideally, individual cells subvert their own relative reproductive success so as to maximize the reproductive success of the organism as a whole

(c) This makes sense in most multicelled situations because individual cells are genetically identical

(d) Therefore, in terms of the creation of progeny multicelled organisms, the reproduction of any one cell is genetically equivalent to the reproduction of any other

(a) On the other hand, if individual cells of a multicelled organism are not genetically identical, then the reproductive success of one cell does not automatically correspond to the reproductive success of any other cell

(b) Such situations destabilize cooperative behavior between cells

(c) Extreme example is a lack of cooperation displayed by an unrelated pathogen

(a) Multicelled organisms have evolved a variety of mechanisms which cause genetically identical cells to act cooperatively

(b) They have also evolved mechanisms which serve to prevent non-genetically identical cells from acting not cooperatively

(c) Finally, they have evolved mechanisms which serve to prevent non-genetically identical cells from arising at all

(d) These mechanisms include

(i) Individual cell division restraint

(ii) Immune system control

(iii) Mechanisms which combat mutation

(iv) Starting babies with one or few cells

(v) Requirement for functionality in baby-generating tissue

(a) Body cells which have managed to escape their own self-imposed restraints on cell division often come to not resemble other body cells, at least to the "eyes" of the immune system

(b) Often these cells are reproducing at either developmentally inappropriate times, or in inappropriate positions in the body

(c) Most such cells are recognized by the immune system and destroyed

(a) Lack of genetic identity results from mutation within individual cells

(b) Oncogenes (cancer-causing genes) are examples of what can result from the occurrence of mutation

(c) Cells possess elaborate mechanisms which usually serve to significantly minimize the occurrence of genetic change

(d) (the absence of such mechanisms themselves are products of cancer-contributing, mutated genes)

(e) We will consider these mechanism in Chapter 15

(a) A typically overlooked mechanism by which the genetic identity of a multicelled organism is maintained is via the initiation of the organism starting with only a single cell

(b) This assures that all of the cells associated with that organism are identical from the start

(c) The alternative mechanism, starting new organisms from many cells, allows any evolution that had occurred in the parent organism (i.e., over-replication of renegade cells) to continue in the offspring

(d) Thus, starting babies from only a single cell actually serves as a cancer-fighting strategy, and in fact, in part helps explain (to some degree) why older people are typically more prone to cancer than are younger people

(e) Note that this mechanism is equivalent, both in terms of execution and utility, to pure culture technique as employed in microbiology

(a) Finally, mutational corruption is minimized by the requirement for some minimal biological functionality in both baby generating tissue (e.g., the gonads in animals) and in the tissue of the progeny offspring

(b) Typically, defects in either eggs, sperm, or genitalia result in offspring (or germ cell) non-viability

(c) Certainly partially defective genitalia (say one gonad versus the other in humans) may have a reduced potential to contribute germ cells (i.e., eggs or sperm) relative to healthy tissue, and thus have a reduced potential to compete with the non-mutationally aberrant cells of the parent organism

(d) This is especially a concern for plants which, unlike many animals, fail to sequester the cells they use to produce the next generation

(e) However, for a mutationally aberrant plant cell to give rise to a progeny plant, that cell must be sufficiently intact to, for example, give rise to a properly functioning flower

(f) Thus, plant mutational aberration, though it may be more tolerated than mutational aberration among animal cells, nevertheless must meet some minimal level of functionality to compete with the parent plant over contributing to the next generation

(a) The emergence and destructiveness of cancer occurs within a backdrop of such evolutionary games as those just discussed

(b) The idea is that a cell which divides more rapidly can out-compete less rapidly dividing cells

(c) More rapid division can stem from mutational deviation from the norm

(d) A more rapidly dividing cell, among solid tissue, will, if left alone by the immune system, produce a tumor

(e) A tumor is an relatively undifferentiated mass of cells that is better at growing than it is at contributing to the health of the parent organism

(g) Then they can mechanically disrupt normal body function (in addition to stealing body nutrients)

(a) One thing that can contribute to tumor growth is a mutationally acquired tendency toward invasiveness

(b) Such tumors tend to invade surrounding tissue

(c) This makes such tumors more disruptive than non-invasive tumors

(a) A tumor which is not invasive is described as benign

(b) Typically are sheathed in connective tissue

(a) A tumor which is invasive is described as malignant

(b) Another name for malignant tumors is cancerous

(a) Cancerous tumors are not too big a deal if they stay in one place

(b) Their removal will require a removal of surrounding tissue since cancerous tumors will be expected to have invaded surrounding tissue

(c) But so long as the entire tumor has remained a single hunk of tissue, treatment can be relatively easy (the exception being when the tumor has invaded tissue which is sufficiently vital to body functioning that it cannot be easily removed)

(d) The bigger problem with cancerous tumors occurs when some cancer cells mutationally lose their ability to remain attached to the primary tumor mass

(e) These cells can migrate into the circulatory system

(f) Other mutations can allow such cells to reemerge from the circulatory system at other locals and found secondary tumors

(g) This process is called metastasis

(a) As tumors grow they tend to mutate (dividing cells mutate more readily than non-dividing cells)

(i) Tumor size tends to be directly proportion to cell number

(ii) Cell number is directly proportional to the number of cell divisions which have occurred

(iii) The number of mutations which have occurred is directly proportional to the number of cell divisions which have occurred

(iv) Invasiveness, metastasis, and other nasty properties of cancer cells (such as anti-cancer drug resistance) occur with mutational change

(b) Thus, the larger a tumor is, the more likely it has evolved to invade other tissue, move to other parts of the body, and house cells which are already (i.e., before treatment even commences) resistant to anti-cancer drugs

(c) This is why it is typically considered to be "good" when tumors are "caught early"

(d) Note, however, that it takes a long time for cell division to occur and mutational change to accumulate; this is why cancers tend to start years or even decades before they are detected

(e) Note additionally that the rate of cancer growth is proportional to cancer cell number, i.e., bigger tumors grow faster than smaller tumors (this is a consequence of the properties of exponential growth)

(f) Thus, very often a person may live with a cancer for years or even decades, yet die only a few months following diagnosis; very often diagnosis isn’t made until tumors are large, and large tumors grow larger very fast and are more likely to have metastasized