In my general biology class we are now reading chapter 5 of Mader et al, on Cell Division. This chapter is a bridge between those chapters on descriptive cell biology and those describing the activities of the cell and how we explain the inheritance of traits from one generation to another.
We focused our attention on Mitosis and Meiosis of diploid Eukaryotic cells and followed how these two types of nuclear division manage the sorting of genetic material into daughter cells ensuring that each cell gets an appropriate set of instructions for life.
The body is comprised of somatic cells that include everything from skin to muscle and nerve cells. These are called somatic because soma comes from the greek word for body.
The other type of cell is the gametic cell, referring to germ-line, or sex, cells.
Each of these type of cells goes through its own type of division in order to end up with the correct amount of genetic material ( or ‘ploidy’) in the resulting cell or organism. Ploidy refers to the number of complete sets of genetic material a cell has. We, humans, are diploid organisms having two sets of genetic material in each cell.
Somatic cells undergo mitosis in a way that maintains the diploid state of cells creating two exact replicas of the parent cell.
This mechanism makes sense, because one skin cell might replace a neighboring skin cell following its demise in order to maintain a confluent layer. We would expect every skin cell to be genetically identical to every other and mitosis delivers just that.
However, if we imagine sex cells that were made from mitosis, these would also be diploid (2n). Then a diploid sperm would fuse with a diploid egg and make a tetraploid offspring. Then that organism would have octaploid offspring and so on. This, of course, does not happen.
Sex cells are instead produced by a different kind of division called meiosis. Meiosis is merely a specialized form of mitosis in which the genetic material (ploidy) is ‘halved’. The resulting cells are then haploid (1n or n). As part of the specialization, meiosis occurs in two steps so instead of producing two cells, it produces four (at least theoretically). Also, instead of being identical, each of the resulting sex cells is unique.
But this discussion is supposed to be about cancer, so let’s ignore meiosis for now. I’ve discussed cancer before here, but I just found a couple of good animations that I wanted to include.
The first is an excellent animation on cell cycle and contact inhibition. See how cancer is defined here as the lack of respect for cell-cell signaling, that would otherwise result in a healthy monolayer of cells.
The second, discusses how cancer cells would need to alter their environment in order to get the nutrients they need to survive. It can be tempting to think that cancer cells don’t need these things, but they certainly do.
Once cells mutate in a way that initiates cancer, the constant struggle with the immune system amounts to a selective pressure allowing only the strongest cells to survive. 😦 A brief article describing this battle can be found on the HHMI website. A more thorough treatment of the subject can be found in a freely available review by my friend Dr. Ezekiel Fuentes-Panana.
Here’s one last animation showing a tumor mass producing metastatic cells that leave the mass and migrate to new locations within the body. I’m not that fond of this video, but it does communicate the message I wanted to get across.
Alltogether, cancer cells are those that no longer obey the rules of polite cellular society and continue to reproduce through unchecked mitosis when such division is not in the best interest of the organism as a whole. One way these cells do this is to cease responding to contact inhibition signals. This results in the production of a tumor mass that will need to obtain energy and will often do so by sending out pro-angiogenic signals resulting in new blood vessel formation. As the tumor continues to grow, it may invade neighboring tissue and ultimately even metastasize into the blood or lymph leading to a number of secondary tumors throughout the body.