2.7Cell Formation—Cell Lineage
Unicellular organisms such as yeast proliferate by producing identical cells through cell division as a survival strategy. However, if multicellular organisms such as humans doubled identical cells by cell division, all of the resulting cells would be identical, and the diversity of cells seen in organisms would not be created. Therefore, multicellular organisms have a mechanism called "differentiation" that works in tandem with cell division to produce diverse cells. One method for studying this phenomenon is to trace the fate of cells one at a time from zygote formation to the formation of the entire body. However, it is impossible to trace the development of 60 trillion cells in humans.
Cell Lineage of Nematodes
Nematodes (C. elegans) are the only organisms thus far that have been successfully used to trace the history of cells from the zygote to the completed body (Fig. 2-9). In 2002, Sydney Brenner received the Nobel Prize in Physiology or Medicine for his achievements in his research using these nematodes. Nematodes are soil organisms with a total length of about 1 mm. One of the advantages of using these organisms is that their eggs and bodies are transparent, which enables observation of their cells when they are still alive. In addition, nematodes have nervous and digestive systems despite having a simple body structure.
They are actually made of only 959 cells, excluding reproductive cells. Fig. 2-10 shows a record (cell lineage) of thorough, cell-by-cell observations over the course of development from one fertilized egg to a variety of cells. The number of cells actually created is 1,090; however, 131 cells die in the process of body formation. About 30 percent of these 959 cells are nerve cells. These results clearly underline the importance of transmission of information. In nematodes, almost all properties of cells are determined by 7 cell divisions, or by a maximum of 14 cell divisions.
This genealogical analysis showed that the diverse cells of nematodes are created according to a predetermined genetic program. In other words, the manner in which the body of a nematode will be formed has been written in its genes. This fact is also true of humans. However, in humans there are many acquired elements that prevent identical twins from being exactly the same. Details about these elements will be discussed in Chapter 4.
This study of cell lineage led to a surprising discovery: Specific cells are genetically programmed to die during the development of the organism
The cell death programmed to occur during development is called "programmed cell death." Until this discovery was made, it had been believed that cell death occurred because the cell was spent and its role had been accomplished. However, it was discovered that cell death is induced even in active cells. Analysis of the mutant nematode in which induction of cell death had stopped in certain cells led to the discovery of the proteins involved in induction and control of cell death. These analogous proteins are present in many organisms, including humans. They act in similar manners in relation to induction and control of cell death. These results show that there is a mechanism of cell death that is common to all organisms (see "apoptosis" in the Column, Section 2 of Chapter 7).
In humans, diseases occur when apoptosis does not occur or occurs excessively. Diseases caused by absence of normal cell death by apoptosis include cancer and autoimmune diseases, and diseases caused by excessive cell death by apoptosis include AIDS. These results show that cell death is important for the world of cells, and that the homeostasis of individual organisms is preserved by the balance between the birth and death of cells.