Mammalian spore-like cells--a reservoir of spare parts for old-age?
Random exogenous environmental effects--such as radiation and virus--break the biopolymers of the cell (e.g., proteins and DNA), deteriorate its biochemical processes, and eventually cause wear-out and death. Worn-out cells are replaced through the process of cell division. However, a limit to the number of times a cell divides has been noted in all fully differentiated human cell types, as well as in other organisms. While stem cells are an exception and may continue to regenerate cells for the entire lifespan of the organism, they are susceptible too to radiation, infection, and other forms of environmental damage. Spore-like cells are small dormant simple cell-like structures which have the ability to differentiate into mature cells of the tissue from which they are isolated or into the cell types of another tissue. They seem to be present in every tissue in the body. Spore-like cells tolerate conditions that kill differentiated or partially differentiated cells, such as complete oxygen deprivation, and exposure to temperatures which are either much higher or much lower than normal body temperature. When awaken, they can proliferate more rapidly and into more types of differentiated cells than do terminally differentiated cells or stem cells. Reliability theory is a general system theory about systems failure that predicts very well the age-related mortality rates in many species. Reliability theory predicts the late-life mortality deceleration as a consequence of the exhaustion of the redundancy at extreme old-age. The main hypothesis is that the purpose of the spore-like cells is to provide redundancy (i.e., spare parts) for the differentiated and partially differentiated cells in a tissue. Since their use may be required dozens of years after their creation, they need to stay in a dormant and robust state that protects them from deterioration due to detrimental environmental effects. This hypothesis can be validated by measuring the age-related concentration of spore-like cells in various types of tissues and organs. It is predicted that the concentration of spore-like cells in various organs is highly correlated with the health of those organs, especially at old-age. It is expected that tissues with an initially large concentration of spore-like cells may live longer than tissues with an initially small concentration of spore-like cells.