In the live thesis building blogxperiment I edit (digest, compile, write, rewrite, delete) my ongoing doctoral thesis in blog posts and put the parts together on thesis live. The title: The physiologic role of stem cells in tissues with different regenerative potential.
1.1 Stem cells and regenerative medicine: basic concepts
/turnover: cellular turnover/
The concept of biological turnover (rate) can be interpreted on many levels: molecules, molecular pathways (signaling), organelles, cells, tissues, organs. The turnover rate by which a biological entity is replaced can be quantified by measuring its half-life. /In abstract form “the half-life of a quantity whose value decreases with time is the interval required for the quantity to decay to half of its initial value” (Wikipedia) I have to check whether it is problematic to explicitly use a Wikipedia entry – I am sure it is used implicitly – in a PhD thesis/ The concept of half-life refers to the time required for an initial quantity of entity E to decay half of its initial value. According to Caplan [reference]: “Every cell in the body has a specific half-life; every cell comes to maturation and will, predictably, drop dead in due course.” For instance erythrocytes have half-lives of 60-90 days and the turnover rate of hepatocytes is 1-2 times/year. On Figure 1 from Caplan the lineage development of a differentiated cell and its replacement cell is delineated. The relative position of these two curves to one another defines growth, steady-state, or atrophy depending on when the first cell dies and when its replacement, the second cell, comes online. /I am not sure here how to solve the problem of legends in the case of figures coming from the literature but I figure it out, here is/
Original legend: Developmentally and genomically regulated changes in tissues occur in postneonatal organisms because every cell has a lifespan. Thus, the solid line represents the lineage progression that results in a cell exhibiting a differentiated function and, at a fixed time, the mature cell naturally expires. To maintain tissue mass and function, the replacement cell must be ascending the lineage progression (dashed line) prior to the expiration of the mature cell. If the dashed curve is shifted to the left, this represents tissue expansion or growth, while if the dashed curve is shifted in time to the right, this represents tissue atrophy.
Literature: Caplan AI (2007) Adult mesenchymal stem cells for tissue engineering versus regenerative medicine. J Cell Physiol. 213(2):341-7.