Thesis live: 1.1 Turnover or Every cell has a lifespan

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/

2 models of embryonic and adult blood formation: Figure by Ueno and Weissman

If you have previously thought (in your spare time) that the conventional wisdom concerning blood formation is that the yolk sac’s embryonic blood-forming cells serve only the embryo, while the source of adult blood-forming stem cells is the region called aorta-gonad-mesonephros (AGM), it’s time to think it again due the elegant experiments of Samokhalov et al.: Cell tracing shows the contribution of the yolk sac to adult haematopoiesis Nature 446, 1056-1061 (26 April 2007)

Legend: a, The ’separate’ model.

First differentiated cell types in mammals are…

…blood cells, “generated in early-stage embryos, for instance in mice in the yolk sac at embryonic day (E) 7.0.-7.5, just 2 or 3 days after the undifferentiated conceptus implants in the uterus.” Source: Ueno-Weissman: Blood lines from embryo to adult Nature 446, 996-997 (26 April 2007)