Here I collect all things related to my live, thesis building trial including the thesis draft:

Posts in reverse chronological order:

Thesis live: 1.2 Liver, regeneration and stem/progenitor cells

Thesis live 1.1 The stem cell niche

Thesis live: 1.2 Kidney and stem cells

Thesis live: 1.1 Turnover or Every cell has a lifespan

Thesis live: 1.2 tissue/stem cell introduction scheme

Thesis live: 1.1 Basic concepts: Let’s roll!

Thesis live: Introduction, “contents” draft

Warming up to write my thesis on the blog

Choosing a proper title for the thesis: The physiologic role of stem cells…

The “live”, thesis building blogxperiment: progress through little steps

Editing my doctoral thesis on stem cells in a blog: Why not?

1. Introduction:

1.1 Stem cells and regenerative medicine

Looking for the exact definition of stem cell is sometimes the source of endless semantical debates, but at least we do know two generally accepted criteria: stem cells are able to renew themselves and could differentiate into other type of cells. First, they are unspecialized, mitotic cells that renew themselves for any (i.e. long) periods through series of cell divisions, which result in similar unspecialized stem cells. This is the so called and overstated “immortality” characteristics.
The other side of the stem cell coin is that under certain physiologic or experimental conditions, they can be induced to become differentiated cells with special functions such as the contractile cells of the striatal muscle or the insulin-producing cells of the pancreas. So stem cells are those cells, which give rise to an identical, undifferentiated, mitotic stem cell and a more specialised cell with another phenotype through an asymmetric cell division. The resulting progenitor cells mature into functional, specialised cells of the organism. What kind of cells they could be, is partly the function of the developmental potential of the cells and the local environment, where these cells anchor.

/Other basic concepts to be explained here: turnover, embryonic, placental, adult stem cells, progenitor cells, niche, components of niche, uni-, multi-, pluripotency, regeneration, tissue engineering/

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.

The concept of the stem cell niche was first proposed theoretically by Schofield exactly 30 years ago in the context of hematopoietic stem cells: “a hypothesis is proposed in which the stem cell is seen in association with other cells which determine its behaviour. It becomes essentially a fixed tissue cell. Its maturation is prevented and, as a result, its continued proliferation as a stem cell is assured. Its progeny, unless they can occupy a similar stem cell ‘niche’, are first generation colony-forming cells, which proliferate and mature to acquire a high probability of differentiation, i.e., they have an age-structure.”

Niches are restricted and specialized tissue microenvironments that integrate local and systemic signals for the regulation and maintenance for resident stem cells. The elements of the stem cell niche include the constraints of the architectural space, cellular components like stromal supporting and descendent/progenitor cells and acellular elements, like soluble and membrane bound molecules, paracrine and endocrine signals from local or distant sources and neural input [Figure by Jonas].

Niches are dynamic entities, could be redistributed and ideally “a candidate niche should be transiently depleted of its full complement of stem cells and then shown to take up and maintain a newly introduced stem cell” [Morrison, Spradling, 2008, thanks for Ouroboros for picking this review]
Jones DL, Wagers AJ. (2007) No place like home: anatomy and function of the stem cell niche. Nat Rev Mol Cell Biol. 2008 Jan;9(1):11-21. Review.
Morrison SJ, Spradling AC. (200 Stem cells and niches: mechanisms that promote stem cell maintenance throughout life. Cell;132(4):598-611.

Regenerative medicine is the science and technology built around stem cells’ regenerative capacity. This is a whole new concept compared to the traditional medicine: the aim is to facilitate and amplify or replace the native regenerative potential of the organism, the targeted tissue or organ based on the results of developmental biology and biotechnology. Classical medicine focuses on the patomechanism of the illness, the elimination of cell death, and tissue protection, while regenerative research does not care about the causes of the injury, and its aim is not to eliminate the harmful effects of the injury, but to replace, and renew the damaged function.

1.2. Tissues, organs with different turnover and regenerative potential

In order to discuss the different adult tissues in a unified manner, from a systemic point of view, I use the following tissue/stem cell introduction scheme where data are available: development of particular tissue, number of cell types, bioenergetics (high/intermediate/low energy demand), turnover (high/intermediate/low), regenerative potential
(high/intermediate/low), resident stem cells, niche, markers, cell sources from other tissues that can contribute to the particular tissue during normal turnover or chronic/acute injury.

Kidney

In the adult kidney the nephrons (approximately 500 000 nephronic units/kidney) develop from the metanephric mesoderm/mesenchyme while the collecting ducts are coming from the ureteric bud. The kidney is a complex structure with at least 26 different cell types. The renal function is particularly age-dependent (loss of functional renal mass up to 25%). The kidney is an active tissue with high energy demand and contains a lot of mitochondria (especially in the proximal tubules). On the other hand, while the turnover rate is low, there is a robust although limited regenerative response to acute kidney injury. The candidate cellular sources of recovery, replacement: adjacent, less damaged tubular cells, resident adult kidney stem/progenitor cells and circulating mesenchymal cells from the bone marrow. Amongst others the following cell surface markers were used for isolating/enriching stem cell/multipotent renal progenitor populations: CD133, stem cell antigen-1 (Sca-1), CD24, CD90, Pax-2, Oct-4, Rex-1 (see table).

One such population was isolated from the cortical interstitium making up 0.8% of all cortical cells and were capable to differentiate into epithelial and endothelial like cells in vitro forming tubular structures in SCID mice [Bussolati et al, 2005]. In the lack of definitive markers of kidney stem cells not much certain could be said on the kidney stem cell niche.

Literature: Gupta S, Rosenberg ME. (200 Do Stem Cells Exist in the Adult Kidney? Am J Nephrol. 19;28(4):607-613

Percy CJ, Power D, Gobe GC. Renal ageing: changes in the cellular mechanism of energy metabolism and oxidant handling. Nephrology (Carlton). 2008 Apr;13(2):147-52.

Bussolati B, Bruno S, Grange C, Buttiglieri S, Deregibus MC, Cantino D, Camussi G. (2005) Isolation of renal progenitor cells from adult human kidney. Am J Pathol. 2005 Feb;166(2):545-55.

/bioenergetics data missing/

Liver

During organogenesis the hepatic endoderm epithelium invades the surrounding mesenchyme to form the liver bud and continued epithelial/mesenchymal interactions stimulate cell proliferation and morphogenesis. Consequently, the liver is largely of endodermal origin - including cells with a mesodermal origin and - and contains many different cell types: two epithelial liver cell types, the hepatocytes and bile duct cells, stellate cells (formerly called Ito cells), Kupffer cells, vascular endothelium, fibroblasts, and leukocytes (Desmet 1994). Hepatocytes are the main funtional liver cells accounting for ~70% of the cells in the liver and form the bulk of the liver weight (90%), yet only ~60% of total liver DNA is hepatocyte-derived (many of them with 2n, 4n, 8n DNA content). An adult human liver contains about 80 x10(9), hepatocytes. Hepatocytes are in a quiescent state and the turnover rate is low, 1-2 times/year[]. The remarkable regenerative potential of liver is well-known, in humans the liver almost completely regenerates in about 1 month after two-thirds (up to 75%) partial hepatectomy and this process can occur repeatedly in contrast to most other parenchymal organs, such as kidney or pancreas. In the literature the term liver or hepatic stem cells is used for precursors of the hepatocytes and the bile duct epithelial cells. On the other hand liver stem/progenitor cells can be define in different ways. The cells which give rise to regeneration after partial hepatectomy and in other liver injuries are differentiated hepatocytes with a virtually unlimited differentiation potential. This type of endogenous liver regeneration is progenitor independent. Transplanted hepatocytes also have the ability to repopulate damaged, injured recipient livers [Rhim et al 1994.] but the hepatocytes do not significantly repopulate normal adult liver following transplantation. Although unique liver stem cell markers are not currently available, “oval cells” are the best candidates for non-hepatocyte, progenitor-dependent liver regeneration. These cells have a high nuclear/cytoplasmic ratio, termed oval cells due to the initial morphology and their parent cells probably reside in the canal of Hering. Oval cells express markers of both bile duct epithelium (CK-7, CK19, OV-6) and hepatocytes (albumin, alpha-fetoprotein) and are bipotential; have the ability to differentiate into both of the major liver cell types (Sirica et al. 1990; Sirica 1995). Oval cells also express also express hematopoietic stem cell (HSC) genes (c-kit, CD34, Sca-1 and Thy-1) however the idea that hematopoietic stem cells in the bone marrow are the ancestors of oval cells seems improbable and several recent studies in mice and rats have shown that transdifferentiation of HCS into “oval cells” is a very rare event probably without physiological significance [Thorgeirsson S, Grisham J 2006]. Oval cells are facultative liver stem cells that is in case of specific chronic injuries, caused by chemicals such as DL-ethionine, galactosamine, and azo dyes they regenerate the liver in murine models.

Literature:

Thorgeirsson S,Grisham J. (2006) Hematopoietic cells as liver epithelial stem cells: a critical review of all the evidence, Hepatology 43 2–11.
http://www.cincinnatichildrens.org/research/div/dev-biology/fac-labs/zorn-lab/liver-dev.htm
Grompe M, Finegold MJ (2001) Liver Stem Cells Stem Cell Biology 455-497 Cold Spring Harbor Laboratory Press
Oertel M, Shafritz DA. Stem cells, cell transplantation and liver repopulation Biochim Biophys Acta. 2008 Feb;1782(2):61-74.
Rhim JA, Sandgren EP, Degen JL, Palmiter RD, Brinster RL. (1994) Replacement of diseased mouse liver by hepatic cell transplantation. Science. 263(5150):1149-52.

/ Gut epithelium,
Blood - hematopoietic system
Epidermis,
Mammary epithelium,
Vascular endothelium,
Adrenal cortex,
Pancreas,
Lung parenchyma,
Brain - central nervous system
Heart,
Skeletal muscle,
Retina,
Spinal Cord/

1.3 Repair/Regenerative mechanisms

/differentiation
fusion
paracrine factors
immunomodulatory effects
anti-inflammatory effects
mitochondrial transfer/

1.4 Functional improvements through stem cells