**2. Regenerative potential of stem cells is compromised with age**

### **2.1 Effect of donor age on regenerative potential of stem cells**

Autologous stem cell-based therapies seem promising for several diseases. As humans get older, stem cell function deteriorate like other cells of the body. Diseases, especially degenerative diseases generally affect elderly people and therefore using autologous cells for such patients may have practical concerns [12]. With aging, superoxide dismutase activity (SOD) declines [13]. Studies indicate that differentiation potential of stem cells is negatively related with age of donor [12, 14–16], and therefore cell potential to form osteoblasts [12, 15–17], cartilage [12, 14–16] and other cell types is compromised. Another important aspect for cell-based therapies, that is, proliferation is also adversely effected with increasing donor age. Choudhery et al. [15] indicated that the number of population doublings decreased while the time of population doublings increased for cells obtained from aged donors as compared to cells from the young donors. Similarly, number of the colony forming units, size of colonies and plating efficiency of aged cells decreases in vitro [13–17]. Overall, growth kinetics and differentiation potential of the cells are inversely proportion to the donor age [15–18].

#### **2.2 Effect of in vitro passaging**

Cell-based therapies require large number of cells to get the favorable results in patients. For this purpose cells are expanded in vitro before use in most of clinical applications [12]. Leonard Hayflick in 1960 described that after a limited number of cell divisions the cells stop dividing. Cell morphology changes and they become enlarged and irregular in shape. The cells undergo a replicative senescence and this limited life span of cells is called as Hayflick's limit [19]. In this way replicative senescence limits the therapeutic potential of stem cells. The differentiation

**139**

**Table 1.**

*Hypoxic Preconditioning as a Strategy to Maintain the Regenerative Potential of Mesenchymal…*

The structural and functional microenvironment in tissues where stem cells reside is known as stem cell niche described for the first time by Schofield [24]. A cell niche maintains the identity and functional characteristics of resident cells [25]. Important identified stem cell niches are in bone marrow [26], vascular vessels [27], liver [28] adult kidneys [29] intestine [30] endometrium [31], oral tissue [32], skin [33] and adipose tissue [34]. **Table 1** shows variable oxygen concentrations in some

Lungs (tracheal, bronchial, bronchiolar and alveolar epithelial cells) 13–14% [35] Subcutaneous 3–8% [36] Adipose tissue 3–10% [37, 38] Heart 2–6% [39] Brain (superficial cortex to deep white matter) 3–5% [40] Brain (hypothalamus, hippocampus, midbrain) 0.5% [40] Liver (parenchyma) 4–7% [41] Kidney (renal cortex) 4–9.5% [42] Kidney (medulla) 2% [42] Pancreas (exocrine) 2.7–4.6% [43] Pancreas (endogenous beta cell) 5–6% [43] Stomach 6–10% [44]

Uterus 2.5% [47] Bone marrow 1–7% [48] Umbilical vein and arteries 2.4–3.8% [35] Blood 5–13% [35]

**values**

2–5% 3–6% 5–9%

0–2% 4–6% **References**

[45]

[46]

**Organ/tissues PO2** 

potential of cells decreases with increasing number of in vitro passages. For example, bone marrow derived MSC showed decreased differentiation towards adipogenesis, osteogenesis and chondrogensis at late passages as compared to initial passages [20]. In addition, the proliferative potential of MSCs decreases after long term passages [21]. Human Wharton's jelly-derived mesenchymal stem cells showed significant decrease in growth kinetics and differentiation when cultured for longer time as compared to the cells in initial passages [22]. Feline adipose tissue derived MSC showed a progressive decrease in pluripotency and proliferation over

*DOI: http://dx.doi.org/10.5772/intechopen.93217*

continuous passaging [23].

important stem cell niches.

Small intestine Lumen Mucosa Serosa

Serosa

Large intestine lumen and mucosa

*Oxygen levels in different tissue in-vivo.*

**3. Oxygen levels vary in tissues**

*Hypoxic Preconditioning as a Strategy to Maintain the Regenerative Potential of Mesenchymal… DOI: http://dx.doi.org/10.5772/intechopen.93217*

potential of cells decreases with increasing number of in vitro passages. For example, bone marrow derived MSC showed decreased differentiation towards adipogenesis, osteogenesis and chondrogensis at late passages as compared to initial passages [20]. In addition, the proliferative potential of MSCs decreases after long term passages [21]. Human Wharton's jelly-derived mesenchymal stem cells showed significant decrease in growth kinetics and differentiation when cultured for longer time as compared to the cells in initial passages [22]. Feline adipose tissue derived MSC showed a progressive decrease in pluripotency and proliferation over continuous passaging [23].
