Barbieri Thesis - BioMedical Materials program (BMM)

Chapter 1 – Introduction
phenotype and de–differentiate into blastemal cells. Such de–differentiated cells then
proliferate and later differentiate again into phenotypes capable to synthesize the
needed tissues eventually regenerating the whole amputated body parts.
Salamanders can regenerate, besides legs and tail, also injured retina and
intestine. [52, 59] Being unable of blastema formation (i.e. the source of cells for
regeneration), larger animals including human beings lost much of their regenerative
potential. [80] In mammals one of the most exciting example of natural tissue
regeneration is given by the liver, which fully regenerates itself when resected. This
process is initiated by the tissue removal, when all cells comprising the left intact
organ are triggered to proliferate until the reconstruction of the missing parts. [81]
Table 1. Overview of stem cells in various body tissues.
Tissue / organ Description
Bone marrow contains two kinds of somatic stem cells, namely
hematopoietic stem cells (HSCs)
Bone marrow
[60] and marrow stromal cells (MSCs, called
also mesenchymal stem cells). [61, 62] HSCs are capable to differentiate into all
blood lineages and can regenerate bone marrow after loss. [60, 63, 64] MSCs
can differentiate into various cellular phenotypes according to the tissue they
migrate, e.g. chondrocytes if they move to cartilage tissue, osteoblasts if in
bone, myoblasts if in muscle. Besides these, they can generate also
adipocytes and endothelial cells. MSCs move to injured tissues and
participate to their repair or regeneration, but they appear having no role in
tissue homeostasis. [61, 62, 65–67] It has recently been proposed to use MSCs
even for lung diseases treatment. [68]
Liver contains stem and progenitor cells that generate oval cells, which then
Liver
differentiate into hepatocytes and biliary cells. [69, 70]
Neural stem cells (NSCs) allow neurogenesis by giving rise to neurons,
astrocytes and oligodendrocytes.
Brain
[43, 71, 72] However, it is still not clear whether
newly formed neurons in adult brain are integrated into its neural circuits and
thus the purpose of neurogenesis is under debate. [73]
The epidermis has a high renewal rate (~3–4 weeks) because of its
exposure to the extra–body environment and thus it has to cope with various
stresses, such as exposure to sunlight and substances or friction. Three skin
Skin
sites for stem cells have been identified in the epidermis, particularly in
interfollicular area, hair follicle bulge and sebaceous glands. The stem cells
lying in the hair follicle bulge participate to the regeneration of surface
epidermal cells after skin injury. [74, 75]
The small intestine hosts stem cells able to regenerate its crypts within few
Intestinal epithelium
Skeletal and
cardiac muscle
days. [76]
Skeletal muscle myocytes never proliferate. Regeneration of injured skeletal
muscle is assured by satellite cells localized beneath the myocyte basal
lamina, which can differentiate into myocytes when triggered by injury. [77]
The presence of stem cells in myocardium is still nowadays debated and it is
proposed that heart may have progenitor cells having capacity to regenerate
myocardium after small injuries, and that this ability decreases with aging. [78]
Cornea The transparency of cornea is maintained by limbal stem cells (LSCs). [79]
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