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STEM CELLS

BASIC CONCEPTS OF STEM CELLS

Stem cells are unspecialized cells that have the capability to constantly self-renew, with unlimited divisions and to differentiate into various types of specialized cells of the human organism, such as blood cells, heart muscle cells brain cells or bone cells.                                                            Most of the time, they stay inactive but under the right condition in the body, by physical or chemical factors they get prompted to proliferate and differenziate into cells with specialized functions.  

STEM CELLS PROPERTIES

Self Renewal

The ability to undergo an unlimited number of replication cycles, without changing its differentiation status.

Potency

The ability to generate progenies for different cell types. 

Hierarchy of Cell Potency

Totipotent Stem Cells

The zygote, that derives from the fusion of the gametes, is a single totipotent cell, which can divide into numerous identical cells. Totipotent stem cells  can give rise to any cell types found in an embryo and they originate all fetal tissues (placental cells and body cells).

Pluripotent stem cells

Pluripotent stem cells can give rise to all cell types of the body and can originate all body tissues: ectoderm (neurons, skin), mesoderm (muscles, cartilage,bones, blood, fat), endoderm (endocrine glands). Pluripotent stem cells cannot develop into extra-embryonic tissue (the placenta or yolk sac, for example) and cannot generate the whole organism. 

Multipotent Stem Cells 

They are cells that can only differentiate into a limited and specialized series of cell lines, present in a specific tissue or organ. This category includes adult stem cells, also called tissue-specific or  somatic stem cells; for example, hematopoietic stem cells that can generate only blood cells.     They remain quiescent until they perceive signals to differentiate into specific type of cells, to replenish damaged and dead cells in the body (tissue maintenance role).                                   They are found in specific microenvironment called stem cell niche.

Unipotent Stem Cells 

They can differentiate into only one cell type.  

STEM CELL CLASSIFICATION BASED ON ORIGIN  

Embrionic Stem Cells   

Embryonic stem cells are pluripotent cells, that are derived from the inner cell mass of a blastocyst and that give rise to the fetus. They are pluripotent stem cells, so they can divide into more stem cells and can become any type of cell in the body.   

Fetal Stem Cells   

Fetal stem cells can be isolated from the fetal blood and bone marrow and from other fetal tissues, including liver, pancreas, spleen and kidneys. After birth they will become unipotent adult stem cells.

Fetal Adnexa Derived Stem Cells  

Present in the umbilical cord, in the placenta and in the amniotic sac.                                          The embryonic and fetal stem cells have ethical and scientific issues that hinder their use; in contrast, stem cells recovered postnatally from the umbilical cord, including amnion/placenta are capable to forming many different cell types because they are multipotent and they possess the characteristics of both embryonic and mesenchymal stromal stem cells.

Adult Stem Cells

Adult stem cells are derived from adult tissue and are multipotent or unipotent cells.                    They are capable of maintaining cellular homeostasis of a mature organism.                       Furthermore, unlike embryonic and fetal stem cells, are free from ethical or religious controversies. 

 

 

 

 

 

MESENCHYMAL STEM CELLS

Mesenchymal stem cells (MSCs) are multipotent, non hematopoietic adult stem cells, that are present predominantly in the stroma of bone marrow, with the ability to differenziate in osteocytes but also into others types of mesodermal origin cells, like chondrocytes, adipocytes and myocytes. In colture, they are easily identified as clonal, plastic, addherent, fibroblastic cells. MSCs represent a valid cell source for tissue regeneration therapy becuse they can be obtained from a large number of tissues (including heart, bone marrow and adipose tissue among others) and they have the ability to differentiate into many cell types, moreover they own immunosoppressive, anti-inflammatory function and the capacity to home to injury site.

Bone Marrow Mesenchymal Stem Cells

The post­natal bone marrow  is an organ composed of two main sectors with distinct lineages, that have a bidirectional crosstalk, represented by the hematopoietic tissue proper and the surrounding stroma. The bone marrow derived hematopoietic stem cells  (HSCs) renew continuously, along the lifespan of an individual, the new mature cell lineages in peripheral circulation, such as erythrocytes, leukocytes and thrombocytes. The stroma of the bone marrow is composed of reticular connettive tissue, which forms a support network rich in growth and stimulating factors, providing a microenvironment that regulates the function of hematopoiesis. Cell types that constitute the bone marrow stroma include fibroblasts, adipocytes, osteoblast and endothelial stem cells that give raise to mature endothelial cells that form the new walls of vessels, in human vascular disorders. The mesenchymal stem cells (MSCs) are founded within the bone marrow stroma and can differentiate into various mesodermal lineages, including osteocytes, adipocytes, chondrocytes, myocytes.

Adipose Derived Mesenchymal Stem Cells

Adipose-derived stem cells (ASCs) are mesenchymal stem cells (MSCs), that are obtained from adipose tissue, and have the ability to differentiate into multiple cell lineages. Unlike bone marrow-derived MSCs, ASCs can be easily obtained from adipose tissue by a minimally invasive procedure. Indeed, harvesting bone marrow mesenchymal stem cells (BMSCs) has several limitations due to the invasive procedure, not without risks and poorly tollerate by the patient because of the possible pain. Another problem is the limited number of MSCs that can be obtained from the harvested bone marrow  because only a small amount (0.001–0.01%) of cells are MSCs. Therefore, ASCs are very important for regenerative medicine, just think that an abundant numbers of adipose-derived stem cells (ASCs) have been reported to be easily isolated from multiple sites of adipose tissue of the human body and one gram of adipose tissue contains about 5000 ASCs, number it represents 500 times greater quantity than those contained in one gram of bone marrow aspirate. ASCs have the ability to undergo multi-lineage differentiation, including osteogenic, chondrogenic, adipogenic, cardiomyocytic, hepatic and neurogenic differentiation and this proliferation and differentiation capacity into different mesenchymal lineages, make them a effective, less invasive alternative to BMSCs for cell-based regenerative medicine.