Stem cell based alternative toxicology testing
(Embryonic) stem cells
Embryonic stem (ES) cells are the origin of all tissues in the mammalian body. They are isolated from the inner cell mass of a 3-4 day old embryo, called the "blastocyst". ES cells are pluripotent, meaning they have the potential to differentiate into all somatic cells found in the body. In their undifferentiated state, they can be cultured indefinitely and - through differentiation in vitro - constitute a potentially unlimited source of somatic cells.
In contrast, adult stem cells are further developed than embryonic stem cells. They are generally considered multipotent, but no longer pluripotent. Adult Stem Cells play vital roles in the body, in particular in the replacement of senescent or damaged cells. In some cases, e.g. for red blood cells or intestinal surfaces, millions of new cells are required every day. Different tissues, such as the highly specialised heart muscle cells or the neurons of the brain, do not possess self-renewal systems, and are therefore not capable of repairing damage caused by e.g. a heart attack or stroke.
From the zygote to ES cells
All higher eukaryotes, including mammals, begin life as a single totipotent cell, the fertilised oocyte, the zygote. Division of the zygote produces an increasing number of smaller cells, called "blastomeres". The zygote divides on its way through the oviduct until it consists of around 100 cells at day 2 to 3 after fertilisation. This stage is called "morula". Now liquid enters the inner region of the morula and forms the "blastocoel" that divides the trophoblast (which later forms the placenta) from the inner cell mass, or "epiblast". This stage of the embryo is called "blastocyst". The inner cell mass (ICM) of the blastocyst can be used to derive ES cells.
In mammals, all cell types that make up the adult are derived from the inner cell mass of the blastocyst. Cells of the ICM can be isolated and give rise to colonies in vitro. ES cell lines such as those used by the ESNATS consortium can be maintained in culture indefinitely.
Self-renewal and differentiation
Depending on culture conditions, undifferentiated, pluripotent ES cells can either be maintained in their pluripotent state in vitro and divide without differentiation (self-renewal) or they develop into specialised, differentiated cells belonging to one of the three germ layers (ectoderm, mesoderm and endoderm).
Differentiation of ES cells
In the embryoid body (EB), different cell types begin to develop without any modulation from outside. Comparable to patterning in the "gastrula stage" of an embryo, we can find cells of all three germ layers in the EB after around 5-7 days, i.e. ectoderm, mesoderm and endoderm. All tissues and cell types in the body result from these three layers.
Alternative Testing Strategies
Alternative testing approaches
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Stem cells and alternative testing text to be added
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