Komórki macierzyste/en

Definiton

Stem cells are primary, unspecialized cells, which are characterized with unlimited self-renewal capacity and differentiation into specialized descendant progenitor cells forming tissues and organs. Stem cells exist both in embryos and adult organism. The proper function of the organism depends on tissue homeostasis, and the maintenance of homeostasis is related with stem cell pool, that balances the number of somatic cells in the body. In every organ of the organism systematically appears new cells, which maturate and differentiate into organ-specific cells, and after fulfilling their specific biological function they undergo into programmed cell death called apoptosis. Stem cells, residing in the cellular niche of a given organ, receive signals from the microenvironment of damaged tissue and initiate the repair process leading to tissue regeneration.

Stem Cell Types

1. Totipotent stem cells – – they are the most primordial cells – the only type of cells capable to differentiate into all types of embryonic somatic cells and form entire human body. They arise as a result of fertilization forming zygote and then in further developmental process they create the blastomers. They are capable to give rise placenta connecting the embryo with the body of the mother.
2. Pluripotent stem cells – these cells are descendants of totipotent stem cells and can give rise to cells of the three germ layers: endoderm, mesoderm, and ectoderm. The cells are capable to organize and form any tissue (except placenta) in the process of embryogenesis. Along with fetal development and formation of individual tissue structures, the cells lose their pluripotent character in favor of tissue-specific character. The pluripotent cells can be found only during embryonic development and they are unable to go back into totipotent cells.
3. Multipotent stem cells– the cells are characterized with ability to differentiate and form tissue from one out of three germ layers
   • The first germ layer (ectoderm) – nerve tissue, epithelium, skin appendages,
   • The second germ layer (endoderm) – digestive system, respiratory system, endocrine system, urinary tract, sensory organs,
   • The third germ layer (mesoderm) – bones, cartilage, skeletal muscle, smooth muscles, heart muscle, tendons, ligaments, adipose tissue. The example of multipotent cells of third germ layer are Mesenchymal Stem/Stromal Cells (MSC).Multipotent cells reside in cellular niches and their number usually decreases with age and with the body's ability to renew itself.
4. Unipotent stem cells– specialized cells capable to division (unlike mature cells), dedicated to differentiate into the cells of given definite tissue. The example of unipotent stem cells satellite cells capable to differentiate into myoblasts and then myocytes forming myofibres of skeletal muscle. Another example are osteoblasts capable to differentiate into osteocytes and form bone tissue. The tissue resident unipotent cells respond to damage signals from local environment by their activation and starting the process of tissue regeneration.

Classification of stem cell types by source of origin

1. Embryonic Stem Cells (ESC) – derived from embryo cells (totipotent stem cells) or inner cell mass (pluripotent stem cells), they are able to differentiate into all type of cells of entire body. However, the use of embryonic stem cells for therapeutic purpose arise arise ethical concerns, therefore their application should not be considered in regenerative medicine.
2. Fetal Stem Cells – can be acquired from cord blood, cord tissue and postpartum placenta - their natural reservoir. In standard conditions the the perinatal tissues constitute are discarded according to medical procedures.
3. Somatic Stem Cells– mature cells residing in the adult body and are responsible for tissues regeneration. These are multipotent stem cells (eg. hematopoietic cells) as well as unipotent cells (eg. satellite cells of skeletal muscle). The acquisition of somatic stem cells is well documented and many of them have found application in cellular therapies eg. hematopoietic cell transplantation in the hematological disorders.