Progenitor cells have pluripotent properties and form the reservoir in various tissues from which somatic tissue cells are formed by proliferation and differentiation. They are generated by asymmetric division of pluripotent stem cells, one of which develops as a progenitor cell and the other of which completes the reservoir of stem cells again. Progenitor cells play a critical role in the formation of new tissue.
What is a progenitor cell?
The term progenitor cells is used to describe precursor cells of certain types of tissue. They arise by asymmetric division from adult multipotent stem cells. In each case, one daughter cell of the divided stem cell develops into a progenitor cell, while the other daughter cell remains in the multipotent stem cell stage and completes the supply of stem cells again. Adult stem cells have been detected in over 20 tissue types to date. After the division of a stem cell, the progenitor cell loses its multipotency in several steps – stimulated by growth factors – and differentiates in each case into the somatic tissue cell of the tissue for which it is intended. This means that the original multipotency first changes into pluripotency, which is necessary for the development of different somatic cells within a tissue, before the cell completely differentiates into a somatic tissue cell with complete loss of its multipotency, pluripotency and ability to divide. Circumstantial evidence suggests that the increasing determinacy of progenitor cells to a specific tissue is still reversible up to a certain stage of development. The differentiation of the cells is controlled by tissue-specific growth factors. The field of research dealing with progenitor cells is subject to dynamic development, so that no universally accepted nomenclature has yet evolved. Some researchers therefore still use the terms progenitor cell and stem cell as synonyms. Because progenitor cells differ from tissue to tissue in terms of their developmental potency, they are also sometimes referred to as determinate stem cells.
Anatomy and structure
The distinctive feature of progenitor cells is their still partial ability to mature into different cells within a tissue. Therefore, they differ from tissue to tissue. For example, a distinction is made between hematopoietic and endothelial progenitor cells. Hematopoietic progenitor cells, which are mainly found in the bone marrow, can develop into white or red blood cells in the course of further differentiation steps. Endothelial progenitor cells circulate mainly in the blood and also originate from the bone marrow. They are used for the repair of blood vessels and for the creation of new vessels (angiogenesis). The endothelial progenitor cells already carry proteins characteristic of vascular endothelia on their surface. In total, progenitor cells have been detected in more than 20 different tissue types, including the brain and the peripheral nervous system. Progenitor cells specialized in certain tissue types are usually referred to as blasts, e.g. osteoblasts, myeloblasts, neuroblasts and many others. They are characterized by the fact that they are usually not yet definitively committed to a particular type of cell. Typical morphological features of blasts include an enlarged nucleus, a high proportion of endoplasmic reticulum, high energy metabolism based on a high number of mitochondria, and many other features.
Function and tasks
As a rule, differentiated somatic cells of a given tissue lose not only their ability to divide but also their ability to regress into progenitor cells. They are also called unipotent because, if they are still capable of division, they can only give rise to cells of the same type with the same properties when they divide. The loss of the ability to divide varies from tissue type to tissue type and is done for safety reasons, since otherwise only minor disturbances could result in the constant formation of new tissue, which can almost inevitably lead to problems.The main task of progenitor cells is therefore to replace tissue cells after injury or after tissue loss due to disease, or to provide the necessary supply of specialized tissue cells during growth processes. The mobilization of progenitor cells occurs as required and is controlled by various cytokines and interleukins. Depending on the tissue type, progenitor cells function as a patrol in the bloodstream or they represent the silent reserve for new tissue cell formation that can be mobilized for repair and growth purposes. For example, endothelial progenitor cells play a special role in overcoming sepsis. Sepsis is usually triggered by bacterial toxins, which leads to increased necrosis and apoptosis (programmed cell death) of endothelial cells in the vessels. It has already been demonstrated that in such cases, increased levels of certain cytokines result in increased release of endothelial progenitor cells from the bone marrow, leading to an increase in the repair mechanism for restoration of damaged inner vessel walls.
Diseases
Progenitor cells, as potential tissue cells, perform the task of stepping in during disease or injury to best repair the tissue damage sustained. The multistep activation and differentiation stages of progenitor cells mean that they themselves can also lead to disease symptoms through acquired or genetic congenital defects. A well-known disease of progenitor cells, which provide the replenishment of white or red blood cells or platelets, is acute leukemia. Malignant progenitor cells spread in the bone marrow and displace the functional progenitor cells. This primarily causes anemia and a lack of platelets. The malignant cells can spread to almost all tissues, including the skin and mucous membranes. In the oral mucosa, they are even palpable as small nodules. Some other types of cancer are also based on altered stem and progenitor cells. In most cases, these are mutated stem cells that give rise to correspondingly altered progenitor cells that have defects in certain protein complexes and therefore divide unchecked, unimpressed by deactivating cytokines.
