Stem cells

Stem cells are undifferentiated cells found in embryonic and adult tissues and have the ability of self-renewal and differentiation in one or several different cell types and subsequently they replenish dying cells and regenerate damaged tissues.

During fertilization fusion of haploid gametes, egg and sperm, forms a single diploid cell called zygote. The zygote undergoes cell division known as mitosis and ends with two identical cells. These cells are totipotent because they have the potential to develop into a new organism. The totipotent cells repeat process of mitosis and around five days after fertilization the mass of cells begins to cavitate in the center, producing the blastocyst. The blastocyst is a sphere of about 150 cells, with an outer layer the trophoblast (which will eventually form the placenta), a fluid-filled cavity - the blastocoel, and a cluster of cells on the interior - the inner cell mass. The organism in this middle stage between zygote to fetus is called an embryo and the cells of inner cell mass are called embryonic stem cells (ESC). These cells are pluripotent and can give rise to three germ layers (endoderm, mesoderm and ectoderm) an estimated 220 different cell types in humans. The main difference between totipotent and pluripotent cells is that totipotent cells can give rise to both the placenta and the embryo.

As the embryo grows these pluripotent ESCs develop into more specialized, multipotent stem cells. Multipotent stem cells are also unspecialized cells and have the same basic features of all stem cells: self-renew for long periods of time and ability to develop into specialized cells (terminally differentiated cells)with specific purpose and functions. However, in contrast to pluripotent cells, multipotent cells are characterized by more limited proliferative and differentiation potential and usually they can only form cell types from the same lineage as the original tissue.

For example haematopoietic stem cell (HSC) can develop into a red blood cells, white blood cell or platelets (all specialized cells). During this process, developing HSCs migrate through several anatomical sites (the yolk sac, the placenta and the fetal liver), which provide signals for their self-renewal and establishment of all blood lineages. After this complex cell migration, proliferation and commitment process HSCs colonize the bone marrow at birth. There many more examples of such multipotent cells in different human organs (neural, mesenchymal) and they known as adult stem cells (ASCs). During homeostasis, most ASCs are quiescent and divide only rarely to maintain an appropriate quantity of differentiated cells. This is possible because various anatomical sites (e.g. bone marrow) contain specialized microenvironments – niches, which sustain ASC self-renewal, multipotency or promote lineage differentiation. Bipotent stem cells are capable of differentiating into two cell types. Lymphoid progenitor cells can develop into T cells and B cells. Unipotent stem cells are capable of differentiating into only one mature, terminally differentiated cell type. Spermatagonial stem cells are unipotent and only capable of developing into sperm cells.

In response to specific signals, stem cells are thought to produce all differentiated cells via the generation of intermediate precursors that arise from stem cells through asymmetric division. Additionally, this process is shaped by an environmental asymmetry, in which one daughter cell leaves the niche that sustains ASC self-renewal and is then exposed to the new signals that promote lineage differentiation. These partially differentiated cells, termed progenitor cells, begin to express differentiation markers while migrating away from the niche and can be distinguished from stem cells both functionally and by the expression of these specific markers. The total number of stem cells present in the any given tissue represents a dynamic balance between self-renewal, differentiation and cell death. This dynamic balance is modulated by environmental signals to meet the demands of the tissue to which the stem cells may supply newly differentiated cells.

The recent ability to identify stem cells by defined set of markers and after their isolation to examine their properties in vitro has allowed a detailed examination of the role of molecules that regulate stem cell self-renewal and differentiation. Both cell extrinsic and intrinsic factors that regulate growth and survival and factors which determine the fate choice between self-renewal and differentiation have been described.

The pioneering work by Dr. James Thomson in 1998 enabled the isolation and culture of non-human primate and human embryonic stem cells (hESC) and has helped to establish and define in vitro culture conditions that maintain human pluripotency ex vivo for extended periods of time under defined cell culture conditions. hESCs are derived from embryos, created for in vitro fertilization and destined to be discarded. Despite the fact that hESC differentiate in various cell types of the body and might have a great therapeutic potential, because they derivation involves the destruction of embryos that raised ethical and religious concerns and many countries, including Lithuania, issued a ban on hESC research.

In 2006 Prof. Shinya Yamanaka reprogrammed adult skin cells into stem like state. His team demonstrated that pluripotency can be induced in skin fibroblast by expression of four transcription factors: OCT4, SOX2, KLF4, and C-MYC, which transformed skin fibroblast into induced pluripotent stem cells (iPSCs) and that earned Yamanaka a Nobel Prize in 2012.

This technology potentially allows for any human cell type to be generated at a scale impossible to obtain from primary sources. Previously inaccessible human cell types (e.g., neurons) can now be generated for investigating basic biological and pathological processes. Differentiated cells derived from patients’ iPSC are being used to generate disease-specific models that can then be applied in drug discovery assays, drug development applications, toxicology screening and biomarker discovery.

In recent years has been agreat deal of scientific and public interest in potential use of adult stem cells for medical application. As the number of clinical trials and the variety of adult cells used in regenerative therapy increases, processing and banking of these cells and regulatory issues remains a primary concern. Clinically applicable cryopreservation and banking of tissues and adult stem cells offers unique opportunities to advance the potential uses and widespread implementation of these cells in clinical applications.

There are several ethically acceptable and accessible sources of autologous adult stem cells that have been used successfully in humans: