11 July 2008

Types of stem cells and their properties

Aging is a complex biological phenomenon, it creeps up imperceptibly, and its results become especially noticeable after 65-70 years. And this is not only a change in appearance, but also specific violations of organizational structures and their functioning. This even gave rise to the creation of rather primitive mechanistic hypotheses that consider aging as a simple deterioration of cells and tissues. These include one of the first general biological theories proposed by N. Rubner (1908). On the other hand, the wear of organs and the disruption of their functioning has given rise to the problem of prosthetics, which is being solved with varying success in dentistry, orthopedics, traumatology, cardiac surgery. Despite the obvious progress in these areas, the problem of the foreignness of the prosthesis remains. Another way of "natural prosthetics" is offered by transplantology, which, on average, has achieved gigantic success in transplanting various organs over half a century: heart, liver, kidneys. But the problem of foreignness also remains, and it can lead to rejection of the transplanted organ. The isolation of stem cells, the development of methods for their cultivation and application has opened up prospects for "autoprosthetics", when the restored organ structure will have a complete biological identity. Based on these laboratory studies, a new direction in medicine has emerged – regenerative medicine based on the methods of cellular technologies.

What biological features of stem cells are attractive for clinical medicine, gerontology and developmental biology, and where do these cells come from?

Stem cells are defined as undifferentiated cells capable of self-renewal and differentiation into specialized cells. Stem cells are not homogeneous, there are several types of them that differ in their ability to differentiate into other more specialized cells.

The unlimited ability to differentiate into all types of cells, tissues and organs is called totipotence. It is obvious that it is the first diploid cell formed from the fusion of two germ cells - the zygote – that is the root of totipotence. During the first divisions of the zygote (at the morula stage) totipotence persists. Subsequent divisions lead to the development of a blastocyst (4-7 days of embryo development before its implantation into the uterine wall), when there are no tissues and organ laying. From the internal pluripotent cell mass of the blastocyst, cellular layers are formed, called germ sheets, each of which (endoderm, ectoderm and mesoderm) form a certain set of organs and tissues. The final link in this hierarchy of possibilities are unipotent specialized cells of the body (Fig. 1).

Cells capable of transforming into various types of biological tissues in the body are called stem cells. There are embryonic and postnatal stem cells. The first - embryonic stem cells - are isolated from embryos from the blastocyst stage.  Postnatal stem cells are present in the organs and tissues of the adult body. Among them, hematopoietic stem cells serve to form new blood cells and are contained in the bone marrow. Stromal stem cells serve to restore all other tissues, a small number of them are found in each tissue, and the main amount is still in the same bone marrow (V.N. Smirnov, 2005). Cells that are precursors of eggs and spermatozoa are called germinogenic stem cells (E.I. Zueva, A.V. Kurtova, L.S. Komarova, 2005). 

Theoretically, stem cells can be obtained from bone marrow – either from one's own or an immunologically compatible donor, from embryos or from umbilical cord blood. The source of stem cells for humans can also be stromal cells of adipose tissue.

Stem cells have some common characteristics that distinguish them from other cells:

- self-maintenance of both its properties and quantity;

- differentiation leading to irreversible development of specialized cells;

- polypotency, which allows the development of differently specialized cells;

- the ability to recycle in the body, or homing.

These features of stem cells make it possible to restore all functioning tissue elements. It is this ability to produce a wide variety of cell types that makes stem cells the most important regenerative reserve in the body, which is used to replace defects that occur for one reason or another.

Biologists were particularly surprised by the presence of stem cells in the central nervous system, whose cells lose their ability to reproduce very early in the course of ontogenesis. It has been shown that stem cells in response to various lesions of the nervous tissue begin to divide, followed by differentiation into nerve and glial cells.

The discovery of stem cells has changed our understanding of the organization of tissues and the mechanisms of regenerative processes in them. The existence of pluripotent stem cells in the adult organism has been proven, the existence of which was postulated only for embryos. These cells are the most important component of reparative processes in the body. The intelligent use of the potential of stem cells opens up great opportunities for reparative medicine.

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