Stem cells

Stem cell classification and potential

Depending on their lifespan in the body, there are two main pools of immature cells that fundamentally differ in their capabilities:

1. Embryonic stem cells (ESCs). Under natural conditions, they are present only during the first two weeks of embryonic development. Their key feature is totipotency. This means absolute universality: ESCs are capable of turning into any of the approximately 260 cell types that make up the human body. It is this property that makes them the main tool for prospective regeneration of any damaged organs and tissues.
2. Postembryonic stem cells. This cell type persists in human tissues throughout life after the completion of the embryonic period. Unlike ESCs, they possess pluripotency. Their potential is narrower: they can give rise to many, but by no means all cell types. A classic example is hematopoietic stem cells of the red bone marrow, which provide continuous hematopoiesis.

The main function of all these cells is to ensure uninterrupted physiological (daily routine tissue renewal) and reparative (recovery after injuries) regeneration.

Therapeutic cloning technology

For transplanted tissues to engraft, they must be completely genetically compatible with the patient. To obtain such structures, therapeutic cloning is used (for example, the somatic cell nuclear transfer method). This makes it possible to create ESCs autologous to the recipient, which eliminates immunological rejection.

The nuclear transfer mechanism consists of several stages:

• The donor oocyte has its own nucleus removed beforehand (obtaining an enucleated oocyte).
• The patient's somatic cell (for example, a skin or intestinal epithelial cell) is placed next to it.
• An electrical pulse is used to stimulate cell fusion and oocyte activation.
• The somatic cell DNA is reprogrammed: it returns to its initial zygote state.
• Active division begins, and after a few days a blastocyst (an embryo of 100–250 cells) is formed, from which the necessary stem cells are isolated.

The most important advantage of this technique is solving the problem of cellular aging. In normal mature cells, with each division cycle, the terminal regions of chromosomes — telomeres — irreversibly shorten, which limits their lifespan. During genome reprogramming, the enzyme telomerase is activated. It extends the telomere regions to their original length, making it possible to obtain completely healthy and "rejuvenated" cells.

Differentiation and application in medicine

Cells extracted from the inner cell mass of the blastocyst are placed in a special culture. At the initial stage, it is important to prevent them from entering early differentiation. Then, the process of directed development is launched to obtain a specific type of tissue (nervous, muscular, endocrine).

The direction of differentiation can be:

• Spontaneous: occurs naturally during cultivation on nutrient media (this is how blood cells or cardiomyocytes are obtained).
• Induced: requires external intervention using chemical regulators. It has been proven that the addition of retinoic acid induces the transformation of embryonic stem cells into neural cells.

Upon successful transplantation, such cells engraft and begin to produce local chemical signals, which stimulates the replacement of the patient's destroyed structures with their own newly grown healthy tissues. The main condition for success is taking into account self-recognition genes (MHC/HLA).

How does it differ from reproductive cloning?

Both areas of science use similar starting technologies—transferring the patient's genetic material into an empty donor oocyte. However, their ultimate goals are radically different.

In therapeutic (non-reproductive) cloning, the resulting early embryo is artificially destroyed to isolate valuable cell cultures. These cultures are grown in vitro and used exclusively for the treatment (autologous transplantation) of a specific individual whose DNA was used.

In the case of reproductive cloning, the embryo is preserved. It is implanted into a surrogate mother for subsequent gestation, and the ultimate goal is the birth of a new full-fledged organism genetically identical to the nucleus donor.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Methods for creating patient-specific ESCs: parthenogenesis and cytoplasmic transfer.
• Immunological barriers: the role of self-recognition genes (MHC/HLA) in transplantation.
• Stages of early human embryogenesis: from zygote formation to blastocyst development.
• Mechanisms of reparative regeneration: how local chemical signals control tissue repair.
• The Hayflick limit and the molecular basis of cellular aging.

Other topics in the "Medical Genetics" section

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