Biology of stem cells: an overview

Abstract

Stem cells are defined as precursor cells that have the capacity to self-renew and to generate multiple mature cell types. Only after collecting and culturing tissues is it possible to classify cells according to this operational concept. This difficulty in identifying stem cells in situ, without any manipulation, limits the understanding of their true nature. This review aims at presenting, to health professionals interested in this area, an overview on the biology of embryonic and adult stem cells, and their therapeutic potential.

Keywords: adult stem cells; biological characteristics; cell therapy; embryonic stem cells; human diseases.

Figure 1

Embryonic stem cell cultivation. The zygote undergoes successive mitotic divisions until a sphere of cells—the blastocyst—is formed. In the blastocyst, the trophoblast at its periphery generates the embryonic membranes and placenta, whereas the inner cell mass develops into the fetus. Embryonic stem cells are immortal in culture, having been established from one pluripotent cell collected from the inner cell mass. These are capable of differentiating into any of the mature cell types present in the adult organism.

Figure 2

Production of induced pluripotent stem (iPS) cells. iPS cells are produced by treating mature cells, such as fibroblasts, with genes that ‘dedifferentiate' them to a pluripotent stage, similar to an embryonic stem cell. Viral vectors, such as retroviruses, are generally used for gene transfer. The transformed cells become morphologically and biochemically similar to pluripotent stem cells, with the advantage of representing autologous cells in therapeutic applications.

Figure 3

Adult or somatic stem cells (ASCs). ASCs are present in all types of organs and tissues in the organism, exemplified here by neuronal stem cells in the subventricular zone of the brain, epithelial stem cells, and hematopoietic stem cells in the bone marrow. They are responsible for replenishing cells that die, either in physiological (wear and tear) or pathological processes.

Figure 4

The niche and asymmetric cell division of stem cells. The fate of stem cells is determined by their interaction with their microenvironment or niche. The niche is composed of other (stromal) cells, extracellular matrix (ECM), and signaling factors, which, in combination with intrinsic characteristics of stem cells, define their properties and potential. Adult stem cells divide asymmetrically to produce two kinds of daughter cells. Whereas one remains in the niche as a self-renewing stem cell, the other one becomes a precursor or progenitor cell, exits the niche, and enters a pathway of proliferation and differentiation, which leads to the formation of a mature cell type.