Tissue and Cell Culture

More than 200 types of cells make up the human body. They are assembled into a variety of tissues, such as skin, bone, and muscle. Most tissues contain a mixture of cell types. Cells are small and complex, which makes it difficult for scientists to see their structures, to discover their molecular composition, and especially to find out how their various components function.

What can be learned about cells depends on the available tools. Culturing (or growing) cells is one of the most basic techniques used by medical researchers. The growth of human cells outside the body enables the investigation of the basic biological and physiological phenomena that govern the normal life cycle and many of the mechanisms of disease. In traditional research methods, mammalian cells are cultured using vessels in which cells settle to the bottom surface of the vessel under the influence of gravity. This gravitational influence results in a thin sheet of cells, with the depth of a single cell, called a monolayer. Cells in human tissues, however, are arranged in complex, three-dimensional structures. When cells are grown in a monolayer, they do not perform all the functions that the original tissue does. Although much valuable information can be gained from monolayer cell cultures, further understanding of the processes that govern gene expression and cellular differentiation is limited because the cells are not arranged as they are in the human body.

When the influence of gravity is decreased, the cells are able to grow in more tissue-like, three-dimensional aggregates, or clusters. On the ground, cells sediment to a surface and interact with it. In space, they do not. That is an advantage because there is a minimization of the cell interaction with inert surfaces. Another advantage is there is no surface to confine the direction in which the cells will grow. This allows for three-dimensional growth, more like actual tissues in the body.

A bioreactor is a device that is used to grow tissues in three-dimensions. The bioreactor allows cells to be cultured in a continuous freefall state, simulating microgravity and providing a unique cell culture environment on the ground. This allows for cell aggregation, differentiation, and growth. The bioreactor affords researchers exciting opportunities to create three-dimensional cell cultures that are similar to the tissues found in the human body. Using both space- and ground-based bioreactors, scientists are investigating the prospect of developing tissues that can be used in medical transplantation to replace failed organs and tissues (Figure 1-23). Bioreactors in space are not only interesting for the production of cells or tissue. They will also be the key element for the treatment of waste products (such as water and C02), the production of food, or the decontamination of the life support system from unwanted bacteria and other microorganisms (Walther 2001).

In addition, investigators are striving to produce models of human disease to be used in the development of novel drugs and vaccines for the treatment and prevention of diseases, to devise strategies to reengineer defective tissues, and to develop new hypotheses for the progression of diseases such as cancer. Growing cultures for long time periods on board the ISS will further advance this research. Finally, cells exposed to simulated and true microgravity respond by making adaptations that give new insights into

Figure 1-23. Specimens of tissue-engineered cartilage grown in space tend to become spherical in space, demonstrating that tissues can grow and differentiate into distinct structures in microgravity. The flight samples (A) were smaller, more spherical, and mechanically weaker than Earth-grown control samples (B). Adapted from Freed et al. (1997).

Figure 1-23. Specimens of tissue-engineered cartilage grown in space tend to become spherical in space, demonstrating that tissues can grow and differentiate into distinct structures in microgravity. The flight samples (A) were smaller, more spherical, and mechanically weaker than Earth-grown control samples (B). Adapted from Freed et al. (1997).

cellular processes, establish a cellular basis for the human response to microgravity and the space environment, and pave the way for cell biology research in space regarding the transition of terrestrial life to low-gravity environments (Figure 1-24).

Figure 1-24. Space biology research benefits humans back on Earth, as crewmembers on board the ISS perform long-duration research that could lead to medical advancements, new materials, and breakthroughs in technology-, including the development of countermeasures to the symptoms of the aging process. Photo courtesy of NASA.

Figure 1-24. Space biology research benefits humans back on Earth, as crewmembers on board the ISS perform long-duration research that could lead to medical advancements, new materials, and breakthroughs in technology-, including the development of countermeasures to the symptoms of the aging process. Photo courtesy of NASA.

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