to the ability that some organisms possess to cope with the deleterious effects of drying and survive in a state of almost complete
dehydration [1]. This phenomenon occurs in all major taxa.
Although widely spread in spores, pollen, and seeds, anhydrobiosis
of whole organisms is relatively rare [2, 3]. Examples of whole
organisms that can survive in an anhydrobiotic state for prolonged
periods include water bears or tardigrades and resurrection plants
like Craterostigma plantagineum (see Fig. 1b). Such organisms can
be dehydrated to water contents less than 10% on a dry weight basis
and resume vital metabolism after rehydration [4, 5]. During storage in the anhydrobiotic state, most chemical reactions and
biological processes are suspended. In dry matrices, however, free
radicals are still able to move and chemically alter biomolecules,
causing damage and defining the life-span (i.e., storage stability).
One thing that many anhydrobiotic organisms have in common is that they synthesize large amounts of non-reducing disaccharides, particularly sucrose and trehalose [2]. Whereas sucrose is
predominantly found in resurrection plants, pollen, and seeds [5–
7], trehalose is found in tardigrades, brine shrimp, and nematodes
[8]. These sugars form a highly viscous glassy state when the water
content is reduced (see Fig. 1a, c). Molecular movement is slowed
down in the glassy state, and chemical reactions almost come to a
standstill. Moreover, it has been postulated that sugars protect
anhydrobiotic cells by replacing water normally surrounding
biomolecules [2].
Sugar synthesis is typically one of the earliest visible biochemical adaptations seen during acquisition of desiccation tolerance.
However, a general reduction in cell metabolism and synthesis of
specific proteins, scavengers and antioxidants, are equally important. In seeds as well as in other organisms such as tardigrades and
Polypedilum vanderplanki, late embryogenesis abundant (LEA)
proteins have been identified as lyoprotective agents [9–
11]. Among other protective effects, LEA proteins increase the
glass transition temperature and strengthen the hydrogen bonding
network of the cytoplasmic glassy matrix in which cellular structures are embedded and preserved [12–14]. Anhydrobiotic organs
and organisms need sufficient time to prepare themselves to survive
drying. Whereas sugar synthesis occurs, e.g., in response to hormonal stimulation, the relative humidity and tissue water content
should be gradually decreased to allow for LEA-protein synthesis
[15, 16].
The complex series of biochemical adaptations that occur during drying of anhydrobiotic cells do not take place during drying of
ordinary mammalian cells, because they lack the necessary adaptation pathways. One of the major hurdles with preservation of cells
and tissues that do not possess the natural ability to survive freezing
and/or drying is to introduce protective components into the
system.
Principles of Cryopreservation and Freeze-Drying
5
dehydration [1]. This phenomenon occurs in all major taxa.
Although widely spread in spores, pollen, and seeds, anhydrobiosis
of whole organisms is relatively rare [2, 3]. Examples of whole
organisms that can survive in an anhydrobiotic state for prolonged
periods include water bears or tardigrades and resurrection plants
like Craterostigma plantagineum (see Fig. 1b). Such organisms can
be dehydrated to water contents less than 10% on a dry weight basis
and resume vital metabolism after rehydration [4, 5]. During storage in the anhydrobiotic state, most chemical reactions and
biological processes are suspended. In dry matrices, however, free
radicals are still able to move and chemically alter biomolecules,
causing damage and defining the life-span (i.e., storage stability).
One thing that many anhydrobiotic organisms have in common is that they synthesize large amounts of non-reducing disaccharides, particularly sucrose and trehalose [2]. Whereas sucrose is
predominantly found in resurrection plants, pollen, and seeds [5–
7], trehalose is found in tardigrades, brine shrimp, and nematodes
[8]. These sugars form a highly viscous glassy state when the water
content is reduced (see Fig. 1a, c). Molecular movement is slowed
down in the glassy state, and chemical reactions almost come to a
standstill. Moreover, it has been postulated that sugars protect
anhydrobiotic cells by replacing water normally surrounding
biomolecules [2].
Sugar synthesis is typically one of the earliest visible biochemical adaptations seen during acquisition of desiccation tolerance.
However, a general reduction in cell metabolism and synthesis of
specific proteins, scavengers and antioxidants, are equally important. In seeds as well as in other organisms such as tardigrades and
Polypedilum vanderplanki, late embryogenesis abundant (LEA)
proteins have been identified as lyoprotective agents [9–
11]. Among other protective effects, LEA proteins increase the
glass transition temperature and strengthen the hydrogen bonding
network of the cytoplasmic glassy matrix in which cellular structures are embedded and preserved [12–14]. Anhydrobiotic organs
and organisms need sufficient time to prepare themselves to survive
drying. Whereas sugar synthesis occurs, e.g., in response to hormonal stimulation, the relative humidity and tissue water content
should be gradually decreased to allow for LEA-protein synthesis
[15, 16].
The complex series of biochemical adaptations that occur during drying of anhydrobiotic cells do not take place during drying of
ordinary mammalian cells, because they lack the necessary adaptation pathways. One of the major hurdles with preservation of cells
and tissues that do not possess the natural ability to survive freezing
and/or drying is to introduce protective components into the
system.
Principles of Cryopreservation and Freeze-Drying
5