[考古] 7月RC动物体内结晶冬眠考古，求确认

是这个~树蛙那段确实说了血糖什么的~~

bale
你629考得怎么样？
我7月来二战了！

Freeze tolerance
The ability to withstand the long-term freezing of body fluids has developed in diverse groups of animals
including some frogs and turtles, many types of insects, and a variety of intertidal marine molluscs and
barnacles (Storey and Storey, 1989, 1996). Freeze tolerance occurs in several species of woodland frogs
that hibernate in the leaf litter of the forest floor including the wood frog (Rana sylvatica) (Figure 3), the
gray tree frog (Hyla versicolor), the spring peeper (Pseudacris crucifer) and the chorus frog (Pseudacris
triseriata). The Siberian salamander (Salamandrella keyserlingii) and two turtle species, the terrestrial box
turtle (Terrapene carolina) and the painted turtle (Chrysemys picta) also survive freezing. Freeze tolerance
by painted turtles is limited to the newly hatched juveniles that stay in their underground nests for their first
winter of life whereas the adults winter under water.
The driving force for freeze tolerance was probably an inability to mount an effective defense
against inoculative freezing by environmental ice. For example, the water-permeable skin of frogs is no
barrier to ice propagation and although frogs chilled to -2°C may stay supercooled if they are sitting on a
dry substrate, they begin to freeze in less than 30 seconds if they touch ice crystals. Since frogs need to
hibernate in the humid the leaf litter to keep from desiccating, they have virtually no chance of avoiding
freezing if ice penetrates into their microenvironment.
Freezing can cause multiple types of damage to unprotected organisms (Figure 4). Ice formation
inside of cells scrambles intracellular architecture and is lethal in virtually all instances so even freeze
tolerant animals take precautions to limit ice formation to extracellular spaces. Extracellular ice can also do
physical damage by squeezing or shearing cells, puncturing membranes or bursting microcapillaries so that
upon thawing, the integrity of cells and organs is destroyed. Ice propagating through extracellular spaces
such as the abdominal cavity, blood stream, gut lumen and bladder also causes severe dehydration of cells.
This is because the formation of ice, which is a crystal of pure water, excludes the solutes that were
dissolved in it and raises the concentration of the remaining unfrozen extracellular fluid. This highly
concentrated fluid puts an osmotic stress on cells and draws water out of them so that they shrink in
volume. If shrinkage exceeds a critical minimum cell volume, irreversible damage is done to the lipid
membranes surrounding the cell and the cells are not viable after thawing. Freezing of blood also halts the
delivery of oxygen and nutrients to organs which most organisms cannot tolerate for long.
Freeze tolerant animals have developed defenses against these possible injuries with adaptations
that fall into several categories: (1) regulation of ice propagation through body tissues, (2) damage repair to
deal with bleeding injuries caused by ice, (3) minimizing cell volume reduction during freezing, (4)
membrane and protein stabilization, (5) resistance to oxygen deprivation, and (6) reactivation of vital signs
(breathing, heart beat, nerve and muscle activity) after thawing (Storey and Storey, 1996).
To control ice formation, freeze tolerant animals use specific nucleators (Figure 4). Instead of
lowering their SCP in winter as freeze avoiding animals do, freeze tolerant animals raise their SCP by using
nucleators so that freezing occurs begins just below the FP. Some species introduce special ice nucleating
proteins into their blood whereas others use contact with environmental ice crystals or the presence of
nucleating bacteria on the skin or in the gut to stimulate ice formation. The slow freeze initiated by
nucleators allows the greatest possible time for organs to make metabolic adjustments before blood
circulation halts and permits a controlled dehydration of organs that sequesters most of the ice in extraorgan
spaces (such as the abdominal cavity). This reduces the chance of internal damage to organs such as
by ice expansion within the lumen of capillaries. Some freeze tolerant animals also appear to have AFPs in
their body fluids which seems contradictory. However, it appears that the function of AFPs in freeze
tolerant systems is to help regulate crystal growth and inhibit recrystallization, the process whereby small
crystals regroup over time into larger crystals. In addition, freeze tolerant animals enhance their damage
repair mechanisms so that bleeding injuries can be dealt with rapidly upon thawing. In wood frogs, for
example, freezing stimulates the production of blood clotting proteins.
Controlled dehydration of cells and organs can minimize ice damage but cell volume reduction
can only go so far before cell membranes collapse under compression stress. Generally, freeze tolerant
animals can endure the conversion of up to ~65% of their total body water into extracellular ice but the
remainder must remain liquid within cells. Water retention in cells is aided by the synthesis of high levels
of glycerol or related carbohydrates which provide the same protection to the intracellular milieu of freeze
tolerant animals that they do for all of the body water of freeze avoiding animals. Frogs use glucose as their
cryoprotectant with levels of this blood sugar rising by 50-100 fold or more whenever body fluids begin to
freeze (Storey and Storey, 1996) . Interestingly, frogs show no evidence of the debilitating effects of
hyperglycemia that are evident at much lower sugar levels (2-10 fold above normal) in diabetics. Other
cryoprotectants are also produced that stabilize the structure of cell membranes so that they can resist
compression stress; the sugar, trehalose, and the amino acid, proline, are widely used for this function.
They intercalate between the headgroups of membrane phospholipids to stabilize the bilayer structure that
is key to biological function and prevent the lipids from collapsing into an amorphous gel.
Freeze tolerant animals have also enhanced their ability to cope with oxygen deprivation for there
is no breathing and no blood circulation while frozen. Again, high glycogen reserves are used to produce
ATP energy via glycolysis with lactate build-up tolerated during the freeze. Freeze tolerant animals also
show enhanced antioxidant defenses that can minimize damage due to the production of oxygen free
radicals when breathing resumes after thawing. The molecular mechanisms that reactivate vital signs
during thawing are still largely unexplored. In frogs, a resumption of heart beat is the first detectable vital
sign, followed soon thereafter by breathing and later by a return of coordinated muscle movements. Studies
of the physiology and biochemistry of natural freezing survival by frogs are revealing numerous secrets that
are being applied in the development of improved cryopreservation technology for the freezing storage of
mammalian cells, tissues and organs.