Cold-induced ultrastructural changes in bull and boar sperm plasma membranes

Cold shock damage is due to lipid phase transitions in cell membranes: a demonstration using sperm as a model

When cells are cooled to temperatures above the freezing point of water at rates greater than a few degrees per minute, they sustain irreversible injury. Reduction of this "cold shock" damage could increase the survival of animals and plants at low environmental temperatures and improve the cryopreservation of plant and animal cells. Leakage of solutes across membranes, associated with thermotropic phase transitions in membrane lipids, is thought to be responsible, but this hypothesis has not been tested directly. Using Fourier transform infrared spectroscopy (FTIR), we measured the lipid phase transitions in intact, living sperm, the animal cell in which cold shock has been studied most extensively. A shift in the CH2 absorbance peaks indicates the transition from liquid-crystalline to gel phase. The phase transition in sperm membranes occurred at a lower temperature for a marine shrimp than for the pig. In each case, potassium leakage, which is a hallmark of cold shock damage, increased abruptly near the end of the phase transition. Human sperm are quite resistant to cold shock, and an abrupt lipid phase transition was not detected. This phase behavior is typical of membranes containing a high proportion of cholesterol, and human sperm have an unusually high sterol content. High cholesterol levels are known to stabilize membranes during cooling. Overall, the lipid phase behavior was consistent with the temperature range over which cooling was damaging for pig and shrimp sperm, and the with the extent of damage produced in pig and human sperm. This is the first direct evidence that cold shock results from lipid phase transitions in cell membranes.

Cryopreservation of boar semen. I: A literature review

The present review summarizes information concerning the methods available to cryopreserve boar semen, covering the historical background, cryobiology and cryoprotecting considerations, technological developments and recent advances in cryopreservation methodologies. Successful methods for cryopreservation of boar semen have not been achieved despite numerous efforts world wide. Improvements in semen preservation technologies have been deterred by lack of in vitro methods that can accurately predict in vivo fertilizing capacity of frozen boar semen. The cell membrane is of crucial importance with regard to freeze-thaw survival of spermatozoa. It is important to optimize the survival of the plasma membrane as this is a non homogenous entity both in structure and function. The boar sperm membrane exhibits extreme sensitivity to freezing treatment. Freezing and thawing results in considerable changes in electrolyte dynamics and damages have mainly been associated with alterations in the head membranes especially at thawing. To date fruitless efforts have been carried out to find a cryoprotectant for the spermatozoa membranes and glycerol still continues to be used despite its harmful effects to the membranes.

Cryopreservation of boar semen. II: Effect of cooling rate and duration of freezing point plateau on boar semen frozen in mini- and maxi-straws and plastic bags

The post-thaw motility and the acrosome integrity of semen from 4 boars frozen with a programmable freezing machine, in mini (0.25 ml) and maxi (5 ml) plastic straws and in 10 x 5 cm Teflon FEP-plastic bags (0.12 mm thick, 5 ml), were compared. The freezing of the semen was monitored by way of thermo-couples placed in the straws and the bags. Three freezing programmes were used, namely A: from +5 degrees C, at a rate of 3 degrees C/min, to -6 degrees C, held for 1 min at -6 degrees C, and followed by a cooling rate of 20 degrees C/min to -100 degrees C; B: a similar curve except that there was no holding time at -6 degrees C and that the cooling rate was 30 degrees C/min, and C: from +5 degrees C to -100 degrees C, with a cooling rate of 35 degrees C/min, followed by storage in liquid N2. Despite the freezing curve assayed, both the mini-straws and the bags depicted much shorter freezing point plateaus as compared to the maxi-straws. Post-thaw sperm motility as well as the amount of normal apical ridges were equally significantly higher when semen was frozen in mini-straws or in bags than in maxi-straws. Significant differences in these post-thawing parameters were obtained between the freezing curves used. The stepwise freezing procedure A appeared as the best alternative for boar semen, considering this in vitro evaluation.

Lipid composition and thermotropic phase behavior of boar, bull, stallion, and rooster sperm membranes

Composition and thermotropic phase behavior of sperm membrane lipids from species ranging in sensitivity to cold shock were determined. Lipids from whole sperm and sperm plasma membrane were fractionated into neutral lipid, glycolipid, and phospholipid fractions. Compositional analyses were completed for free sterols, phospholipids and phospholipid-bound fatty acids. Phase transition temperatures were determined for phospholipid and glycolipid fractions using differential scanning calorimetry. Cholesterol was the major sterol in sperm lipids of all species. Cholesterol to phospholipid molar ratios were 0.26, 0.30, 0.36, and 0.45 for sperm plasma membrane of the boar, rooster, stallion, and bull, respectively. Choline and ethanolamine phosphoglycerides and sphingomyelin were the major phospholipid classes in sperm and their proportions differed across species. Phospholipid-bound fatty acyl compositions of choline and ethanolamine phosphoglycerides were characterized by a high proportion of docosapentanoyl and docosahexanoyl groups in mammalian sperm and shorter, more saturated groups in rooster sperm. Glycolipids represented less than 10% of total polar lipids for all species. Thin-layer chromatographic analysis indicated that the major glycolipid component of rooster sperm was different from that of mammalian sperm. Peak phase transition temperatures (Tm) for sperm membrane phospholipids were 24.0, 25.4, 20.7 and 24.5, for the boar, stallion, and rooster, respectively. Corresponding Tm's for glycolipids were 36.2, 42.8, and 33.4 with no exotherm for rooster sperm glycolipids. These results demonstrate a difference in both composition and thermotropic phase behavior of glycolipids between rooster and mammalian sperm which may be related to the greater tolerance of rooster sperm to rapid cooling.

Assessment of ultrastructure in isolated cochlear hair cells using a procedure for rapid freezing before freeze-fracture and deep-etching

Separated cochlear outer hair cells and isolated strips of organ of Corti containing hair cells and supporting cells have been rapidly frozen before freeze-fracture and deep-etching by immersion of samples sandwiched between two copper plates into liquid nitrogen-cooled propane: isopentane. Assessment of this procedure has shown that no significant freezing damage occurs. The ultrastructure of the hair cells revealed by freeze-fracture of these non-chemically fixed preparations was generally very similar to that seen in fixed material. This indicates that the processing of cochlear tissue normally used for electron microscopy produces few obvious structural artefacts. It also demonstrated that procedures for isolating cochlear hair cells generally do not affect cell structure significantly. However, some isolated hair cells did show abnormalities within the membranes of the lateral cisternae. Such membrane alterations, which would not be identified by light microscopy, occurred to a variable extent but were more commonly present after prolonged periods in maintenance medium. Deep-etching of the preparations to examine extracellular features around stereocilia revealed clearly lateral cross-links between stereocilia. However, tip-links could not be positively identified in either unfixed or prefixed preparations.