The abundance of data pertaining to the metabolism of lipids in relation to mammalian fertilization has warranted an effort to assemble a molecular membrane model for the comprehensive visualization of the biochemical events involved in sperm capacitation and the acrosome reaction. Derived both from earlier models as well as from current concepts, our membrane model depicts a lipid bilayer assembly of space-filling molecular models of sterols and phospholipids in dynamic equilibrium with peripheral and integral membrane proteins. A novel feature is the possibility of visualizing individual lipid molecules such as phosphatidylcholine, phosphatidylethanolamine, lysophospholipids, fatty acids, and free or esterified cholesterol. The model illustrates enzymatic reactions which are believed to regulate the permeability and integrity of the plasma membrane overlying the acrosome during interactions between the male gamete and capacitation factors present in fluids of the female genital tract. The use of radioactive lipids as molecular probes for monitoring the metabolism of cholesterol and phosphatidylcholine revealed the presence of (1) steroid sulfatase in hamster cumulus cells, (2) lecithin: cholesterol acyltransferase in human follicular fluid, (3) phospholipase A2, and (4) lysophospholipase in human spermatozoa. These enzymatic reactions can be integrated into a pathway that provides a link between the concepts of lysophospholipid accumulation in the sperm membranes and alteration of the cholesterol/phospholipid ratio as factors involved in the preparation of the membranes for the acrosome reaction. Capacitation is viewed as a reversible phenomenon which, upon completion, results in a decrease in negative surface charge, an efflux of membrane cholesterol, and an influx of calcium between the plasma and outer acrosomal membranes. Triggered by the entry of calcium, the acrosome reaction involves phospholipase A2 activation followed by a transient accumulation of unsaturated fatty acids and lysophospholipids implicated in membrane fusion which occurs during the formation of membrane vesicles in spermatozoa undergoing the acrosome reaction.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/mrd.1120120209

A molecular membrane model of sperm capacitation and the acrosome reaction of mammalian spermatozoa

When sperm of the ram, bull, boar and stallion are cold-shocked by rapid cooling to near freezing point, motility and metabolic activity are irreversibly depressed and the acrosome and plasma membrane disrupted. Ram sperm become susceptible to cold shock in the proximal corpus region of the epididymis when the cytoplasmic droplet has moved backwards to the distal portion of the sperm midpiece. The membrane constituents phospholipids and cholesterol are important in cold shock which causes loss of lipid from sperm. The susceptibility of sperm to cold shock is linked with a high ratio of unsaturated:saturated fatty acids in the phospholipids and a low cholesterol content. The high unsaturated fatty acid content of sperm also makes them susceptible to damage from peroxidation which adversely affects motility, metabolism, ultrastructure and fertility. Hydroxynonenal, a product of fatty acid peroxidation, depresses the motility and oxygen uptake of ram sperm in vitro and may react with the -SH groups of the axonemal microtubules. High calcium concentrations in the external medium may decrease the motility and metabolism of sperm and 'calcium intoxication' may be a factor in cold shock. Lowering the environmental temperature increases calcium uptake by sperm and the effect is aggravated if the rate of cooling is rapid. Phospholipids, particularly those in egg yolk, protect sperm to some extent from cold shock and also prevent increased calcium flux into the sperm. Suggestions are made for increasing the life span of sperm during preservation and microencapsulation by adding agents that may stabilize membranes, counter peroxidation and decrease calcium uptake.

Lipids and calcium uptake of sperm in relation to cold shock and preservation: a review

The lipid composition of the sperm membrane has a significant effect upon the functional characteristics of spermatozoa. In the present study we investigated the fatty acid (FA) composition of subpopulations of spermatozoa separated on a discontinuous Percoll gradient (47:90%) and the FA composition of phospholipids (PL) of sperm heads and tails in both normal and abnormal semen samples. In normozoospermic samples, polyunsaturated fatty acids (PUFA) represented 34.0 +/- 1.3 (mean +/- SE, mole %) and 25.6 +/- 1.2% of total FA of PL of the 47 and 90% Percoll fractions respectively. Docosahexaenoic acid (22:6omega3, DHA) contributed to more than 60% of total PUFA. DHA was significantly lower in both the 47% (P < 0.05) and the 90% (P < 0.01) Percoll fractions of oligozoospermic samples and in the 90% Percoll layer of asthenozoospermic samples (P < 0.01), compared with normozoospermic samples. The omega6/omega3 ratio was significantly increased in both Percoll fractions of samples with oligozoospermia (47%, P < 0.001 and 90%, P < 0.001) or with asthenozoospermia (47%, P < 0.05 and 90%, P < 0.001) compared with normozoospermic samples. The oxidative potential index (OPI) of spermatozoa recovered from the 47% Percoll layer was significantly higher (P < 0.0001) than of those recovered from the 90% Percoll. Mean melting point (MMP), an index of membrane fluidity, was significantly lower in head than in tails (P < 0.01) of spermatozoa, and also in both the 47% (P < 0.01) and 90% (P < 0.001) Percoll fractions of normozoospermic samples in comparison with oligozoospermic samples. The MMP was significantly higher (P < 0.05) in samples of patients with idiopathic oligo/asthenozoospermia, varicocele, and male accessory gland infection (MAGI). These differences in FA composition of PL in subpopulations of human spermatozoa, and in their heads and tails may be related to sperm maturity and to differences in physiological function.

/pdf/the-fatty-acid-composition-of-phospholipids-of-spermatozoa-2i97zsywff.pdf

The fatty acid composition of phospholipids of spermatozoa from infertile patients

The aim of the present study was to characterize the effects of feeding tuna oil on the lipid and fatty acid composition of boar spermatozoa and to relate changes in composition to boar semen characteristics. Ten boars were paired by age and allocated to one of two diets (five boars per diet). The diets, which were offered for 6 weeks, consisted of a basal diet that was either unsupplemented or supplemented with 30 g tuna oil kg(-1) diet. Adding tuna oil to the diet increased the ether extract concentration of the diets fed from 65 to 92 g kg(-1) dry matter and supplied 10.5 g long chain polyunsaturated (n-3) fatty acids per 100 g total fatty acids. There were no changes in semen fatty acid composition after 3 weeks of feeding tuna oil. However, after 5 and 6 weeks, the proportions (g per 100 g total fatty acids) of 22:6(n-3) in sperm phospholipid fatty acids were increased from 34.5 to 42.9 g by feeding tuna oil and 22:5(n-6) decreased from 29.8 to 17.9 g. No changes were observed in other sperm lipids or seminal plasma phospholipids as a result of the diets fed. Feeding tuna oil increased the proportion of spermatozoa with progressive motility and with a normal acrosome score and reduced the proportion of spermatozoa with abnormal morphologies.

/pdf/effects-of-feeding-tuna-oil-on-the-lipid-composition-of-pig-3hkfbgmjpp.pdf

Effects of feeding tuna oil on the lipid composition of pig spermatozoa and in vitro characteristics of semen

The composition, structure and function of lipids in the tissues of ruminant animals.

https://rep.bioscientifica.com/downloadpdf/journals/rep/18/3/jrf_18_3_006.pdf

B. Ahluwalia

Papers

Fatty acid composition of lipids of bull, boar, rabbit and human semen