Cell motility in viscous fluids is ubiquitous and affects many biological processes, including reproduction, infection and the marine life ecosystem. Here we review the biophysical and mechanical principles of locomotion at the small scales relevant to cell swimming, tens of micrometers and below. At this scale, inertia is unimportant and the Reynolds number is small. Our emphasis is on the simple physical picture and fundamental flow physics phenomena in this regime. We first give a brief overview of the mechanisms for swimming motility, and of the basic properties of flows at low Reynolds number, paying special attention to aspects most relevant for swimming such as resistance matrices for solid bodies, flow singularities and kinematic requirements for net translation. Then we review classical theoretical work on cell motility, in particular early calculations of swimming kinematics with prescribed stroke and the application of resistive force theory and slender-body theory to flagellar locomotion. After examining the physical means by which flagella are actuated, we outline areas of active research, including hydrodynamic interactions, biological locomotion in complex fluids, the design of small-scale artificial swimmers and the optimization of locomotion strategies. (Some figures in this article are in colour only in the electronic version) This article was invited by Christoph Schmidt.

/pdf/the-hydrodynamics-of-swimming-microorganisms-bonwl3c7w2.pdf

The hydrodynamics of swimming microorganisms

The molecular basis of mammalian sperm capacitation, defined functionally as those processes that confer on the sperm the acquisition of fertilization-competence either in vivo in the female reproductive tract or in vitro, is poorly understood. We demonstrate here that capacitation of caudal epididymal mouse sperm in vitro is accompanied by a time-dependent increase in the protein tyrosine phosphorylation of a subset of proteins of M(r) 40,000-120,000. Incubation of sperm in media devoid of bovine serum albumin, CaCl2 or NaHCO3, components which individually are required for capacitation, prevent the sperm from undergoing capacitation as assessed by the ability of the cells to acquire the pattern B chlortetracycline fluorescence, to undergo the zona pellucida-induced acrosome reaction and, in some cases, to fertilize metaphase II-arrested eggs in vitro. In each of these cases the protein tyrosine phosphorylation of the subset of capacitation-associated proteins does not occur. Protein tyrosine phosphorylation of these particular proteins, as well as sperm capacitation, can be recovered in media devoid of each of these three constituents (bovine serum albumin, CaCl2 or NaHCO3) by adding back the appropriate component in a concentration-dependent manner. The requirement of NaHCO3 for these phosphorylations is not due to an alkalinization of intracellular sperm pH or to an increase in media pH. Caput epididymal sperm, which lack the ability to undergo capacitation in vitro, do not display this capacitation-dependent subset of tyrosine phosphorylated proteins in complete media even after extended incubation periods, and do not fertilize metaphase II-arrested eggs in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Capacitation of mouse spermatozoa. I. Correlation between the capacitation state and protein tyrosine phosphorylation.

At coitus, human sperm are deposited into the anterior vagina, where, to avoid vaginal acid and immune responses, they quickly contact cervical mucus and enter the cervix. Cervical mucus filters out sperm with poor morphology and motility and as such only a minority of ejaculated sperm actually enter the cervix. In the uterus, muscular contractions may enhance passage of sperm through the uterine cavity. A few thousand sperm swim through the uterotubal junctions to reach the Fallopian tubes (uterine tubes, oviducts) where sperm are stored in a reservoir, or at least maintained in a fertile state, by interacting with endosalpingeal (oviductal) epithelium. As the time of ovulation approaches, sperm become capacitated and hyperactivated, which enables them to proceed towards the tubal ampulla. Sperm may be guided to the oocyte by a combination of thermotaxis and chemotaxis. Motility hyperactivation assists sperm in penetrating mucus in the tubes and the cumulus oophorus and zona pellucida of the oocyte, so that they may finally fuse with the oocyte plasma membrane. Knowledge of the biology of sperm transport can inspire improvements in artificial insemination, IVF, the diagnosis of infertility and the development of contraceptives.

/pdf/sperm-transport-in-the-female-reproductive-tract-1wd76o2ooi.pdf

Sperm transport in the female reproductive tract

Computer assisted semen analyzers in andrology research and veterinary practice

The genetic improvement of agriculturally important species and disease control are of fundamental importance to the success of a sustainable agri-food industry. In this sense, artificial insemination (AI) is arguably the most important tool contributing to the advancement of modern animal production. Through AI, 1 ejaculate from a genetically superior male can be used to impregnate multiple females to maximize the distribution of agriculturally favorable genes. As well, AI eliminates physical contact between animals, thus limiting the spread of sexually transmitted diseases. Successful semen cryopreservation enhances these advantages of AI over natural breeding. Long-term storage facilitates semen transport over distances, permits the quarantine of semen, and enables extended use of superior germplasm, even after the sire’s death. For agriculturally important animals, semen cryopreservation is an established industry worldwide, particularly for dairy cattle. Genome resource banking to preserve the biodiversity of endangered species or valuable transgenic lines also would benefit from sperm cryopreservation. For many mammals; however, effective semen cryopreservation is not a reality because a large number of sperm are apparently infertile following freezing and thawing. Compared to fresh, 8 times more cryopreserved bovine sperm were required to achieve equivalent fertilization rates in vivo (Shannon and Vishwanath, 1995). Although AI in pigs with fresh, cooled semen is increasingly popular, the fertility of cryopreserved semen remains about half that of fresh semen (Crabo, 1991). Following either transuterine or oviductal insemination of superovulated ewes, conception rates and percentage of fer-

https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.1939-4640.2000.tb03268.x

Semen cryopreservation in domestic animals: a damaging and capacitating phenomenon.

A movement pattern known as hyperactivation has been observed among sperm recovered from the periovulatory oviduct of several species. In culture medium, hyperactivated sperm swim in a pattern that is far less progressive than that of freshly ejaculated sperm. In the oviduct, sperm encounter highly viscoelastic substances, such as mucus and the cumulus matrix. We have previously reported that hyperactivated hamster sperm become more progressive in vitro when the viscosity of medium is increased. In the present study, we tested the effect of increasing the viscosity and viscoelasticity of the medium on the swimming progressiveness of mouse sperm. Caudal epididymal sperm were incubated in a medium that produced hyperactivated motility in 60 min. Swimming velocities of sperm incubated for 60 min were compared with those of fresh sperm after addition of one of the following to culture medium: solutions of 1.8% methylcellulose (high viscosity), 1.8% long chain polyacrylamide (high viscoelasticity), or culture medium alone (low viscosity). In culture medium, hyperactivated sperm had significantly lower mean straight-line velocities than fresh sperm (p = 0.004); this difference disappeared in methylcellulose (p = 0.085) and was reversed in polyacrylamide (p = 0.004). This and other velocity measurements indicated that hyperactivated mouse sperm penetrate viscoelastic media more efficiently than fresh sperm and therefore may be more efficient at penetrating oviductal mucus and cumulus matrix in vivo.

/pdf/hyperactivation-enhances-mouse-sperm-capacity-for-5f0qiggkfo.pdf

Hyperactivation enhances mouse sperm capacity for penetrating viscoelastic media.

At some time before fertilization, mammalian sperm undergo a change in movement pattern, termed hyperactivation. There is evidence that hyperactivation offers an advantage to sperm for detaching from the oviductal mucosa, for penetrating viscoelastic substances in the oviduct, and for penetrating the zona pellucida. Hyperactivation is known to require extracellular calcium, but little else is known about the mechanisms by which calcium affects sperm movement. The calcium-sensitive fluorescent dye indo-1 was used to follow intracellular calcium levels ([Ca2+]i) in individual moving sperm. Sperm were loaded with 10 microM of the acetoxymethyl ester form of the dye and then rinsed. The dye was excited at 340 nm by using a filtered xenon stroboscope, and images at the 405-nm and 490-nm excitation maxima were simultaneously digitized at 30 per sec for 2.1 sec. [Ca2+]i was significantly higher in the acrosomal and postacrosomal regions of the head and in the flagellar midpiece (the principal piece could not be measured) in hyperactivated than in nonhyperactivated sperm (P < 0.0001). [Ca2+]i oscillations were detected in the proximal half of the midpiece that were identical in frequency to the flagellar-beat-cycle frequency in 12 of 17 hyperactivated sperm (median, 3.5 Hz). Rapid [Ca2+]i oscillations were also detected in the acrosomal and postacrosomal regions, as well as in the distal midpiece. Oscillations were not eliminated by dampening the flagellar bending with methyl cellulose. The [Ca2+]i oscillations detected in sperm are significantly more rapid than oscillations detected in other cell types.

https://www.pnas.org/content/pnas/90/10/4660.full.pdf

Intracellular calcium increases with hyperactivation in intact, moving hamster sperm and oscillates with the flagellar beat cycle.

The objectives of this study were to describe hyperactivated motility in boar sperm and to determine the incidence of hyperactivation among boar sperm flushed from the oviduct. Oviducts were surgically removed from 13 gilts 32 hours after mating them to fertile boars. The majority of the sperm flushed from the oviducts was immotile, weakly motile, or stuck to mucus or cellular debris. The mucus could not be penetrated by the sperm. The remaining 3% to 19% of the flushed sperm was free-swimming. Only five hyperactivated sperm were recovered, all from the ampulla of the oviduct. The remainder of the free-swimming sperm travelled in linear trajectories and possessed significantly higher flagellar curvature ratios (the flagella were less bent) than boar sperm measured in diluted semen. Hyperactivated motility was induced in washed ejaculated boar sperm, using a 1-minute pulse of 4 mumol/L calcium ionophore A23187. The ionophore-treated sperm had significantly lower straight-line velocities, linearities, and flagellar curvature ratios than controls, as would be expected for hyperactivated sperm. They were vigorous and swam in circles. It was concluded that, although few hyperactivated boar sperm could be recovered from the oviduct, boar sperm are capable of undergoing hyperactivation.

Xiaobing Dai

Papers

Hyperactivation enhances mouse sperm capacity for penetrating viscoelastic media.

Intracellular calcium increases with hyperactivation in intact, moving hamster sperm and oscillates with the flagellar beat cycle.

Movement characteristics of boar sperm obtained from the oviduct or hyperactivated in vitro.