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Pregnancy Timeline by SemestersFetal liver is producing blood cellsHead may position into pelvisBrain convolutions beginFull TermWhite fat begins to be madeWhite fat begins to be madeHead may position into pelvisImmune system beginningImmune system beginningPeriod of rapid brain growthBrain convolutions beginLungs begin to produce surfactantSensory brain waves begin to activateSensory brain waves begin to activateInner Ear Bones HardenBone marrow starts making blood cellsBone marrow starts making blood cellsBrown fat surrounds lymphatic systemFetal sexual organs visibleFinger and toe prints appearFinger and toe prints appearHeartbeat can be detectedHeartbeat can be detectedBasic Brain Structure in PlaceThe Appearance of SomitesFirst Detectable Brain WavesA Four Chambered HeartBeginning Cerebral HemispheresFemale Reproductive SystemEnd of Embryonic PeriodEnd of Embryonic PeriodFirst Thin Layer of Skin AppearsThird TrimesterSecond TrimesterFirst TrimesterFertilizationDevelopmental Timeline
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Home | Pregnancy Timeline | News Alerts |News Archive Oct 29, 2013

 

Sperm are not fertile until they spend time in the female reproductive tract, removing the sperm head membrane through a series of biochemically delicate stages that promote binding of sperm and egg.







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What's new in the future of male contraceptive

A "pill for men" may still be a long way down the road,but new knowledge of how sperm binds to an egg is at the heart of being able to control fertilization, and will lead to scientists being able to block or enhance the process.

Reproductive biologist Pablo Visconti at the University of Massachusetts Amherst (UMass) reports advances in understanding the basic processes of sperm capacitation that may improve success rates of in vitro fertilization (IVF), and eventually lead to a male contraceptive.

The findings appear in early online editions this month of Proceedings of the National Academy of Sciences (PNAS) and the Journal of Biological Chemistry (JBC), which named the study a finalist for "Paper of the Week."


It was the discovery of sperm capacitation in the 1950s, which means destabilizing the sperm head membrane to allow greater binding between sperm and egg, that made IVF possible.

Sperm are not fertile until they spend time in the female reproductive tract, moving through a series of delicate stages that promote capacitation.

Over the last 50 years it has become clear that capacitation not only involves many stages, but that each mammal species has a unique capacitation.


Species-specific requirements are being established on a case-by-case basis. Despite years of research using IVF in horses is prohibitively difficult because so little is known about requirements for horse sperm capacitation. Despite a growing interest in using IVF to help preserve endangered species, the hurdles are immense as each species requires unique procedures, Visconti adds.

In the PNAS study, researchers experimented with increasing intracellular calcium using calcium ionophore, A23187. Calcium is known to play a role late in sperm capacitation, after pathways that depend on soluble adenylyl cyclase, cyclic adenosine monophosphate (cAMP), protein kinase A (PKA) and other enzyme cascades have taken place. A major drawback of adding calcium ionophore, however, is that it overwhelms sperm, quickly immobilizing them.


Visconti credits his co-authors Hiroyuki Tateno in Japan and Ryuzo Yanagimachi in Hawaii with the idea of washing ionophore away after use.


Visconti: "Until they conceived it, no one had thought of this trick. They did the first experiments. Later, more experiments confirmed that when the ionophore is washed away, the sperm retain just the calcium they need. They self-regulate their optimal calcium concentration and are ready to go on."

"Our laboratory is mainly interested in the basic science of how sperm acquire fertilizing capacity, and this shortcut offers some translational possibilities for calcium ionophore use in IVF. This shortcut may address many of the difficult situations we encounter with IVF in other species."

Under these conditions, ionophore-treated sperm fertilized 80 percent of eggs, which developed into normal offspring. The data indicates that ionophore-treated mouse sperm can fertilize ova even when the cAMP/PKA signaling pathway is inhibited.

Using biochemical analysis methods in knockout mice, Visconti and colleagues showed that while soluble adenylyl cyclase (sAC) is present in the sperm tail, transmembrane adenylyl cyclases (tmAC) are present in the sperm head, another unexpected finding.

Proceedings of the National Academy of Sciences, Significance
Sperm capacitation enables spermatozoa to undergo the acrosome reaction and to exhibit vigorous motility called hyperactivation. At the molecular level, capacitation is associated with activation of a cAMP-dependent pathway and with the increase of intracellular pH and Ca2+ concentrations. Ca2+ ionophore A23187 elevates intracellular Ca2+ and induces the acrosome reaction but renders the spermatozoa motionless. However, when the ionophore was washed away, spermatozoa recovered motility, showed hyperactivation, and were able to fertilize cumulus-intact eggs. In these conditions, sperm acquired fertilizing capacity even when the cAMP pathway was inactivated. Fertilized oocytes with A23187-treated sperm developed into normal offspring. These data indicate that a short elevation of intracellular Ca2+ overcomes other necessary signaling pathways during capacitation and renders sperm fertile.

Abstract
Ca2+ ionophore A23187 is known to induce the acrosome reaction of mammalian spermatozoa, but it also quickly immobilizes them. Although mouse spermatozoa were immobilized by this ionophore, they initiated vigorous motility (hyperactivation) soon after this reagent was washed away by centrifugation. About half of live spermatozoa were acrosome-reacted at the end of 10 min of ionophore treatment; fertilization of cumulus-intact oocytes began as soon as spermatozoa recovered their motility and before the increase in protein tyrosine phosphorylation, which started 30–45 min after washing out the ionophore. When spermatozoa were treated with A23187, more than 95% of oocytes were fertilized in the constant presence of the protein kinase A inhibitor, H89. Ionophore-treated spermatozoa also fertilized 80% of oocytes, even in the absence of HCO3−, a component essential for cAMP synthesis under normal in vitro conditions. Under these conditions, fertilized oocytes developed into normal offspring. These data indicate that mouse spermatozoa treated with ionophore are able to fertilize without activation of the cAMP/PKA signaling pathway. Furthermore, they suggest that the cAMP/PKA pathway is upstream of an intracellular Ca2+ increase required for the acrosome reaction and hyperactivation of spermatozoa under normal in vitro conditions.

Journal of Biological Chemistry Abstract
Fertilization competence is acquired in the female tract in a process known as capacitation. Capacitation is needed for the activation of motility (e.g. hyperactivation) and to prepare the sperm for an exocytotic process known as acrosome reaction. While the HCO3--dependent soluble adenylyl cyclase Adcy10 plays a role in motility, less is known about the source of cAMP in the sperm head. Transmembrane adenylyl cyclases (tmACs) are another possible source of cAMP. These enzymes are regulated by stimulatory heterotrimeric Gs proteins; however, the presence of Gs or tmACs in mammalian sperm has been controversial. In this manuscript, we used Western blotting and cholera toxin-dependent ADP ribosylation to show Gs presence in the sperm head. Also, we showed that forskolin, a tmAC specific activator, induces cAMP accumulation in sperm from both WT and Adcy10 null mice. This increase is blocked by the tmAC inhibitor SQ-22536 but not by the Adcy10 inhibitor KH7. While Gs immunoreactivity and tmAC activity are detected in the sperm head, PKA is only found in the tail, where Adcy10 was previously shown to reside. Consistent with an acrosomal localization, Gs reactivity is lost in acrosome reacted sperm, and forskolin is able to increase intracellular Ca2+ and induce the acrosome reaction. Altogether, these data suggest that cAMP pathways are compartmentalized in sperm, with Gs and tmAC in the head and Adcy10 and PKA in the flagellum.

Experimenting with mouse sperm in vitro, Visconti worked with colleagues at UMass Amherst, Weill Cornell Medical College, University of Hawaii Medical School, Asahikawa Medical University Japan, Universidad Nacional de Rosario and Facultad de Medicina Argentina and Universidad Nacional Autónoma de México.

Original press release:http://www.eurekalert.org/pub_releases/2013-10/uoma-rbm102813.php