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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
CLICK ON weeks 0 - 40 and follow along every 2 weeks of fetal development
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Home | Pregnancy Timeline | News Alerts |News Archive July 3, 2014

 

Sperm (blue) latch onto a control egg (left)
but can't bind to an egg lacking the glycoprotein ZP2 (right).
Image credit: Avella et al., 2014

 






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How sperm bore into the egg

A key protein is discovered which sperm need to fertilize an egg. Before it can fertilize an egg, a sperm has to bore through the outer egg layer known as the zona pellucida. Despite decades of research, some of the mechanisms in this process were unclear.

A study in The Journal of Cell Biology now identifies the protein in the zona pellucida that sperm latch onto.

The zona pellucida protects the egg and the early embryo before implantation. Its structure seems simple—in humans it contains four kinds of glycoproteins, and in mice it only contains three.

But researchers haven't been able to identify the sperm's binding partner in the layer, although their suspicions have fallen on two of the glycoproteins, ZP2 and ZP3.

To find out more, Jurrien Dean and colleagues from the National Institute of Diabetes and Digestive and Kidney Diseases engineered mice to produce various combinations of human and mouse zona pellucida glycoproteins.


Mouse sperm didn't bind to the zona pellucida if it was missing ZP2, and female mice lacking the protein were sterile. Researchers also found that sperm couldn't latch onto eggs if ZP2 was missing a key region at the beginning of the protein.

This result jibes with a previous finding that fertilization triggers the release of an enzyme that severs ZP2 in this region, thus preventing additional sperm from attaching to the zona pellucida..


The team also tested the binding of human sperm to mouse eggs surrounded by a zona pellucida harboring human glycoproteins. Human sperm adhered to the mouse zona pellucida if it contained human ZP2 but not if it carried human ZP3, confirming the importance of ZP2.

Abstract
The extracellular zona pellucida surrounds ovulated eggs and mediates gamete recognition that is essential for mammalian fertilization. Zonae matrices contain three (mouse) or four (human) glycoproteins (ZP1–4), but which protein binds sperm remains controversial. A defining characteristic of an essential zona ligand is sterility after genetic ablation. We have established transgenic mice expressing human ZP4 that form zonae pellucidae in the absence of mouse or human ZP2. Neither mouse nor human sperm bound to these ovulated eggs, and these female mice were sterile after in vivo insemination or natural mating. The same phenotype was observed with truncated ZP2 that lacks a restricted domain within ZP251–149. Chimeric human/mouse ZP2 isoforms expressed in transgenic mice and recombinant peptide bead assays confirmed that this region accounts for the taxon specificity observed in human–mouse gamete recognition. These observations in transgenic mice document that the ZP251–149 sperm-binding domain is necessary for human and mouse gamete recognition and penetration through the zona pellucida.

Avella, M.A., et al. 2014. J. Cell Biol. doi:10.1083/jcb.201404025

The Journal of Cell Biology (JCB) is published by The Rockefeller University Press. All editorial decisions on manuscripts submitted are made by active scientists in conjunction with our in-house scientific editors. JCB content is posted to PubMed Central, where it is available to the public for free six months after publication. Authors retain copyright of their published works, and third parties may reuse the content for non-commercial purposes under a creative commons license. For more information, please visit http://www.jcb.org.

Research reported in the press release was supported by the Intramural Research Program of the National Institutes of Health, National Institute of Diabetes and Digestive and Kidney Diseases (ZIA-DK015603).



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