Can a single sperm travel alone to an egg and fertilize it?

Question

WebMD writes in its Sperm FAQ:

It takes just one sperm to fertilize an egg and achieve a pregnancy, but for each sperm that reaches and fertilizes an egg, there are millions that don't.

Parenting.com comes writes in Misconception: A Low Sperm Count is No Big Deal:

MYTH: A low sperm count is no big deal. After all, it only takes one to fertilize an egg.

FACT: Men need more than one sperm—lots, lots more than one—to fertilize an egg after intercourse. In a typical ejaculate (about a teaspoon), 250 million sperm are released. But of those 250 million, only hundreds make it as far as the fallopian tubes. Functioning as a kind of synchronized sports team, most of the sperm release an enzyme that helps clear the pathway to the egg so that one sperm can penetrate it.

Is a single sperm enough or do multiple sperms have to release an enzyme that clears the pathway and a single sperm can't release enough of that enzyme on its own?

Answer 1

No, it takes more than one sperm. The egg has multiple barriers which take up to 100 sperm, co-operating, to penetrate.

The fusion of the egg with its three barriers (cumulus-oocyte complex, zona pellucida and finally the plasma membrane) takes approximately 10 sperm to clear the first two barriers of the oocyte referring to De Jonge and up to 100 sperm to clear a path through these barriers. Chance determines which of these sperm is the final single one that fertilizes the oocyte.

There have been cases where sperm have not been rendered non-functional as is evidenced by the numerous case reports of human tubal pregnancies that arose in spite of lack of access of sperm from the uterus into the oviduct on the side of ovultation.

Research evidence shown here shows that probably only 10 or 20 spermatozoa reach the zona pellucida and there are probably no more than tens of sperm that have reached and begun to penetrate through the cumulus in the humans.

The COC serves as the final sperm filter and probably only 10 or 20 spermatozoa make it through the cumulus to reach the zona pellucida. The race to the finish will be governed by effective ligand-receptor interactions, functionally active signal transduction cascade pathways

Upon contact with the zona there may only be slightly more than a handful of sperm that completely fulfil the preceding elements; perhaps chance is the final determinant for which of these is the fertilising sperm.

The spermatocyte must penetrate two main oocyte barriers which are 1) corona radiata and 2) zona pellucida and binding of sperm to oocyte occurs with the help of a metalloprotease protein in the sperm plasma membrane fusing with a receptor on the oocyte membrane.

1) Corona Radiata penetration-The coronal cells do not constitute a real barrier since they are very loosely attached to the zona and are partially disrupted by the action of the oviduct fluid and enzymes.

2) Zona pellucida penetration- Digestion of the layer in the immediate vicinity of the sperm by the released acrosomal enzymes such as acrosin. The egg can be fertilized by many sperm if the zona pellucida is removed.

Referring to B.M. Gadella et.al. in 2015, ejaculated mammalian sperm cells are initially unable to fertilize the oocyte. Sperm are activated by a process called sperm capacitation in the female genital tract in vivo and acquire fertilizing potential once reaching the isthmus of the oviduct.

Runner-up sperm that have not finished travelling through the zona pellucida by the time the hardening occurs are stopped in their path. Successful fertilization requires not only that a sperm and egg fuse, but that not more than one sperm fuses with the egg. Fertilization by more than one sperm which is polyspermy almost inevitably leads to early embryonic death.

1. Dr. Charles Lindemann who has studied mechanisms of sperm motility extensively gives the following information about the entry of many sperms to break down the corona to let one sperm get through to the egg.

The egg is usually covered by a thick layer of cells called the corona radiata that serve as a blockade to restrict sperm from getting into the egg. Sperm cells contain enzymes that break this barrier down. It may actually require an assault of many sperm to break down the corona sufficiently to let one sperm get through to the egg.

2. A combination of many sperms (a.k.a helper sperms) attacking the structures surrounding the oocyte makes it possible for one single sperm to unite with the oocyte.

Summarizing, we have here to do with a directed "attack" of many sperm cells on the structures surrounding the oocyte, with the final goal of making it possible for one single spermatozoon to unite with the oocyte.

Answer 2

Sperm are intricately adapted to existing alongside millions of their peers at all times, but is simply unknown if they can survive and achieve success without these peers.

There simply is not enough research to know. Specifically, no one has ever tried a series of billions of trials in which they introduced a single sperm and checked if it created a pregnancy.

But, there is some research into sperm cooperation which gives us a method though which one might theorize that the answer to that question could be NO.

Reproductive success is best predicted by the mobility of sperm [1], which is known to be affected by sperm cooperation [2]. So we know for a fact, at least in many species, a single sperm introduced into a female will be facing a greater challenge than if it were one of many. Furthermore, some researchers have even theorized about the existence of different classes of sperm which exist specifically to help the fertilizing sperm succeed [3]. Whether, in some species, this journey taken alone is simply insurmountable or not has never been researched (as far as I can tell).

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1. Birkhead TR, Martinez JG, Burke T, Froman DP. 1999 Sperm mobility determines the outcome of sperm competition in the domestic fowl. Proc. R. Soc. Lond. B 266, 1759–1764. (doi:10.1098/rspb.1999.0843)
2. Higginson DM, Pitnick S. 2010 Evolution of intra-ejaculate sperm interactions: do sperm cooperate? Biol. Rev. 86, 249–270. (doi:10.1111/j.1469-185X.2010.00147.x)
3. http://goaskalice.columbia.edu/answered-questions/why-does-it-take-millions-sperm-fertilize-egg