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Dr. Lindemann's Fun Sperm Facts!
Average volume and content of human ejaculate # The volume and content of the ejaculate depend on the length of time between ejaculations. The average volume of semen is 2.75 ml, ranging from 2-6 ml, the higher volumes following periods of abstinence. An average human ejaculate contains about 180 million sperm (66 million/ml), but some ejaculates contain as many as 400 million sperm. Both quantity and quality of the sperm are important determinants of fertility. A man is considered clinically infertile if his sperm concentration falls below 20 million/ml semen. # (Just remember though- it only takes ONE to make a baby) The average sperm count fell from 113 million sperm/ml of semen in 1940 to 66 million/ml in 1990. The volume of a single ejaculate has declined from 3.40 ml to 2.75 ml. This means that men on average are now ejaculating less than half the number of sperm as men did 50 yrs ago. A drop from more than 380 million sperm to about 180 million sperm per ejaculate. Furthermore, the number of motile sperm has also dipped. Importantly, the sperm count has not declined in the less polluted areas of the world during the same time period. #
(In other words the time it takes for them to get where they need to go)
The QUESTION most frequently asked to Dr. Lindemann via e-mail: How long can sperm live on environmental surfaces? ANSWER: They die as they dry out, so it depends on how quickly they dry. They also are killed by fresh water due to osmotic shock. Soap and detergents such as those used for laundering and hand washing also kill them, as these substances strip off the cell membrane of the sperm. Something to consider: They are the haploid half of your life cycle so be careful where you leave them.
Motile (moving sperm) have been found in the human Fallopian tube up to 85 hours after coitus, although the ability to fertilize an egg is usually lost before motility is lost.
Who you calling Diploid? ^ Sperm are the male gamete produced from the germ cell line. The germ cells of the male are called spermatogonia, which oddly means sperm-eggs. Spermatogonia differentiate into spermatocytes in the testes, and it is the spermatocytes that divide by a process called meiosis. Meiosis is a type of cell division that reduces the chromosome number from 46 to 23. For this reason the sperm cells that are produced after meiosis have only half the full number of chromosomes. They carry half of the genetic material needed to make a new organism. Since they only have half the normal number of chromosomes they are referred to as haploid cells as opposed to the normal body cells that are said to be diploid. Some life forms, such as seaweed and ferns, spend a large part of their life in the haploid state and actually have a haploid body as well as a diploid body. Higher animals, including humans, have a life cycle where only a short time is spent in the haploid state and the haploid organism is limited to single cells we call eggs and sperm. In this generalized view of things, the sperm and egg are the haploid phase of the human life cycle. The vast majority of multicellular organisms have a sexual reproductive phase in their life cycle during which haploid gametes are produced. In the non-vascular plants (like the seaweed) the gametes can look pretty much alike. This is called isogamy. The gametes of these organisms usually have two flagella that serve for swimming and also as arms to grab and stick to each other for fertilization. One of the green algae that does this is called Chlamydomonas and is a fresh-water single-celled plant. Chlamydomonas has become the leading experimental model system to study the genetics of flagella. It is especially good for this because it can re-grow flagella quickly, it can be grown by the bucket-full in pond water, and it can be mutated extensively to isolate flagella with missing parts. The National Science Foundation maintains a repository of Chlamydomonas strains at Duke University as part of the Chlamydomonas Genetics Center. In this resource facility many useful mutant strains of Chlamydomonas are available for research on the assembly and motility of the flagellum. Believe it or not, structural proteins important to human sperm have actually been identified first in the flagella of this green water plant! This tells us all something about how well nature has conserved the gamete part of the life cycle. It also shows us how much we share with the other living things on this earth. Higher animals and plants generally have developed an uneven size of the eggs and the sperm (heterogamy) with only the sperm retaining a flagellum and the ability to swim. In the higher plants (herbs, trees and shrubs) the sperm have lost their flagella and are delivered to the egg through a pollen tube. This hidden sperm feature is called cryptogamy, which literally means hidden gamete. Higher animals, including man, also have heterogamy with a big egg and small sperm, but unlike the higher plants the sperm retain the flagellum for swimming. Human sperm literally swim up the oviduct to meet and fertilize the egg, which is the other gamete. Most multicellular animals have swimming sperm with flagella, although a few like the nematode worm Caenorhabditis elegans have lost the flagella for swimming and move by crawling. Marine animals, like sea urchins and fish, often release the sperm and eggs right into the seawater. This often means the sperm must "home in" on the eggs from a long distance. The sperm of sea squirts and sea urchins are known to have a chemical homing mechanism called chemotaxis that attracts sperm to chemicals released from the egg mass. Recent work published in the prestigious journal Science suggests the bizarre possibility that such a homing system has been retained by the sperm even in humans! The chemoattractant molecules that seem to work for human sperm are odorant molecules. The sperm smell their way to the egg! Fact can sometimes be stranger than fiction. Does size matter? ^ One would think that a big animal would have bigger gametes, but nothing could be further from the truth. The biggest sperm I have ever seen came from a fruit fly! The sperm of a fruit fly can be as long as the body of the male fly, about 1.1 mm. On the other hand, of the vast number of mammals humans have one of the smallest sperm cells, measuring only 40 microns long. Rats produce one of the largest sperm at 170 microns long. When we talk of mice and men we may be the bigger animals in the diploid phase of life, but it is the mice that have the bigger haploid phase of life with 80-micron long sperm. How many is enough? ^ This question always comes up in regard to human fertility. If it takes only one sperm to fertilize and egg, why does a low sperm count make a man infertile? A fertile male human ejaculates between 2 and 5 ml of semen (on average about a teaspoon). In each ml there are normally about 100 million sperm. If the concentration falls below 20 million sperm/ml there is usually some trouble with fertility. Twenty million still seems to be a lot, so why the problem? Part of the problem may have to do with attrition, since only a small fraction of the sperm deposited in the woman's vagina end up in the uterus. From those that make it to the uterus, only a small fraction of the sperm find their way to the oviducts. Usually the egg is all the way up at the other end of the oviduct. Of those that are in the oviduct only a small fraction make their way from the lower to the upper oviduct. So, in fact, the number of sperm successfully arriving at where the egg is located is actually very small. The second problem is that the egg is not just waiting to be fertilized by the first sperm to come along. The egg is usually covered by a thick layer of cells called the cumulus oophoricus that serve as a blockade to restrict sperm from getting into the egg. It may actually require an assault of many sperm to break down the cumulus sufficiently to let one sperm get through to the egg. So the whole process is somewhat like a marathon run in a maze filled with mucus followed by an obstacle course. That one sperm that finally makes it is the champion of Mother Nature's triathlon. If you don't have enough competitors to start, none are left at the finish. We don't know for sure but this may be a way of selecting for a healthy sperm to do the job of passing genes to the next generation. Modern in-vitro fertilization techniques can by-pass this selection process and achieve fertilization with much fewer sperm. Successful fertilization is now even accomplished with sperm that can't swim. This is accomplished by directly injecting sperm into the egg with a tiny glass pipette (tube). Only time will tell what bypassing nature's triathlon will do to the human gene pool.
* Austin and Short (1982) Reproduction in mammals: Germ cells and fertilization. Cambridge University Press, NY, NY # Lauralee Sherwood (2001) Human Physiology: From cells to systems. Brooks/Cole, Pacific Grove, CA ^ Charles B. Lindemann |
