You might remember Monty Python’s irreverent musical tribute to sperm “Every Sperm is Sacred“. While perhaps not sacred, per se, sperm are pretty fascinating little cells. Did you know it takes approximately seventy days to make a sperm which is no bigger than half the width of a human hair?  Sperm are a renewable resource in men, because they arise from stem cells, which replicate continuously by mitosis to produce more cells, some of which become more stem cells and others become mature sperm. The scientific name for sperm production is spermatogenesis.

Almost every cell type in the human body  has a total complement of 46 chromosomes total (23 paired chromosomes), making it diploid.  Diploid cells have a pair of sex chromosomes, either XX (female)  or XY (male) which determine gender and 22 autosomal chromosomes numbered 1-22 that encode for everything else.

Sperm (and eggs) are unusual because unlike every other cell type in the body, they are genetically halved or haploid because they have 23 chromosomes, one unpaired chromosome for each of the 23 chromosomes types.   When haploid sperm and haploid egg come together at fertilization, they combine their genetic material to restore the normal chromosome number to 46 (23 matched pairs), becoming a diploid cell again.

Looking at a karyotype may be helpful. A karyotype is like a scrap book of your chromosomes. The chromosomes are processed from a cell sample and splatted out on a piece of paper so that individual chromosomes can be photographed and  the images laid out and matched so that it is obvious if someone has two many or two few of a type of chromosome.  This Make Your Own Karyotype link is useful because it shows you how chromosomes are matched. Imagine the pile of unmatched chromosomes on the left belonging to the sperm and the numbered chromosomes on the right belonging to the egg. At fertilization, they find their partner and DO-SI-DO, restoring the total number of chromosomes to 46 total or 23 pairs.

Aneuploidy is a fancy name for having the wrong number of chromosomes, either too many or two few. During the meiosis stage of sperm cell production, chromosomes pairs separate. Failure to separate  (non-disjunction) during meiosis is a common cause of aneuploidy. Aneuploidy results in an abnormal embryo which may or may not be compatible with life. Trisomy 21 is obviously compatible with life. Other trisomies are not, and can result in a miscarriage.

Aneuploidy in sperm can occur as a consequence of chemotherapy. Men who have been exposed to chemotherapy for cancer treatment often stop producing sperm for a period of months. Chemotherapy targets rapidly dividing cells for destruction. Stem cells and cancer cells are both rapidly dividing cells. Chemotherapy kills the rapidly dividing stem cells called spermatagonia.

Sperm banks store sperm for men (and pubertal boys) before chemotherapy just in case sperm production doesn’t recover. When chemotherapy is done, if there are spermatagonia remaining, they often will start making sperm again, but the sperm produced as the factory reopens is prone to quality problems, namely increased tendency to produce aneuploid sperm, which can cause abnormal embryos. For this reason, men are often counseled by their doctor to wait two years after their chemotherapy is done before trying to father a child.

Although this is a “sperm post”- non-disjunction occurs when eggs are produced too. As women age, we are more likely to produce aneuploid eggs. This is the main reason that older women have more difficulty getting pregnant and statistically more likely to lose the baby if they do conceive. Aneuploidy in the sex chromosomes can cause Turner’s Syndrome, in which the child has a single X chromosome.

Pre-implantation genetic screening can be used to detect embryos that have chromosomal aneuploidy, derived from either the sperm or the egg. Embryos are produced using in vitro fertilization. On day 3 of culture, one cell from the 8 cell embryo is removed (biopsied) and sent off for testing. Two days later, on day 5 of culture, the IVF lab gets a lab report on each tested embryo, identifying those with an abnormal number of chromosomes. These embryos are not transferred.

Using PGS for aneuploidy screening is somewhat controversial, especially in older women. Older women are most likely to have aneuploid embryos and are also most likely to have very few embryos. So the patient population that is most likely to benefit from aneuploidy detection is also least likely to have enough embryos so that a normal one can be found to transfer. Sometimes, older patients decide it’s not worth the expense, particularly if they will follow-up with pre-natal testing. Of course, then you may detect an abnormality that might force you to consider the difficult option of terminating the pregnancy.

As usual, the technology is actually the easy part. The information we gain from the technology can force us to make hard choices.

© 2010, Carole. All rights reserved.

Posted on June 6, 2010 at 7:52 pm
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Categorised under : Inside the Lab , Repro Bio 101
Tagged with : diploid , egg aneuploidy , haploid , meiosis , mitosis , preimplantation genetic screening , sperm aneuploidy , spermatogenesis