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The process of meiosis in oocytes is confusing to everybody. You are not alone! O.K., here's how it works:

Phase I

During fetal life, the oocytes enter meiosis and stop at the diplotene stage of meiosis I. They display a large nucleus - the germinal vesicle- which encloses the decondensed oocyte chromatin. These immature oocytes are diploid, but in the process of becoming haploid.

Phase II

From puberty on, the preovulatory LH surge (or hCG in the case of IVF/IUI) induces reinitiation of oocyte meiosis which will then arrest again at the metaphase II stage. Metaphase II oocytes are characterized by the presence of the first polar body which contains half of the oocyte chromosomes. The oocytes are now haploid. The oocytes are ovulated (or retrieved in the case of IVF) at the metaphase II stage.

Phase III

Each chomosome is made up of two homologs (copies of the DNA). When the egg is penetrated by the sperm, it induces resumption of meiosis. In this phase, the homologs (copies) separate and one copy of each chromosome goes into the oocyte and the other into the second polar body. This completes the meiotic division process. Because sperm and egg undergo the same process, the fertilized egg (zygote) is now diploid. The DNA replicates to form two homologs for each chromome prior to the next mitotic division (one cell to two cell).

Oocyte "maturation" has two components. Nuclear maturation (resumption of meiosis) and cytoplasmic maturation (making everything the egg needs to activate following sperm penetration and to get it through the first two division cycles). What I was referring to in my previous post was nuclear maturation induced by the hCG (i.e. completion of metaphase II of meosis and the extrusion of the first polar body - a haploid oocyte). However, failure of fertilization in oocytes that are achieved nuclear maturity (metaphase II) is usually caused by inadequate cytoplasmic maturation. The sperm gets in, but the egg is a dud. Based on your first post, the oocytes may have had a problem in both areas of maturation.

If we get a hCG level below 100 mIU/ml the morning after the injection, we suspent the donor took the shot later than expected (which is usually the case when things go wrong) or they injected it incorrectly (unusual). We then question (interogate!) the donor to find out what time they really took the shot and how they took it. They usually fess up and the cycle can be saved. It can mean doing egg retrievals at inconveneint times, but at least the eggs have an opportunity to mature before retrieval.

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Thanks so much. I have vague recollections of biology class, and although we certainly learned about the stages of meiosis, I don't think we ever learned when these stages actually occur.

So does the first polar body contain the "complement" (so to speak) of the chromosomes that the mother contributes to the child while the second polar body contains the "extra" copy of the very chromosomes that the mother contributes to the child? I have this vague thought that there are a total of three polar bodies. Is that just a sign that my memory of biology class so many years ago has faded? And when you say the same process occurs in the sperm, I take it you mean that when the sperm enters the egg it is haploid but contains two copies of the 23 chromosomes it will contribute to the child, and that after fertilization the extra copy is somehow gotten rid of. Where does it go?

About testing the donor's hCG level the day after the shot, I had been thinking that the hCG entered one's bloodstream fairly quickly, and then would start to "fade out" of it, so that someone who took the shot on time would have a lower hCG level than someone who took it late. But if your program tests to see that a certain hCG level has been reached, I must be wrong about that. When does the hCG level from the shot "max out"?

As always, many thanks. (I can hardly believe you took the time to explain meiosis to me. I've looked at a number of university websites trying to get this straight, but all I've seen is the usual explanation of the stages with little information about when they occur. Thank you so VERY much. It is so nice to feel less in the dark about these things.)

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You haven't got it quite right yet. Not to worry. As I said, the whole thing is complicated and unless you work with it daily as I do, it is impossible to remember.

When the egg gets to meiosis II, there are only half the usual (diploid) number of chromosomes. However, which half of the chromosmes the eg ends up with is random. For each pair of the 23 pairs, it could have one of the chromosomes contributed by the father or one of the chromosomes contributed by the mother. Mix and match, as it were. With regard to sex chromosomes, the egg is always left with an X chromosome because females always have two X chromomses. Half the chromosomes (which half is random) go to the first polar body and they do not contribute to resulting embryo. Instead of having the diploid number of chromomses (two copies of each chromosome), the egg now contain one copy of each of the 23 chomosomes.

The sperm determines the sex of the resulting embryo as sperm carry eith an X chromomse (that it got from the mother) or a Y chromomse that it got from the father. XX = girl and XY = boy.

Each chromomes has two "sister" chromatids. After the sperm enters, but before the sperm and egg chromosomes are combined in the nucleus of the resulting embryo, the egg excrudes a second polar body containing half of the sister chromatids for each of the chromosmes. The sister chromatids contain vertually identicle copies of the DNA. They are not completely identical becasue of "crossing over which occurs at an earlier stage. The crossing over story is for another day. The result is that the egg now contains half the number of chromosomes (23 single chromomes instead 23 pairs) and half the number of sister chromatids (each chromosome split at the centromere to produce two single chromatids - half a chromosome). This completes the meiotic process for the egg.

So, in the end, there are two polar bodies, not three.

The sperm has completed the meiotic process before entering the egg, so it brings in half the number of chromosomes and half the number of chromatids. The chromatids from each parent then replicate before the first division so that each chromosome has two chomatids. After the replication, the result is the original diploid number of paired chromosomes (23 paired chromomses - half from the sperm, half from the egg) and, after replication of the DNA, each chromosome now has two sister chromatids. The haploid pronucleus from the sperm and the haploid pronucleus of the egg then migrate towards each other and fuse creating the diploid embryonic nucleus. Now the embryo has the correct amount of chromsomes (2n) and chomatids to begin dividing (1 cell to 2 cell, 2cell to 4 cell and so on).

The maximum hCG level is reached about 24 hours after the shot. So, when we test the hCG level about 12 hours after the shot, it is still rising. If its low, that means the time of the injection was later than it should have been, since it hasn't had time to rise appropriately. Consequently, the follicles haven't been exposed to enough hCG yet and that will delay maturation, which is completed around 36 hours after the hCG injection. Ovulation (if allowed, which its not in IVF) would occur around 39-42 hours after the hCG injection, when the eggs have had a sufficient time to undergo the final maturation.

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Wow, thanks! We were miscommunicating a bit, I think, because when I talked about the chromosomes that the mother would contribute to the child, I meant the chromosomes that the egg would later contribute to the zygote (if the egg got fertilized by a sperm). I think that you took me to mean the chromosomes that the mother of the person whose egg we're talking about already contributed to that person (the one whose egg we're talking about). Also when I said "copy" of a chromosome I meant _exact copy_ where I think you you would call, say the 22nd chromosome that came from the father and the 22nd chromosome that came from the mother of the person whose egg we're talking about "copies" of each other.

This is all so interesting. (Too bad that knowing about the process won't make it any more likely that I'll be able to complete it successfully. Oh, to be eighteen, or indeed thirty-eight, again!) You've been just great. Thanks.

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