|MadSci Network: Cell Biology|
Thanks for such an interesting question Jason. Lets take a look at some of the issues in your question, and I think you will see that there is alot more going on during the formation of a zygote than just the alignment of chromosomes.
First off, lets clarify some definitions, because your question mentions a 'female egg' that has 23 pairs of chromosomes, and thats a bit confusing. Almost all eukaryotic cells have 2 copies of each chromosome. Humans have 23 pairs of chromosomes. Most of these are numbered from 1 - 22, and there is also an X and a Y chromosome. Human females have 2 copies of chromosomes 1-22 and two X chromosomes. Human males have 2 copies of chromosomes 1-22, one X chromosome and one Y chromosome. The two copies of a given chromosome present in a cell are referred to as a 'chromosome pair'. Cells with two copies of the chromosomes are called 'diploid' cells.
Almost all multi-cellular amimals go through a stage in their life-cyle in which they produce gametes which fuse (in a process called fertilization) to form a new cell called a zygote (not an egg). The zygote then develops into a complete organism. Gametes differ from all of the organism's other cells in that they only have one copy of each chromosome from each pair. These cells are called 'haploid' cells. In humans, gametes made by males are called 'sperm', and gametes made by females are referred to as 'oocytes' or 'ovae' (eggs). Because diploid male cells have both an X and a Y chromosome, sperm can have either an X chromosome or a Y chromosome in addition to one copy each of chromosomes 1 - 22. Because diploid female cells only have X chromosomes, oocytes can only have X chromosomes. So a person's gender is determined by the nature of sperm that fertilized the oocyte.
So, in your question, where you are asking about taking a 'female egg' from 2 donors, I am going to assume that you are asking about oocytes, because there are no male or female oocytes, and because while diploid zygotes can be male or female, they cannot be fused together to form a baby boy or girl. Your question then becomes, "Why can't you form a (female) zygote by fusing two oocytes, and why doesn't the body allow such an occurrence."
With that in mind, lets look at some of the issues involved. First of all, the oocyte is surrounded by a thick jelly-like barrier called the zona pellucida (filmy region). This barrier has to be digested by special enzymes before anything can get close to the oocyte's cell membrane. The enzymes that are capable of digesting the zona pellucida are found in a special structure on the tip of the sperm cell, called the acrosome. When a sperm cell encounters the zona pellucida, the acrosome releases its enzymes, and these spread into the area surrounding the sperm. Each sperm has to reach the oocyte's cell membrane by enzymatically cutting its own path through the zona pellucida. The enzymes found in the acrosome are not found in oocytes, so it would be difficult in nature for two oocytes to meet and fuse to form a zygote; their respective zonae pellucidae would keep them apart.
Once a sperm cell penetrates the oocyte's cell membrane, the membrane changes so that no more sperm can enter the cell, and this change sets in motion the entire series of events that result in the fertilized zygote developing into an embryo. This process is initiated by an unidentified protein found in the cytoplasm of the sperm cell. When a sperm cell fuses with an oocyte, the presence of this protein causes a calcium cascade, which changes the oocyte's cell membrane as it propagates around the surface, and this signal starts the processes that result in a new organism being formed. Oocytes don't contain this protein, so the process would not be initiated even if you stripped them of their protective barriers and fused them.
In addition, the chromosomes that are found in mamalian sperm (paternal chromosomes) and the chromosomes that are found in mamalian oocytes (maternal chromosomes) are marked in a process called imprinting. Some of the genes found on paternal chromosomes are activated during development, while the same genes on maternal chromosomes remain inactive. Similarly, some maternal genes are active during development, while the corresponding paternal genes remain inactive, so you can see that if you had two copies of maternally imprinted genes but no paternally imprinted genes, you might run into some problems; not all of the genes you needed would be active.
Taking the long evolutionary view, the reason that organisms don't fuse two oocytes to form a zygote is because sexual reproduction functions to increase a species' genetic diversity by combining and mixing genetic characteristics from different individuals. If you reproduce simply by fusing oocytes derived from the same organism, you will wind up with a new organism that is identical to its parent at best, and which has lost half of the parent's genes at worst. That probably isn't very good for the species in the long run.
Still, there are a few rotifers, fish, reptile and bird species that reproduce through a process called parthenogenesis. The offspring in these species are genetically identical to their parents (the best case scenario above), but even these species don't generate new zygotes through the random fusion of oocytes; the haploid nucleii in the oocytes of these species duplicate, resulting in a diploid zygote.
What follows is largely speculation on my part, but it seems possible that, perhaps billions of years ago, organisms did reproduce by fusing identical oocyte-like gametes, because there were no sperm. There was probably a distant time, in the early days of eukaryotic life, when all organisms were "female". If the gametes of these hypothetical organisms managed to fuse with the gametes of a different organism, then the goals of sexual reproduction (increased genetic diversity) would have been achieved, but more often than not zygotes would probably have been formed from two gametes from the same parent, resulting in a loss of genetic diversity in the next generation. It would not be until some organisms started producing specialized sperm and/ or oocyte gametes that the advantages of sexual reproduction would really become apparent.
So, I hope you can see what some of the barriers are to the generation of a viable zygote from a pair of oocytes. There are physical barriers such as the zona pellucida, biochemical barriers such as the absense of a fertilization-triggering enzyme in oocytes, and genetic barriers such as genetic imprinting, all of which are the result of billions of years of evolution favoring sexual reproduction.
Sperm protein that signals fertilization
Wolny YM, Fissore RA, Wu H, Reis MM, Colombero LT, Ergun B, Rosenwaks Z, Palermo GD (1999) Human glucosamine-6-phosphate isomerase, a homologue of hamster oscillin, does not appear to be involved in Ca2+ release in mammalian oocytes. Molecular Reproduction and Development 52 277-87
Reduced genetic diversity in parthenogenetic reptiles
Fu J, Murphy RW, Darevsky IS (1999) Limited genetic variation in Lacerta mixta and its parthenogenetic daughter species: evidence from cytochrome b and ATPase 6 gene DNA sequences. Genetica 105227-31
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