so we got the AMAZINGLY GOOD news yesterday that all three embryos made it to blastocyst phase, meaning they were happy in the lab and grew from single cells to mulberry-ish clumps of cells to big hollow fluid-filled beachballs. the outer ring of the beachball, the “trophoblast” (greek: “immature cells for feeding”) will eventually become the placenta that “feeds” the embryo/fetus, and inside the trophoblast layer, there’s a clump of cells off to one side called the “endoderm” (greek: “inside skin”–kinda creepy) that will eventually become an embryo (2-8 weeks after conception) and then a fetus (8 weeks from conception to birth). the clump will differentiate into three separate cell layers (think a stack of pancakes), then roll up into a tube (think an enchilada made of 3 pancakes), then curl into the classic embryo shape (think a macaroni noodle made of a 3-pancake enchilada). that’s you and me. we’re glorified macaroni noodles. just tubes with attitude.
this morning, at 6 days of age, our embryos were biopsied (a couple of cells were pinched off the trophoblast–not as risky as it sounds) and then sent into cryostorage. they’ll stay frozen on day 6 of development until we thaw them for transfer, and in the mean time, those three sets of cell samples are being sent to a lab for genetic testing. lots of folks have asked me about that, so i figured i’d expound a little in today’s post. stay with me 🙂
genetic testing isn’t “choosing eye color and hair color” and all of the things that we associate with high school learning about expression of dominant and recessive genes. each of the samples we sent will undergo karyotype testing for what are called “aneuploides” meaning too many or too few chromosomes. these errors can occur during meiosis (the cell division that produces eggs and sperm) or mitosis (the cell division that happens after fertilization).
at my age (40), there’s a whopping 86% chance that any given egg will contain a genetic error caused during meiosis. if you think waaaaaaay back to your last a&p class (which was likely high school bio), you’ll remember that during cell replication, the soupy, unorganized chromatin in a cell’s nucleus organizes itself into the “chromosome shape” that we think of when we think of genes, does some replicating and some gene swapping, and then spindle fibers pull the chromosomes to their own side of the cell before cleavage and division happen (massively simplified–here’s a good image of meiosis). as XX folks age, our spindle fibers get tired (a lifetime of paying taxes and fixing everyone else’s problems will do that, y’all), and they may pull all the genes to one side before division, creating one cell with too few genes and one with too many.
so lets say you have an egg with too many or too few genes, and it combines with a sperm that has its own set (23 chromosomes, or half a hooman). if your egg has an extra copy of one of the chromosomes, you wind up with one of the “x shaped” chromosome pairs that now has three sets (looks like XI) instead of two (looks like X). this is called a “trisomy” (greek for “three bodies”). most trisomies are not compatible with life, meaning cell division will arrest at some point or that an embryo, fetus, baby, or child will die as a result. a few trisomies are compatible with life: trisomy 21 (three copies of the 21st chromosome, or down syndrome) and sex chromosome trisomies (shout out to my XXX friends!) are examples.
if the egg with too few chromosomes is fertilized, it results in what is known as a “monosomy” (greek for “one body”, and on a karyotype you’d see the “I” of one set of genetic information where you’d expect to see the “X” of two sets. only one type of monosomy is compatible with life, and that’s monosomy at the 23rd pair, or XO sex chromosomes (X nada, compared to XX, or XY). this monosomy is called turner syndrome, and while compatible with life, it causes short stature, infertility, and often heart and other organ system problems. actress linda hunt has turner syndrome, for stature reference.
all of this to say that because my old-ass spindle fibers aren’t so much interested in reproduction this late in life, only 1 in 7 eggs that i produce will be free of trisomies and monosomies. that’s 14%, y’all. sperm are much less likely to contain these errors because they’re batch-made fresh daily (sorry), but it is possible. it’s also possible that egg and sperm are fine that but a trisomy or monosomy is caused early in mitosis after fertilization, and that error replicates in a way that will eventually cause arrested development of an embryo or a situation incompatible with life for a fetus or baby.
fun fact: these errors can also produce “mosaic” embryos that are a combination of normal (“euploid”) and abnormal (“aneuploid”) cells, and embryos can sometimes correct this error and junk the bad cells!
our genetic testing, then, isn’t about eye color. it isn’t about intelligence, handedness, or ability to curl a tongue. what cooper genomics will be looking for in our wee three will be complete sets of chromosomes. question: are all of the instructions to make a human there with nothing missing and no competing copies? 46 chromosomes in 23 pairs. goldilocks style–too many? too few? just right? when we get our report back in nine days (DID I MENTION WE HAVE TO WAIT NIIIIIIINE DAYS?), we’ll know whether any/some/all of our embryos are genetically viable. each has a 14% chance, which seems abysmal, but i keep reminding myself that stats like this only really matter on a population scale. they could all be perfect and normal. i’m voting for that one.
fun side note: because the 23rd set of chromosomes tells us XX or XY, we’ll know the assigned sex of our embryos. even cooler: there still gets to by mystery because we won’t know how they would grow up, look, develop, express, be, feel, and identify. even weirder: if all are viable and there’s variety, we can choose an XX or XY kid. even meaner: if we do, we aren’t going to tell anyone! mwahahaha.
so that’s where we are today. the snowseeds are on ice, samples are en route, and scientists will be doing scientist shit with them over the next week.
folks, if we clear this hurdle, even with one of the embryos, there’s almost an 85% chance that embryo transfer will result in a viable pregnancy. each hurdle has felt enormous, and this one feels biggest of all. thanks for being along for the ride!
yours truly,
a hopeful mama