And we're back with part 4 of Sci's brand spanking new series on female reproduction!! This one's going to be a whopper, fertilization and pregnancy is a lot to cover, and so it will be very basic and divided into two posts, though less basic than the "baby in your tummy" bit that Sci learned when she was small.
(Sci had this book when she was small. It made her very popular with the local kids, and I still giggle every time I think of the part where they tell you that an orgasm is a like a big, wonderful sneeze.)
So here we go, basic fertilization and early pregnancy, starting with your friend, the sperm:
When a daddy and a mommy love each other very much...
So when a man and a woman get it on, use IVF, or any other method used to get sperm up inside a lady, you run the risk of fertilization occurring.
And entering the ring, on this side, THE SPERM!
As you can see, the sperm is basically just a cap full of DNA and enzymes (the enzymes are in the acrosome), and a long tail full of mitochondria. The mitochondria provide the power to make this dude a powerful little swimmer. About 1-3mm/min. Speedy. Due to the mitochondria, it only swims one direction. Not one step back for these guys.
(Sperm are powerful swimmers)
And in this corner...THE EGG!
The proper name for the egg here is the oocyte, but that's annoying to type so I'm going to call it the egg. The egg here (as seen in the previous post on the menstrual cycle) has already gone through ovulation, and is hanging out usually somewhere in the vicinity of the fallopian tube (which tube is up for debate, on average, women alternate each month, ovulating left or right, so you have a 50-50 chance. In the case of fraternal twins, two eggs are released and fertilized. In the case of identical twins, one egg is released, is fertilized, and splits soon after to end up with two fetuses).
You will notice in the picture above that the egg is surrounded by what appear to be little bubbles. These are granulosa cells, cells that could have been eggs, but did not respond fast enough to the spike in FSH and LH in the menstrual cycle, and are now doomed to play this egg's entourage, called the corona radiata. They will play a part in the protection of the egg.
So now you have the egg and the sperm, LET THEIR POWERS COMBINE!
(Couldn't resist. It's late and Sci is punchy. *sings* Captain Planet, he's our hero, gonna take pollution down to zero...)
So fertilization: it's an obstacle course of epic proportions for the sperm, and this is all to the female's evolutionary advantage. She needs to bear some fit offspring, and the fittest male to do the job is going to have the best swimmers, who can get past the vaginal mucosa, the cervix, and who happen to take the right turn into the right fallopian tube. And once a lucky, lucky sperm hits the egg, the REAL work begins:
(It's even more complicated than this. Hold on to your hats)
1) The sperm approaches the egg. To get in close to the egg, your chosen sperm has to weave its way past all the granulosa cells in the corona radiata. Like trying to wiggle your way through to the bar at a St. Patrick's Day celebration.
2) The sperm wiggles up to the bar and binds to a coating on the outside of the egg known the zona pellucida, this binding triggers the release of the enzymes in the acrosomal cap of the sperm, which start chewing their way through the zona pellucida.
3) Enzymes: Om nom nom
4) The sperm heads in toward the oocyte membrane in the gap created by the enzymes. Once it hits the membrane of the egg, microvilli (little fingers of membrane) reach out and surround the sperm head. He's got an in.
5) When the microvilli surround the sperm head, the two cell membranes of the egg and sperm fuse. The contents of the sperm (the DNA) enter the egg.
6) You'd think that was all there way, right? Nope! The moment of the sperm contents going into the egg triggers the egg to harden up the defenses. You don't want to get fertilized twice, and so the egg causes the zona pellucida to harden, preventing other sperm cells from entering. She's made the commitment.
7) Male and female nuclei fuse. We have fertilization.
(A side note on sex determination: Did you know that it's the male that determines the sex of the offspring? Of course you know that women are XX and men are XY. When sperm and eggs go though meiosis, they end up with exactly half of the chromosomes, so women end up with X or X, while men end up with X or Y. This means that the male must donate the sperm with the Y in order to end up with a male fetus. This also means that Henry VIII divorced or beheaded all those women due to their inability to bear him a prince when it was ALL HIS FAULT. That jerk.)
And now we're on to the next step.
The fertilized egg will implant in the wall of the uterus, to form the placenta (which is SO fascinating and we'll get to it in a second) and in order to allow the blastocyst, and eventually the fetus, to get nutrition from the mother.
So first, say your egg is released and fertilized on day 1. We can now call it a zygote. It's still in the fallopian tube and is going to take another 3-4 days to make it down to the uterus (eggs aren't fast movers). During this time, it's already been fertilized and will start undergoing divisions. Zygote will become the morula. The morula reaches the uterus and divides some more to become the blastocyst. The blastocyst will actually hang out in the uterus for another 2-5 days before finally implanting in the uterine wall, which it will need to do to continue to draw the nutrients necessary for growth.
Prior to the implantation in the uterine wall, the little ball of cells is obtaining nutrients from secretions in the fallopian tubes and uterus, provided to it by the increases in progesterone during this part of the cycle.
But now, it's big, it's bad, and it needs to implant.
But it needs to implant in the MOTHER. The blastocyst now has half its DNA from the mother, and half from the father, so it's possible that the mother's immune system will recognize it as foreign tissue and reject it. To prevent this, the blastocyst secretes immunsuppressive hormones, including the one called human chornionic gonadotropin (hCG). hCG is the important one here, it maintains the corpus luteum in the ovary, which helps keep progesterone levels high in the mother to promote nutrients for the blastocyst and to keep the uterine wall nice and think. hCG is ALSO important because it is the hormone that is detected in early detection pregnancy tests, if you test high for hCG, you must have a blastocyst in there.
Then the blastocyst develops an outer layer of cells called trophoblasts. These trophopblasts secrete enzymes that digest the endometrial wall of the uterus. The digested stuff goes back to feed the blastocyst, and the blastocyst itself invades the endometrial wall of the uterus.
These trophoblasts have another role to play: they become part of the...
Placentas are cool. Really cool. Some people eat them, which is kind of gross. But what Sci thinks is awesome about the placenta is that it is an amazing merging of fetal and maternal cells, all without any direct contact, so the maternal tissue won't react to the fetal tissue inside it. It's a truly brilliant system.
Though it looks really gross:
The placenta is a multi-layered affair arising from the trophoblast cells of the blastocyst, and the endometrial wall of the mother. It's the interface through which EVERYTHING is going to go, oxygen, nutrients, waste, etc, until the baby is born.
You can see above how the blastocyst sends out trophoblastic cells, and these will recruit maternal blood vessels to the area of the blastocyst. In the early months of development, the permeability of the placenta is low, because the membranes between fetal and maternal tissue haven't been reduced to their minimum thickness, but as the baby gets older and requires more oxygen, permeability increases.
The fetal part of the placenta (as well as the maternal part) are full of capillaries, which are in little finger-like things called villi. The capillary walls are very thin, and surrounded by several cell layers.
The fetus's blood flows through two umbilical arteries to the capillaries in the villi, and back through an umbilical vein to the fetus. At the area of the villi, the fetal blood interfaces with the placenta, where the maternal vessels are, and oxygen exchange occurs through a portal system.
ARGH. This is going to be effing hard to explain without a picture, and I cannot seem to find a pic of the portal gas exchange system. Anyone wanna help me out?
In the meantime, here goes:
The portal system works like this: the mother's blood comes in to her artery, all high in oxygen and nutrients. It comes close to the fetus' vein, which is very LOW in oxygen and nutrients. As they pass each other, the things in the maternal artery will diffuse into the fetus' vein, because things in nature tend to diffuse from higher to lower concentrations. Once they are equal, diffusion ends, and that's all the fetus is going to get.
Luckily for the fetus, fetal hemoglobin (the chemical we use to carry oxygen) has a higher affinity for oxygen than adult hemoglobin, and so the fetus is capable of gleaning a lot more oxygen out of the maternal blood than it might otherwise.
What is really cool about this system is that the fetal blood and the maternal blood never touch! This is why you can have a different blood type from your mother, since they never touch, she can't have an immune response to you (though in some cases this still happens, and it apparently makes for a really miserable pregnancy).
Now, you might know that the fetus and the placenta do not touch each other directly, they are connected via the umbilical cord. As the fetus grows, it floats in amniotic fluid which is produced by the kidneys of the fetus, which helps protect the fetus from physical stress, and which also (towards the end) serves as a place for waste to build up. That fluid will be lost via the vagina during birth (this is your water breaking, and since it's full of baby crap by then, the water breaking doesn't sound very nice).
And now, we've got amniotic fluid, we've got a placenta, we've got a fetus.
It's time for pregnancy. But Sci is exhausted, and we're going to come back to pregnancy at a later date. Like Wednesday. Stay tuned!