Implantation Behavior MCAT Khan Academy
So it's been about a week since you've been fertilized. You used to be an egguntil a sperm found you, you went through a zygote stage and you cleaved, dividinginto multiple cells, and you've finally become a blastocyst. You've been bouncing aroundin the fallopian tube for a little while,but you finally made it
into the endometrialcavity within the uterus, and you start bumping against this nice endometrial lining,and that looks like a great place to get nutrients. In fact, you'd like to start the process of implantation. But at this point, you're still stuck inside this shell, this thick layer
of glycoproteins that are surrounding you called the zona pellucida. Well a good thing, about this time your zona pellucida is starting to finally disintegrate away, so I'm going to erase bits of it here, you can see it's starting to fall apart, and your outer cells are becoming exposed,
you're hatching out ofyour zona pellucida. But the endometrium isn'tsitting still either, in fact, it's anticipating your arrival, and the endometriallining is proliferating. In fact, it's developingthese valleys here, and you come to rest in one of these valleys, and that's called a crypt. And here, your zona pellucida is still
disintegrating away, and your outer layers of cells can come directly in contact, and this contact between the two cell layers is called apposition. Now at this point, you're not really firmly embedded at the endometrium, you're just kind of resting on top of it. You could be easily dislodged.
What you'd like to do is get really stuck in there, so that you can start the process of nutrient transfer. And this outer cell layer that we've mentioned before, theseare called trophoblasts. They start to multiply, in fact, they don't only multiply, but they start invading in, and you can see them here,