[NOISE] Okay, we just went through several stages and what I've done here is to redraw the bud stage. Because I want to do some other stages as we go along here. So again, remember, these mammory primordia that form along this mammary line on the embryo and the fetus. Mammary bud stage is a critical, kind of pivotal stage. Remember again, we have the ectodermal cells. They're basically, at this point, epithelial cells. The mammary mesenchyme right around them, and then the fat pad precursor. And these things will start to become ever more important as we go along. The next stage is called the mammary bulb stage. And here, again, ectodermal cells. Except it kind of starts looking almost like a light bulb, in a sense. The same kind of idea, mammary mesenchyme around here, and then the fat pad precursor down here. So this is called the bulb stage, and that's at about day 14. There's a slight difference, actually, in the cells that are in this neck area, even compared to these. So things are actually, again, starting to kind of differentiate and move along, in terms of forming what we're going to end up with in terms of the mammary gland. [COUGH] So a couple of things start happening next. So okay, we can go draw an arrow like this. Now we're going to go down to the next stage. After this stage things start to happen, not simultaneously, but very quickly. A whole bunch of things start to happen. One of those things, we start getting nipple, or teat, formation. And I'll try to draw this. What's happening here is that the mesenchyme cells, the mesenchymal cells, are growing and pushing this way. So we're starting to push this out of the surface. So you can actually see something growing basically out of the surface of the embryo. You've got a little mammary groove there, or mammary pit is what they call it. Again, you still have the mesenchymal, mammary mesenchyme. And you still have what's going to be a fat pad precursor down here. So we start seeing nipple formation, nipple or teat formation. Around the same time, maybe just a little bit after that, let's see. Teat formation's occurring at about, for the mouse, about 16.5 days. So again, a little bit beyond the bulb stage. Then things start to get really kind of interesting. because you start to see some structures that you can project ahead and say okay, other than the nipple, we can kind of see what these things are going to end up being. So now we start getting into what's called the sprout formation. So I'll kind of replicate what I just drew a little bit. So we have this, this is very similar to what we have here. Again, the mesenchymal cells, ectodermal cells. But what happens is, down at the bottom here, this starts sending down what's called a sprout. This is called sprout formation. And what's happening is that this is growing down in that mammary fat pad precursor. So once it starts dropping down in there, again remember, this is still a solid core of cells here. This is a solid core of cells. Once it starts dropping down in there, now all of a sudden this thing starts elongating. The second thing that starts happening is you start getting these side branchings coming off of here. So some of them might come off of here. Again, this is called branching or ductal branching. And how that's happening is again, this idea of a communication between the cells that are here in mesenchyme, and the cells that are in the solid cores of cells that are making up the primary sprouts. These offshoots here, they are the ductal branching, are sometimes called secondary sprouts. Again, at this point, this is a solid core of cells. So again, we've gone from starting a nipple formation to primary sprout, secondary sprouts, the ductal branching. Sometimes they call that the ductal tree, or glandular tree. Then, so this is going to be the nipple. This is going to be the part right under the nipple. We're starting to get the major ducts being formed. But we don't have a lumen yet. So after this stage, we start to see lumen formation. So in the middle of this thing, again remember, this was a solid core of cells. Right down about this area, the cells in this part, in the middle start to die. They undergo a process called apoptosis. Apoptosis is programmed cell death. The residual part of those cells is phagocytized, or eaten by the cells that are still surrounding this thing, the live cells that are there. And you start to get a lumen formed. As time then progresses, this lumen expands this way and expands this way. So by the time the animal's born, he has a nipple. Except for the ones, of course, the male mice is one example. But most of them have a nipple, the males and females have a nipple. You're going to have the major area right below the nipple. You're going to have, the major ducts have formed, the very, very initial major ducts have formed just a little bit. And then you have a lumen having been formed there at this point. So we're going through all these stages. And now these things are, you can actually see something on the gland or on the fetus. And then looking inside of them, it goes just a little bit interior to the animal, to the surface of the animal. [NOISE]
