[SOUND] Now we go into the various stages of mammary gland development during the fetal phase of development of the individual. What I want to do is kind of compare some different species here. So this is what we just really talked about with regard to the mouse. So mouse gestation of 19 to 20 days. Placode started appearing about 11.5 days, something like that. The mammary bud about a day later. A few days later, nipple formation. Primary spout very shortly there after. And then canalization, I talk about lumen formation, and sometimes people use the work canalization for the canal being formed in the center of that solid ball of cells. That's about day 18, so it's getting real close to the young being born, for this is kind of wrapping up at this point. So let's take another look at another species and let's talk about pigs, or swine. And what do we got here? We got about 114 to 115 days. And again, all these numbers are going to be in days, I've indicated that up here. We're talking about 23 days for what would be equivalent to a placode being formed. Just a few days later, mammary buds being formed. And that's not unlike that, there's a lot of difference between these two stages. T formation is starting at about day 40, and primary sprout formation at about day 55. And canalization's occurring about Day 85. So it still has about a month left, about 30 days left of pregnancy when canalization's occurring. So this is getting close to being all the structures are there, so very shortly thereafter that would be the case for this particular species. Okay, let's take a look at another species that might be of interest to some of us and that is our own species, humans. Okay, we have about 270 day gestation or pregnancy. When does the placode appear? Just past the first month. The mammary bud is just, again, instead of a week and a half later, almost two weeks later. The nipple formation occurs at approximately 56 days. About 84 days is for the primary sprout being formed. And then canalization is occurring a bit later in our species, at about 150 days. So there's still four months of gestation by the time, again, we get to this point of where essentially the nipples form, the rudiments of the gland are all pretty much there. Then we want to compare that with a species that we used a lot in this course and talk about, and that's cattle. 280 day gestation, in this case. About 35 days, so not that dissimilar from our own species, but 43 days here. About 65 days here, For nipple formation, or teat formation. About 80 days, For primary sprout formation, and then the beginning in at about 100 days for canalization. So some similarities, some differences. Obviously the longer gestation goes, the more it's stretched out. Again, humans and cattle, in terms of total length of gestation, there's a lot of similarities to these numbers. Cattle seem to be forming canalization a little bit quicker or a little bit earlier than our own species. So these next several images are bovine. This particular image is about 33 days, 35 days of gestation to the attachments occurred there. You can see in, just to the left of the center there, is where the actual embryo or the fetus is. 33 days, 35 days, this animal would be starting to form a placode. So it's about that timeframe. A bovine fetus, again, way over to the left is the fetus. You can see the the full placenta there having been formed at about 95 days. So this animal would be past the primary sprout stage. It might be a few secondary sprouts being formed and really just has started or it's just about to start the canalization stage of mammary development. And this one is a 180 day fetus, bovine fetus. Large again, placenta is very well formed there. The animal is really getting pretty big at this point. It still has 100 days of gestation to go. 180 days, everything that's going to form is formed. They'll continue getting a bit bigger but pretty much all of the structures that are going to form during the fetal stage have formed. And I think that's a really critical point is that by the time the animal is born, everything is there. The nipple is there, what would be the primary sprout or what comes from the primary sprout, some of the really, really major ducts right at the base of the nipple are there and those structures are there. The fat pad, again, also had been formed at that point. So we can kind of summarize that, during the fetal period, the mammary gland goes through, is formed by these reciprocal interactions between ectoderm and mesoderm. Again, I talked about the idea that the ectodermal cells are communicating with the mammary mesenchyme. And that communication's going on constantly. And then when the primary sprout dips down into the fat pad precursor, again, you've got a lot of communication going on between what's mesenchymal cells or mesodermal cells, and what were originally ectodermal cells. And that's what's causing the gland structures to form. Mammary primordia, undergo multiple stages of development. So we've talked about of the placode, the bud, the bulb, the primary sprout formation, the canalization we talked about a whole range of different stages of relatively distinct stages. And again, to emphasize, at birth then the primary tissues that have formed, the teat or nipple, limited branching of the ducts, and this fat pad that we'll keep talking about for the next several stages. So that is a quick summary of the fetal period in terms of mammary development. [SOUND]
