Lactation and especially milk, which is the product of that unique mammalian process, are routinely encountered within our daily lives. Nevertheless, they often are poorly understood by many, even including many who are engaged in the business of producing milk. The overall course goal is to introduce fundamental concepts that form the basis for understanding the biology of lactation, the biology of the mammary gland, and the products of that important physiological process. As a learner in this course, you will be provided with a series of easily understood presentations that collectively will help you build a foundation for greater understanding of lactation. You will be able to engage with other learners so that you can extend your learning beyond the video presentations. Ultimately, you will be able to construct your own mental model for understanding the wide range of topics that relate to the biology of lactation. Upon completion of the course, you will be prepared to expand your knowledge and understanding of lactation from other sources and experiences as you pursue your individual interests. Before you start the course, I suggest that you identify a question or several questions about lactation that you already have on your mind. This could be from your own experiences, something you read about or saw, or something you have wondered about. Write down your question(s) and use that to help you decide how to engage with the content of this course. You might engage with the modules in the order they are presented, or start with a module that is of particular interest to you, or pick and choose modules in any order. I encourage you to engage in all of the types of learning activities that this course has to offer, including but not limited to, the discussion forums, quizzes, peer-review assignments, and concept maps and other learning aids.
Microstructure - 2D and 3D
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Module 2: Mammary Gland Structure
One of the most fundamental requirements for understanding any aspect of biology is to appreciate the relationships that exist between form and function, or anatomy and physiology. In the case of lactation and the mammary gland, it is important to be able to visualize the various levels of mammary gland structure in order to understand how the mammary gland grows and functions. This module presents an overview of mammary gland anatomy and macrostructure using the dairy cow as our primary example. In addition, the tissue, cellular, and subcellular levels of mammary structure are presented. The module provides a conceptual foundation that will allow you to individually explore mammary gland anatomy and microstructure in other mammalian species.
教學方
Dr. Walter Hurley, PhD
Professor Emeritus
[SOUND] As a thin slice through the tissue,
essentially we're kind of looking at this in two dimensions,
but it is a three dimensional tissue.
So what I want to do is to kind of use some props here to explain,
get a better idea what this thing might really look like in three dimensions.
So let's get rid of that slide and use one of my props here.
This is a tube, would be not unlike the tubes that are in the lobule.
If we cut the end of the tube off, just straight through this, and
look at that in two dimensions, what are we going to see?
Essentially we're going to see, put it down here, a circle.
So there's the lumen in the middle and then this would be the epithelial cells.
That's an alveolus.
On the other hand, if we happen to cut this thing and
if this thing was curved like this and we happen to cut it up here, just right,
graze the surface here, again, what would we see?
We'd see an oval, something like that.
So instead of a circle, it would be a slightly different shape.
If on the other hand, we cut down closer,
down to the bottom of this kink right here, what would we see?
Well, we kind of see almost a figure eight.
Depending upon, how, where exactly it was cut.
Same tubule, just cut in different planes.
If we did something like this,
had a tubule that was really undulating like that and cut through it here,
we'd see just a bunch of, we'd see four circles.
And again, it depends upon the plane that we're cutting through as to
what kind of shape we would see.
So, let me get one more prop out here.
So if you can imagine, I've got a little one of these little things,
whatever they're called.
Got a lot of projections coming out,
just like all the tubules coming radiating out of that lobule as it's developing.
So if you can kind of imagine, if we kind of smoosh some of these up,
they're all in there.
They're all independent, they're all separate.
But they kind of go in all different kinds of directions.
And we took a two-dimensional slice through that.
We're going to see all kinds of things together.
And that's really kind of what we're seeing as we think about what a lobule is,
and what we're seeing in those pictures.
So let's go to the next slide and I'll try to explain this again.
So, these are serial sections through the same developing lobule.
Again early in pregnancy in a pig, the first pregnancy in a pig.
So, let's examine something.
So again, these are serial sections for the same lobule.
So, you can see right here as, I'm sorry,
the duct comes in, radiates off in all these different directions.
Right in here on this slide, we've just kind of hit,
it's kind of like this demonstration over here I drew,
just kind of grazing the inner edge of the duct.
The next, this is several sections later.
These are about five to eight micron wide sections.
We're kind of right in the middle of that duct.
And then down here it's starting to close,
we're starting to get to the other side of that.
So if we go back to this guy, the top one in that particular duct,
we're kind of hitting this edge.
The middle picture, we're kind of hitting right in the middle and then this picture
down here, right here, we're hitting the bottom edge of that particular duct.
So again, you can kind of see the interpretation.
If we look at this up here, it looks like it's something entirely separate.
But if we come down here,
we realize it's part of the same thing, it's just radiating off.
So it depends upon what section we're in cutting through this that we actually see.
Again, you can see that right there,
it's continuous and you kind of guess that it's continuous in here.
It would look like it was something discrete.
So we have to keep that in mind as we're thinking about
the kinds of images I'm going to show you and interpreting those images.
Let's go to the next slide.
This is in a lobule.
What we would think of these is a lot of aveoli,
whole bunches of alveoli here, that is those discreet circles.
But go to the next slide, what I've done is to try to draw a line here to
indicate this is all one tubule, actually branches here and goes up.
And so just to kind of illustrate that,
if we kind of lay my tube behind that, you can see it's all the same tube.
But again, it's branching here, it's probably branching here.
Some of these other ones may be continuous with it, or
they may be a separate tubule that's kind of undulating, and you just happen to be
catching different components of it as it's cut through in two dimensions.
Go to the next slide.
But again, one of the keys is that these lobules are discrete.
So again, very early pregnancy,
these are just four separate lobules that are developing here.
They probably all go in to the same duct, which is not in the plane of this
particular section, this histological section, but again, they are discrete.
So as they develop,
this is going to develop this separately from this, from this and so on.
The other thing I want to point is if we look carefully at this,
what we can see is there are several kinds of connective tissue.
It depends upon the point in development of the animal.
We have a lot of adipocytes here, these fat cells.
We have the heavy connective tissue down here, that really dark pink stain material
that you've seen in some of the other images here earlier on,
here's some more right here.
But if you look inside the lobule,
what you find is there really aren't adipoctyes inside the lobule.
And you don't really find this really hard, heavily pink stained,
really thick connective tissue bands running inside the lobule.
So there, the connective tissue between all these tubules and this alveoli,
as we've called them, is a little bit different then the connective tissue
out in these other areas.
And we'll see that in some other slides as we go through this.
Here's one example, this is a very late pregnancy, so
just a few days prior to when this particular pig would have their young.
So there's the pink stained material here,
the really dark pink stained material is essentially colostrum.
We can see the connective tissue sheaths that surround these lobules,
so there's one lobule here, two, three, four, five and six lobules.
And see where the fat cells are, these are the adipocytes, they're between them,
so they haven't gone away quite yet.
They will go away when that animal starts lactating there.
So again the lobules are discreet and we think of these as alveoli,
they're just again part of the same structure, that lobular structure.
And just basically depends upon the plane that you're cutting through the lobule.
So let's review very briefly,
just again this introduction again to the idea to the lobules, to alveoli.
Let's have the slide here we just saw a few moments ago, again this idea of
lobule, these are the discrete structures, surrounded by connective tissue.
There is connective tissue within the lobule, but
this is going to be a structure that develops independent of this structure,
independent of that structure and so on.
And within each of those many, many, when they start lactating at any rate, many,
many of what we call alveoli.
And there again we get back this idea of depends upon
the plane at which you're cutting through that tubule as to whether you see circles,
like you do right here or whether you see other kinds of shapes.
Like we might see, here's more of an oval-shaped one right here,
again it kind of just depends upon the plane.
So an alveolus is really in that sense,
an artifact of the fact that you've cut through this thing and
how it's again, a very thin, two-dimensional slice through the tissue.
