Lobes, Lobules and Cytogenesis

00:01 It has a long duct that opens into the duodenum some distance away. Well, when we look at large glands, we can see that they have a certain type of organization. Here is a section through a large salivary gland. When you look at from this magnification, it is rather deeply pink staining and you can identify or you will able to identify the different types of salivary glands because the three types of salivary glands and the pancreas have a preponderance of a certain sort of secretory product. Some are purely mucous, some such as a parotid gland and the pancreas are purely serous. Some are mixed. And using that criteria, you can distinguish the three different salivary glands and the pancreas. But all these large compound glands are divided into lobes in smaller components called lobules. Look very carefully at this section here and you can see large connective tissue spaces dividing this gland into large segments.

01:17 The septa of connective tissue are dividing these glands into lobes. Smaller segments of the connective tissue can divide the lobe into a lobule. So look carefully at this slide and you can see lots of tiny little divisions. They are separated by wide spaces here, which is artifactual caused by shrinkage of the tissue, but it is a good way of really illustrating that all these compound glands are dividing into large components called lobes and each lobe has subdivisions called lobules. And later on we are going to learn that ducts and also blood vessels acts as the secretory units first of all by coming in through these large connective tissue spaces between the lobes and then through the smaller connective tissue spaces going to each lobule and therefore to the secretory units.

02:22 Let me just diverge slightly and introduce you to the myoepitheleal cells. These are contractile cells. And as you can see here, if you look very very carefully, this is a section through a sweat gland. And what you see the pilar regions, the pilar clusters of cells are the secretory units of the sweat gland. The small darker little tube you see is the duct portion of the sweat gland. And I will talk to you in more detail about these glands when we look at skin.

02:58 But what I want to bring your attention to, is the myoepithelial cell. As I've said, they contract themselves and they wrap around the secretory units. They hug the secretory units like an octopus.

03:15 And you can see very very fine little red lines on the outside surfaces of these secretory cells.

03:22 They are the processes of the myoepithelial cells wrapping aorund the secretory units.

03:31 Myoepithelial cells are very important in exocrine glands, because they wrap around the secretory units and help with the secretion moving out of the cell into the luminal space.

03:44 There are sometimes glands secrete cells. They have stem cells and the product in these glands are other cells, to replace cells higher up in epithelial surfaces, above where the glands are. They don't produce proteins or mucous. They produce other cells. If we have a look at these three images, they illustrate secretory cells and also stem cells. This picture here on the left hand side illustrates a number of finger like projections that protrude into the lumen of the small intestine. These finger like projections are called intestinal villi.

04:34 And labelled here is a single intestinal villus. Where that label is pointing is the underlying connective tissue, the lamina propria. But on the surface of these villus or villi is an epithelium and that epithelium is involved with absorbing material from the lumen of the duct into various blood vessels and lymphatic channels that running at lamina propria. But on the middle section, you can see some red stained cells, that live in the glandular part of this epithelial surface, labeled here as small invaginations into the underlying lamina propria. That is where the secretory cells live deep in the glands and not on the surface.

05:31 Well besides these secretory cells, there are also stem cells shown here. A different stains used to show up clumps of chromosomes in some of these cells that are undergoing mytotic activity. Well these stem cells, at the base of these gland,s produce other cells that move up along the surface of the little glands and they differentiate into secretory cells that you see in the middle section or they can differentiate into the surface epithileal cells and be involved in the absorption of products from the lumen. These stem cells are very important in epithelia because epithelial cells are continually being replaced. And they are replaced because of these stem cells being present in all the epithelial surfaces that we are going to come across when we look at the organ systems in later lectures.