Cells: Adult Stem Cell Niches, Lymphocyte and Plasma Cell

00:01 One of the other cells that we see in connective tissue is the stem cell. On the left hand side, there are certain stem cells in epithelial tissue. These epithelia shown here happen to be lining the intestinal area in the small intestine and epithelial cells are continually lost or that need to be replaced. So in epithelia, you have these stem cells. While on the right hand side, you have a stem cell called the pericyte. The pericyte is associated with blood vessels, little blood capillaries and they do a number of things. They can control the growth of endothelial cells, in other words the growth of blood vessels, but they are relatively undifferentiated.

00:58 But they can be stimulated to become advent tissue cells or paravascular cells to strengthen the capillary wall. In some circumstances, they can develop into smooth muscle cells as the blood vessel network develops further. And here you see some smooth muscle cells around the very very small lumen of a blood vessel, that perhaps could have been derived from one of these pericytes. You know in actual fact, the pericytes are surrounded by the external lamina of the endothelial cells that lines the blood vessel, so it is seperated from the connective tissue. So really or probably not, by definition, a connective tissue cell, what we tend to talk about pericytes when we talk about very small blood capillaries lying in connective tissue spaces. Well, one of the other connective tissue cells that I have mentioned before, are the immune cells in the lamina propria. Here is an image of the lamina propria supporting an epithelial surface. Sometimes these immune cells such as the lymphocyte here can wander off into the epithelium and they are really on sentry duty.

02:22 They are really wandering through taking out surveillance activities to make sure that they can identify antigens, they are programed to identify and then alert the body to the invading antigens that they detect. Well finally, let's look at the plasma cell.

02:43 The plasma cell operates in connective tissues, but as we learn when we deal with the lymphoid tissues, it is derived from the lymphocyte. On the right hand side, you can see the plasma cell labeled, lying within connective tissue. It is easy to identify because it has a characteristic clock face nucleus with the heterochromatin is arranged around the periphery of the nucleus creating what appears to be a clock face. These are very very busy cells. They are making antibody. And so when you look at the cytoplasm, sometimes it has its basophilic appearance and this reflects the protein factory within the cytoplasm that is making the antibody, the protein. The little clear halo you often see around the nucleus is the golgi complex, the part of the cytoplasm, that packages of these proteins, these antibodies for export.

03:50 On the left hand side, it is very difficult to see the plasma cell because they are embedded amongst other nuclei or other cells close to the epithelial surface. But again, if you look very carefully through these cells, you can identify them by having the very characteristic clock face nucleus. Well, as a summary, let's go back to this diagram and my challenge for you is to now review the sorts of cells that I have just described that are resident in connective tissue and they are all derived from the mesenchyme cell. And make sure you can also differentiate in this diagram, cells that are originating from blood cells because in a later lecture on blood, which is another form of connective tissue, I am going to discuss these cells as well. Remember the resident cells that are derived from mesenchymes are always present in connective tissues whereas the wandering cells derived from blood wander in and out, undergoing a surveillance role, looking for foreign proteins, foreign cells and antigens. And we will talk about all of these cells when we look at the lymphoid tissue. And also before we move on to the lectures on cartilage and bone, make sure you have a good understanding of the sorts of fibres that make up connective tissue. The collagen fibres, reticular fibres and elastic fibres because those fibres are going to be very important and dominate in certain types of cartilage and also in bone.

05:45 And also have a general understanding of the extracellular matrix because again that often determines the type of properties that the connective tissues have, that we will talk about later on cartilage, bone and blood. So I hope you enjoyed this lecture.

06:05 I hope you are looking forward to the subsequent lectures on cartilage and also bone.