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Lecture 6

LECTURE 6: Tissue Interaction in Development.doc

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Biological Sciences
Ian Brown

LECTURE 6: TISSUE INTERACTIONS IN DEVELOPMENT Slide 4 • The internal organs are formed as a result of the two interacting tissue layers: epithelium and mesenchyme • Epithelium cells  are cuboidal (cube-shaped)  held together very firmly by junctional complexes and cell adhesion molecules  exhibit polarity. Top surface that faces into lumen is called apical surface and the other surface which is next to the basement membrane is called basal surface. Apical can change in shape, may develop cilia and villi (cellular protrusions), and can be differentiated into different shapes • Basement membrane separates epithelium and mesenchyme layer, is not composed of cells, instead it's composed of extracellular molecules, and is rich in collagen and laminin Slide 5 • The image on the left is of epithelium cells in columnar (column-like shape). In between the two columnar epithelium cell layers are mesenchyme cells • Both the epithelium and mesenchyme give off inducing factors that cause lining by internal epithelium cells to develop into very specific structures that give internal organs their unique appearance and structure. Slide 6 • They are scattered, irregular cells • ECM is between mesenchyme cells • MAKE THIS CORRECTION ON SLIDE: secrete, not secret Slide 7 • it's acellular • not composed of cells, but have composed on fibrous molecules and proteins Slide 8 • Present in basement membrane • In mesenchyme cell layer, they usually have a hole in the middle which called lumen • IEC lines the lumen internally • MES is outside the IEC layer coating it Slide 10 • MES = Mesenchyme • IEC = Internal Epithelial Cells • What is the digestive tract? It is a hollow tube, formed by 2 cell layers, going from your mouth to your behind. The tube passes by esophagus, stomach, and intestines. • The inner lining of the hollow tube is the IEC layer and on the outside, we have the mesenchyme layer • The wavy lines on the diagram represent MES layers, straight lines represent IEC layers, and the space between the straight lines is lumen • They all look very similar but they give off different kinds of inducing factors • MESe gives off inducing factors that causes IEC to differentiate into IEC of esophagus which are cuboidal shaped cells • MESs gives off inducing factors that causes IEC to differentiate into IEC of stomach which are cells with ruffled surface at the apical surface and villi (tube-like structures) coming out of the surface • MESi near the top gives off inducing factors that causes IEC to differentiate into IEC of intestine which are columnar shaped cells • There are variation of MES that give off different inducing factors, causing different specialized epithelium developing at the right places Slide 11 • First, we isolate some MES cells from the esophagus and grow them alone in the petri dish. So, you'll have a layer of MES cells on the bottom of the petri dish. Then, we add-in non-gut IEC and see what they differentiate into. Result: they differentiate into IECe. • Similar experiment with MESi • This is one-step induction. There are also two-step induction examples out there like the two-step induction of liver. First step involves the inducing factor from the heart MES cells and then, an inducing factor from liver MES tissue. Hence, a two step induction Slide 12 • Pancreas develops from the digestive tract • The bulge or outfold of gut, detected on embryonic day 9, of very undifferentiated cells develops into pancreas • From embryonic day 9 to 12, inducing factors given off by the mesenchyme affects the adjacent IEC to trigger rapid cell division and on day 12, you get finger-like process growing into the MES layer and a branch-like structure Slide 13 • MES layer also has blood vessels and that's where the blood vessels network of internal organs is Slide 14 • The Y-axis label should be "Levels of pancreas-specific proteins in the IECp layer" • E9 means embryonic day 9 • Phase 2 - IEC cells went under very rapid cell division stimulated by a factor given off by the MES cells which is called Mesenchyme factor (MF) • Cell division stops when we move from pahse 2 to 3 and induction of pancreatic-specific proteins goes up Slide 16 • How do we know MF from MES is boosting ICE into rapid cell division? By testing the role of MF • We first isolate pancreatic potential cells (IECp) and grow them on a petri dish without MESp layer and cells don't enter phase 2 because they don't undergo rapid cell division • In experiment C), instead of growing a layer of MESp cells, MESp cells were broken down and only MESp cell lysate was added to IECp cells Slide 17 • No, the MF does not need to enter the cell to function. It just need to interact with the cell surface receptors and that's enough to trigger IEC cells into phase 2 Slide 18 • The question at the top of the slide can also be word like this: "Do we always need MESp for development or can we use MES from other tissues?" Slide 19 • Trachea is tube-like structure that branches into right and left bronchus • Air comes in through the mouth and to trachea and then down the trachea branches out to right and left bronchus • Bronchus branches out to smaller and smaller air tubes embedded in the wall of the lung where we get the exchange of oxygen in air and blood system • The picture shown on slide 19 is a picture of a differentiated lung Slide 20 • This is a picture of an embryonic lung • Components of lung develop when IEC and MES interact • We also have basement membrane between IECt and MESt & between IECb and MESb • We have 2 parts developing here: 1) Tube-like trachea 2) Branch-like bronchus Slide 21 • How do we know if IEC will become trachea or bronchus? By conducting tissue culture experiments and organ culture experiments • Tissue culture
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