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Lecture 6

BIOC19H3 Lecture Notes - Lecture 6: Petri Dish, Bronchus, Lysis

Biological Sciences
Course Code
Ian Brown

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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

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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
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