2. • How does a cell, tissue, or organ develop its shape?
• How does our body make muscle tissue in both our legs and neck? How does it
know to make special muscle for the heart?
• These are topics of morphogenesis- this happens through cell-to-cell
communication. It is how differentiated cells become more organized into tissues
and organs.
• In order to develop this specialized tissue, it is necessary to understand three
mechanisms requiring cell-to-cell communication:
• Cell adhering
• Cell shape changing
• Cell signaling
3. Cell interaction
(communication)
• Cell interaction starts at the cell
membrane.
• There are various proteins either
embedded in the cell membrane,
attached to it, or secreted through it.
• These cells are responsible for how cells
interact.
• A receptor id one type of protein. It
grabs other proteins.
• They could be proteins sent
from another cell (like a
chemical)
• They could be a protein
attached to another cell
(another receptor)
4. Receptors
• When the protein receptors are the same on both cells it is called
homophilic binding. When they are different, it is referred to as
heterophilic binding.
• Proteins send from cells to go to others are call signaling proteins
(ligands)
• When cells are neighboring (adjacent), it is called juxtacrine
signaling
• When cells are a short distance away it is called paracrine
signaling
5. Adhesion and
Sorting
• Embryo cells display selective
affinity.
• In lab experiments, they self-
segregate.
• Certain cells will get together
with others
• Groups of cells will segregate
themselves as well
6. Thermodynamic Model of Cell Interactions
• Cells don’t sort by random. It is in a certain order
• The differential adhesion hypothesis states that:
• How cells sort themselves depends on the differences they have
in adhesion
• Cells move to be near cells with similar adhesion strength to
maximize their bonding.
• Recall the three germ layers. In a lab setting, cells from the three
layers were mixed in a solution.
• Not only did endoderm cells stay with other endoderm cells, they
were internal to mesoderm cells.
• The cells still grouped and made their spatial boundaries
7. Cadherins and Cell
Adhesion
• Cadherins- these are
adhering proteins that
adhere to cadherins on
adjacent cells.
• They are attached to the
membrane and anchored
inside the cell by proteins
called catenins.
• A good example of this
phenomenon is in your
skin (epithelial tissue)
8. The Extracellular Matrix as a Source of
Developmental Signals
• An extracellular matrix (EM) is merely the environment around cells.
The EM is very important for cell movement, adhesion, and
development of epithelial tissue.
• The EM can be made up of various materials:
• Proteoglycans- help cells send signals to other cells (especially
paracrine factors)
• Laminin- forms part of the Basal Lamina, which is a sheet that
epithelial cells connect to
• Fibronectin- this is like paving the street on which cells migrate
9. Integrins
• Integrins are receptors
embedded in cells that can bind
proteins on the inside of the
cell with proteins of the
extracellular matrix (like
fibronectin)
10. Epithelial-Mesenchymal Transition (EMT)
• EMT is a series of events where epithelial cells are transferred to mesenchymal
cells
• How does this happen?
• Paracrine factors from neighboring cells cause the down regulation of the
expression of cadherins (downregulating mean reducing the receptors)
• They then release their attachment to other cells, or integrins. They also
become unattached from the basal lamina as well.
11. Cell Signaling
• Induction and Competence
• Development depends on the exact arrangement of tissues and
cells.
• Induction- when cells with different properties interact. This
has two components:
• Inducer-the tissue that makes the signal that changes cell
behavior
• Responder- the target tissue (the tissue receiving the
signal)
• Competence- The ability of a cell to respond to an induction
12. Paracrine Factors: Inducer Molecules
• How are inducer signals transmitted?
• Juxtacrine interaction- adjacent cells (they are touching)
• Paracrine interaction- cells within a roughly 15 cell radius
• Autocrine interaction- when the same cell is the recipient (this is
a rare occurrence)
• Morphenogens- these are paracrine chemicals that cause different
forms of genetic expression based on their concentration.
13. Signal Transduction
Cascades
• A signal transduction cascade
is the process that starts
when the ligand is bound to
the receptor.
• It causes a series of
reactions within the
cytoplasm of the cell
14. Juxtacrine Signaling for Cell Identity
• Proteins from the inducting cell are going to interact with adjacent
responding cells.
• There are three juxtacrine factors widely used:
• Notch proteins
• Cell adhesion molecules
• Eph receptors/ephrin ligands
• Notch- when this receptor is activated on the outside of the cell, the
end on the cytoplasmic side breaks off and goes to the nucleus to
make changes in gene expression.
