1. Cell Differentiation: Cell interactions in Development
Patricia Zuk, PhD
Research Director
Regenerative Biogengineering and Repair (REBAR) Lab
Department of Surgery
David Geffen School of Medicine at UCLA

2. so cells can interact with each other and with their environment
this interaction turns specific signaling paths ON or OFF
these pathways become important for mediating proliferation, differentiation and apoptosis
all three are crucial to development

3. Differentiation: Stem cells
so fertilization of the egg takes place in the oviduct
the fertilizes zygote travels to the uterus for implantation
along the way – the zygote begins to divide (mitosis)
2-cell, 4-cell, 8-cell embryonic stages etc….
the embryo reaches a stage called the morula = ball of small cells (embryo has not enlargened)
by the end of the first week the second embryonic stage – the blastocyst - forms

4. Differentiation: Embryonic Stem cells
the blastocyst is a hollow ball of cells containing an outer rings of progenitor cells = trophoblast and an inner mass of cells at one end of the embryo = inner cell mass
it is these ICM cells that are the source for the derivation of embryonic stem (ES) cells
the ES cells are said to be totipotent – have the ability to specialize or differentiate into ALL cells of the embryo
the blastocyst then begins a process of differentiation and these ES cells form populations of stem cells with more restricted potentials
the ES cells first differentiate into two layers called the embryonic disc – divides the blastocyst cavity into an amniotic cavity and a yolk sac (primitive hematopoietic organ)
these two layers then continue to differentiate into the three germ layers of the embyro
ectoderm, mesoderm and endoderm
the formation of these germ layers marks the gastrula embryonic stage

5. Germ Layers
the ectoderm, mesoderm and endoderm are thought to be made up of stem cells with a more restricted phenotype when compared to ES cells BUT still capable of forming multiple cell types within that lineage
e.g. pluripotent stem cells
interactions between signaling molecules produced by these germ layers and with the developing ECM around these tissues results in specific developmental events = patterning
patterning requires the exposure of cells to a succession of signals and subsequent activation of their associated pathways

6. Cellular interactions in development: Induction
interactions between the cells of the germ layers influence the fate of the stem cells within these layers
can affect their differentiation paths
induction = mechanism where one cell population influences the development of neighbouring cells
e.g. mesoderm induces the overlying ectoderm to form neural tissue
embryonic development is a series of inductive events
binary – have a choice between one fate or another (presence of one signal – development down one path, absence of signal – development down another path
gradient – multiple fates may result – dependent upon the level or threshold of the signaling molecule (these signaling molecules are called morphogens)
relay – a signal induces a cascade which determines the fate of cells in proximity – these cells than produce additional signals which affect the fate of their neighbours

7. Cellular Interactions: Patterning and TGFb superfamily signaling
a number of extracellular signaling molecules play roles in determining the fates of developing cells and where these cells will develop
TGFb superfamily: TGFb, BMPs, activin (BMP2 and BMP4)
wide role in development
mediate proliferation of stem cells
can determine stem cell fate – e.g. BMP4 = mesoderm
also mediate the production of CAMs, growth factors, ECM molecules – all which play roles in cell signaling and resulting differentiation responses
homologs found in multiple vertebrates
binding of these homologs to their receptors results in their activation
this activation phosphorylates/activates downstream adaptors called Smads
a cascade of Smads are activated resulting in eventual nuclear translocation and regulation of gene expression
these Smads are specific to the TGFb superfamily growth factor (Smad 1 = BMP2, Smad 2 = TGFb)
this signaling path is fine-tuned by the action of several adaptor proteins (chordin, xolloid, noggin)
this fine tuning can result in the induction of neural tissue as opposed to mesodermal tissue and results in the patterning of tissues within the embryo

8. Other Patterning Signal Paths
numerous other patterning pathways comprised of multiple signaling proteins
e.g. hedgehog pathway – patterning of local tissues and determination of cell fates
initial studies in Drosophila
homologs now characterized in vertebrates
hedgehog (sonic and indian hh in vertebrates), wingless (Wnt in vertebrates), patched, smoothened, dishevelled
limb patterning – FGF, sonic hedghog,
neural tube differentiation – sonic hedgehog gradients

9. Want to know how ridiculous this gets??
other signaling molecules in cell differentiation/tissue fate determination
brachyury – mesoderm
goosecoid
toll, spaetzle, cactus & dorsal
dorsal targets: snail, twist, rhomboid
delta, notch, jagged, suppressor of hairless
hunchback, Kruppel
even-skipped, fushi-tarazu