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By the fly hunnys
 Morphogenesis
in animals involves specific
changes in cell shape, position and adhesion
 The developmental fate of cells depends on
their history and on inductive signals
 Major
aspect of development in both animals
and plants, but only in animals does it
involve the movement of cells (changes in
cell shape and position)
 Changes in cell shape and position are
involved in cleavage, gastrulation, and
organogenesis

Changes in cell shape involve reorganization of the
cytoskeleton.


Cytoskeleton also drives cell migration (active
movement of cells from one place to another) Cells
crawl within the embryo by using cytoskeletal fibers to
extend and retract cellular protrusions.
Changes in cell shape



Invagination (inpocketings) caused by contractions and
bending inward
Evaginations (outpocketings) caused by contractions
bending outward.
Convergent Extension: type of morphogenic movement
in which the cells of a tissue layer rearrange themselves
so that the sheet becomes narrower (Converges) while it
becomes longer (extends)

Is important in early embryonic dev.
ECM: The mixture of secreted glycoproteins lying
outside the plasma membrane of cells.
 Known to help guide cells in many types of
morphogenetic movements

ECM Fibers may function as tracks, directing
migrating cells along particular routes
 Several kinds of extracellular glycoproteins (including
fibronectin) promote cell migration by providing
specific molecular anchorage for moving cells
 Other substances in the ECM keep cells on the correct
path by inhibiting migration in certain directions.
Depending on which substances they secrete,
nonmigratory cells situated along migration pathways
may promote or inhibit movement of other cells.

As migrating cells move along pathways,
receptor proteins on their surfaces pick up
directional cues from the immediate
environment.
 Such signals from the ECM can direct the
orientation of cytoskeletal elements in a way
that propels the cell in the proper direction.
 CELL ADHESION MOLECULES: glycoproteins which
are located on the surface of cells and bind to
CAMs on other cells.


Vary in amount, chemical identity, or both from one
type of cell to another. These differences help
regulate morphogenetic movements and tissue
building.
 Class
of CAM that requires calcium ions for
proper function.
 Many different cadherins and the gene for
each cadherin is expressed in specific
locations at specific times during embryonic
development.
 Also involved in the tight adhesion of cells in
the mammalian embryo that first occurs at
the 8 cell stage (when cadherin production
begins).
 The
developmental fate of cells depends on
their history and on inductive signals

Development also requires the timely
differentiation of many kinds of cells in specific
locations
 During
early cleavage divisions, embryonic
cells must somehow become different from
one another
 Initial
differences between cells result from
the uneven distribution of cytoplasmic
determinates in the unfertilized egg


By partitioning the heterogeneous cytoplasm of a
polarized egg, cleavage parcels out different
mRNAs, proteins, and other molecules to
blastomeres in a type of asymmetrical cell
division.
Resulting differences in the cell’s cytoplasmic
composition help specify body axes and influence
the expression of genes that affect the
developmental fate of the cells.
 Local
environmental differences play the
major role in establishing early differences
between embryonic cells.

For example-Cells of the inner cell mass are
located internally in the early human embryo,
whereas trophoblast cells are located on the
outside surface of the blastocyst. The different
environments of these two groups of cells appear
to determine their vey different fates.
 Once
initial cell asymmetrics are set up,
subsequent interactions among the
embryonic cells influence their fate, usually
by causing changes in gene expression.

The mechanism this occurs by is called
INDUCTION.


Eventually brings about the differentiation of the
many specialized cell types making up an animal.
May be mediated by diffusible chemical signals or, if
the cells are in contact, by cell surface interactions.
FATE MAPS: general territorial diagrams of
embryonic development.
 How is this done?


Mark an individual blastomere during cleavage then
follow the marker as it was distributed to all the
mitotic descendents of that cell
Can a cell’s fate be changed by moving it?
 2 important conclusions

1. In most animals, specific tissues of the older
embryo can be attributed to certain early “founder
cells”
 2. As development proceeds, a cell’s DEVELOPMENTAL
POTENTIAL (the range of structures it can give rise to)
becomes restricted.

 Establishing
the basic body plan is a fist step
in morphogenesis and is a prerequisite for
the development of tissues and organs.

In nonamniotic vertebrates, basic instructions for
establishing the body axes are set down early,
during oogenesis or fertilization


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The animal-vegetal axis indirectly determines the
anterior-posterior body axes.
Once any 2 axes are established the third is default
In amniotes the body axes are not fully
established until later

Gravity, pH differences

TOTIPOTENT: is capable of developing into all
the cell types found in the adult
In many species that have cytoplasmic determinates
only the zygote is totipotent
 The fates of embryonic cells can be affected not only
by the distribution of cytoplasmic determinants but
also by how their distribution is affected by the
zygote’s characteristic pattern of cleavage
 Cells of mammalian embryos remain totipotent until
the 16 cell stage
 Regardless of how similar or different early embryonic
cells are in particular species, the progressive
restriction of potency is a general feature in the dev.
Of all animals!

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