doc-20190402-waaaaa0028-190407100309.pdf

MORPHOGENSIS
“ It is the biological process that causes an
organism to develop its shape.”
MORPHOGENETIC MOVEMENTS;
“During morphogenesis , individual cells or mass of cells
move relative to each other. They fold, roll, bend , extend,
spread and these movements are together called
morphogenetic movement.”

CELLULAR PROCESS OF
MORPHOGENESIS
Cell intrinsic process
1. Cell division
2. Change in cell shape and size
3. Cell fusion
4. Cell death ( apoptosis )
Cell extrinsic process
5. Cell matrix interaction
6. Cell adhesion

1. Role of cell division
Cell division in morphogenesis involves two coordinated
processes:
1. Growth ( differential cell proliferation results differential
growth of some parts of embryo )
2. Increase cell number ( number of cell increase by division of
cell )
 Cleavage determined by position and orientation of mitotic
spindle.

2. Change in cell shape and size
 Cell shape can be changed by
reorganization of cytoskeleton.
 Change in size changes the
volume.
 Changes in volume help cell
to dilate in specific direction.

3. Role of cell fusion
 Cell fusion is an important cellular process in which
several uninuclear cells ( cells with a single nucleus )
combine to form a multinuclear cell, known as a
syncytium.
 The first step of embryonic development ( fertilization ) is
a process of cell fusion

 Myoblasts ( undifferentiated
proliferating cells ) aligned
and form myotubes ( multi-
nucleated fibers ).
 Actin and myosin ( muscle
specific proteins ) are
synthesized and then
assembled into muscles
fibers.

4. Role of cell death
 Programmed cell division play
role to generate the specific
structure.
 Apoptosis ( cell death ) during
limb development in vertebrates
help in separation of digits .

5. Role of cell-matrix interaction
 It is the material (or tissue) in animal or plant cells, in which
more specialized structures are embedded.
 Matrix provides a substratum upon which cells can migrate.
 Many animal cells show extension migration individually or
in group such as neural crest cells, germ cells etc.
 Loss of cell-matrix may result delamination of cells from its
basal layers.

6. Role of cell adhesion
 Cell adhesion is the process
by which cells form contacts
with each other through
specialized protein complexes.
 It help the cells to organize
into tissue and organs.

 Cell adhesion molecules (CAM) bind animal cells
together.
 These molecules determine whether some cells can
adhere to some other cell or not.
 Gain in cell-cell adhesion results aggregation of cells.
 Loss of cell-cell adhesion results delamination of cells
from an original layer.

Cell adhesion
“Aggregation of cells following a genetic interaction
and cells form tissues and organs is called the cell
adhesion “
 It is an important force in morphogenesis as well as
morphogenesis movements.
 It is an organized making and breaking of cell contacts.

DIFFERENTIAL CELL AFFINITY
 Differential Cell Affinities is a dominant process that drives
morphogenesis.
 In 1955 Towner and Holtfreter were the first to demonstrate
differences in the cell surfaces of distinct cell types.
Experiment: make single cell suspensions from different amphibian
tissues soon after neural tube formation.

 Using an alkaline solution, they made single cell
suspensions from the ectoderm, endoderm and mesoderm
layers of an embryo soon after neural tube formation.
 Two or more of the cell suspensions were combined. When
the pH was normalized, cells formed aggregates on agar-
coated petri dishes. Analysis of these aggregates
demonstrated that:
1. Reaggregated cells become spatially segregated. Cells do
not remain mixed. They sort out according to cell type.

2. Typically, the final position of the reaggregated cells
reflects their embryonic position.
 Holtfreter interpreted this as evidence for selective affinity.
The inner surface of the ectoderm has a positive affinity
for mesoderm and a negative affinity for endoderm.
Mesoderm has positive affinity for both ectoderm and
endoderm.
3. The selective affinity of cells changes during development.

 On the basis of their sorting behavior in mixed aggregate,
different types of cells can be given a rank according to
their ability to occupy the central position.
 Example :
 Heart cells and liver cells are mixed , heart cells are
always in the center surrounded by the liver cells.
 Again cartilage cells mixed with heart cells or liver cells,
occupy the central position.

 Tissues from chick form
hierarchies of adhesiveness.
a. When heart and liver cells
are mixed ,heart cells take
central position.
b. When cartilage cells are
mixed with liver or heart
cells then cartilage cells
take central position.

Differential adhesion hypothesis
Differential adhesion hypothesis (DAH) is a hypothesis that
explains cellular movement during morphogenesis with
thermodynamic principles (closed system move naturally
towards a state of minimum free energy).
In DAH tissues are treated as liquids consisting of mobile cells
whose varying degrees of surface adhesion cause them to
reorganize spontaneously to minimize their interfacial free energy.

 Steinberg in 1964 proposed that cells rearrange
themselves into most thermodynamic stable pattern.
Sorting of cells need difference in the strength of adhesion
among those cell type.
 Cell types having greater surface tension sort within those
cell types having lesser surface tension ( the attraction of
the particles in the surface layer by the bulk of the liquid,
which tends to minimize surface area ).

Cell adhesion molecules ( cam )
Cell adhesion molecules (CAMs) are a subset of cell
adhesion proteins located on the cell surface involved in
binding with other cells or with the extracellular matrix
(ECM) in the process called cell adhesion.
 In essence, cell adhesion molecules help cells stick to
each other and to their surroundings.

CAM is glycoprotein with three domains:
 Larger extracellular domain ( CAM-CAM )
 Transmembrane domain ( CAM-plasma membrane )
 Cytoplasmic domain ( microfilaments via linker protein )

Homophilic binding :
CAM binds with same type
of CAM.
Heterophilic binding :
Binding b/w different CAM.
 Binding specificity of
CAM depend on protein
and carbohydrate
groups.

Cell-cell adhesion molecules
1. Cadherins
( Ca++ depending molecules )
1. Immunoglobulin-like CAMs
( Ca++ independent molecules )
1. Selection
( sugar adding process )

1. cadherins
 Transmembrane proteins that mediate cell–cell adhesion
in animals. ( homophilic )
 It is a type of cell adhesion molecule (CAM) that is
important in the formation of junctions to bind cells with
each other. ( Ca++ depending )
 Catenin: The complex protein that anchored the
cadherins and form cadherin-catenin complex.

 Cadherin-catenin-action interaction holds the cells together
strongly.

2. immunoglobulin-like CAM
 They are cell adhesion molecules that belong to
Immunoglobulin superfamily of protein. ( antibodies )
 They are either homophilic or heterophilic ( characterized
by extracellular domains ).
 N-CAM (Neural Cell-Adhesion Molecule ) help in
development of chicken retina, also stop connection b/w
optic nerve and brain.

3. selectins
 The selectins are a family of heterophilic CAMs that are
depending on heterophilic mechanism ).
Types of selectins:
a. L-selectin (express on white blood cells)
b. P-selectin (on platelets cells)
c. E-selectin (on activated endothelial cells)

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