Presentation: Differential Cell Affinity

1. S Y E D M U H A M M A D K H A N
( B S H O N S . Z O O L O G Y )
Differential Cell Affinity

2. Introduction
 Morphogenesis is the formation of organized
animal bodies.
 Different cells of an organism show differential
affinity (more close to similar kind as compared to
dissimilar kinds).
 For multi-cellularity to be possible, cells must be
held together. This is done via cell adhesion
molecules (i.e. cadherins).
 They helps in creating boundaries between different
tissues.

3. Differential Cell Affinity
 The cells of an embryo show selective affinities (close
to similar cells and not so close to dissimilar cells).
 For example, the inner surface of the ectoderm has a
positive affinity for mesodermal cells and a negative
affinity for the endoderm.
 The mesoderm has positive affinities for both ectodermal
and endodermal cells.
 Selective affinities change during development.
 For development to occur, cells must interact differently
with other cell populations at specific times.

4. Differential Adhesion Hypothesis
 Cells do not sort randomly.
 They can actively move to create tissue organization.
 In 1964, Malcolm Steinberg proposed the
differential adhesion hypothesis.
 It states that cells show more affinity or closeness to
other cells of similar type but not to those of a
different type.

5. Thermodynamic Model of Cell Interactions
 Proposed in 1964 by Steinberg.
 Thermodynamic model of cell interactions
proposes that the cells rearrange themselves into the
most thermodynamically stable pattern.

6. Thermodynamic Model of Cell Interactions
 Consider that cell types A and B have different strengths
of adhesion.
 If the strength of A-A connections is greater than the
strength of A-B or B-B connections, sorting will occur,
with the A cells becoming central.
 If the strength of A-A connections is less than or equal to
the strength of A-B connections, then the aggregate will
remain as a random mix of cells.
 If the strength of A-A connections is far greater than the
strength of A-B connections (if A and B cells show
essentially no adhesivity toward one another) then A cells
and B cells will form separate aggregates.

7. Thermodynamic Model of Cell Interactions
Arrangement occurs in the most thermodynamically stable pattern

8. Changes In Equilibrium of Embryo
 The early embryo can be viewed as existing in an
equilibrium state.
 It remains so, until some change in gene activity changes
the cell surface molecules.
 As a result, cellular movements occur to restore the cells
to a new equilibrium configuration.
 For sorting to occur, cell types must have different
strengths of their adhesion.