AAYUSHMAN CHAKRABORTY
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CONDITIONAL SPECIFICATION
DEFINITION:
The ability of cells to achieve their respective fate by interaction with other cells.
What a cell becomes depends on an array of interactions with its neighbors, which include:
i. Cell-to-cell contact (Juxtacrine factors).
ii. Secreted signals (Paracrine factors).
iii. Physical properties and its natural environment (Mechanical stress).
Fig. 1: Conditional
specification. (A)
What a cell becomes
depends on its
position in the
embryo. Its fate is
determined by the
interaction with its
neighboring cells. (B)
If cells are removed
from the embryo, the
remaining cells can
regulate & compensate for the missing part.
Weismann’s Hypothesis: When the first cleavage division separated the future ‘right’ & future ‘left’
half of the embryo, there would be separation of ‘right’ determinants from ‘left’ determinants.
Wilhelm Roux’s test on Weismann’s Hypothesis: Roux killed one of t he two cells in a 2-cell stage
frog embryo, with the help of a hot needle. He observed that only the unaffected side of the embryo
developed into larva, while the damaged side degenerated and gave rise to a dead mass of cells.
Fig. 2: Roux’s
attempt to
demonstrate
autonomous
specification.
Destroying one
of a 2-cell frog
embryo
resulted in the
development of
only one half of the embryo.
He claimed that specification was autonomous and all the instructions for normal development were
present inside each cell.
