Question

Provide an example of conditional specification via a paracrine factor

Answer 1

Conditional specification is non-autonomous specification seen in vertebrates. This specification is caused by interactions between cells. There is massive cell rearrangements and migration may be seen during or before specification.

Cell death in neurons due to synaptic connections is an example of conditional specification of cell fate during development. Signals are induced from different cells which indicate which cell dies or survives. In paracrine control, a secreted molecule which is diffusible will act on a different cell type and induce its death. Neutrotrophic factors (NTFs) regulate cell death in neurons and glial during conditional specification. NTFs for neurons include Neutrotrophins (Nerve growth factor, NT-3, NT-4), glial cells derived NTFs (GDNF, neuroturin) and ciliary derived NTFs (CNTF, CT-1). These NTFs bind to specific receptors on the target cells. As a result, signal transduction cascades such a phosphoinositiol-3 kinase/Akt, protein kinase pathways, protein kinase B etc are initiated which lead to neuronal death. Akt can induce Bad expression, which prevents activation of Bcl-2 and Bcl-X.