anterior posterior patterning (class 4)

anteriorizing signals

• DKK1, Noggin, and Cerberus

  • antagonists of WNTs, BMPs, and nodal

  • secreted from anterior visceral endoderm and prechordal plate protect the rostral neural plate from posteriorizing influences

posteriorizing signals

• FGF, WNT, and retinoic acid

  • specify hindbrain and spinal cord regional identity

role of SHH in forebrain patterning

• secreted from prechordal plate and ventral midline of neural tube

• induces formation of ventral midline of forebrain, specifying left and right cerebral hemispheres

effects of SHH signaling loss

• when mutated, ventral midline signaling fails

• leads to failure of prosencephalon to bifurcate, resulting in single, fused brain (holoprosencephaly)

isthmic organizer (MHB)

sharp boundary that matures, located between mesencephalon and rhombomere 1 of hindbrain

• FGF8 expressed to specify regional identities in mesencephalon and metaencephalon

patterning: rostral FGF8 / WNT1

this environment patterns the mesencephalon / midbrain along both dorsovenetral and anteroposterior axes

patterning: FGF8

positions and scales the superior and inferior colliculi

patterning: SHH from floor plate

w/ rostral FGF8 promotes specification of mesencephalic dopaminergic (mdDA) neurons.

• correct FGF8 dosage and timing are crucial for size and placement of dopamine meurons

patterning: WNT1

keeps mesencephalon and metencephalon develop as separate compartments

• loss of WNT1 or FGF8 results in nearby complete absence of the cerebellum