•  Cross-repression between brain regions 

•  Diffusible molecules produced by signaling centers 

•  Regions with unique transcription factor expression profiles 

•  Timing of gene/protein expression! 

• Connectivity of neurons 

•  Phenotype of neurons 

(anatomical and neurochemical) 

• Patterns of gene expression 

At neural tube closure, the telencephalic vesicles (cerebral hemispheres) are not 

morphologically detectable. 

Rapid expansion of the telencephalon then begins, except in the dorsal midline roof 

plate, which results in the apparent 'cleavage' of the forebrain (Pax-6?).

initially 3 primary brain structures are prosencephalon mesencephalon and 

rhombencephalon. Prosencephalon becomes diencephalon and telencephalon. 

Mesencephalon becomes midbrainand rhombencephalon becomes cerebellum, 

pons, and medulla 

- midbrain-hindbrain (MHB) or Isthmus is located at the boundary of Otx2 and Gbx2 expression

(1) In-situ hybridization (ISH): expression of Otx2 in chick brain 

Otx2 (Orthodenticle homeobox gene) is expressed in the 

anterior neural plate (A) and remains expressed in most 

brain regions throughout development (B-E) 

Gbx2 (Gastrulation Brain Homeobox 2), expressed in 

rhombomeres 1-3 throughout development (expressed at caudal part of brain) 

(2) Mechanisms at work in boundary formation: 

• Cross-repression (boundaries are suppressed by 2 different TF) 

• Cell-sorting & differential adhesion 

• Regulation of cell migration 

also produce Wnt1 and FGF8 and expressed engrailed (en1), a transcription factor. 

*In situ histochemistry (ISH) and immunohistochemistry (IHC) are the same 

process, except the former is done in whole mounts (e.g. whole embryos) and 

the latter is done in histological tissue sections.

- 

- directs the formation of ectopic midbrain: organizer

•  Graft from the quail MHB to the chick forebrain 

•  Results: 

•  Ectopic cerebellum: formed from host and donor cells 

•  Ectopic midbrain: formed from host cells (of the prospective forebrain)

** this depends on the donor tissue and time of transplantation. If you do this in the late stage, you will not get this transformation. Gene expression has shut off so that it is hard for them to transform. 

they do not develop the midbrain and cerebllum so this suggest that the midbrain and cerebellum requires wnt for development. 

BUT, the expression of transcription factor En1 (engrailed) at the MHB rescues the Wnt1 KO phenotype. In Wnt1 KO, En1 expression is initiated normally, but is lost as development progresses. This suggests that one role of Wnt1 at the MHB is maintenance of En1 expression. But when wnt1 KO, en1 is lost but if you can overexpress en1, it can rescue the wnt1 KO phenotype. EN1 is also expressed in human CNS. 

- induces formation of ectopic midbrain 

Experiment: you place a bead coated with FGF8 into the chick forebrain. This induces formation of ectopic, mirror-duplicated midbrains (suggests that FGF8 is sufficient for midbrain induction) 

- However, FGF8 may do different things when you do experiments at different times because proteins and gene expressions would have changed over this time. 

Signaling 

molecules expressed at patterning centers regulate the graded expression of specific transcription factors 

(Emx2, Pax6, Coup-TFI, and Sp8). These factors are involved in the early regionalization of the cortical 

primordiumand, thus, in the position, size and identity of functionally distinct cortical areas in the postnatal 

animal. They also control proliferation and/or differentiation of cortical progenitors.

FGF8, expressed at the 

rostral patterning center, and TGFβ, expressed at the caudal patterning center, induce the production of 

Cajal–Retziussubtypes which modulate early cortical patterning by transporting signaling molecules over a 

long distance. A new class of migrating neurons, cortical plate (CP) transient cells, as well as the meninges 

and the ingrowing vasculature also play a role in growth and differentiation of the cerebral cortex. CoP: 

commissural plate; PSB: pallial–subpallialboundary or anti-hem; hem: cortical hem; RA: retinoic acid; R: 

rostral; C: caudal; M: frontal/motor area; S: somatosensory area; A: auditory area; and V: visual area

At early stage of development 

Cajal-Retzius cells migrate out 

and run laterally over the outer 

surface of the cortex just 

beneath the meninges. The 

cortical plate (CP) cells also 

migrate from the medial part of 

the cortex and then proliferate 

to generate many of the layers 

of the cortex. These cells 

express and transport 

transcription factors to distinct 

regions of the cerebral 

hemispheres 

Later in development, ANR becomes the 

commissural plate (CoP), which connects 

the left and right hemispheres in adult as 

the corpus callosum (myelinated axons) 

ANR produces FGF8 which 

induces Cajal-Retziuscells to 

proliferate 

gf8 start CR to start migrating. They will have interactions with other cells. 

The ant. neural ridge is important in forming the corpus callosum 

if cut corpus callosum to stop epilepsy (split brain patients) 

a distorted representation of the human body within 

the sensory and motor cortex. The cortical scale for each part of the 

body is related to the functional importance of that body region

mouse use whiskers to detect their surroundings and tell them their dimensions. There is some communication from base to top of brain. 

When get to cerebral hem., is that there's a pattern which is highly sterotypical and masks the position of the whiskers. SO if know position of whiskers, can tell where it is in the brain that controls it. 

Like a map, a cortical topographic 

representation is highly ordered 

and to scale, relative to importance 

of that body function (important 

sensory information comes to mice 

via whiskers) 

- FGF8 from the ANR governs the formation of anterior regions of the cortex. 

Most apparent example: 

development of whisker 

barrels in the 

somatosensory area 

• When FGF8 is ectopically 

expressed in the posterior 

forebrain (via in vivo 

electroporation, which 

permeabilizescell 

membranes), a partial 

duplication of the S1 barrel 

field is observed 

if ectopically express it in post forebrain, you can get a duplicated map. 

if take fgf8 and fire into sensory, you get a memory image. 

Fgf8 from the ANR governs the formation of anterior regions 

Mouse embryos (P6, postnatal day 6) electroporated at 

E11.5 (embryonic day 11.5) with FGF8 to produce a 

second mirror image homolog of whisker barrels 

Horizontal section (dorsal-ventral plane), processed 

with cytochrome oxidase histochemistry, which shows 

cells with increased metabolic activity (correspond to 

whisker barrel locations in cortex) 

Ectopic whisker pad field (Wp2) forms a mirror-image of 

the normal whicker barrel field (Wp1)

one of metabolic enzymes when cells are hghly active and firing lots of AP will use more cytochome oxidase if want to look at surface of brain, wiggle the whisker of mouse and will activate whiskers and send impulses to brain and activate those neurons of that region of crotex involved with that whiskers. 

 Fgf8 from the ANR dictates the position of anterior regions of forebrain 

Anterior electroporation of Fgf8or sFGFR3causes opposite shifts of the barrel 

fields. (sFGFR3 is a soluble human recombinant FGF) 

• In this case, electroporation created either a source of Fgf8 or Fgf8 

antagonist at the anterior pole of the forebrain 

(white arrows mark the A-P midpoint of the cortex in each section) 

- inhibits the expression of emx2 and coup-TF1 (transcripion factors) 

Emx2 is a homeodomain transcription factor 

Coup-TF1 (chick ovalbumin upstream 

transcription factor 1) 

Both are expressed high posteriorly (but in 

contrasting medial to lateral gradients) 

govern the formation and position of posterior cortical structures. 

Serotonin IHC (neuronal marker) in P7 mice with reduced (A) or enhanced (C,D) 

expression of Emx2 

A.  Mice with reduced Emx2 expression exhibit a reduction in size of posterior 

structures (V1=visual cortex) and increase of anterior structures (A1=auditory 

cortex and S1=somatosensory cortex): both territories shift posteriorly 

B.  Wild type mouse for comparison (F/M = frontal/motor cortex) 

C, D. Mice with enhanced expression of Emx2 in neural progenitors exhibit the 

opposite phenotype (large V1, reduced F/M and S1) 

- govern the formation and position of posterior structures

Serotonin IHC in P7 wild type (wt) mice (A) and mice with KO of Coup-TF1 (B) 

A.  Serotonin IHC reveals the primary somatosensory areas; F/M = frontal/motor cortex 

B.  Conditional elimination of Coup-TF1 at E10 and later produces a reduction in size 

of posterior structures; F/M cortex is expanded posteriorly 

At caudal levels of the neural tube (i.e. in the spinal cord), Shh (sonic hedgehog) is 

expressed by the notochord and floor plate and governs neural fate. 

Does Shh govern ventral identity in the telencephalon? 

To investigate the role of Shh, explants of prospective rostral telencephalon were 

prepared from chick gastrula (R): 

Dorsal view of the chick gastrula: 

• Primitive streak analagous to the frog blastopore 

• Hensen’s node analagous to the dorsal lip of the 

blastopore in frog