**1. Introduction**

The mammalian telencephalon, which comprises the cerebral cortex, olfactory bulb, hippo‐ campus, basal ganglia (striatum and globus pallidum), and amygdala, is a highly complex and evolutionarily advanced brain structure. All higher brain functions including the inte‐ gration and processing of sensory and motor information, the memory storage and retrieval, and the regulation of emotional and drive states take place at the telencephalic level. In hu‐ mans, the telencephalon also governs the ability to make rational decisions, to plan for the future and to have the creative impulses [1].

At cellular level, the telencephalon is populated by a large diversity of neurons, including **glutamatergic** projection neurons, **GABA (γ-aminobutyric acid)-ergic** interneurons and projection neurons, as well as **cholinergic** interneurons and projection neurons.

Many neurological pathologies are caused by malfunction of telencephalic neurons, as a re‐ sult of neurodegenerative processes (e.g. Alzheimer disease), genetic mutations (e.g. Hun‐ tington disease), or abnormal development (e.g. autism, schizophrenia and epilepsy), all with devastating consequences for the normal brain function.

During the past ten years much progress has been made in elucidating the mechanisms that orchestrate the generation of different telencephalic neuronal subtypes. A combination of fate-mapping studies with genetic loss-of-function and gain-of-function experiments has been successfully used to uncover important molecular players in the development of the rodent telencephalon.

At early stages of its development, the telencephalon is divided into two main regions: **dorsal** (pallium) and **ventral** (subpallium). The pallium is further subdivided into three longitudinal zones: dorsal, medial, and lateral. The dorsal pallium gives rise to the **neo‐**

**cortex -** the most complex structure of the mammalian brain. The medial pallium devel‐ ops into the hippocampal formation (archicortex), cortical hem and the choroid field. The lateral pallium matures into the paleocortex (olfactory and some limbic areas). From the **ventral** telencephalon medial, lateral and caudal ganglionic eminences (**MGE, LGE** and **CGE**) emerge giving rise to the **basal ganglia** and parts of the **amygdala**, but also to neurons that migrate into the cortex and olfactory bulbs. The progenitor domains in the embryonic telencephalon generate specific types of neurons which finally form the com‐ plex neural networks of the mature telencephalon.

Understanding the developmental ontogeny of the diverse telencephalic neuronal popula‐ tions provides an essential framework for the design of rational approaches towards pluri‐ potent stem cell differentiation for *cellular models* and *cell replacement therapies* for telencephalic related diseases.

Schematic mouse mature (left side) and embryonic (right) brain; telencephalic subdivision: olfactory bulb (Ob), neo‐ cortex (Ncx), palleocortex (Pcx), archicortex (Acx), hippocampus (Hi), striatum (St) and globus pallidum (GP), pallium (pink) medial ganglionic eminence (MGE-violet), lateral ganglion eminence (LGE-light green), caudal ganglionic emi‐ nence, (CGE-dark green); domains and origins (as arrows) of telencephalic glutamatergic (red), GABAergic (green) and

Telencephalic Neurogenesis Versus Telencephalic Differentiation of Pluripotent Stem Cells

http://dx.doi.org/10.5772/54251

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**Glutamatergic projection (pyramidal) neurons** comprise the majority (70-80%) of cortical neurons, they are generated in the dorsal telencephalon; have an excitatory role in the corti‐ cal and include many subtypes. Each subtype is characterized by a specific combination of

Cajal-Retzius neurons are a transient population expressing reelin and playing a key role in

**GABAergic neurons** are generated in the ventral telencephalon and also include many sub‐

GABAergic cortical interneurons, comprising 20-30% of the cortical neurons, have an inhibi‐ tory role in the cortical circuits; they originate in the ventral telencephalon and subsequently

Other types of GABAergic neurons include the interneurons and projection neurons that

Different subclasses of GABAergic interneurons arise from different progenitor domains in the subpallium: *somatostatin (Sst)* subclasses of GABAergic interneurons that ultimately re‐ side in the cortex and the basal ganglia are generated in dorsal MGE (dMGE) [7;8]. *Parvalbu‐ min (Pv)* subclasses of GABAergic interneurons, constituting the majority of the cortical interneurons, are generated in ventral MGE (vMGE) [9;10]. *Calretinin (Carl), NPY and reelin*expressing GABAergic cortical interneurons are produced primarily in CGE [11;12]. Calr ex‐ pressing GABAergic interneurons, which ultimately reside in the olfactory bulbs and

GABAergic projection neurons, such as the medium spiny neurons (MSN) which constitute the majority of the striatal neurons, express DARPP32 and Calr and arise from the ventral

populate the striatum, pallidum, olfactory bulb and other forebrain ventral regions.

laminar position, morphology, marker expression and connectivity pattern [4;5].

the formation of the cerebral cortex. They die during the first postnatal week [6].

**Figure 1.** The domains of telencephalic neurons in adult and embryonic mouse brain

types of *interneurons* and *projection neurons*.

migrate dorsally into the developing cortex.

amygdala, arise from the dorsal LGE (dLGE) [13].

LGE (vLGE) [13;14].

cholinergic (blue) neurons.

In the first part, we review the stages of the mouse telencephalic development, the morpho‐ gens and the transcription factors (TF) that are intimately involved in the telencephalic pat‐ terning and neuronal subtype specification (**Sections 2-5**).

In the second part, we present recently reported protocols for differentiation of mouse and human pluripotent stem cells into telencephalic populations, following the development principles and reflecting the *in vivo* signaling pathways; we point on the relevant morpho‐ gens and TF in each stage, where the level of expression of relevant sets of TF can be consid‐ ered as a milestone between each differentiation step *in vitro* [2] (**Sections 6-8**).

Finally, we describe our model system in which the *in vitro* differentiation of human and mouse embryonic stem (ES) cells are temporally aligned to each other and compared with mouse telencephalic neurogenesis *in vivo* [3]. Since the telencephalic development has been extensively studied in animal models, it is important to strengthen the interspecies compara‐ tive approaches in order to gain further insights into the human telencephalic development. We provide evidence for differences in the default differentiation of mouse and human plu‐ ripotent stem cells that proves the utility of the comparative system for optimizing the di‐ rected telencephalic differentiation of human pluripotent stem cells. We also exemplify how Hedgehog (Hh) signaling pathway is implied in telencephalic neuronal fate decision *in vitro* and *in vivo* (**Section 8**).
