Zona limitans intrathalamica

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The zona limitans intrathalamica (ZLI) is a

prethalamus
(ventral thalamus).

anterior tegmental structures.[2] The ZLI together with the prethalamus and thalamus make up the mid-diencephalic territory (MDT).[3]

prethalamus and thalamus
depicts the ZLI.

Discovery

Cell lineage restriction boundaries, across which replicating cells cannot migrate, were first discovered in

anterior regions of the brain were examined in search of other cell lineage restriction boundaries, and multiple potential boundaries continue to be studied (see Developmental Boundaries).[4]

Layout of Hox genes in the fruitfly leading to differential development of segments

The importance of these compartments as local signaling centers, areas which chemically influence surrounding tissue, was elucidated by first observing differential expression of Hox genes in various compartments and second by observing mutant D. melanogaster and corresponding phenotypic (physical) changes.[4]

The ZLI was first discovered in the

genetic markers in culture (see Signaling).[5] These experiments confirmed the ZLI as a signaling center. In lineage-labeling experiments, cells were genetically marked, so that each time a labeled cell replicated, its progeny were marked as well. Cells that were marked in the developing ZLI and their progeny remained restricted to the zone. These experiments demonstrated the ZLI as a cell-lineage restriction boundary.[6]

Not only a boundary, the ZLI is also a

telencephalon and diencephalon), inducing a ZLI-like region that induces thalamic fate.[7]

Developmental boundaries

During

shh from the ZLI, are often released from these boundaries and compartments in a concentration gradient (the chemicals are in much higher concentrations closer to the source) and confer identity to flanking regions. Other genes differentially expressed
in these flanking regions aid in ensuring proper differentiation (see Signaling).

Main structures of the developing brain: the prosencephalon (forebrain) consists of the telencephalon and diencephalon, the mesencephalon is the midbrain, and the rhombencephalon is the hindbrain.

Many developmental boundaries have been studied: within the forebrain alone, the confirmed cell lineage restriction boundaries are the pallial-subpallial boundary (PSB) dividing the dorsal and ventral telencephalon, the diencephalon-midbrain boundary (DMB) posterior to the ZLI, and the ZLI. The ZLI, like each

posterior regions. Other developmental boundaries serve as cell-lineage restriction boundaries but not signaling centers, while others are signaling centers to and from which cells can migrate. Despite discoveries of cell lineage restriction boundaries and compartments in the brain, many of the regions studied have been disproven as segmental boundaries. These areas have potential as signaling centers, which have influence over the development of neighboring tissues.[4]

These boundaries have great influence over other regions of the brain: the placement of the ZLI not only affects the size of adjacent regions but also the size of the

telencephalon
. The same is true for other developmental boundaries in the brain and throughout the body: shifts in boundaries responsible for allocating a certain amount of tissue to a certain function result in drastic changes in the adult structure. These boundaries are of crucial importance for proper differentiation.

Formation

Initial axis patterning

After

midbrain-hindbrain boundary (MHB), rhombomeres, and the ZLI aid in anteroposterior organization.[5]

Emergence of Shh expression

Shortly after the beginning of

prethalamus and thalamus, respectively.[3]

The ZLI is also characterized by a lack of

Shh expression does not extend dorsally until a few hours later.[5]

Positioning

The factors influencing the formation and location of the ZLI are widely studied but still disputed. Differences between different

genetic pathways, specifically between chicks/ mammalian models and zebrafish
.

Wnt (the family wingless) genes are crucial for the development of the ZLI both directly and indirectly in all animal systems. Along with the role of Wnt genes in patterning the anteroposterior axis through gradient polarization, Wnt8b is expressed within the ZLI itself and may help guide dorsal movement of Shh expression.[9] The Wnt polarization gradient has been linked to induction of ZLI-patterning genes IRX3 and SIX3, which border the ZLI posteriorly and anteriorly, respectively. However, these genes have been shown to be non-essential for ZLI formation in zebrafish and have been reevaluated in other models.[1][3]

Specification of the ZLI may also involve the

telencephalon, with the epichordal plate posterior
to it.

Studies of the formation of the ZLI performed in

posteriorly.[3] Other genes crucial for differentiation in the brain, including Fgf genes responsible for patterning of the midbrain-hindbrain boundary (MHB), have been implicated in positioning of the ZLI.[9]

Studies on the role of Shh signaling in the ZLI were difficult to study for many years, because mutants lacking expression have many developmental deficits including lack of a diencephalon.[12] Explant and lineage-labeling experiments previously described aided in elucidation of the role of Shh and other genes in differentiation of these tissues. More recently, the mouse Shh;Gli3 double mutant was found to have an enlarged diencephalon with a ring of Fgf8 and Wnt in place of the ZLI, indicating a complex interaction between Shh and these genes at the ZLI.[13] This also indicates that other patterning cues are able to establish Fgf8 and Wnt signaling domains at the ZLI in the absence of Shh and Gli3.

Differentiation after ZLI degradation

After differentiation of the

ventral thalami to merge into one functional unit, as shown by replication-incompetent retroviral experiments that marked cells and showed their migration throughout the diencephalon.[9]

Signaling

After establishment of the ZLI,

shh
competence help to pattern the ZLI.

Signaling from the ZLI cooperating with

prethalamus and rostral thalamus differentiate into GABAergic inhibitory neurons, whereas her negative cells become glutamatergic relay neurons. Both cell types depend on Shh signal as trigger to initiate the developmental programme.[14]

References

  1. ^
    PMID 20541814
    .
  2. PMID 15063186
    .
  3. ^
    PMID 17670791
    .
  4. ^
    S2CID 10869618
    .
  5. ^
    PMID 17999760
    .
  6. S2CID 26205097
    .
  7. S2CID 3987564
    .
  8. PMID 16452095
    .
  9. ^
    S2CID 20491034
    .
  10. ^
    PMID 16026780
    .
  11. PMID 9753681
    .
  12. PMID 14568100
    .
  13. PMID 21925158
    .
  14. ^ Scholpp S, Delogu A, Gilthorpe J, Peukert D, Schindler S, Lumsden A. Her6 regulates the neurogenetic gradient and neuronal identity in the thalamus. Proc Natl Acad Sci U S A. 2009 Nov 24;106(47):19895-900 [1]
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