The ''zona limitans intrathalamica'' (ZLI) is a
lineage-restriction compartment and primary developmental boundary in the
vertebrate forebrain (which is analogous to the human
cerebrum) that serves as a signaling center and a restrictive border between the
thalamus (also known as the dorsal thalamus) and the
prethalamus (ventral thalamus).
Sonic hedgehog
Sonic hedgehog protein (SHH) is encoded for by the ''SHH'' gene. The protein is named after the character ''Sonic the Hedgehog''.
This signaling molecule is key in regulating embryonic morphogenesis in all animals. SHH controls organogenesis and ...
(shh) signaling from the ZLI is crucial in the development of the ''
diencephalon
The diencephalon (or interbrain) is a division of the forebrain (embryonic ''prosencephalon''). It is situated between the telencephalon and the midbrain (embryonic ''mesencephalon''). The diencephalon has also been known as the 'tweenbrain in ol ...
'',
which develops into the
thalamus, the
pretectum, and the
anterior tegmental structures.
The ZLI together with the prethalamus and thalamus make up the mid-diencephalic territory (MDT).
Discovery
Cell lineage restriction boundaries, across which replicating cells cannot migrate, were first discovered in
invertebrates, where the expression of various
Hox genes in each compartment confer the
differentiation of observable segments in the adult body of
Drosophila melanogaster. Analogous structures were discovered in the developing
vertebrate brain.
Rhombomeres, which extend down the
embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
from the
hindbrain, contain clear
boundaries and each express various Hox genes that are necessary for
differentiation of structures within the body. More
anterior
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
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).
The importance of these
compartments as local signaling centers, areas which
chemically
A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...
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.
The ZLI was first discovered in the
chick
Chick or chicks may refer to:
*Chick (young bird), a bird that has not yet reached adulthood
People
* Chick (nickname), a list of people
* Chick (surname), various people
* Chick McGee, stage name of radio personality Charles Dean Hayes (born 19 ...
using
explant In biology, explant culture is a technique to organotypically culture cells from a piece or pieces of tissue or organ removed from a plant or animal. The term ''explant'' can be applied to samples obtained from any part of the organism. The extracti ...
and lineage-labeling experiments. In the explant experiments, cells in the region that will become the ZLI, the
prethalamus, and the
thalamus in the chick were removed and placed into separate
cultures; the cells continued to grow and retained their identity (the ZLI began
transcribing ''
shh'', whereas the
prethalamus and
thalamus did not). Necessity of the ZLI and its corresponding ''
shh'' expression were evidenced by lack of thalamic and prethalamic
genetic markers in culture (see Signaling).
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.
Not only a boundary, the ZLI is also a
compartment with separate cell lineage restriction boundaries both
anterior
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
and
posterior of a section of ''
shh'' expression. The importance of the ZLI was once more confirmed by
ectopic expression of ''
shh'' in other regions of the
forebrain, known as the
prosencephalon during development (both the
telencephalon and
diencephalon
The diencephalon (or interbrain) is a division of the forebrain (embryonic ''prosencephalon''). It is situated between the telencephalon and the midbrain (embryonic ''mesencephalon''). The diencephalon has also been known as the 'tweenbrain in ol ...
), inducing a ZLI-like region that induces thalamic fate.
Developmental boundaries
During
development in both
vertebrates and
invertebrates, cell lineage restriction boundaries and signaling centers are formed in order to ensure proper
differentiation of the body.
Chemical signals, like ''
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).
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
rhombomere, serves as an independent
compartment that confers the identity of
diencephalon
The diencephalon (or interbrain) is a division of the forebrain (embryonic ''prosencephalon''). It is situated between the telencephalon and the midbrain (embryonic ''mesencephalon''). The diencephalon has also been known as the 'tweenbrain in ol ...
in
anterior
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
and
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.
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. A
posterior shift in the ZLI allows more cells to be allocated to 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
gastrulation
Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of cells), or in mammals the blastocyst is reorganized into a multilayered structure known as the gastrula. Be ...
, the
embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
is completely
undifferentiated and requires many different cues to initiate proper
differentiation of the body. The top (roof plate, on the dorsal side of the embryo) and bottom (floor plate, on the ventral side) play a crucial role in these first steps: each acts through global
signaling
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...
(signaling throughout the entire embryo) for
dorsoventral neural patterning. After completion of the
development of the
dorsoventral axis, more local signaling occurs in the developing brain: developmental boundaries such as the
midbrain-hindbrain boundary (MHB),
rhombomeres, and the ZLI aid in
anteroposterior organization.
Emergence of ''Shh'' expression
Shortly after the beginning of
dorsoventral patterning,
Shh is expressed along the
basal plate (the bottom) of the
embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
, which functions in "
ventralization of the
neural tube, promotion of growth and
proliferation
Proliferation may refer to:
Weapons
*Nuclear proliferation, the spread of nuclear weapons, material, and technology
*Chemical weapon proliferation, the spread of chemical weapons, material, and technology
* Small arms proliferation, the spread of ...
, and formation of the
hypothalamus".
As the
embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
continues to
develop, ''Shh'' expression characteristic of the ZLI extends
dorsally to form a wedge that eventually narrows to a strip at approximately 22
somites (the number of developed
rhombomeres) or less than one day in
zebrafish. Although ''Shh'' expression extends
dorsally from the
basal plate, the ZLI is capable of forming even without the
basal plate or
mesoderm
The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical E ...
al tissue. ''
Shh'' cooperates with
''dlx2'' and ''fezl''
anteriorly
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
and ''
IRX3
Iroquois-class homeodomain protein IRX-3, also known as Iroquois homeobox protein 3, is a protein that in humans is encoded by the ''IRX3'' gene.
Discovery and name
The Iroquois family of genes was discovered in ''Drosophila'' during a mutag ...
'' and ''dbx1a'' posteriorly (genes expressed vary among different organisms), which are
genes that are expressed in the
prethalamus and
thalamus, respectively.
The ZLI is also characterized by a lack of
lunatic fringe (''lfng''), which is observable even before ''Shh'' expression is observed in the forming ZLI. This indicates that cells are fated for ZLI formation before induction by ''Shh'' (n.b. ''Shh]' signals from the ZLI and is characteristic of the
compartment during and after formation). ''Lfng'' is expressed very early (soon after
gastrulation
Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of cells), or in mammals the blastocyst is reorganized into a multilayered structure known as the gastrula. Be ...
) in the region that will become the ZLI, but shortly thereafter expression recedes through migration of ''lfng''-expressing cells to form the ''lfng''-free wedge characteristic of the developing ZLI. ''
Shh'' expression does not extend
dorsally until a few hours later.
Positioning
The factors influencing the formation and location of the ZLI are widely studied but still disputed. Differences between different
animal models further complicate elucidation of the
genetic pathways, specifically between chicks/
mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
ian 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.
The
Wnt polarization gradient has been linked to induction of ZLI-patterning genes ''IRX3'' and ''
SIX3'', which border the ZLI
posteriorly and
anteriorly
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
, respectively. However, these
genes have been shown to be non-essential for ZLI formation in
zebrafish and have been reevaluated in other models.
Specification of the ZLI may also involve the
ventral tissues of the
embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
: the
prechordal and epichordal plates, or neuroepithelial tissue (see figure). Interactions between these tissues may be responsible for the recession of expression of
''lfng'' which allows
dorsal movement ''Shh'' expression.
The specific prechordal and epichordal plates, characterized by expression of ''SIX3'' and ''IRX3'', respectively, may influence positioning of the ZLI more so than the genes themselves.
The prechordal plate
ventrally borders the
telencephalon, with the epichordal plate
posterior to it.
Studies of the formation of the ZLI performed in
zebrafish have revealed the significance of ''
OTX2'' and ''
IRX1'' in ZLI positioning. ''OTX2'' expression characterizes the developing
optic tectum, which is responsible for sight processing. Expression extends
anteriorly
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
and ends sharply at the ZLI, with high
expression along the line where ''Shh'' is expressed.
Before the ZLI is formed, ''OTX2]'' is expressed ubiquitously throughout the
forebrain, and begins to recede to the position of the putative ZLI. Experiments where ''OTX2]'' expression was repressed showed no
Dorsum (biology), dorsal movement of ''Shh'' expression and no ZLI formation.
''Irx1'', which is analogous to ''IRX3'' in birds and mammals, is expressed posterior to the ZLI. Studies have suggested that while ''OTX2'' positively restricts the ZLI
anterior
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
ly (''
Shh'' cannot be expressed where ''OTX2'' is not), 'IRX1'' negatively restricts the ZLI
posteriorly.
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.
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
The diencephalon (or interbrain) is a division of the forebrain (embryonic ''prosencephalon''). It is situated between the telencephalon and the midbrain (embryonic ''mesencephalon''). The diencephalon has also been known as the 'tweenbrain in ol ...
.
Explant In biology, explant culture is a technique to organotypically culture cells from a piece or pieces of tissue or organ removed from a plant or animal. The term ''explant'' can be applied to samples obtained from any part of the organism. The extracti ...
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. 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
progenitor cells (at a precise
stage yet to be fully determined), the ZLI and its lineage restriction disappears, allowing cells to migrate across the former boundary and the
Dorsum (biology), dorsal and
ventral thalami
The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all directions, ...
to merge into one functional unit, as shown by replication-incompetent
retroviral experiments that marked cells and showed their migration throughout the diencephalon.
Signaling
After establishment of the ZLI, ''
shh'' has been shown to induce
expression of
thalamic
The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all directions, ...
and
prethalamic markers
The term Marker may refer to:
Common uses
* Marker (linguistics), a morpheme that indicates some grammatical function
* Marker (telecommunications), a special-purpose computer
* Boundary marker, an object that identifies a land boundary
* Marke ...
, ''gbx2'' and
''dlx2''/ ''nkx2.1'', respectively. This differential induction most likely is due to the expression of genes such as ''IRX3'' in the
thalamus:
ectopic expression experiments showed that if ''IRX3'', which is normally expressed in the developing
thalamus, is expressed
anterior
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
to the ZLI, then the developing
prethalamus will change identity.
[ Note that these genes which help confer '' shh'' competence help to pattern the ZLI.
Signaling from the ZLI cooperating with ]thalamic
The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all directions, ...
and prethalamic markers
The term Marker may refer to:
Common uses
* Marker (linguistics), a morpheme that indicates some grammatical function
* Marker (telecommunications), a special-purpose computer
* Boundary marker, an object that identifies a land boundary
* Marke ...
ensures the migration of post-mitotic (neural progenitor) cells to the mantle zone where the cells assemble into nuclei characteristic of the thalamus. These nuclei are the mechanism of the relay of information from the thalamus to the cortex. The thalamus itself is highly diverse, with each nucleus having distinct morphologies and physiologies according to the region of the brain to which it is connected. These differences are thought to originate in differential gene expression
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, protein or non-coding RNA, and ultimately affect a phenotype, as the final effect. The ...
in the thalamus and prethalamus, which allows for a single structure with multiple different and separate functions once the two have merged and completed growth and differentiation. So a functional interplay between Shh from the ZLI and the bHLH factor Her6 (homolog to HES1) determines the neuronal identity within the thalamus: Her6 positive cells in the 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.[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):19895-90]
/ref>
References
{{DEFAULTSORT:Zona Limitans Intrathalamica
Diencephalon
Developmental neuroscience