Tanycytes
Encyclopedia
Tanycytes are special ependymal cells located in the floor of the third ventricle
having processes extending deep into the hypothalamus
. It is possible that their function is to transfer chemical signals from CSF
to CNS
.
The term “Tanycyte” comes from the Greek word tanus which means elongated.
Recent research has shown that these bipolar cells bridge the gap between the central nervous system (CNS) via cerebrospinal fluid (CSF)
to the portal blood. This may also link the CSF to neuroendocrine events.
Currently, it is thought that there are two different mechanisms by which tanycytes participate in the release of Gn-Rh. One includes the cyclic remodeling of the spatial relationship between Gn-RH terminals, the tanycytes, and the perivascular space. The second is the cell to cell signaling mechanism mediated by specific tanycyte compounds. Recent evidence supports both mechanisms, and also the possibility that both are part of a single mechanism.
During the 1970s and 1980s, tanycytes were the subject of many research publications, ranging from their morphology to function. But the lack of advanced methodological tools stalled research and led to disagreements between researchers about the full role of tanycytes. Recent advances in immunocytochemistry have allowed for new research in this area.
Even so, tanycytes also display certain characteristics that distinguish them from radial glia cells. Tanycytes in rats begin to develop in the last two days of pregnancy and continue on until they reach their full differentiation in the first month of life. Radial glia cells on the other hand, are a key component of the embryonic brain. Tanycytes also contain many proteins not found in radial glia cells. Thus, evidence now suggests that tanycytes are genealogical descendants of radial glia cells that do not develop into astrocytes, but rather into their own subpopulation.
α1 tanycytes line the area of the ventromedial nucleus and part of the dorsomedial nucleus and project their basal processes to these nuclei. α2 tanycytes line the area of the arcuate nucleus and most of them project their process within this nucleus.
ß1 tanycytes line the lateral envaginations in the infundibular recess and project their processes to the lateroexternal region of the medial eminence and land on the perivascular space of the portal capillaries located here.
ß2 tanycytes line the floor of the infundibular recess and their basal processes end on the portal capillaries of the median zone of the median eminence. Therefore a tanycytes bridge the lumen of the third ventricle with the blood vessels of the medial basal hypothalamus, and the ß tanycytes establish the anatomical link between the ventricular CSF and the portal blood.
ß2 tanycytes form a barrier between the CSF and neuropil of the median eminence. Other functional difference include both a-subtypes and ß1 tanycytes contain the glucose-1 transporter while ß2 tanycytes do not, also while ß1 tanycytes do express insulin-like growth factor
binding protein ß2 tanycytes do not.
Tanycytes have been linked to many neuroendocrine mechanisms by the fact that they produce enzymes involved in their production. Many enzymes responsible for steroid synthesis are found in CSF.
5a-reductase which is responsible for the conversion of testosterone
and progesterone
to dihydrotestosterone and dihydroprogesterone respectively has been found to be present in rat tanycytes. The enzymes thyroxine deiodinase type I and II generate T3 fromT4 thus activating thyroid hormone. Deiodinase type II predominates in the CNS and appears to be localized in all subtypes of tanycytes. This enzymes messenger is also present throughout the tanycyte cytoplasm. Researchers propose that deiodinase type II in tanycytes plays a relevant role by providing T3 locally to the hypothalamus and other regions of the CNS via CSF.
Tanycytes contain the glucose-transporting molecules GLUT-1 and GLUT-2. No other ciliated ependyma of the third ventricle do. Studies have shown that these transporters are functional. This leads researchers to postulate that tanycytes are possibly involved in the detection of glucose levels in the CSF. Plasma and CSF glucose levels are known to be in a fixed ratio which is species dependent (3:1 in rats, 3:2 in rabbits, and 5:4 in humans). It is thus speculated by some researchers that by sensing CSF glucose levels the cells are also monitoring plasma glucose levels. This would mean that tanycytes could play a role in the complex mechanism controlling insulin secretion.
In laboratory tests, tanycytes have been transplanted into the damaged spinal cord of rats and have been shown to support regeneration of lesioned axons.
Third ventricle
The third ventricle is one of four connected fluid-filled cavities comprising the ventricular system within the human brain. It is a median cleft between the two thalami, and is filled with cerebrospinal fluid ....
having processes extending deep into the hypothalamus
Hypothalamus
The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...
. It is possible that their function is to transfer chemical signals from CSF
Cerebrospinal fluid
Cerebrospinal fluid , Liquor cerebrospinalis, is a clear, colorless, bodily fluid, that occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord...
to CNS
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...
.
The term “Tanycyte” comes from the Greek word tanus which means elongated.
Function
Recent text suggests that tanycytes are involved in transferring selected molecules from CSF to blood.Recent research has shown that these bipolar cells bridge the gap between the central nervous system (CNS) via cerebrospinal fluid (CSF)
Cerebrospinal fluid
Cerebrospinal fluid , Liquor cerebrospinalis, is a clear, colorless, bodily fluid, that occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord...
to the portal blood. This may also link the CSF to neuroendocrine events.
The role in the release of Gonadotropin-releasing hormone (Gn-RH)
Recent research found that tanycytes participate in the release of gonadotropic hormone-releasing hormone (Gn-RH). Gn-RH is released by neurons located in the rostral hypothalamus. These nerve fibers are concentrated in the region that exactly matches the distribution of ß1 tanycytes.Currently, it is thought that there are two different mechanisms by which tanycytes participate in the release of Gn-Rh. One includes the cyclic remodeling of the spatial relationship between Gn-RH terminals, the tanycytes, and the perivascular space. The second is the cell to cell signaling mechanism mediated by specific tanycyte compounds. Recent evidence supports both mechanisms, and also the possibility that both are part of a single mechanism.
History
The term tanycyte was coined by Horstmann in 1954 when he described a distinct structural feature of the cell, which is a single, long basal process that projects to a distinct region of the hypothalamus.During the 1970s and 1980s, tanycytes were the subject of many research publications, ranging from their morphology to function. But the lack of advanced methodological tools stalled research and led to disagreements between researchers about the full role of tanycytes. Recent advances in immunocytochemistry have allowed for new research in this area.
Morphology
Tanycytes share some features with radial glia cells and astrocytes. Their form and location have led some authors to regard them as radial glia cells that remain in the hypothalamus throughout life. This has led some to believe that these cells share the same lineage.Even so, tanycytes also display certain characteristics that distinguish them from radial glia cells. Tanycytes in rats begin to develop in the last two days of pregnancy and continue on until they reach their full differentiation in the first month of life. Radial glia cells on the other hand, are a key component of the embryonic brain. Tanycytes also contain many proteins not found in radial glia cells. Thus, evidence now suggests that tanycytes are genealogical descendants of radial glia cells that do not develop into astrocytes, but rather into their own subpopulation.
Location
Tanycytes in adult mammals are found in the ventricular system and the circumventricular organs. They are most numerous in the third ventricle of the brain, but can also be seen in the spinal cord radiating from the ependyma of the central canal to the spinal cord surface.Subtypes
There are 4 distinct subtypes of tanycytes found in the rat each with their own distinct characteristics such as location, spatial relationship, morphology, cytochemistry, ultrastructure, and certain functions. These subtypes are α1, α2, ß1 and, ß2.α1 tanycytes line the area of the ventromedial nucleus and part of the dorsomedial nucleus and project their basal processes to these nuclei. α2 tanycytes line the area of the arcuate nucleus and most of them project their process within this nucleus.
ß1 tanycytes line the lateral envaginations in the infundibular recess and project their processes to the lateroexternal region of the medial eminence and land on the perivascular space of the portal capillaries located here.
ß2 tanycytes line the floor of the infundibular recess and their basal processes end on the portal capillaries of the median zone of the median eminence. Therefore a tanycytes bridge the lumen of the third ventricle with the blood vessels of the medial basal hypothalamus, and the ß tanycytes establish the anatomical link between the ventricular CSF and the portal blood.
ß2 tanycytes form a barrier between the CSF and neuropil of the median eminence. Other functional difference include both a-subtypes and ß1 tanycytes contain the glucose-1 transporter while ß2 tanycytes do not, also while ß1 tanycytes do express insulin-like growth factor
Insulin-like growth factor
The insulin-like growth factors are proteins with high sequence similarity to insulin. IGFs are part of a complex system that cells use to communicate with their physiologic environment...
binding protein ß2 tanycytes do not.
Research
ß tanycytes appear to be polarized transport cells responsible for moving molecules from CSF to the portal blood. The absorption of molecules in tanycytes is mediated by clathrin-dependent and caveolin-dependent endocytosis. Evidence of this polarity is found in the fact that clathrin is present in the ventricular cell pole and virtually absent in the basal cell pole.Tanycytes have been linked to many neuroendocrine mechanisms by the fact that they produce enzymes involved in their production. Many enzymes responsible for steroid synthesis are found in CSF.
5a-reductase which is responsible for the conversion of testosterone
Testosterone
Testosterone is a steroid hormone from the androgen group and is found in mammals, reptiles, birds, and other vertebrates. In mammals, testosterone is primarily secreted in the testes of males and the ovaries of females, although small amounts are also secreted by the adrenal glands...
and progesterone
Progesterone
Progesterone also known as P4 is a C-21 steroid hormone involved in the female menstrual cycle, pregnancy and embryogenesis of humans and other species...
to dihydrotestosterone and dihydroprogesterone respectively has been found to be present in rat tanycytes. The enzymes thyroxine deiodinase type I and II generate T3 fromT4 thus activating thyroid hormone. Deiodinase type II predominates in the CNS and appears to be localized in all subtypes of tanycytes. This enzymes messenger is also present throughout the tanycyte cytoplasm. Researchers propose that deiodinase type II in tanycytes plays a relevant role by providing T3 locally to the hypothalamus and other regions of the CNS via CSF.
Tanycytes contain the glucose-transporting molecules GLUT-1 and GLUT-2. No other ciliated ependyma of the third ventricle do. Studies have shown that these transporters are functional. This leads researchers to postulate that tanycytes are possibly involved in the detection of glucose levels in the CSF. Plasma and CSF glucose levels are known to be in a fixed ratio which is species dependent (3:1 in rats, 3:2 in rabbits, and 5:4 in humans). It is thus speculated by some researchers that by sensing CSF glucose levels the cells are also monitoring plasma glucose levels. This would mean that tanycytes could play a role in the complex mechanism controlling insulin secretion.
In laboratory tests, tanycytes have been transplanted into the damaged spinal cord of rats and have been shown to support regeneration of lesioned axons.
External links
- http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Nervous/Nervous.htm
- NIF Search - Tanycyte via the Neuroscience Information FrameworkNeuroscience Information FrameworkThe Neuroscience Information Framework is a repository of global neuroscience web resources, including experimental, clinical, and translational neuroscience databases, knowledge bases, atlases, and genetic/genomic resources.-Description:...