The Full Wiki

More info on Tanycytes

Tanycytes: Wikis

Advertisements

Note: Many of our articles have direct quotes from sources you can cite, within the Wikipedia article! This article doesn't yet, but we're working on it! See more info or our list of citable articles.

Encyclopedia

From Wikipedia, the free encyclopedia

Neuron: Tanycytes
Tanycytes - Third ventricle wall in the brain of an immature rat. A tanycyte coexpressing CuZn SOD and GFAP is marked by the arrow.
Third ventricle wall in the brain of an immature rat. A tanycyte coexpressing CuZn SOD and GFAP is marked by the arrow.
NeuroLex ID sao1149261773

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.

Recent text suggests that tanycytes are involved in transferring selected molecules from blood to CSF.The term “Tanycyte” comes from the Greek word tanus which means elongated. This term 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. 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.

During the 70’s and 80’s tanycytes were the subject to many research publications ranging from morphology to function. But the lack of advanced methodological tools stalled research and led to disagreements between researchers about the full role that tanycytes play. But recent advances in immunocytochemistry has allowed for new research into the functional role these cells play.

Tanycytes share some features with radial glia cells and astrocytes. Their form and location has 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. But 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 available now leads to the idea that tanycytes are genealogical descendants of radial glia cells that do not develop into astrocytes, but rather into their own subpopulation.

Tanycytes in adult mammals are found in the ventricular system and the circumventricular organs. They are most numerous in the third ventral 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.

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.

One functional difference in the subtypes is that both a-subtypes do not possess barrier properties, whereas ß1 tanycytes have been show by immunocytochemical staining to not allow the passage markers beyond their region of the . 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.

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.

One of the more exciting finds in recent research is 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. There is thought to be 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. Most recent evidence both supports both mechanism but also the possibility they are both part of a single mechanism.

Tanycytes have been linked to many neuroendocrine mechanisms by the fact that the 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 dihydroprogesteron 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.

One other promising aspect of tanycyte research is that in laboratory test, tanycytes have been transplanted into the damaged spinal cord of rats and have been shown to support regeneration of lesioned axons.

Although advances have been made in science allowing for tanycyte research to advance, it seems that every conclusion reached only seems to lead to more questions. Although most of the morphological features of tanycytes have been recorded, much is still to be learned about their role in the link between CSF and the portal blood. The microscopic structures of the tanycyte may for the most part be realized and understood, but the full understanding of the chemical function may truly never be known.

References

Rodriguez E. M., Blázquez J. L., Pastor F. E., Peláez B., Pena P., Peruzzo B., and Amat P. (2005). Hypothalmic Tanycytes: A Key Component of Brain-Endocrine Interaction. International Review of Cytology, Vol. 247 89-164.

Toshihiko Kubota, Kazufumi Sato, Hidetaka Arishima, Hiroaki Takeuchi, Ryuhei Kitai, and Takao Nakagawa. (2006). Astroblastoma: Immunohistochemical and ultrastructural study of distinctive epithelial and probable tanycytic differentiation. Neuropathology 26, 72-81.

Guandano-Ferraz A., Obregon M., St. Germain D., Bernal J. (1997) The type 2 iodothyronine deiodinase is expressed primarily in glial cells in the neonatal rat brain . Proc. Natl. Acad. Sci. USA Vol. 94, 10391-10396.

M. Prieto, N. Chauvet, and G. Alonso. (2000). Tanycytes Transplanted into the Adult Rat Spinal Cord Support the Regeneration of Lesioned Axons. Experimental Neurology 161,27-37.

External links

Advertisements

Advertisements






Got something to say? Make a comment.
Your name
Your email address
Message