||This article is written like a personal reflection or opinion essay rather than an encyclopedic description of the subject. (October 2008)|
||This article may be too technical for most readers to understand. (October 2008)|
Third ventricle wall in the brain of an immature rat. A tanycyte coexpressing CuZn SOD and GFAP is marked by the arrow.
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 text suggests that tanycyte 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.
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 β1 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.
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.
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.
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. Tanycytes represent approximately 0.6% of the population of the lateral ventricular wall, as described by Doetsch et al. 
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, β1 and, β2.
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.
β1 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. β2 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 α tanycytes bridge the lumen of the third ventricle with the blood vessels of the medial basal hypothalamus, and the β tanycytes establish the anatomical link between the ventricular CSF and the portal blood.
β2 tanycytes form a barrier between the CSF and neuropil of the median eminence. Other functional difference include: both α-subtypes and β1 tanycytes contain the glucose-1 transporter, while β2 tanycytes do not; also, while β1 tanycytes do express insulin-like growth factor binding protein, β2 tanycytes do not.
β 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 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.
- Doetsch F, Garcia-Verdugo JM, Alvarez-Buylla A (1997) Cellular composition and three-dimensional organiza-tion of the subventricular germinal zone in the adult mammalian brain. J Neurosci 17:5046–61
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.
- NIF Search - Tanycyte via the Neuroscience Information Framework