Tanycytes are special

ependymal cells The ependyma is the thin neuroepithelial ( simple columnar ciliated epithelium) lining of the ventricular system of the brain and the central canal of the spinal cord. The ependyma is one of the four types of neuroglia in the central nervous sys ...

found in the

third ventricle The third ventricle is one of the four connected ventricles of the ventricular system within the mammalian brain. It is a slit-like cavity formed in the diencephalon between the two thalami, in the midline between the right and left lateral v ...

of the

brain A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...

, and on the floor of the

fourth ventricle The fourth ventricle is one of the four connected fluid-filled cavities within the human brain. These cavities, known collectively as the ventricular system, consist of the left and right lateral ventricles, the third ventricle, and the fourth ve ...

and have processes extending deep into the

hypothalamus The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalamu ...

. It is possible that their function is to transfer chemical signals from the

cerebrospinal fluid Cerebrospinal fluid (CSF) is a clear, colorless body fluid found within the tissue that surrounds the brain and spinal cord of all vertebrates. CSF is produced by specialised ependymal cells in the choroid plexus of the ventricles of the bra ...

to the

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all par ...

. The term ''tanycyte'' comes from the Greek word tanus which means elongated.

Structure

Tanycytes share some features with

radial glial cell Radial glial cells, or radial glial progenitor cells (RGPs), are bipolar-shaped progenitor cells that are responsible for producing all of the neurons in the cerebral cortex. RGPs also produce certain lineages of glia, including astrocytes and ol ...

s and

astrocyte Astrocytes (from Ancient Greek , , "star" + , , "cavity", "cell"), also known collectively as astroglia, are characteristic star-shaped glial cells in the brain and spinal cord. They perform many functions, including biochemical control of e ...

s. Their form and location have led some authors to regard them as radial glial cells that remain in the

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

gestation Gestation is the period of development during the carrying of an embryo, and later fetus, inside viviparous animals (the embryo develops within the parent). It is typical for mammals, but also occurs for some non-mammals. Mammals during pregna ...

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 The ventricular system is a set of four interconnected cavities known as cerebral ventricles in the brain. Within each ventricle is a region of choroid plexus which produces the circulating cerebrospinal fluid (CSF). The ventricular system is con ...

and the

circumventricular organs Circumventricular organs (CVOs) ( circum-: around ; ventricular: of ventricle) are structures in the brain characterized by their extensive and highly permeable capillaries, unlike those in the rest of the brain where there exists a blood� ...

. They are most numerous in the third ventricle of the brain, are also found in the

, and can also be seen in the

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spi ...

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. Tanycytes have also been shown ''in vivo'' to serve as a diet-responsive neurogenic niche.

Function

Recent work suggests that tanycyte cells bridge the gap between the central nervous system (CNS) via cerebrospinal fluid (CSF) to the hypophyseal portal blood. This may also link the CSF to neuroendocrine events.

Role in the release of gonadotropin-releasing hormone

Researches in 2005 and 2010 found that tanycytes participate in the release of

gonadotropin-releasing hormone Gonadotropin-releasing hormone (GnRH) is a releasing hormone responsible for the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary. GnRH is a tropic peptide hormone synthesized and released ...

(GnRH). GnRH 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. β1 and β2 tanycytes are found nearer the

arcuate nucleus The arcuate nucleus of the hypothalamus (also known as ARH, ARC, or infundibular nucleus) is an aggregation of neurons in the mediobasal hypothalamus, adjacent to the third ventricle and the median eminence. The arcuate nucleus includes several ...

and the

median eminence The median eminence, part of the inferior boundary of the hypothalamus in the brain, is attached to the infundibulum. The median eminence is a small swelling on the tuber cinereum, posterior to and atop the pituitary stalk; it lies in the area rou ...

. Currently, it is thought that there are two different mechanisms by which tanycytes participate in the release of GnRH. One includes the cyclic remodeling of the spatial relationship between GnRH 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.

References

;Bibliography * * * *

External links

*http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Nervous/Nervous.htm
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