Tanycytes are highly specialized

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 cerebral 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 lef ...

of the

brain The brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head (cephalization), usually near organs for ...

, 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 ...

. Each tanycyte has a long basal process that extends deep into the

hypothalamus The hypothalamus (: hypothalami; ) is a small part of the vertebrate brain that contains a number of nucleus (neuroanatomy), nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrin ...

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

cerebrospinal fluid Cerebrospinal fluid (CSF) is a clear, colorless Extracellular fluid#Transcellular fluid, transcellular body fluid found within the meninges, meningeal tissue that surrounds the vertebrate brain and spinal cord, and in the ventricular system, ven ...

to the

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

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

Structure

Tanycytes are highly specialized ependymal cells (also called ependymoglial cells) with long basal processes. Tanycytes in adult mammals are found in the

ventricular system In neuroanatomy, 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 ventric ...

, and the

median eminence The median eminence is generally defined as the portion of the ventral hypothalamus from which the portal vessels arise. The median eminence is a small swelling on the tuber cinereum, posterior to and on top of the pituitary stalk; it lies in th ...

, a

circumventricular organ 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–b ...

. They are most numerous in the

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 that extends from the medulla oblongata in the lower brainstem to the lumbar region of the vertebral column (backbone) of vertebrate animals. The center of the spinal c ...

radiating from the

central canal The central canal (also known as spinal foramen or ependymal canal) is the cerebrospinal fluid-filled space that runs through the spinal cord. The central canal lies below and is connected to the ventricular system of the brain, from which it r ...

(also known as the ependymal canal), to the spinal cord surface. The long processes extend through the layer of

astrocytes Astrocytes (from Ancient Greek , , "star" and , , "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 end ...

to cross the median eminence and form end-feet on neuropil, and blood vessels near the portal perivascular space.

Processes

Along the process from each tanycyte are some unusual protrusions as spikes, and swellings, and at their end feet are boutons or claws. The different protrusions contain blood vessels, and contact different neurons, and cells. The protrusions contain

ribosome Ribosomes () are molecular machine, macromolecular machines, found within all cell (biology), cells, that perform Translation (biology), biological protein synthesis (messenger RNA translation). Ribosomes link amino acids together in the order s ...

mitochondria A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...

, varied vesicles, and transporters.

Subtypes

In the third ventricle four different subtypes of tanycte populations with differing structure, morphology, genetics, and function have been defined. These are named beta 2 (β2) and beta 1 (β1), and alpha 2 (α2) and alpha 1 (α1). β2 tanycytes line the median eminence; β1 tanycytes line part of the

infundibular recess The pituitary stalk, also known as the infundibular stalk, infundibulum, or Fenderson's funnel, is the connection between the hypothalamus and the posterior pituitary, the posterior lobe of the pituitary gland. The floor of the third ventricle is ...

. α2 tanycytes line the dorsomedial arcuate nucleus; α1 tanycytes line the ventromedial nucleus and the dorsomedial nucleus.

Function

A tanycyte has a single long basal process that crosses into the hypothalamic parenchyma to make contact with blood vessels and an array of neurons. The different components of the tanycyte process protrusions that include ribosomes, mitochondria, and transporters, indicate communication between the tanycytes and blood vessels, and between the tanycytes and neurons. These interactions are associated with the regulation of different

neuroendocrine Neuroendocrine cells are cells that receive neuronal input (through neurotransmitters released by nerve cells or neurosecretory cells) and, as a consequence of this input, release messenger molecules ( hormones) into the blood. In this way they b ...

functions. Tanycytes have been shown ''in vivo'' to serve as a diet-responsive neurogenic niche. The targeted neurons in the hypothalamus are in the

arcuate nucleus The arcuate nucleus of the hypothalamus (ARH), or ARC, is also known as the infundibular nucleus to distinguish it from the arcuate nucleus of the medulla oblongata in the brainstem. The arcuate nucleus is an aggregation of neurons in the medio ...

, the

ventromedial nucleus The ventromedial nucleus of the hypothalamus (VMN, VMH or ventromedial hypothalamus) is a nucleus of the hypothalamus. In 2007, Kurrasch ''et al''. found that the ventromedial hypothalamus is a distinct morphological nucleus involved in termina ...

, and the dorsomedial nucleus.The neurons contacted by the tanycyte processes include

orexigenic An orexigenic, or appetite stimulant, is a drug, hormone, or compound that increases appetite and may induce hyperphagia. This can be a medication or a naturally occurring neuropeptide hormone, such as ghrelin, orexin or neuropeptide Y, which inc ...

, and

anorexigenic An anorectic is a drug that reduces appetite, resulting in lower food consumption, leading to weight loss. These substances work by affecting the central nervous system or certain neurotransmitters to create a feeling of fullness or reduce the desi ...

neurons that control energy balance. Studies suggests that tanycyte cells bridge the gap between the central nervous system (CNS) via cerebrospinal fluid (CSF) to the hypophyseal portal blood. Tanycytes provide a link that is both structural and functional between the CSF and the perivascular space of the hypophyseal portal vessels. Unlike regular ependymal cells there are tight junctions between them, and between their adjacent ependymal cells. There are also desmosomes present which together with the tight junctions provide structural support.

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 rele ...

(GnRH). GnRH is released by GnRH neurons located in the

in the

. These nerve fibers are concentrated in the region that exactly matches the distribution of β1 tanycytes. β1 and β2 tanycytes are found nearer the

and the

References

;Bibliography * * * *

External links

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