Tanycytes are specialized cells that act as a bridge between the brain’s fluid-filled cavities and its functional tissues. This unique position allows them to perform diverse roles in bodily regulation, from sensing nutrients to creating new neurons. Their versatility in managing the body’s resources makes them a subject of intense study for understanding the brain’s complex operations.
Location of Tanycytes in the Brain
Tanycytes are positioned deep within the brain, lining the walls of the third ventricle. This ventricle is a central cavity filled with cerebrospinal fluid (CSF) and is situated within the hypothalamus, a control center for functions like hunger, thirst, and hormone release.
This placement allows them to act as gatekeepers between the CSF and the neural circuits of the hypothalamus. They monitor the CSF’s contents and communicate that information to the brain’s metabolic control centers, influencing neurons that regulate energy balance.
The Structure and Types of Tanycytes
The name “tanycyte” is derived from a Greek word meaning “elongated,” which describes their shape. Each cell has a main body along the edge of the third ventricle in contact with the cerebrospinal fluid. From this cell body extends a single, long projection that reaches deep into the hypothalamus.
Scientists classify tanycytes into four main subtypes based on their location along the ventricle wall:
- Alpha-1
- Alpha-2
- Beta-1
- Beta-2
These subtypes are distributed from the upper to the lower parts of the third ventricle. Alpha tanycytes are in the lateral walls, while beta tanycytes are located more ventrally in the floor. This segregation suggests each subtype interacts with different neuronal populations, giving them distinct roles.
The Functions of Tanycytes
Tanycytes manage the brain’s internal environment by forming a selective barrier, controlling the passage of molecules between the CSF, the bloodstream, and brain neurons. The region of the hypothalamus where beta tanycytes reside has leaky capillaries, unlike other brain areas. This structure gives them privileged access to hormones and nutrients circulating in the blood.
A primary function is nutrient sensing, as tanycytes can detect metabolic molecules like glucose and amino acids. When you eat, the rise in blood glucose is sensed by tanycytes, which transmit this information to hypothalamic circuits. This process allows the brain to directly monitor the body’s energy state.
This sensory information is then used to regulate metabolism. By communicating with neurons that control appetite, tanycytes help adjust feelings of hunger and fullness. For instance, after detecting high glucose, they can signal to appetite-suppressing neurons. They also transport hormones like leptin from the bloodstream into the brain.
Tanycytes and Neural Stem Cell Activity
Beyond transport and sensing, a subpopulation of tanycytes can act as neural stem cells, which create new neurons and other supportive glial cells. For a long time, it was believed the adult brain had a limited capacity to generate new neurons, especially in the hypothalamus.
Research has revealed that certain tanycytes can divide and give rise to new cells in the adult brain through a process called adult neurogenesis. This suggests the hypothalamus is not static but has a capacity for self-repair and adaptation. These newly generated cells can integrate into the existing hypothalamic circuits that regulate energy balance.
The discovery of tanycytes as a source of new neurons has opened new avenues for understanding brain plasticity. It implies the brain’s control center for metabolism can physically change in response to physiological needs or environmental cues, such as changes in diet. This challenges previous assumptions about long-term adaptation in the regulation of body weight.
Connection to Health and Disease
The functions of tanycytes place them at a crossroads of health and disease. Impairments in their ability to sense nutrients or transport hormones are investigated as factors in metabolic disorders. If tanycytes become less effective at detecting glucose or transporting appetite-regulating signals, the brain may not receive accurate information, potentially leading to overeating and obesity.
Disruptions in tanycyte function are also linked to the development of type 2 diabetes. The same mechanisms that regulate energy balance are tied to glucose homeostasis, and faulty signaling from tanycytes can contribute to this disease. These cells are also involved in regulating hormones that control puberty and reproduction, linking metabolic status with fertility.
The stem cell properties of tanycytes present therapeutic possibilities. The ability to generate new neurons in the hypothalamus could be harnessed to treat brain injuries or neurodegenerative conditions. Understanding how to stimulate this capacity might offer strategies for repairing damaged neural circuits or counteracting age-related decline.