Tanycytes are a specialized type of cell found within the brain, recognized for their roles in brain function and overall health. These cells are important in how the brain interacts with the rest of the body, offering new avenues for understanding complex biological processes. Their characteristics set them apart from other brain cells, hinting at specialized functions that are still being uncovered.
What Are Tanycytes?
Tanycytes are specialized ependymal cells that line the brain’s ventricular system. They are primarily located in the third ventricle, especially in the hypothalamus. The term “tanycyte” comes from the Greek word “tanus,” meaning “stretch” or “elongated,” describing their shape.
These cells have an elongated shape, with a cell body in the ventricular wall and a long process extending deep into the surrounding brain tissue. This structure allows them to bridge the cerebrospinal fluid (CSF) within the ventricle and the brain parenchyma. This positioning at the interface between the CSF and brain tissue distinguishes tanycytes from other glial cells, such as astrocytes or oligodendrocytes.
Tanycytes as Brain Gatekeepers
Tanycytes regulate the passage of substances between the cerebrospinal fluid (CSF) and the brain. Their location allows them to monitor the composition of both the CSF and blood. They transport molecules, including hormones and nutrients, from the CSF into the brain tissue.
Beta-tanycytes, a subtype, interact directly with fenestrated capillaries in the median eminence, a region where the blood-brain barrier is less restrictive. This interaction allows them to regulate the access of circulating molecules to the hypothalamus. Tanycytes form a specialized barrier between the CSF and the hypothalamus, contributing to the selective permeability of this brain region.
Studies involving tanycyte removal show increased permeability of the CSF-hypothalamus barrier, indicating their role in maintaining its integrity. The loss of tight junction proteins like ZO-1 in these cells further supports their role in controlling molecular exchange. This barrier function helps shield hypothalamic neurons from various CSF signals.
Tanycytes and Metabolic Control
Tanycytes have a role in regulating metabolism, appetite, and the body’s energy balance. They function as sensors for circulating nutrients like glucose and amino acids, and hormones such as leptin and ghrelin. This sensory information is then relayed to specific neurons within the hypothalamus, influencing behaviors related to feeding, body weight, and glucose homeostasis.
Leptin, a hormone produced by fat cells, signals satiety and helps regulate food intake. Tanycytes in the median eminence can take up leptin from the blood, facilitating its entry into the brain. In conditions like diet-induced obesity, impaired leptin transport by tanycytes can contribute to leptin resistance, where the brain no longer responds effectively to high circulating leptin levels.
Ghrelin, primarily produced in the stomach, stimulates appetite. The balance between ghrelin and leptin, influenced by tanycytes, maintains metabolic homeostasis. Disruptions in this balance, such as elevated ghrelin or leptin resistance, can contribute to metabolic disorders like obesity and type 2 diabetes. Understanding these interactions may offer new strategies for managing these conditions.
Tanycytes and Brain Regeneration
Tanycytes also possess properties of adult neural stem cells or progenitor cells, particularly within the hypothalamus. These cells have the capacity to divide and differentiate into new neurons or glial cells in the adult brain. This regenerative potential contributes to brain plasticity and repair mechanisms.
In response to neural injury, tanycytes can transition from a quiescent state to actively dividing cells, promoting self-renewal and regeneration. Studies show that tanycytes enter the cell cycle upon neural injury to facilitate tissue repair. This regenerative capacity suggests a role for tanycytes in the brain’s response to damage or disease, offering avenues for future research in regenerative medicine.