Tanycytes and pituicytes: morphological and functional aspects of neuroglial interaction.

The hypothalamo-hypophyseal system is supplied with two types of specialized glial cells that interact in neuroendocrine functional dynamics: the tanycytes and the pituicytes. Tanycytes are the dominating glial cells within the median eminence. Similar to radial glia, they extend from the floor of the third ventricle to the neurohemal surface of the median eminence. Pituicytes, as specialized astrocytes, are the main glial cells of the neural lobe. They are in intimate contact with the perivascular space of the sinusoidal vessels. Morphological similarities between the two cell types focus on their interaction with terminal branches of hypothalamic neurons in both regions of the neurohypophysis, the median eminence and the neural lobe. Release of hypothalamic hormones is apparently influenced by pituicytes and tanycytes. For instance, both types of cells are capable of closing or opening the access to the vessels. Thereby, in contrast to the "blood-brain-barrier" function of astrocytes, pituicytes and tanycytes display "brain-blood-barrier" functions. Pituicytes are characterized by the expression of specific membrane-bound receptors for opioids, vasopressin, and beta-adrenoceptors, indicating that they receive input by numerous neuroactive substances. Integration of these incoming signals may result in a regulation of neurosecretion, especially by morphological changes and by modulation of extracellular ion concentrations. Comparable modulatory mechanisms of tanycytes have not yet been elucidated in a convincing manner. Besides possible regulatory functions, tanycytes are considered to possess guiding functions for hypothalamic axons and to be involved in transport mechanisms between ventricle and blood vessels of the portal system.