Ultrastructure of the organum vasculosum laminae terminalis in the advanced teleost Chelon labrosus (Risso, 1826).

The advanced teleost Chelon labrosus shows a well-developed organum vasculosum laminae terminalis (OVLT). Together, the vascular organization and ultrastructure have many similarities with those found in mammals, though it is somewhat simpler. To date, the dense network of fenestrated capillaries in the nervous parenchyma of the OVLT observed in Chelon labrous has not been reported in other fishes, and resembles the porta system of mammals. However, the OVLT does not present intraventricular neuronal processes, as occurs in the corresponding organ of most vertebrates. Moreover, between the numerous neurons and the neuropil that surround these capillaries, terminal buttons with clear and dense-core vesicles can be appreciated. Most striking is the contact of neurons and even terminal buttons with the basal lamina of the external limiting membrane without interpose glial prolongations that separate the nervous parenchyma from the perivascular space; this represents windows in the blood brain barrier. The above characteristics and the absence of cilaited or chemorecptors structures suggest that neurohemal function of the OVLT, possibly under the control of neurotransmiters secreted by the terminal buttons dispersed throughout the whole organ.