Mesencephalic trigeminal midline ridge formation in sharks, a proposed circumventricular organ: developmental aspects

In scanning electron microscopic (SEM) examination of 30 shark brains, a distinctive midline ridge formation on the ventricular surface of the optic tectum was found in all juveniles and adults of the three carcharhinid species (sandbar, dusky, and smooth hound) and the one lamnid species (mako) investigated. This formation is part of the mesencephalic trigeminal (Mes V) complex of these animals, and one of its remarkable features is a population of very large cerebrospinal fluid (CSF)-contacting and supraependymal Mes V neuronal somas. These cells, whose presence in the CSF compartment is heralded by distinct premonitory ependymal bulges, are not seen in contact with CSF until the sharks are well into their first postnatal year or later. Once established, the population of CSF-contacting and supraependymal Mes V neurons remains in substantial numbers over the life span of the animals into the period of sexual maturity. Age-related changes in patterns of ependymal ciliation of the midline ridge formation, tending toward a state of oligociliation, are also apparent in all species examined. Similarities between the midline ridge formation of the Mes V complex and known circumventricular organs (CVOs) are discussed and it is suggested that the formation be considered as a previously unrecognized CVO. It is hypothesized that the midline ridge formation with its contingent of CSF-immersed neurons, by monitoring one or more CSF factors, might serve to alter the excitability of the Mes V complex as a whole and, thus, regulate intensity of biting reflexes in sharks.