Median eminence equivalence of the neurohypophysis of the hagfish, Eptatretus burgeri

Three substances of different molecular size were injected into the ventricular space (III) of the neurohypophysis of anesthetized hagfish, Eptatretus burgeri, to learn whether such substances could diffuse across the connective tissue barrier between the neuro- and adenohypophysis. The substances tested were trypan blue, a vital dye that is a fine particulate colloid; horseradish peroxidase (HRP), an enzyme protein whose distribution in tissue can be revealed through its chromogenic action on a substrate; and ferric ion, whose distribution can be revealed through the Prussian blue reaction. All three substances were transferred across the neurohypophyseal wall to the extracerebral area within a few minutes, the trypan blue and HRP apparently through action by tanycytes. The trypan blue, within a 15-min period, moved no further. The HRP protein diffused through connective tissue as far as the dorsal (2 min) and ventral (5 min) borders of the adenohypophysis, but did not enter the secretory cells themselves. The ferric ion reached the adenohypophysis quickly (2 min) and entered the secretory cells in higher concentration than it was in the surrounding connective tissue. These data indicate that the hagfish ventral neurohypophysis is functionally capable of supplying neurosecretory regulatory factors to the adenohypophysis. It is thus a "diffusional median eminence." We can propose that the cyclostome median eminence, lacking a portal relation to the adenohypophysis, represents the phylogenetically primitive brain--pituitary relationship.