Evolution of the Brain–Pituitary Relationship: Evidence from the Agnatha

Abstract

In the brain of the lamprey there are neurosecretory cells that manufacture and carry known hormonal materials to the neurohypophysis, which is that part of the diencephalic floor that is adjacent to the adenohypophysis. The adenohypophysis is differentiated into a pars distalis and a pars intermedia. However, unlike all other vertebrates, there is no vascular or nervous connection between the neurohypophysis and the pars distalis. If there is any regulation of the adenohypophysis by the hormones released from the endings of neurosecretory axones in the neurohypophysis, it would have to be by diffusion across the intervening connective tissue. Such regulation remains to be demonstrated.It is difficult to decide whether this relationship between neurohypophysis and pars distalis in the lamprey (and hagfish) is primitive or degenerate in its evolution. However, in three groups of more primitive fishes an intermediate condition is found in which a vascular connection exists between the neurohypophysis and a part of the adenohypophysis. This being so, we may speculate that the lamprey and other cyclostomes represent the simplest and most primitive anatomical relationship between brain and adenohypophysis, one in which the two organs are merely apposed, allowing neurosecretions to diffuse between the two. In the elasmobranchs, holocephalians, and the coelacanth, part of the pars distalis has been vascularized by portal vessels beginning in the neurohypophysis. In all other vertebrates the entire pars distalis is so vascularized.Key words: brain, pituitary gland, neurosecretion, hormones, evolution, Lampetra richardsoni, L. tridentata, Petromyzon marinus