1. THE PITUITARY GLAND
89
ous portal vessels from the elongated median eminence supply all parts
of the dorsal lobe. No portal vessels enter the neurointermediate lobe in
this fish, in contrast to sharks and rays, and the putative ventral lobe receives no portal blood at all (Jasinski and Gorbman, 1966). As these
authors point out, this may well be the most primitive of all vertebrate
hypophysial portal systems.
All authors agree on the absence of direct neurosecretory innervation
of the pars distalis, in contrast to teleosts, so that the portal system would
seem to be the only route for hypothalamic control (see Follenius,
196%). The existence of a portal supply to the neurointermediate lobe is
puzzling in view of the rich neurosecretory innervation of this region,
and the functional significance of this part of the portal system awaits
future work (Mellinger, 1960a,b; Chevins, 1968). The portal supply to
the pars distalis is clearly similar to that in primitive actinopterygians and
dipnoans, and as in those groups is to be correlated with the absence of
direct innervation of the pars distalis.
VI. THE PITUITARY GLAND IN CYCLOSTOMES
In this group, we find that the most primitive of all pituitary glands
has a much simpler structure than that of gnathostomes. Both neural and
glandular components can be recognized, but direct or vascular communication between the two is very limited. Many of the cells in the adenohypophysis appear chromophobic, especially in young animals, which
hampers investigation of the histophysiology. Another great difficulty in
studies on these animals is that their primitive evolutionary position
makes it hazardous to assume that all the hormones secreted by the
gnathostome pituitary are also produced by the cyclostome gland. In fact,
evidence for the secretion of hormones other than gonadotropin( s ) , MSH,
and AVT in lampreys is very insecurely based (see Lanzing, 1954; Evennett, 1963; Evennett and Dodd, 1963; Adam, 1963; Sawyer, 1966b; Larsen,
The pituitary in myxinoids (hagfishes) appears to be more primitive
than that of lampreys (Petromyzontidae) . The adenohypophysis of
Myrine has been described by Olsson ( 1959), Matty ( 1960), Adam
( 1960, 1963), and Olsson et al. (1965). It consists of follicles and clusters
of cells embedded in connective tissue below the hypothalamus, and
there is no clear cytological differentiation between pars distalis and pars
intermedia (Fig. 25A). Some workers report that the connective tissue
septum between neurohypophysis and adenohypophysis may be missing
1965).
89
ous portal vessels from the elongated median eminence supply all parts
of the dorsal lobe. No portal vessels enter the neurointermediate lobe in
this fish, in contrast to sharks and rays, and the putative ventral lobe receives no portal blood at all (Jasinski and Gorbman, 1966). As these
authors point out, this may well be the most primitive of all vertebrate
hypophysial portal systems.
All authors agree on the absence of direct neurosecretory innervation
of the pars distalis, in contrast to teleosts, so that the portal system would
seem to be the only route for hypothalamic control (see Follenius,
196%). The existence of a portal supply to the neurointermediate lobe is
puzzling in view of the rich neurosecretory innervation of this region,
and the functional significance of this part of the portal system awaits
future work (Mellinger, 1960a,b; Chevins, 1968). The portal supply to
the pars distalis is clearly similar to that in primitive actinopterygians and
dipnoans, and as in those groups is to be correlated with the absence of
direct innervation of the pars distalis.
VI. THE PITUITARY GLAND IN CYCLOSTOMES
In this group, we find that the most primitive of all pituitary glands
has a much simpler structure than that of gnathostomes. Both neural and
glandular components can be recognized, but direct or vascular communication between the two is very limited. Many of the cells in the adenohypophysis appear chromophobic, especially in young animals, which
hampers investigation of the histophysiology. Another great difficulty in
studies on these animals is that their primitive evolutionary position
makes it hazardous to assume that all the hormones secreted by the
gnathostome pituitary are also produced by the cyclostome gland. In fact,
evidence for the secretion of hormones other than gonadotropin( s ) , MSH,
and AVT in lampreys is very insecurely based (see Lanzing, 1954; Evennett, 1963; Evennett and Dodd, 1963; Adam, 1963; Sawyer, 1966b; Larsen,
The pituitary in myxinoids (hagfishes) appears to be more primitive
than that of lampreys (Petromyzontidae) . The adenohypophysis of
Myrine has been described by Olsson ( 1959), Matty ( 1960), Adam
( 1960, 1963), and Olsson et al. (1965). It consists of follicles and clusters
of cells embedded in connective tissue below the hypothalamus, and
there is no clear cytological differentiation between pars distalis and pars
intermedia (Fig. 25A). Some workers report that the connective tissue
septum between neurohypophysis and adenohypophysis may be missing
1965).