1. THE PITUITARY GLAND
73
in the regulation of adenohypophysial activity with the indications that
the peptides which these fibers probably secrete may have peripheral
activities. At the present time too little evidence is available to permit
further analysis of this problem, More work is needed, particularly to
demonstrate definitively the functional significance of the neurohypophysial invasion of the pars distalis, so characteristic of teleosts. Direct innervation of the pars intermedia is found throughout the vertebrates
(Iturizza, 1964; Knowles, 1965; Wingstrand, 1966b), but the anterior
neurohypophysis-pars distalis association is a teleostean speciality, and it
may be that the extensive neurosecretory innervation of the pars distalis
cells in this group evolved as an adjunct or supplement to the capillaries
of the secondary centrifugal plexus. It is worth recalling the comparison,
frequently made ( e.g., Green, 1951; Wingstrand, 1959, 1966a; Follenius,
1965a,b), between the median eminence of other gnathostomes and the
anterior neurohypophysial core of teleosts, a comparison all the more apt
in the light of recent information, both being regions where hypothalamic
neurosecretory fibers terminate on capillaries which convey blood to the
pars distalis; it may be that the anterior capillaries of the secondary
centrifugal plexus are functionally equivalent to the hypophysial portal
vessels, conveying products of neurosecretory cells (mainly Type B) to
the pars distalis, the median eminence and portal plexus being as it were
enclosed within the pars distalis (see Kerr, 1968).
Comparisons between the tetrapod and fish pituitary regions are complicated by the presence in nearly all fishes of the saccus vasculosus, a
thin-walled folded sac growing out from the third ventricle (infundibular
recess) posterior to the pituitary. This structure, despite its close proximity to the pituitary (particularly marked in Acipenser and elasmobranchs) seems to have no functional connection with the gland. In the
vertebrate embryo there arises from the postero-ventral floor of the
hypothalamus a posteriorly directed downgrowth, the saccus infundibuli.
In fishes, this gives rise to the saccus vasculosus, and the fish neurohypophysis arises more anteriorly from an area of the hypothalamic floor
that in tetrapods and lungfishes gives rise to the median eminence. In
tetrapods, the saccus infundibuli gives rise to the pars nervosa, the main
part of the neurohypophysis ( Wingstrand, 1966a). Thus embryologically,
the neurohypophysial core of fishes is not comparable to the tetrapod
pars nervosa, which is more closely comparable to the fish saccus vasculosus; but in functional terms the pars nervosa may be likened to the
posterior part of the fish neurohypophysis, particularly considering the
extensive innervation of the pars intermedia by neurohypophysial fibers
in many tetrapods ( Wingstrand, 1966b; Section 11, C ) . On both embryological and functional grounds, the tetrapod median eminence may be
73
in the regulation of adenohypophysial activity with the indications that
the peptides which these fibers probably secrete may have peripheral
activities. At the present time too little evidence is available to permit
further analysis of this problem, More work is needed, particularly to
demonstrate definitively the functional significance of the neurohypophysial invasion of the pars distalis, so characteristic of teleosts. Direct innervation of the pars intermedia is found throughout the vertebrates
(Iturizza, 1964; Knowles, 1965; Wingstrand, 1966b), but the anterior
neurohypophysis-pars distalis association is a teleostean speciality, and it
may be that the extensive neurosecretory innervation of the pars distalis
cells in this group evolved as an adjunct or supplement to the capillaries
of the secondary centrifugal plexus. It is worth recalling the comparison,
frequently made ( e.g., Green, 1951; Wingstrand, 1959, 1966a; Follenius,
1965a,b), between the median eminence of other gnathostomes and the
anterior neurohypophysial core of teleosts, a comparison all the more apt
in the light of recent information, both being regions where hypothalamic
neurosecretory fibers terminate on capillaries which convey blood to the
pars distalis; it may be that the anterior capillaries of the secondary
centrifugal plexus are functionally equivalent to the hypophysial portal
vessels, conveying products of neurosecretory cells (mainly Type B) to
the pars distalis, the median eminence and portal plexus being as it were
enclosed within the pars distalis (see Kerr, 1968).
Comparisons between the tetrapod and fish pituitary regions are complicated by the presence in nearly all fishes of the saccus vasculosus, a
thin-walled folded sac growing out from the third ventricle (infundibular
recess) posterior to the pituitary. This structure, despite its close proximity to the pituitary (particularly marked in Acipenser and elasmobranchs) seems to have no functional connection with the gland. In the
vertebrate embryo there arises from the postero-ventral floor of the
hypothalamus a posteriorly directed downgrowth, the saccus infundibuli.
In fishes, this gives rise to the saccus vasculosus, and the fish neurohypophysis arises more anteriorly from an area of the hypothalamic floor
that in tetrapods and lungfishes gives rise to the median eminence. In
tetrapods, the saccus infundibuli gives rise to the pars nervosa, the main
part of the neurohypophysis ( Wingstrand, 1966a). Thus embryologically,
the neurohypophysial core of fishes is not comparable to the tetrapod
pars nervosa, which is more closely comparable to the fish saccus vasculosus; but in functional terms the pars nervosa may be likened to the
posterior part of the fish neurohypophysis, particularly considering the
extensive innervation of the pars intermedia by neurohypophysial fibers
in many tetrapods ( Wingstrand, 1966b; Section 11, C ) . On both embryological and functional grounds, the tetrapod median eminence may be