1. THE PITUITARY GLAND
71
connections was reviewed; from the work on pituitary transplants and
in vitro culture, some degree of hypothalamic control of the gland was
indicated, although the control seems less important than in higher vertebrates (Ball et al., 1965). From Sections 11, C and 11, D, it will be seen
that anatomically there appear to be at least two routes by which hypothalamic control could be imposed on the adenohypophysis.
1. NEUROSECRETORY TERMINATIONS WITHIN THE ADENOHYPOPHYSIS
From ultrastructural and histochemical information, it appears that
neurosecretory fibers of Type A or Type B come into more or less intimate contact with all the adenohypophysial cells, Type A apparently predominating in the pars intermedia, and Type B in the pars distalis, but
with overlapping distributions in each case. Type A fibers probably originate in the NPO, and most Type B fibers probably originate in the NLT.
The nature of the contact between the nerve fiber and endocrine cells
ranges from immediate physical contact, synaptic in some cases, as in the
pars distalis of Hippocampus, Tilapia, Phoxinus, and the guppy, and the
pars intermedia of Conger, Gadus, Phoxinus, and the guppy; through
cases where the nerve fiber ends on a single or double basement membrane which separates the neuron from the endocrine cells (Tinca, Perca,
Salmo, and Tilapia); to the condition (Fig. 23) in which the nerve fiber
ends in an extravascular space which separates the fiber terminal
from the endocrine cells (Anguilla, Perca, and Salmo, Vollrath, 1967).
It should be noted, however, that Knowles and Vollrath (1966a) have
questioned the nature of the boundary between endocrine cells and nerve
fibers that Follenius and Porte (1962) termed a “basement membrane,”
suggesting that it may in fact be a double membrane enclosing an extravascular space, as in Anguilla (Fig. 23).
2. NEUROSECRETORY TERMINATIONS ON CAPILLARIES IN THE
NEUROHYPOPHYSIAL CORE
Apart from the neurosecretory terminations within the adenohypophysis, many workers have concluded that neurosecretory fibers make synaptic contact with the walls of the capillaries of the primary longitudinal
plexus and its branches into the secondary centrifugal plexus (Follenius
and Porte, 1962; Follenius, 1965a,b; Bhargava, 1966). According to Follenius (1965a,b) it is Type B fibers that terminate in the anterior part
of the primary capillary plexus, close to the pars distalis, while Type A
fibers from the NPO mostly terminate on capillaries in the more posterior
part of the neurohypophysial core, close to the pars intermedia. Nucleus
preopticus fibers terminate on capillaries in the neurohypophysial core in
71
connections was reviewed; from the work on pituitary transplants and
in vitro culture, some degree of hypothalamic control of the gland was
indicated, although the control seems less important than in higher vertebrates (Ball et al., 1965). From Sections 11, C and 11, D, it will be seen
that anatomically there appear to be at least two routes by which hypothalamic control could be imposed on the adenohypophysis.
1. NEUROSECRETORY TERMINATIONS WITHIN THE ADENOHYPOPHYSIS
From ultrastructural and histochemical information, it appears that
neurosecretory fibers of Type A or Type B come into more or less intimate contact with all the adenohypophysial cells, Type A apparently predominating in the pars intermedia, and Type B in the pars distalis, but
with overlapping distributions in each case. Type A fibers probably originate in the NPO, and most Type B fibers probably originate in the NLT.
The nature of the contact between the nerve fiber and endocrine cells
ranges from immediate physical contact, synaptic in some cases, as in the
pars distalis of Hippocampus, Tilapia, Phoxinus, and the guppy, and the
pars intermedia of Conger, Gadus, Phoxinus, and the guppy; through
cases where the nerve fiber ends on a single or double basement membrane which separates the neuron from the endocrine cells (Tinca, Perca,
Salmo, and Tilapia); to the condition (Fig. 23) in which the nerve fiber
ends in an extravascular space which separates the fiber terminal
from the endocrine cells (Anguilla, Perca, and Salmo, Vollrath, 1967).
It should be noted, however, that Knowles and Vollrath (1966a) have
questioned the nature of the boundary between endocrine cells and nerve
fibers that Follenius and Porte (1962) termed a “basement membrane,”
suggesting that it may in fact be a double membrane enclosing an extravascular space, as in Anguilla (Fig. 23).
2. NEUROSECRETORY TERMINATIONS ON CAPILLARIES IN THE
NEUROHYPOPHYSIAL CORE
Apart from the neurosecretory terminations within the adenohypophysis, many workers have concluded that neurosecretory fibers make synaptic contact with the walls of the capillaries of the primary longitudinal
plexus and its branches into the secondary centrifugal plexus (Follenius
and Porte, 1962; Follenius, 1965a,b; Bhargava, 1966). According to Follenius (1965a,b) it is Type B fibers that terminate in the anterior part
of the primary capillary plexus, close to the pars distalis, while Type A
fibers from the NPO mostly terminate on capillaries in the more posterior
part of the neurohypophysial core, close to the pars intermedia. Nucleus
preopticus fibers terminate on capillaries in the neurohypophysial core in