2. THE NEUROHYPOPHYSIS
133
granules (Polenov and Belenki, 1965). The nerve axons are free of connective tissue or blood vessels, but they are accompanied by neurogliatype pituicytes. These pituicytes are closely associated with the nerve
terminals. The neurosecretory axons terminate as large swellings, which
contain irregular mitochondria, vesicles resembling synaptic vesicles, and
fine “elementary vesicles” which represent the neurosecretory granules
of light microscopy. These elementary vesicles are 11W2800A in
diameter, and it seems probable that they possess an outer lipoprotein
membrane with electron dense material inside. Within the nerve terminals
there are similar structures with contents of lower electron density, and
others which appear empty, except for a few granules; it is probable that
these represent various stages in the loss of neurosecretory materials.
Since no granules can be found outside the terminals, Polenov and
Belenki ( 1965) suggest that the neurosecretory neurons lose components
of low molecular weight from their elementary vesicles. Many of the
neurosecretory terminals make close contact with intermedia cells; none
appears to associate directly with the blood sinusoids in Raiu claoatu and
Tygon pastinaca (Polenov and Belenki, 1965).
There is no doubt that neurosecretory nerves make direct contact with
the cells of the pars intermedia in many species of elasmobranchs. However, vascular connections could also be important. The neurosecretory
axons deposit dense neurosecretion close to the neural lobe blood plexus
of species such as Squalus acanthias and Etmopterus spinux. The neural
lobe plexus drains into the sinusoids of the pars intermedia (Meurling,
1967a), and this could allow neurosecretory materials to pass from the
pars nervosa into the pars intermedia. In those species which lack a
neural lobe plexus, direct neural contacts may be more important, but
it is interesting to note that these are also the species which possess a
neurointermediate lobe component of their pituitary portal system, so
that humoral materials could be supplied directly from the median
eminance. In Scyliorhinus caniculus, the neurointermediate lobe receives
its entire blood supply from veins which leave the brain, and there is no
direct arterial supply ( Meurling, 1967a). Neurosecretory materials may
not only reach the sinuses of the pars intermedia, but they may pass on
from the sinuses, through the interorbital vein, into the general circulation. However, in some species, such as Raia clavata, loss into the systemic
circulation does not appear to be as easy, since there is no neural lobe
plexus, and the neurosecretory axons do not appear to contact the blood
sinusoids of the pars intermedia (Polenov and Belenki, 1965). Nevertheless, the nerve terminals may well be sufficiently close to the rich blood
sinuses of the intermedia tissue for their active agents to reach the general
circulation.
133
granules (Polenov and Belenki, 1965). The nerve axons are free of connective tissue or blood vessels, but they are accompanied by neurogliatype pituicytes. These pituicytes are closely associated with the nerve
terminals. The neurosecretory axons terminate as large swellings, which
contain irregular mitochondria, vesicles resembling synaptic vesicles, and
fine “elementary vesicles” which represent the neurosecretory granules
of light microscopy. These elementary vesicles are 11W2800A in
diameter, and it seems probable that they possess an outer lipoprotein
membrane with electron dense material inside. Within the nerve terminals
there are similar structures with contents of lower electron density, and
others which appear empty, except for a few granules; it is probable that
these represent various stages in the loss of neurosecretory materials.
Since no granules can be found outside the terminals, Polenov and
Belenki ( 1965) suggest that the neurosecretory neurons lose components
of low molecular weight from their elementary vesicles. Many of the
neurosecretory terminals make close contact with intermedia cells; none
appears to associate directly with the blood sinusoids in Raiu claoatu and
Tygon pastinaca (Polenov and Belenki, 1965).
There is no doubt that neurosecretory nerves make direct contact with
the cells of the pars intermedia in many species of elasmobranchs. However, vascular connections could also be important. The neurosecretory
axons deposit dense neurosecretion close to the neural lobe blood plexus
of species such as Squalus acanthias and Etmopterus spinux. The neural
lobe plexus drains into the sinusoids of the pars intermedia (Meurling,
1967a), and this could allow neurosecretory materials to pass from the
pars nervosa into the pars intermedia. In those species which lack a
neural lobe plexus, direct neural contacts may be more important, but
it is interesting to note that these are also the species which possess a
neurointermediate lobe component of their pituitary portal system, so
that humoral materials could be supplied directly from the median
eminance. In Scyliorhinus caniculus, the neurointermediate lobe receives
its entire blood supply from veins which leave the brain, and there is no
direct arterial supply ( Meurling, 1967a). Neurosecretory materials may
not only reach the sinuses of the pars intermedia, but they may pass on
from the sinuses, through the interorbital vein, into the general circulation. However, in some species, such as Raia clavata, loss into the systemic
circulation does not appear to be as easy, since there is no neural lobe
plexus, and the neurosecretory axons do not appear to contact the blood
sinusoids of the pars intermedia (Polenov and Belenki, 1965). Nevertheless, the nerve terminals may well be sufficiently close to the rich blood
sinuses of the intermedia tissue for their active agents to reach the general
circulation.