86
J. N. BALL AND BRIDGET I. BAKER
NLT ( Mellinger, 1962a), or arising within the neurohypophysis ( Knowles,
1965), terminating on the release pole of the cell. Knowles interpreted
these findings as indicating that hormone synthesis and hormone release
are under independent neurosecretory control. Other workers have observed terminations of Type A fibers on the pars intermedia cells
(Mellinger, 1962a; Pasqualino and Santoro, 1964) and on blood vessels
in this region (Jasinski and Gorbman, 1966), and Meurling (1963)
described A F - ve fibers, possibly equivalent to Knowles’ Type B, ending
on pars intermedia cells and blood vessels in Etrnopterus. However,
Mellinger (1962a) described only Type A fibers in the neurointermediate
lobe of Scylliorhinus, and further showed that destruction of the NPOhypophysial tract led to activation of the pars intermedia cells and release
of excessive MSH, indicating that the lesion had removed inhibitory
control of both synthesis and release. Chevins (1968) found in Raia
that not only tract section and ectopic transplantation of the neurointermediate lobe but also destruction of the NPO itself led to excessive MSH
secretion, which suggests either a single inhibitory control of both synthesis and release, contrary to Knowles’ hypothesis, or that both Type A
and Type B neurons originate in the NPO, which is contrary to the histological and ultrastructural evidence.
Large osmiophilic and acidophilic globules occur in the elasmobranch
pars intermedia, as in other vertebrates (Section 11, B, 6), and have been
interpreted as the products of cellular degeneration (Della Corte, 1961;
Meurling, 1963; Mellinger, 1963a) or as a hormone store (Knowles,
1965).
As in other groups, the pars intermedia has been shown to secrete
MSH, the evidence coming from surgical removal of the neurointermediate lobe (Hogben, 1936; Waring, 1942), bioassay of different pituitary regions ( Hogben, 1936; Landgrebe and Waring, 1941 ), and lesioning
of the NPO-neurohypophysial tract, which caused melanin dispersion
associated with hyperactivity of the pars intermedia cells ( Mellinger,
1963b). Apart from this last study, there have been virtually no observations on natural or experimentally induced changes in the pars intermedia
cells other than reports of the appearance of giant cells in this region
in female Scylliorhinus and Torpedo in relation to the sexual cycle (Della
Corte, 1961; Della Corte and Chieffi, 1962). Knowles (1965) did not
observe these cells in his work on Scylliorhinus, and their significance is
unknown.
The anatomy of the neurohypophysis has been studied in many
elasmobranchs, key references being Scharrer ( 1952), Meurling ( 1962,
1963, 1967a), Mellinger (1962a, 1963a), Mellinger et al. (1962), Follenius ( 1965b), Knowles ( 1965) , and Polenov and Belenky ( 1965). As in
J. N. BALL AND BRIDGET I. BAKER
NLT ( Mellinger, 1962a), or arising within the neurohypophysis ( Knowles,
1965), terminating on the release pole of the cell. Knowles interpreted
these findings as indicating that hormone synthesis and hormone release
are under independent neurosecretory control. Other workers have observed terminations of Type A fibers on the pars intermedia cells
(Mellinger, 1962a; Pasqualino and Santoro, 1964) and on blood vessels
in this region (Jasinski and Gorbman, 1966), and Meurling (1963)
described A F - ve fibers, possibly equivalent to Knowles’ Type B, ending
on pars intermedia cells and blood vessels in Etrnopterus. However,
Mellinger (1962a) described only Type A fibers in the neurointermediate
lobe of Scylliorhinus, and further showed that destruction of the NPOhypophysial tract led to activation of the pars intermedia cells and release
of excessive MSH, indicating that the lesion had removed inhibitory
control of both synthesis and release. Chevins (1968) found in Raia
that not only tract section and ectopic transplantation of the neurointermediate lobe but also destruction of the NPO itself led to excessive MSH
secretion, which suggests either a single inhibitory control of both synthesis and release, contrary to Knowles’ hypothesis, or that both Type A
and Type B neurons originate in the NPO, which is contrary to the histological and ultrastructural evidence.
Large osmiophilic and acidophilic globules occur in the elasmobranch
pars intermedia, as in other vertebrates (Section 11, B, 6), and have been
interpreted as the products of cellular degeneration (Della Corte, 1961;
Meurling, 1963; Mellinger, 1963a) or as a hormone store (Knowles,
1965).
As in other groups, the pars intermedia has been shown to secrete
MSH, the evidence coming from surgical removal of the neurointermediate lobe (Hogben, 1936; Waring, 1942), bioassay of different pituitary regions ( Hogben, 1936; Landgrebe and Waring, 1941 ), and lesioning
of the NPO-neurohypophysial tract, which caused melanin dispersion
associated with hyperactivity of the pars intermedia cells ( Mellinger,
1963b). Apart from this last study, there have been virtually no observations on natural or experimentally induced changes in the pars intermedia
cells other than reports of the appearance of giant cells in this region
in female Scylliorhinus and Torpedo in relation to the sexual cycle (Della
Corte, 1961; Della Corte and Chieffi, 1962). Knowles (1965) did not
observe these cells in his work on Scylliorhinus, and their significance is
unknown.
The anatomy of the neurohypophysis has been studied in many
elasmobranchs, key references being Scharrer ( 1952), Meurling ( 1962,
1963, 1967a), Mellinger (1962a, 1963a), Mellinger et al. (1962), Follenius ( 1965b), Knowles ( 1965) , and Polenov and Belenky ( 1965). As in