1. THE PITUITARY GLAND
13
in fishes to suggest an experimental approach to defining the function of
the 7 cells.
More recently, following the demonstrations by Burden ( 1956) , Pickford and Phillips (1959), and Pickford et al. (1965) that prolactin is the
only mammalian pituitary hormone that will promote tolerance of freshwater in hypophysectomized Fundulus heteroclitus, the 7 cells have been
investigated experimentally, especially in Poecilia latipinnu. Details of
the physiological background will be found in the chapter by Ball, this
volume. For the present, the important point is that evidence indicates
the secretion by the pituitary in both F. heteroclitus and P. latipinnu of a
prolactinlike hormone ( fish prolactin, paralactin) that specifically promotes survival in freshwater by limiting the outflux of sodium from the
body (Maetz et al., 1967; Ensor and Ball, 1968; Ball, 1969; Ball and Ensor,
1969). Fish prolactin is not essential to Fundulus in seawater nor to
Poecilia in dilute seawater (Pickford et al., 1965; Ball and Olivereau,
1964), but it is secreted, presumably at a low rate, by Poecilia in dilute
seawater, with the effect of reducing the rate of sodium exchanges (Ball
and Ensor, 1969).
Studies on the pituitary in these two fishes have shown that only the
7 cell displays cytological evidence of changes in secretory activity such
as would be predicted of the cells secreting fish prolactin. Thus, in
Poecilia these cells are always more active in freshwater than in dilute
or full-strength seawater (Ball and Olivereau, 1964; Olivereau and Ball,
1964; Ball, 1969; Figs. 10 and 11); and these cells, but no others, are
rapidly activated when Poecilia enters freshwater from dilute seawater,
in correlation with reversal of plasma sodium loss, a marked curtailment
of sodium outflux from the body, and an increase in pituitary prolactin
content (Ball and Ensor, 1967, 1969; Ball, 1969; Ensor and Ball, 1968,
1969). Similarly, in F . heteroclitus the 9 cells are consistently more active
and numerous in freshwater than in seawater (Ball and Pickford, 1964;
Emmart et al., 1966), and a regenerated pituitary remnant in an incompletely hypophysectomized individual, experimentally shown to secrete
fish prolactin, consisted almost entirely of active p cells (Ball, 1965a). In
more direct experimental approaches, it was shown that removal of part
of the zone of 7 cells impaired freshwater tolerance of P . latipinnu (Ball,
1965b), and that ectopic pituitary transplants in P. formosa and P.
latipinnu are able to secrete fish prolactin, the transplants always containing active 7 cells (Ball and Kallman, 1962; Ball et al., 1965; Ball and
Olivereau, 1965; Olivereau and Ball, 1966). The identification was
clinched when Ball (1965~) demonstrated that ectopic pituitary transplants of the rostra1 part of P . latipinnu pituitary, composed mainly of 7
cells, secreted fish prolactin in response to entering freshwater, but that
13
in fishes to suggest an experimental approach to defining the function of
the 7 cells.
More recently, following the demonstrations by Burden ( 1956) , Pickford and Phillips (1959), and Pickford et al. (1965) that prolactin is the
only mammalian pituitary hormone that will promote tolerance of freshwater in hypophysectomized Fundulus heteroclitus, the 7 cells have been
investigated experimentally, especially in Poecilia latipinnu. Details of
the physiological background will be found in the chapter by Ball, this
volume. For the present, the important point is that evidence indicates
the secretion by the pituitary in both F. heteroclitus and P. latipinnu of a
prolactinlike hormone ( fish prolactin, paralactin) that specifically promotes survival in freshwater by limiting the outflux of sodium from the
body (Maetz et al., 1967; Ensor and Ball, 1968; Ball, 1969; Ball and Ensor,
1969). Fish prolactin is not essential to Fundulus in seawater nor to
Poecilia in dilute seawater (Pickford et al., 1965; Ball and Olivereau,
1964), but it is secreted, presumably at a low rate, by Poecilia in dilute
seawater, with the effect of reducing the rate of sodium exchanges (Ball
and Ensor, 1969).
Studies on the pituitary in these two fishes have shown that only the
7 cell displays cytological evidence of changes in secretory activity such
as would be predicted of the cells secreting fish prolactin. Thus, in
Poecilia these cells are always more active in freshwater than in dilute
or full-strength seawater (Ball and Olivereau, 1964; Olivereau and Ball,
1964; Ball, 1969; Figs. 10 and 11); and these cells, but no others, are
rapidly activated when Poecilia enters freshwater from dilute seawater,
in correlation with reversal of plasma sodium loss, a marked curtailment
of sodium outflux from the body, and an increase in pituitary prolactin
content (Ball and Ensor, 1967, 1969; Ball, 1969; Ensor and Ball, 1968,
1969). Similarly, in F . heteroclitus the 9 cells are consistently more active
and numerous in freshwater than in seawater (Ball and Pickford, 1964;
Emmart et al., 1966), and a regenerated pituitary remnant in an incompletely hypophysectomized individual, experimentally shown to secrete
fish prolactin, consisted almost entirely of active p cells (Ball, 1965a). In
more direct experimental approaches, it was shown that removal of part
of the zone of 7 cells impaired freshwater tolerance of P . latipinnu (Ball,
1965b), and that ectopic pituitary transplants in P. formosa and P.
latipinnu are able to secrete fish prolactin, the transplants always containing active 7 cells (Ball and Kallman, 1962; Ball et al., 1965; Ball and
Olivereau, 1965; Olivereau and Ball, 1966). The identification was
clinched when Ball (1965~) demonstrated that ectopic pituitary transplants of the rostra1 part of P . latipinnu pituitary, composed mainly of 7
cells, secreted fish prolactin in response to entering freshwater, but that