1. THE PITUITARY GLAND
53
odds with the conclusions of most workers at the light microscope level;
the independence of the two cell types is indicated by their different
reactions in the eel kept in deionized water (see below and Figs. 21
and 22).
Evidence for the Functions of the Pars Intermedia Cells. To understand the difficulties attending any allocation of function to the pars
intermedia cells, it is necessary to appreciate that there is at present no
general agreement about the total physiological role of this part of the
gland. By homology with the tetrapod pars intermedia, it is to be expected that this region is the site of synthesis of melanophore-stimulating
hormone (MSH, intermedin), and there is now good experimental evidence to support this idea. Extracts of teleostean pituitaries affect the
distribution of pigment in both the melanophores and erythrophores of
fishes (see Pickford and Atz, 1957), and experiments with separate extracts of anterior and posterior halves of the gland show MSH activity
to be most abundant in the pars intermedia region (Hewer, 1926; Kent,
1959; Chavin, 1959; Baker, 1965b). Long-term in vitro culture of anterior and posterior halves of the trout pituitary showed that 11-12 times
more MSH is produced by the pars intermedia than by the rest of the
gland; the small amount of MSH secreted in vitro by the anterior region
may be attributed to the inclusion of some pars intermedia cells in these
fractions (Baker, 196513).
The existence of two distinct cell types suggests the production of
two distinct hormones by this region. On other grounds, it has been proposed that two antagonistic melanophorotropic hormones are secreted
by the fish pituitary, one (MSH) causing melanophore dispersion, the
other ( melanophore-concentrating hormone, MCH ) inducing melanin
concentration in the middle of the pigment cell (Pickford and Atz, 1957;
Waring, 1963). Stahl (1958) suggested that the two pars intermedia cell
types might produce these two factors. This two-hormone hypothesis is
not universally accepted, and some workers consider the experimental
data, which originally led to the two-hormone concept in amphibians
and fishes, are explicable in terms of only one hormone (Kent, 1961;
Waring, 1963; see chapter by Fujii, Volume 111).
This lack of certainty about whether the pars intermedia produces one
or two melanotropic factors is reflected by uncertainty about the functions of the two cell types of this region: At present, if only one hormone
is secreted by the pars intermedia we cannot say which cell produces it;
and if the other cell type does indeed secrete a separate hormone, we
cannot be sure of its functions, except to say that there are some indications that such a second pars intermedia factor might have function( s )
53
odds with the conclusions of most workers at the light microscope level;
the independence of the two cell types is indicated by their different
reactions in the eel kept in deionized water (see below and Figs. 21
and 22).
Evidence for the Functions of the Pars Intermedia Cells. To understand the difficulties attending any allocation of function to the pars
intermedia cells, it is necessary to appreciate that there is at present no
general agreement about the total physiological role of this part of the
gland. By homology with the tetrapod pars intermedia, it is to be expected that this region is the site of synthesis of melanophore-stimulating
hormone (MSH, intermedin), and there is now good experimental evidence to support this idea. Extracts of teleostean pituitaries affect the
distribution of pigment in both the melanophores and erythrophores of
fishes (see Pickford and Atz, 1957), and experiments with separate extracts of anterior and posterior halves of the gland show MSH activity
to be most abundant in the pars intermedia region (Hewer, 1926; Kent,
1959; Chavin, 1959; Baker, 1965b). Long-term in vitro culture of anterior and posterior halves of the trout pituitary showed that 11-12 times
more MSH is produced by the pars intermedia than by the rest of the
gland; the small amount of MSH secreted in vitro by the anterior region
may be attributed to the inclusion of some pars intermedia cells in these
fractions (Baker, 196513).
The existence of two distinct cell types suggests the production of
two distinct hormones by this region. On other grounds, it has been proposed that two antagonistic melanophorotropic hormones are secreted
by the fish pituitary, one (MSH) causing melanophore dispersion, the
other ( melanophore-concentrating hormone, MCH ) inducing melanin
concentration in the middle of the pigment cell (Pickford and Atz, 1957;
Waring, 1963). Stahl (1958) suggested that the two pars intermedia cell
types might produce these two factors. This two-hormone hypothesis is
not universally accepted, and some workers consider the experimental
data, which originally led to the two-hormone concept in amphibians
and fishes, are explicable in terms of only one hormone (Kent, 1961;
Waring, 1963; see chapter by Fujii, Volume 111).
This lack of certainty about whether the pars intermedia produces one
or two melanotropic factors is reflected by uncertainty about the functions of the two cell types of this region: At present, if only one hormone
is secreted by the pars intermedia we cannot say which cell produces it;
and if the other cell type does indeed secrete a separate hormone, we
cannot be sure of its functions, except to say that there are some indications that such a second pars intermedia factor might have function( s )