1. THE PITUITARY GLAND
37
identified by their responses to thiourea (Section 11, B, 5 ) . On the ventral
surface of this region, one finds many small chromophobic cells, with
inactive nuclei, presumably undifferentiated cells. In the sexually mature
fish, differentiated basophils are always found in this region (Fig. 2 ) , even
during periods of sexual quiescence, when these cells assume an inactive
appearance but are nevertheless recognizable as differentiated basophils.
Thus, these ventral basophils would appear to be gonadotrops.
Confirmation of this identification comes from observations of changes
in the ventral basophils during the monthly female cycle. The fully
developed oocytes are fertilized in the ovarian follicles by spermatozoa
which may be stored embedded in the ovarian epithelium for long
periods. The ovarian follicle acts as a brood chamber for the developing
embryo. The fertilized egg is not ovulated but is retained in the follicle,
and the embryo then develops on top of the yolky egg. Many embryos
usually develop synchronously, forming a brood. When the fry are fully
developed the follicles rupture, and the brood is expelled via the oviduct
over a short period, a few hours at most. Under laboratory conditions,
with 9 hr illumination per day and at 25OC, female fish undergo regular
cycles of brood production, giving birth about every month, although
the cycle is readily interrupted by unfavorable external conditions.
During the greater part of pregnancy the ovary contains only previtellogenic. (first phase) oocytes, along with the developing embryos; but at
birth, or a few days earlier, some of these juvenile oocytes embark on
rapid vitellogenic (second phase) growth and mature as a new crop of
eggs ready for fertilization within 5-8 days of birth of the previous brood.
Hypophysectomy experiments have shown that pituitary hormones are
not essential for gestation or parturition, but are essential, as in oviparous
forms, for the second phase oocyte growth (Ball, 1960, 1962). Since this
phase is usually completed during the cycle within a few days starting at
or just before parturition, one would predict that elevated pituitary
gonadotropic activity is confined to this limited period of the cycle and is
curtailed or absent during the much longer period occupied by pregnancy. Study of the gonadotrops tends to confirm this prediction, these
ventral basophils displaying marked cytological and morphological
changes correlated with the ovarian cycle. These changes are summarized
in Tables I1 and 111, based on hitherto unpublished observations on
scores of female P . latipinnu together with systematic study and measurement on 30 fish sacrificed at known stages in the monthly cycle. The range
of morphological alterations displayed by the gonadotrops in the cycle
is impressive and is illustrated in Figs. 5 and 6. As in many teleosts (see,
e.g., Olivereau, 1954, 1967a; Sokol, 1961; Schreibman, 1964; van Overbeeke and McBride, 1967) maximal activity in Poeciliu gonadotrops is
37
identified by their responses to thiourea (Section 11, B, 5 ) . On the ventral
surface of this region, one finds many small chromophobic cells, with
inactive nuclei, presumably undifferentiated cells. In the sexually mature
fish, differentiated basophils are always found in this region (Fig. 2 ) , even
during periods of sexual quiescence, when these cells assume an inactive
appearance but are nevertheless recognizable as differentiated basophils.
Thus, these ventral basophils would appear to be gonadotrops.
Confirmation of this identification comes from observations of changes
in the ventral basophils during the monthly female cycle. The fully
developed oocytes are fertilized in the ovarian follicles by spermatozoa
which may be stored embedded in the ovarian epithelium for long
periods. The ovarian follicle acts as a brood chamber for the developing
embryo. The fertilized egg is not ovulated but is retained in the follicle,
and the embryo then develops on top of the yolky egg. Many embryos
usually develop synchronously, forming a brood. When the fry are fully
developed the follicles rupture, and the brood is expelled via the oviduct
over a short period, a few hours at most. Under laboratory conditions,
with 9 hr illumination per day and at 25OC, female fish undergo regular
cycles of brood production, giving birth about every month, although
the cycle is readily interrupted by unfavorable external conditions.
During the greater part of pregnancy the ovary contains only previtellogenic. (first phase) oocytes, along with the developing embryos; but at
birth, or a few days earlier, some of these juvenile oocytes embark on
rapid vitellogenic (second phase) growth and mature as a new crop of
eggs ready for fertilization within 5-8 days of birth of the previous brood.
Hypophysectomy experiments have shown that pituitary hormones are
not essential for gestation or parturition, but are essential, as in oviparous
forms, for the second phase oocyte growth (Ball, 1960, 1962). Since this
phase is usually completed during the cycle within a few days starting at
or just before parturition, one would predict that elevated pituitary
gonadotropic activity is confined to this limited period of the cycle and is
curtailed or absent during the much longer period occupied by pregnancy. Study of the gonadotrops tends to confirm this prediction, these
ventral basophils displaying marked cytological and morphological
changes correlated with the ovarian cycle. These changes are summarized
in Tables I1 and 111, based on hitherto unpublished observations on
scores of female P . latipinnu together with systematic study and measurement on 30 fish sacrificed at known stages in the monthly cycle. The range
of morphological alterations displayed by the gonadotrops in the cycle
is impressive and is illustrated in Figs. 5 and 6. As in many teleosts (see,
e.g., Olivereau, 1954, 1967a; Sokol, 1961; Schreibman, 1964; van Overbeeke and McBride, 1967) maximal activity in Poeciliu gonadotrops is