2. THE PINEAL ORGAN
103
an increasing day length had its greatest stimulatory effect on the
reproductive system (Fenwick, 1 9 7 0 ~ ) .
Further, the increase in gonad
size in goldfish subjected to increasing periods of daily light exposure
could be prevented by the daily injection of melatonin (Fenwick, 1970a).
These results may help to explain the earlier contradictory reports. In
the first place it seems probable that the pineal gland may not function
to a similar extent, or even in an identical manner, during different
periods of the life cycle. As seen previously, the invagination of the
pineal gland increascs with age (Hoar, 1955) and there is a decrease
in sensory cell number (Hafoez and Ford, 1967) together with a marked
degeneration of nervous structures ( Ariens Kappers, 1965). Further,
the pineal gland becomes less accessible to light with increase in body
size; even the effect of pinealectomy on pigment distribution differed
in young and older lampreys (Young, 1935). Second, if the endocrine
function of the pineal organ is dependent upon melatonin as suggested
by Fenwick (1970a,c), and if the level of melatonin is depressed by exposure to long periods of bright light as reported in the mammals (Wurtman and Axelrod, 1965a), then the experimental design must be considered before interpreting the results of pinealectomy. If such controls
are not carefully enforced, the intensity and duration of the experimental photoperiod may be sufficient to lower the melatonin level to a
point where even animals with a pineal are effectively “physiologically
pinealectomized.”
C. The Pineal Body as a Sensory cum Secretory Organ
The apparent disagreement between workers supporting a sensory
function as opposed to those who argue for a secretory function does
not seem justified. Rather, it seems likely that both functions are linked
and that the pineal organ should be viewed neither as an entirely sensory
structure nor as a wholly secretory gland; both sensory and secretory
functions seem likely. This hypothesis is supported by recent observations
made on the chelonian epiphysis. Vivien and Roels (1967) noted that
certain cells within the chelonian epiphysis could be seen to alternate
between characteristics of reception and secretion; it is not difficult to
develop a similar theory for the pineal organ of the fishes.
V. SYNOPSIS
The pineal body of fishes should be reexamined in the light of recent
knowledge concerning the physiology of the pineal in higher vertebrates.
103
an increasing day length had its greatest stimulatory effect on the
reproductive system (Fenwick, 1 9 7 0 ~ ) .
Further, the increase in gonad
size in goldfish subjected to increasing periods of daily light exposure
could be prevented by the daily injection of melatonin (Fenwick, 1970a).
These results may help to explain the earlier contradictory reports. In
the first place it seems probable that the pineal gland may not function
to a similar extent, or even in an identical manner, during different
periods of the life cycle. As seen previously, the invagination of the
pineal gland increascs with age (Hoar, 1955) and there is a decrease
in sensory cell number (Hafoez and Ford, 1967) together with a marked
degeneration of nervous structures ( Ariens Kappers, 1965). Further,
the pineal gland becomes less accessible to light with increase in body
size; even the effect of pinealectomy on pigment distribution differed
in young and older lampreys (Young, 1935). Second, if the endocrine
function of the pineal organ is dependent upon melatonin as suggested
by Fenwick (1970a,c), and if the level of melatonin is depressed by exposure to long periods of bright light as reported in the mammals (Wurtman and Axelrod, 1965a), then the experimental design must be considered before interpreting the results of pinealectomy. If such controls
are not carefully enforced, the intensity and duration of the experimental photoperiod may be sufficient to lower the melatonin level to a
point where even animals with a pineal are effectively “physiologically
pinealectomized.”
C. The Pineal Body as a Sensory cum Secretory Organ
The apparent disagreement between workers supporting a sensory
function as opposed to those who argue for a secretory function does
not seem justified. Rather, it seems likely that both functions are linked
and that the pineal organ should be viewed neither as an entirely sensory
structure nor as a wholly secretory gland; both sensory and secretory
functions seem likely. This hypothesis is supported by recent observations
made on the chelonian epiphysis. Vivien and Roels (1967) noted that
certain cells within the chelonian epiphysis could be seen to alternate
between characteristics of reception and secretion; it is not difficult to
develop a similar theory for the pineal organ of the fishes.
V. SYNOPSIS
The pineal body of fishes should be reexamined in the light of recent
knowledge concerning the physiology of the pineal in higher vertebrates.