410
HOWARD A. BERN
Acid violet (Takasugi and Bern, 1962) has been successfully used, especially to delineate the structure of the tracts and the urophysis. However,
the cell soma is generally unreactive. The cytoplasm seldom shows signs
of much secretion storage, although the huge neurons of some teleost and
elasmobranch species may show large droplets. When the neurosecretory
neurons are small and unimpressive, the staining reaction may be so insignificant as barely to allow their distinction.
The ultrastructural morphology of the caudal neurosecretory neurons
is essentially that characteristic of neurosecretory cells in general (Bern,
1966; Bern and Knowles, 1966). Their neuronal nature is modsed, owing
to the lack of organization of the endoplasmic reticulum into Nissl substance and the extensive development of the Golgi membranes associated
with vesicle and granule production (Figs. 2 and 5). Nevertheless, dendrites, axons, and neurofibrils are all in evidence, along with both morphological and physiological evidence for axodendritic, axosomatic, and
axo-axonic synapses.
These caudal cells were the first neurosecretory cells shown to possess
the electrophysiological properties of neurons (Fig. 7). Synaptic transmission to these cells has been established (Morita et al., 1961; Ishibashi,
1962), and spikes characteristic of impulse conduction are generated. The
duration of the action potential of all caudal neurosecretory neurons so
far studied has been found to be several to 10 or 15 times longer than
that of nearby motor neurons, suggesting some particular properties of
these secreting neurons (cf. Bern and Yagi, 1965), possibly related to
sustained, nonquantal release of hormone. Teleost and elasmobranch
neurons, despite the limited axonal development of the latter, show similar electrical properties (Bennett and Fox, 1962).
The granules produced in the cell body and transported distally [toward the cerebrospinal fluid apparently by dendrites (Fridberg and
Nishioka, 1966), as well as to the neurohemal area] are membrane-limited
electron-dense bodies, varying in diameter from lo00 to 3OOO A (Figs. 2,
3, and 5 ) . Apparently these granules may coalesce into larger masses,
visible as homogeneous droplets with the light microscope. Study of terminals indicates that more than one type of secretory granule may exist in
individual fish species, suggesting the possibility of more than one hormonal product (see also Sano et al., 1966).
To date, almost nothing is known of the chemistry of the urophysial
principles, other than their presumed protein or peptide nature and the
absence of the disul61de bonds so characteristic of the hypothalamic octapeptides. It seems reasonable to propose, however, that the active hormonal principle( s) may be transported in the cell to the terminal point
of release by a protein carrier analogous to the neurophysin of the hypo-
HOWARD A. BERN
Acid violet (Takasugi and Bern, 1962) has been successfully used, especially to delineate the structure of the tracts and the urophysis. However,
the cell soma is generally unreactive. The cytoplasm seldom shows signs
of much secretion storage, although the huge neurons of some teleost and
elasmobranch species may show large droplets. When the neurosecretory
neurons are small and unimpressive, the staining reaction may be so insignificant as barely to allow their distinction.
The ultrastructural morphology of the caudal neurosecretory neurons
is essentially that characteristic of neurosecretory cells in general (Bern,
1966; Bern and Knowles, 1966). Their neuronal nature is modsed, owing
to the lack of organization of the endoplasmic reticulum into Nissl substance and the extensive development of the Golgi membranes associated
with vesicle and granule production (Figs. 2 and 5). Nevertheless, dendrites, axons, and neurofibrils are all in evidence, along with both morphological and physiological evidence for axodendritic, axosomatic, and
axo-axonic synapses.
These caudal cells were the first neurosecretory cells shown to possess
the electrophysiological properties of neurons (Fig. 7). Synaptic transmission to these cells has been established (Morita et al., 1961; Ishibashi,
1962), and spikes characteristic of impulse conduction are generated. The
duration of the action potential of all caudal neurosecretory neurons so
far studied has been found to be several to 10 or 15 times longer than
that of nearby motor neurons, suggesting some particular properties of
these secreting neurons (cf. Bern and Yagi, 1965), possibly related to
sustained, nonquantal release of hormone. Teleost and elasmobranch
neurons, despite the limited axonal development of the latter, show similar electrical properties (Bennett and Fox, 1962).
The granules produced in the cell body and transported distally [toward the cerebrospinal fluid apparently by dendrites (Fridberg and
Nishioka, 1966), as well as to the neurohemal area] are membrane-limited
electron-dense bodies, varying in diameter from lo00 to 3OOO A (Figs. 2,
3, and 5 ) . Apparently these granules may coalesce into larger masses,
visible as homogeneous droplets with the light microscope. Study of terminals indicates that more than one type of secretory granule may exist in
individual fish species, suggesting the possibility of more than one hormonal product (see also Sano et al., 1966).
To date, almost nothing is known of the chemistry of the urophysial
principles, other than their presumed protein or peptide nature and the
absence of the disul61de bonds so characteristic of the hypothalamic octapeptides. It seems reasonable to propose, however, that the active hormonal principle( s) may be transported in the cell to the terminal point
of release by a protein carrier analogous to the neurophysin of the hypo-