and produces an antidiuresis. Conversely the mammalian hormones when injected
into lower vertebrates may act like vasotocin, and promote retention of water in
the amphibians, reptiles and birds.
The neurohypophysial peptide hormones are stored in greatest concentration
in the median eminence and pars nervosa (or its equivalent region in fishes), in proximity to the blood vessels supplying these regions. Contrary to earlier suggestions,
the peptides do not originate in these regions but travel down the nerve axons of
the hypothalamo-neurohypophysial tract after their formation in the regions of
the praeoptic or supraoptic and paraventricular nuclei. They are merely stored at
the termini of the axons. The elucidation of this role of the nerve cells in the formation and storage of the neurohypophysial hormones arose from careful histological observations of this tissue under a variety of physiological circumstances. Certain nerve cells, widely distributed among the invertebrates, as well as in the
neurohypophysial region of vertebrates and caudal parts of the spinal cord of some
fishes (urophysis), have been shown to contain granules, that can be stained by
methods such as the GOMORI chrome-alum haematoxylin procedure. The quantities and distribution of this stainable material can be related to the physiological
circumstances in the animal, and in the instance of the neurohypophysis with the
hormones present. The formation in nerve cells of histologically stainable material
that may be secreted into the blood stream, is termed 'neurosecretion' and was developed by ERNST and BERTA SCHARRER (1945). BARGMANN (1949) related the presence of GOMORI stainable substances in the hypothalamo-neurohypophysial region, to that of the neurohypophysial hormones. He was able to trace its formation
to the region of the supraoptic nucleus, and to follow its movement down the axons
to storage sites at the nerve termini in the pars nervosa (see BARGMANN, 1968). The
hormones are present in membrane-bounded granules attached to a protein called
neurophysin (M. W. 14000) (GINSBURG and IRELAND, 1966) and it is these granules
that constitute the stainable material. The neurosecretory cells, like ordinary nerve
cells, can transmit impulses that are involved in the release of the hormones, but
differ in that they do not enter into synaptic contact with other nerves, but release
their products directly into the circulation (for a review see BERN and KNOWLES,
1966). The hormones of the median eminence are, no doubt, also of neurosecretory
origin, but the neurons here are not readil y stainable, except in the birds (FARNER
and OKSCHE, 1962).
d) Release and Metabolism of Pituitary Hormones
The levels of the adenohypophysial and neurohypophysial hormones in the blood
are influenced by a variety of specific stimuli, that can be related to the particular
roles of the secretions in physiological homeostasis. In addition, release may also
be initiated by many stimuli (non-specific) that bear no apparent relationship to
physiological adjustments by the animal (for a recent review see GINSBURG, 1968).
Thus, the physiological significance of a relationship between a stimulus and the
release of a hormone must be interpreted with some caution. The release of pituitary hormones may be affected by changes in internal conditions such as temperature, hydrostatic pressure, and the concentrations of solutes and metabolites
56
into lower vertebrates may act like vasotocin, and promote retention of water in
the amphibians, reptiles and birds.
The neurohypophysial peptide hormones are stored in greatest concentration
in the median eminence and pars nervosa (or its equivalent region in fishes), in proximity to the blood vessels supplying these regions. Contrary to earlier suggestions,
the peptides do not originate in these regions but travel down the nerve axons of
the hypothalamo-neurohypophysial tract after their formation in the regions of
the praeoptic or supraoptic and paraventricular nuclei. They are merely stored at
the termini of the axons. The elucidation of this role of the nerve cells in the formation and storage of the neurohypophysial hormones arose from careful histological observations of this tissue under a variety of physiological circumstances. Certain nerve cells, widely distributed among the invertebrates, as well as in the
neurohypophysial region of vertebrates and caudal parts of the spinal cord of some
fishes (urophysis), have been shown to contain granules, that can be stained by
methods such as the GOMORI chrome-alum haematoxylin procedure. The quantities and distribution of this stainable material can be related to the physiological
circumstances in the animal, and in the instance of the neurohypophysis with the
hormones present. The formation in nerve cells of histologically stainable material
that may be secreted into the blood stream, is termed 'neurosecretion' and was developed by ERNST and BERTA SCHARRER (1945). BARGMANN (1949) related the presence of GOMORI stainable substances in the hypothalamo-neurohypophysial region, to that of the neurohypophysial hormones. He was able to trace its formation
to the region of the supraoptic nucleus, and to follow its movement down the axons
to storage sites at the nerve termini in the pars nervosa (see BARGMANN, 1968). The
hormones are present in membrane-bounded granules attached to a protein called
neurophysin (M. W. 14000) (GINSBURG and IRELAND, 1966) and it is these granules
that constitute the stainable material. The neurosecretory cells, like ordinary nerve
cells, can transmit impulses that are involved in the release of the hormones, but
differ in that they do not enter into synaptic contact with other nerves, but release
their products directly into the circulation (for a review see BERN and KNOWLES,
1966). The hormones of the median eminence are, no doubt, also of neurosecretory
origin, but the neurons here are not readil y stainable, except in the birds (FARNER
and OKSCHE, 1962).
d) Release and Metabolism of Pituitary Hormones
The levels of the adenohypophysial and neurohypophysial hormones in the blood
are influenced by a variety of specific stimuli, that can be related to the particular
roles of the secretions in physiological homeostasis. In addition, release may also
be initiated by many stimuli (non-specific) that bear no apparent relationship to
physiological adjustments by the animal (for a recent review see GINSBURG, 1968).
Thus, the physiological significance of a relationship between a stimulus and the
release of a hormone must be interpreted with some caution. The release of pituitary hormones may be affected by changes in internal conditions such as temperature, hydrostatic pressure, and the concentrations of solutes and metabolites
56